[ { "text": "Spin-induced localized density excitations in quantum plasmas: In this paper the dominant effect of electron inertia on the dynamics of\nlocalized density excitations is studied in a quantum plasma in the presence of\nelectron spin effects. Using the quantum magnetohydrodynamics (QMHD) model\nincluding electron tunneling and spin polarization phenomena, it is revealed\nthat the quantum effects such as plasma paramagnetism and diamagnetism play\ninevitable role on soliton existence criteria in quantum plasmas. Furthermore,\nit is shown that the magnetosonic localized density-excitation stability\ndepends strongly on the quantum system dimensionality. Two distinct region of\nsoliton stability is shown to exist depending on the value of the electron\neffective mass, where, the soliton amplitude variation with respect to the\nexternal magnetic field strength is quite opposite in these regions. Current\nfindings can be important in the study of dynamical nonlinear wave features in\ndense laboratory or inertial-confined plasmas.", "category": "physics_plasm-ph" }, { "text": "Laboratory observation of plasmoid-dominated magnetic reconnection in\n hybrid collisional-collisionless regime: Magnetic reconnection, breaking and reorganization of magnetic field\ntopology, is a fundamental process for rapid release of magnetic energy into\nplasma particles that occurs pervasively throughout the universe. In most\nnatural circumstances, the plasma properties on either side of the reconnection\nlayer are asymmetric, in particular for the collision rates that are associated\nwith a combination of density and temperature and critically determine the\nreconnection mechanism. To date, all laboratory experiments on magnetic\nreconnections have been limited to purely collisional or collisionless regimes.\nHere, we report a well-designed experimental investigation on asymmetric\nmagnetic reconnections in a novel hybrid collisional-collisionless regime by\ninteractions between laser-ablated Cu and CH plasmas. We show that the growth\nrate of the tearing instability in such a hybrid regime is still extremely\nlarge, resulting in rapid formation of multiple plasmoids, lower than that in\nthe purely collisionless regime but much higher than the collisional case. In\naddition, we, for the first time, directly observe the topology evolutions of\nthe whole process of plasmoid-dominated magnetic reconnections by using\nhighly-resolved proton radiography.", "category": "physics_plasm-ph" }, { "text": "An in-depth numerical study of exact laws for compressible Hall\n magnetohydrodynamic turbulence: Various exact laws governing compressible magnetohydrodynamic (MHD) and\nHall-MHD (CHMHD) turbulence have been derived in recent years. Other than their\nfundamental theoretical interest, these laws are generally used to estimate the\nenergy dissipation rate from spacecraft observations in order to address\ndiverse problems related, e.g., to heating of the solar wind (SW) and\nmagnetospheric plasmas. Here we use various $1024^3$ direct numerical\nsimulation (DNS) data of free-decay isothermal CHMHD turbulence obtained with\nthe GHOST code (Geophysical High-Order Suite for Turbulence) to analyze two of\nthe recently derived exact laws. The simulations reflect different intensities\nof the initial Mach number and the background magnetic field. The analysis\ndemonstrates the equivalence of the two laws in the inertial range and relates\nthe strength of the Hall effect to the amplitude of the cascade rate at sub-ion\nscales. When taken in their general form (i.e., not limited to the inertial\nrange) some subtleties regarding the validity of the stationarity assumption or\nthe absence of the forcing in the simulations are discussed. We show that the\nfree-decay nature of the turbulence induces a shift from a large scale forcing\ntowards the presence of a scale-dependent reservoir of energy fueling the\ncascade or dissipation. The reduced form of the exact laws (valid in the\ninertial range) ultimately holds even if the stationarity assumption is not\nfully verified.", "category": "physics_plasm-ph" }, { "text": "Internal disruptions and sawtooth like activity in LHD: LHD inward-shifted configurations are unstable to resistive MHD\npressure-gradient-driven modes. These modes drive sawtooth like events during\nLHD operation. In this work, we simulate sawtooth like activity and internal\ndisruptions in order to improve the understanding of these relaxation events\nand their effect over the device efficiency to confine the plasma, with the aim\nto improve the LHD present and future operation scenarios minimizing or\navoiding the disadvantageous MHD soft and hard limits. By solving a set of\nreduced non-linear resistive MHD equations, we have studied the evolution of\nperturbations to equilibria obtained before and after a sawtooth like event in\nLHD. The equilibrium $\\beta$ value is gradually increased during the simulation\nuntil it reaches the experimental value. Sawtooth like events and internal\ndisruption events take place in the simulation for $\\beta_{0}$ values between\n$1\\%$ and $1.48\\%$. The main driver of the sawtooth like events is the resonant\nand non-resonant effect of the $(n=1, m=3)$ mode. The instability is stronger\nfor resonant events, and they only appear when $\\beta_{0} = 1.48 \\%$. Internal\ndisruptions are mainly driven by the $(n=1, m=2)$ mode, and they extend\nthroughout the whole plasma core. Internal disruption events do not show up\nwhen resonant sawtooth like events are triggered.", "category": "physics_plasm-ph" }, { "text": "Viscosity and mutual diffusion in strongly asymmetric binary ionic\n mixtures: We present molecular dynamics simulation results for the viscosity and mutual\ndiffusion constant of a strongly asymmetric binary ionic mixture (BIM). We\ncompare the results with available theoretical models previously tested for\nmuch smaller asymmetries. For the case of viscosity we propose a new predictive\nframework based on the linear mixing rule, while for mutual diffusion we\ndiscuss some consistency problems of widely used Boltzmann equation based\nmodels.", "category": "physics_plasm-ph" }, { "text": "A family of Vlasov-Maxwell equilibrium distribution functions describing\n a transition from the Harris sheet to the force-free Harris sheet: We discuss a family of Vlasov-Maxwell equilibrium distribution functions for\ncurrent sheet equilibria that are intermediate cases between the Harris sheet\nand the force-free (or modified) Harris sheet. These equilibrium distribution\nfunctions have potential applications to space and astrophysical plasmas. The\nexistence of these distribution function had been briefly discussed in by\nHarrison and Neukirch 2009, but here it is shown that their approach runs into\nproblems in the limit where the guide field goes to zero. The nature of this\nproblem will be discussed and an alternative approach will be suggested that\navoids the problem. This is achieved by considering a slight variation of the\nmagnetic field profile, which allows a smooth transition between the Harris and\nforce-free Harris sheet cases.", "category": "physics_plasm-ph" }, { "text": "Magnetohydrodynamic Turbulence in the Plasmoid-Mediated Regime: Magnetohydrodynamic turbulence and magnetic reconnection are ubiquitous in\nastrophysical environments. In most situations, these processes do not occur in\nisolation, but interact with each other. This renders a comprehensive theory of\nthese processes highly challenging. Here, we propose a theory of\nmagnetohydrodynamic turbulence driven at large scale that self-consistently\naccounts for the mutual interplay with magnetic reconnection occurring at\nsmaller scales. Magnetic reconnection produces plasmoids that grow from\nturbulence-generated noise and eventually disrupt the sheet-like structures in\nwhich they are born. The disruption of these structures leads to a modification\nof the turbulent energy cascade, which, in turn, exerts a feedback effect on\nthe plasmoid formation via the turbulence-generated noise. The energy spectrum\nin this plasmoid-mediated range steepens relative to the standard inertial\nrange and does not follow a simple power law. As a result of the complex\ninterplay between turbulence and reconnection, we also find that the length\nscale which marks the beginning of the plasmoid-mediated range and the\ndissipation length scale do not obey true power laws. The transitional magnetic\nReynolds number above which the plasmoid formation becomes statistically\nsignificant enough to affect the turbulent cascade is fairly modest, implying\nthat plasmoids are expected to modify the turbulent path to dissipation in many\nastrophysical systems.", "category": "physics_plasm-ph" }, { "text": "The Electric Discharge in Superhigh Density Gas at Current Amplitude up\n to 5x10^5 A: The investigations of powerful discharge in high density hydrogen were\ncarried out on an installation with preliminary adiabatic compression. The\nexperiments were performed under the following conditions: stored energy of the\ncapacitive storage - 140-450 kJ, charging voltage - 8.0-14.0 kV, discharge\ncurrent amplitude- 200-600 kA, current rise rate - 10^9-10^10 A/s. The maximum\nparticle density nmax, achieved just before the discharge, was about 2.0x10^22\ncm-3. It was observed one or several compression of the discharge channel. The\nmoments of contractions corresponds a increase of voltage on discharge gap and\nfeature on a curve of a current. Arc parameters were calculated on the basis of\nthe channel model. It was established that increase of the initial density of\nparticles at increase of a current rate of rise results in reduction of arc\ndiameter with increase of channel temperature.", "category": "physics_plasm-ph" }, { "text": "Effects of oblique incidence and colliding pulses on laser-driven proton\n acceleration from relativistically transparent ultrathin targets: The use of ultrathin solid foils offers optimal conditions for accelerating\nprotons from laser-matter interactions. When the target is thin enough that\nrelativistic self-induced transparency (RSIT) sets in, all of the target\nelectrons get heated to high energies by the laser, which maximizes the\naccelerating electric field and therefore the final ion energy. In this work,\nwe first investigate how ion acceleration by ultraintense femtosecond laser\npulses in transparent CH$_2$ solid foils is modified when turning from normal\nto oblique ($45^\\circ$) incidence. Due to stronger electron heating, we find\nthat higher proton energies can be obtained at oblique incidence but in thinner\noptimum targets. We then show that proton acceleration can be further improved\nby splitting the laser pulse into two half-pulses focused at opposite incidence\nangles. An increase by $\\sim 30\\,\\%$ in the maximum proton energy and by a\nfactor of $\\sim 4$ in the high-energy proton charge is reported compared to the\nreference case of a single normally incident pulse.", "category": "physics_plasm-ph" }, { "text": "Nonlocal adiabatic theory. I. The action distribution function: In this paper, we address the motion of charged particles acted upon by a\nsinusoidal electrostatic wave, whose amplitude and phase velocity vary slowly\nenough in time for neo-adiabatic theory to apply. Moreover, we restrict to the\nsituation when only few separatrix crossings have occurred, so that the\nadiabatic invariant, $\\mathcal{I}$, remains nearly constant. We insist here on\nthe fact that $\\mathcal{I}$ is different from the dynamical action, $I$. In\nparticular, we show that $\\mathcal{I}$ depends on the whole time history of the\nwave variations, while the action is usually defined as a local function of the\nwave amplitude and phase velocity. Moreover, we provide several numerical\nresults showing how the action distribution function, $f(I)$, varies with time,\nand we explain how to derive it analytically. The derivation is then\ngeneralized to the situation when the wave is weakly inhomogeneous.", "category": "physics_plasm-ph" }, { "text": "Neutrino orbital angular momentum in a plasma vortex: It is shown that an electron-neutrino beam, propagating in a background\nplasma, can be decomposed into orbital momentum (OAM) states, similar to the\nOAM photon states. Coupling between different OAM neutrino states, in the\npresence of a plasma vortex, is considered. We show that plasma vorticity can\nbe transfered to the neutrino beam, which is relevant to the understanding of\nthe neutrino sources in astrophysics. Observation of neutrino OAM states could\neventually become possible.", "category": "physics_plasm-ph" }, { "text": "Simulations of electromagnetic emissions produced in a thin plasma by a\n continuously injected electron beam: In this paper, electromagnetic emissions produced in a thin beam-plasma\nsystem are studied using two-dimensional particle-in-cell simulations. For the\nfirst time, the problem of emission generation in such a system is considered\nin the realistic formulation allowing for the continuous injection of a\nrelativistic electron beam through the plasma boundary. Specific attention is\ngiven to the thin plasma case in which the transverse plasma size is comparable\nto the typical wavelength of beam-driven oscillations. Such a case is often\nimplemented in laboratory beam-plasma experiments and has a number of\npeculiarities. Emission from a thin plasma does not require intermediate\ngeneration of electromagnetic plasma eigenmodes, as in the infinite case, and\nis more similar to the regular antenna radiation. In this work, we determine\nhow efficiently the fundamental and second harmonic emissions can be generated\nin previously modulated and initially homogeneous plasmas.", "category": "physics_plasm-ph" }, { "text": "Background Field Method in Thermo Field Dynamics Theory for Wave\n Propagation in Unmagnetized Spinor QED Plasmas: In this work, a many body relativistic quantum field theory for the\ncollective modes of spinor quantum electrodynamic theory (QED) plasmas is\ndeveloped. We introduce the thermo field dynamics into the QED plasma study.\nThe nontrivial background field method is used to take account of the\nnon-perturbativity of background charged plasma particles and radiation field.\nIt is an extension of \"Furry picture\" which is first introduced by Yuan Shi, et\nal. [Phys. Rev. A 94, 012124 (2016)] in their scalar QED plasma study. However,\ntheir wave function method in evaluating the background field of ideal system\nis hard to extend to the general many body cases. We propose a classical limit\nmethod that most perturbative high energy and quantum many body aspects can be\nincluded in a practical way. As an example, the wave propagation in\nunmagnetized electron-positron plasma is discussed. In the low energy limit\ncase, the well known wave dispersion relations for non-relativistic degenerate\nplasma are recovered. In addition, mass increase of plasma particles due to the\nrelativity, effective charge decrease due to the vacuum polarization, finite\nlight velocity influence on the dispersion relation, and temperature influence\non plasma system are discussed. Besides, new phenomenons including the zero\nsound of the electron-positron pair plasma and the particle production induced\nby the plasma oscillation are first reported. At last, the high energy limit\ncase is studied.", "category": "physics_plasm-ph" }, { "text": "The interaction of intense ultrashort laser pulses with cryogenic He\n jets: We study the interaction of intense ultrashort laser pulses with cryogenic He\njets using 2d/3v relativistic Particle-in-Cell simulations (XOOPIC). Of\nparticular interest are laser intensities $(10^{15}-10^{20})$ W/cm$^2$, pulse\nlengths $\\le 100$ fs, and the frequency regime $\\sim 800$ nm for which the jets\nare initially transparent and subsequently not homogeneously ionized. Pulses\n$\\ge 10^{16}$ W/cm$^2$ are found to drive ionization along the jet and outside\nthe laser spot, the ionization-front propagates along the jet at a fraction of\nthe speed of light. Within the ionized region, there is a highly transient\nfield, which may be interpreted as two-surface wave decay and as a result of\nthe charge-neutralizing disturbance at the jet-vacuum interface. The ionized\nregion has solid-like densities and temperatures of few to hundreds of eV,\ni.e., warm and hot dense matter regimes. Such extreme conditions are relevant\nfor high-energy densities as found, e.g., in shock-wave experiments and\ninertial confinement fusion studies. The temporal evolution of the ionization\nis studied considering theoretically a pump-probe x-ray Thomson scattering\n(XRTS) scheme. We observe plasmon and non-collective modes that are generated\nin the jet, and their amplitude is proportional to the ionized volume. Our\ntheoretical findings could be tested at free-electron laser facilities such as\nFLASH and the European XFEL (Hamburg) and the LCLS (Stanford).", "category": "physics_plasm-ph" }, { "text": "Dynamic Procedure for Filtered Gyrokinetic Simulations: Large Eddy Simulations (LES) of gyrokinetic plasma turbulence are\ninvestigated as interesting candidates to decrease the computational cost. A\ndynamic procedure is implemented in the GENE code, allowing for dynamic\noptimization of the free parameters of the LES models (setting the amplitudes\nof dissipative terms). Employing such LES methods, one recovers the free energy\nand heat flux spectra obtained from highly resolved Direct Numerical\nSimulations (DNS). Systematic comparisons are performed for different values of\nthe temperature gradient and magnetic shear, parameters which are of prime\nimportance in Ion Temperature Gradient (ITG) driven turbulence. Moreover, the\ndegree of anisotropy of the problem, that can vary with parameters, can be\nadapted dynamically by the method that shows Gyrokinetic Large Eddy Simulation\n(GyroLES) to be a serious candidate to reduce numerical cost of gyrokinetic\nsolvers.", "category": "physics_plasm-ph" }, { "text": "A novel hydrogenic spectroscopic technique for inferring the role of\n plasma-molecule interaction on power and particle balance during detached\n conditions: Detachment, an important mechanism for reducing target heat deposition, is\nachieved through reductions in power, particle and momentum; which are induced\nthrough plasma-atom and plasma-molecule interactions. Experimental research in\nhow those reactions precisely contribute to detachment is limited. In this\nwork, we investigate a new spectroscopic technique to utilise Hydrogen Balmer\nline measurements to 1) disentangle the Balmer line emission from the various\nplasma-atom and plasma-molecule interactions; and 2) quantify their\ncontributions to ionisation, recombination and radiative power losses. During\ndetachment, the observed $H\\alpha$ emission often strongly increases, which\ncould be an indicator for plasma-molecule interactions involving $H_2^+$ and/or\n$H^-$. Our analysis technique quantifies the $H\\alpha$ emission due to\nplasma-molecule interactions and uses this to 1) quantify the Balmer line\nemission contribution due to $H_2^+$ and/or $H^-$; 2) subsequently estimate its\nresulting particle sinks/sources and radiative power losses. Its performance is\nverified using synthetic diagnostic techniques of both detached TCV and MAST-U\nSOLPS-ITER simulations. Experimental results of this technique on TCV data show\na bifurcation occurs between the measured total $H\\alpha$ and the atomic\nestimate of $H\\alpha$ emission, indicative of the presence of additional\n$H\\alpha$ due to plasma-molecule interactions with $H_2^+$ (and/or $H^-$). An\nexample analysis shows that the hydrogenic line series, even $Ly\\alpha$ as well\nas the medium-n Balmer lines can be significantly influenced by plasma-molecule\ninteractions by tens of percent during which significant Molecular Activated\nRecombination (MAR) is expected.", "category": "physics_plasm-ph" }, { "text": "Quantumlike description of the nonlinear and collective effects on\n relativistic electron beams in strongly magnetized plasmas: A numerical analysis of the self-interaction induced by a relativistic\nelectron/positron beam in the presence of an intense external longitudinal\nmagnetic field in plasmas is carried out. Within the context of the Plasma Wake\nField theory in the overdense regime, the transverse beam-plasma dynamics is\ndescribed by a quantumlike Zakharov system of equations in the long beam limit\nprovided by the Thermal Wave Model. In the limiting case of beam spot size much\nlarger than the plasma wavelength, the Zakharov system is reduced to a 2D\nGross-Pitaevskii-type equation, where the trap potential well is due to the\nexternal magnetic field. Vortices, \"beam halos\" and nonlinear coherent states\n(2D solitons) are predicted.", "category": "physics_plasm-ph" }, { "text": "Efficient training of artificial neural network surrogates for a\n collisional-radiative model through adaptive parameter space sampling: Reliable plasma transport modeling for magnetic confinement fusion depends on\naccurately resolving the ion charge state distribution and radiative power\nlosses of the plasma. These quantities can be obtained from solutions of a\ncollisional-radiative (CR) model at each time step within a plasma transport\nsimulation. However, even compact, approximate CR models can be computationally\nonerous to evaluate, and in-situ evaluations of these models within a coupled\nplasma transport code can lead to a rigid bottleneck. A way to bypass this\nbottleneck is to deploy artificial neural network surrogates for rapid\nevaluations of the necessary plasma quantities. However, one issue with\ntraining an accurate artificial neural network surrogate is the reliance on a\nsufficiently large and representative data set for both training and\nvalidation, which can be time-consuming to generate. In this study we further\nexplore a data-driven active learning and training scheme to allow autonomous\nadaptive sampling of the problem parameter space that ensures a sufficiently\nlarge and meaningful set of training data assembled for the surrogate training.\nOur numerical experiments show that in order to produce a comparably accurate\nCR surrogate, the proposed approach requires a total number of data samples\nthat is an order-of-magnitude smaller than a conventional approach.", "category": "physics_plasm-ph" }, { "text": "ELM control with RMP: plasma response models and the role of edge\n peeling response: Resonant magnetic perturbations (RMP) have extensively been demonstrated as a\nplausible technique for mitigating or suppressing large edge localized modes\n(ELMs). Associated with this is a substantial amount of theory and modelling\nefforts during recent years. Various models describing the plasma response to\nthe RMP fields have been proposed in the literature, and are briefly reviewed\nin this work. Despite their simplicity, linear response models can provide\nalternative criteria, than the vacuum field based criteria, for guiding the\nchoice of the coil configurations to achieve the best control of ELMs. The role\nof the edge peeling response to the RMP fields is illustrated as a key\nindicator for the ELM mitigation in low collisionality plasmas, in various\ntokamak devices.", "category": "physics_plasm-ph" }, { "text": "Quantum effects on plasma screening for thermonuclear reactions in\n laser-generated plasmas: A quantum plasma screening model based on the density matrix formalism is\nused to investigate theoretically the thermonuclear reactions\n$^{13}$C($\\alpha$, $n$)$^{16}$O and $^2$H($d$, $n$)$^3$He in laser-generated\nplasmas over a large range of densities and temperatures. For cold and dense\n(solid-state density) plasmas, our results show that quantum effects can\nenhance the plasma screening for thermonuclear reactions up to one order of\nmagnitude compared to the classical case. This result can have impact on\nnuclear astrophysics predictions, and also may play a role for fusion energy\ngain prospects. Our simulations allow us to identify the laser-generated plasma\nexperimental setting in which the quantum effects on plasma screening could be\nconfirmed at existing high-intensity laser facilities.", "category": "physics_plasm-ph" }, { "text": "Equilibrium reconstruction for Single Helical Axis reversed field pinch\n plasmas: Single Helical Axis (SHAx) configurations are emerging as the natural state\nfor high current reversed field pinch (RFP) plasmas. These states feature the\npresence of transport barriers in the core plasma. Here we present a method for\ncomputing the equilibrium magnetic surfaces for these states in the force-free\napproximation, which has been implemented in the SHEq code. The method is based\non the superposition of a zeroth order axisymmetric equilibrium and of a first\norder helical perturbation computed according to Newcomb's equation\nsupplemented with edge magnetic field measurements. The mapping of the measured\nelectron temperature profiles, soft X-ray emission and interferometric density\nmeasurements on the computed magnetic surfaces demonstrates the quality of the\nequilibrium reconstruction. The procedure for computing flux surface averages\nis illustrated, and applied to the evaluation of the thermal conductivity\nprofile. The consistency of the evaluated equilibria with Ohm's law is also\ndiscussed.", "category": "physics_plasm-ph" }, { "text": "Gyrokinetic theory of magnetic structures in high-beta plasmas of the\n Earths magnetopause and of the slow solar wind: Nonlinear effects of the trapping of resonant particles by the combined\naction of the electric field and the magnetic mirror force is studied using a\ngyrokinetic description that includes the finite Larmor radius effects. A\ngeneral nonlinear solution is found that is supported by the nonlinearity\narising from the resonant particles, trapped by the combined action of the\nparallel electric field and the magnetic mirror force. Applying these results\nto the space plasma conditions, we demonstrate that in the magnetosheath\nplasma, coherent nonlinear magnetic depression may be created associated with\nthe nonlinear mirror mode and supported by the population of trapped ions\nforming a hump in the distribution function. These objects may appear either\nisolated or as the train of weakly correlated structures (the cnoidal wave). In\nthe Solar wind and in the Earths magnetopause, characterized with anisotropic\nelectron and ion temperatures that are of the same order of magnitude, we find\ncoherent magnetic holes of the same form that are attributed to the two\nbranches of the nonlinear magnetosonic mode, the electron mirror and the field\nswelling mode, including also the kinetic Alfven mode, and supported by the\npopulation of trapped electrons. The localized magnetic holes may have the form\nof a moving oblique slab or of an ellipsoid parallel to the magnetic field and\nstrongly elongated along it, that propagates along the magnetic field and may\nbe convected in the perpendicular direction by a plasma flow. While the ion\nmirror structures are purely compressional magnetic, featuring negligible\nmagnetic torsion and electric field, the magnetosonic and kinetic Alfven\nstructures possess a finite electrostatic potential, magnetic compression, and\nmagnetic torsion, but the ratio of the perpendicular and parallel magnetic\nfields remains small.", "category": "physics_plasm-ph" }, { "text": "Electron Weibel Instability in Relativistic Counter-Streaming Plasmas\n with Flow-Aligned External Magnetic Fields: The Weibel instability driven by two symmetric counter-streaming relativistic\nelectron plasmas, also referred to as current-filamentation instability, is\nstudied in a constant and uniform external magnetic field aligned with the\nplasma flows. Both the linear and non linear stages of the instability are\ninvestigated using analytical modeling and Particle-In-Cell (PIC) simulations.\nWhile previous studies have already described the stabilizing effect of the\nmagnetic field, we show here that the saturation stage is only weakly affected.\nThe different mechanisms responsible for the saturation are discussed in detail\nin the relativistic cold fluid framework considering a single unstable mode.\nThe application of an external field leads to a slighlt increase of the\nsaturation level for large wavelengths, while it does not affect the small\nwavelengths. Multi-mode and temperature effects are then investigated. While at\nlarge temperature the saturation level is independent of the external magnetic\nfield, at small but finite temperature the competition between different modes\nin the presence of an external magnetic field leads to a saturation level lower\nwith respect to the unmagnetized case.", "category": "physics_plasm-ph" }, { "text": "Simulating the dynamics of complex plasmas: Complex plasmas are low-temperature plasmas that contain micrometer-size\nparticles in addition to the neutral gas particles and the ions and electrons\nthat make up the plasma. The microparticles interact strongly and display a\nwealth of collective effects. Here we report on linked numerical simulations\nthat reproduce many of the experimental results of complex plasmas. We model a\ncapacitively coupled plasma with a fluid code written for the commercial\npackage comsol. The output of this model is used to calculate forces on\nmicroparticles. The microparticles are modeled using the molecular dynamics\npackage lammps, which we extended to include the forces from the plasma. Using\nthis method, we are able to reproduce void formation, the separation of\nparticles of different sizes into layers, lane formation, vortex formation, and\nother effects.", "category": "physics_plasm-ph" }, { "text": "New tests for a singularity of ideal MHD: Analysis using new calculations with 3 times the resolution of the earlier\nlinked magnetic flux tubes confirms the transition from singular to saturated\ngrowth rate reported by Grauer and Marliani \\cite{GrauerMar99} for the\nincompressible cases is confirmed. However, all of the secondary tests point to\na transition back to stronger growth rate at a different location at late\ntimes. Similar problems in ideal hydrodynamics are discussed, pointing out that\ninitial negative results eventually led to better initial conditions that did\nshow evidence for a singularity of Euler. Whether singular or near-singular\ngrowth in ideal MHD is eventually shown, this study could have bearing on fast\nmagnetic reconnection, high energy particle production and coronal heating.", "category": "physics_plasm-ph" }, { "text": "On the importance of parallel magnetic-field fluctuations for\n electromagnetic instabilities in STEP: This paper discusses the importance of parallel perturbations of the\nmagnetic-field in gyrokinetic simulations of electromagnetic instabilities and\nturbulence at mid-radius in the burning plasma phase of the conceptual\nhigh-${\\beta}$, reactor-scale, tight-aspect-ratio tokamak STEP. Previous\nstudies have revealed the presence of unstable hybrid kinetic ballooning modes\n(hKBMs) and subdominant microtearing modes (MTMs) at binormal scales\napproaching the ion Larmor radius. It was found that the hKBM requires the\ninclusion of parallel magnetic-field perturbations to be linearly unstable.\nHere, the extent to which the inclusion of fluctuations in the parallel\nmagnetic-field can be relaxed is explored through gyrokinetic simulations. In\nparticular, the frequently used MHD approximation (dropping ${\\delta}\\!\nB_\\parallel$ and setting the ${\\nabla}B$ drift frequency equal to the curvature\ndrift frequency) is discussed and simulations explore whether this\napproximation is useful for modelling STEP plasmas. It is shown that the MHD\napproximation can reproduce some of the linear properties of the full STEP\ngyrokinetic system, but nonlinear simulations using the MHD approximation\nresult in very different transport states. It is demonstrated that it is\ndifficult to improve the agreement even in conditions that appear to be more\ncompatible with the assumptions that underlie the MHD approximation.\nFurthermore, it is shown that the sensitivity of STEP to\n${\\delta}B_{\\parallel}$ fluctuations is primarily because the plasma sits close\nto marginality and it is shown that in slightly more strongly driven conditions\nthe hKBM is unstable without ${\\delta}\\! B_{\\parallel}$. Crucially, it is\ndemonstrated that the state of large transport typically predicted by local\nelectromagnetic gyrokinetic simulations of STEP plasmas is not solely due to\n${\\delta}\\! B_{\\parallel}$ physics", "category": "physics_plasm-ph" }, { "text": "Dynamics of two-dimensional complex plasmas in a magnetic field: We consider a two-dimensional complex plasma layer containing charged dust\nparticles in a perpendicular magnetic field. Computer simulations of both\none-component and binary systems are used to explore the equilibrium particle\ndynamics in the fluid state. The mobility is found to scale with the inverse of\nthe magnetic field strength (Bohm diffusion) for strong fields. For bidisperse\nmixtures, the magnetic field dependence of the long-time mobility depends on\nthe particle species providing an external control of their mobility ratio. For\nlarge magnetic fields, even a two-dimensional model porous matrix can be\nrealized composed by the almost immobilized high-charge particles which act as\nobstacles for the mobile low-charge particles.", "category": "physics_plasm-ph" }, { "text": "Wakes in inhomogeneous plasmas: The Debye shielding of a charge immersed in a flowing plasma is an old\nclassic problem in plasma physics. It has been given renewed attention in the\nlast two decades in view of experiments with complex plasmas, where charged\ndust particles are often levitated in a region with strong ion flow. Efforts to\ndescribe the shielding of the dust particles in such conditions have been\nfocused on the homogeneous plasma approximation, which ignores the substantial\ninhomogeneity of the levitation region. We address the role of the plasma\ninhomogeneity by rigorously calculating the point charge potential in the\ncollisionless Bohm sheath. We demonstrate that the inhomogeneity can\ndramatically modify the wake, making it non-oscillatory and weaker.", "category": "physics_plasm-ph" }, { "text": "Production and dynamics of positrons in ultrahigh intensity laser-foil\n interactions: The electron-positron pair production accompanying interaction of a\ncircularly polarized laser pulse with a foil is studied for laser intensities\nhigher than $10^{24}$W cm$^{-2}$. The laser energy penetrates into the foil due\nto the effect of the relativistic hole-boring. It is demonstrated that the\nelectron-positron plasma is produced as a result of quantum-electrodynamical\ncascading in the field of the incident and reflected laser light in front of\nthe foil. The incident and reflected laser light makes up the circularly\npolarized standing wave in the reference frame of the hole-boring front and the\npair density peaks near the nodes and antinodes of the wave. A model based on\nthe particle dynamics with radiation reaction effect near the magnetic nodes is\ndeveloped. The model predictions are verified by 3D PIC-MC simulations.", "category": "physics_plasm-ph" }, { "text": "Dynamical equations and transport coefficients for the metals at high\n pulse electromagnetic fields: We offer a metal model suitable for the description of fast electrophysical\nprocesses in conductors under influence of powerful electronic and laser\nradiation of femto- and picosecond duration, and also high-voltage\nelectromagnetic pulses with picosecond front and duration less than 1 ns. The\nobtained dynamic equations for metal in approximation of one quasineutral\nliquid are in agreement with the equations received by other authors formerly.\nNew wide-range expressions for the electronic conduction in strong\nelectromagnetic fields are obtained and analyzed.", "category": "physics_plasm-ph" }, { "text": "Developing a Launch Package for the PEGASUS Launcher: Railguns are capable to far exceed the muzzle energies of current naval deck\nguns. Therefore one of the most promising scenario for the future application\nof railguns in naval warfare is the long range artillery. Hypervelocity\nprojectiles being propelled to velocities above 2 km/s reach targets at\ndistances of 200 km or more. At the French-German Research Institute the\nPEGASUS launcher is used for investigations with respect to this scenario. The\n6 m long barrel has a square caliber of 40 mm. The power supply unit is able to\ndeliver 10 MJ to the gun. Within this investigation, a complete launch package\nis being developed and experiments are performed that aim at showing that this\npackage can be accelerated to velocities ranging from 2000 m/s to 2500 m/s. A\nlaunch package consists out of an armature, a sabot and the projectile. The\narmature ensures the electrical contact during launch and pushes the sabot with\nits payload through the barrel. The sabot guides and protects the payload\nduring the acceleration. At the same time the accelerating forces generated at\nthe armature needs to be transferred to the projectile. After the launch\npackage has left the barrel, the sabot should open and release its payload, the\nprojectile into free-flight. Here the current status of the launch package\ndevelopment and results from experiments with the PEGASUS railgun are\npresented.", "category": "physics_plasm-ph" }, { "text": "In Search of a Data Driven Symbolic Multi-Fluid 10-Moment Model Closure: The inclusion of kinetic effects into fluid models has been a long standing\nproblem in magnetic reconnection and plasma physics. Generally the pressure\ntensor is reduced to a scalar which is an approximation used to aid in the\nmodeling of large scale global systems such as the Earth's magnetosphere. This\nunfortunately omits important kinetic physics which have been shown to play a\ncrucial role in collisionless regimes. The multi-fluid 10-moment model on the\nother-hand retains the full symmetric pressure tensor. The 10-moment model is\nconstructed by taking moments of the Vlasov equation up to second order, and\nincludes the scalar density, the vector bulk-flow, and the symmetric pressure\ntensor for a total of 10 separate components. Use of the multi-fluid 10-moment\nmodel requires a closure which truncates the cascading system of equations.\nHere we look to leverage data-driven methodologies to seek a closure which may\nimprove physical fidelity of the 10-moment multi-fluid model in collisionless\nregimes. Specifically we use the Sparse Identification of Nonlinear Dynamics\n(SINDy) method for symbolic equation discovery to seek the truncating closure\nfrom fully kinetic particle-in-cell simulation data, which inherently retains\nthe relevant kinetic physics. We verify our method by reproducing the 10-moment\nmodel from the PIC particle data and use the method to generate a closure\ntruncating the 10-moment model which is analyzed through the nonlinear phase of\nreconnection.", "category": "physics_plasm-ph" }, { "text": "Basic concept in plasma diagnostics: This paper presents the basic concept of various plasma diagnostics used for\nthe study of plasma characteristics in different plasma experiments ranging\nfrom low temperature to high energy density plasma.", "category": "physics_plasm-ph" }, { "text": "Experimental investigations of quasi-coherent micro-instabilities in\n Ohmic plasmas: The ITG and TEM instabilities with quasi-coherent spectra have been\nidentified experimentally, by the newly developed far-forward collective\nscattering measurements in J-TEXT tokamak Ohmical plasmas. The ITG mode has\ncharacteristic frequencies in the range of 30-100kHz and wavenumber of\nk_\\theta\\rho_s<0.3. After the plasma density exceeds at critical value, the ITG\nmode shows a bifurcation behavior, featured by frequency decrease and amplitude\nenhancement. Meanwhile, the ion energy loss enhancement and confinement\ndegradation are also observed. It gives the direct experimental evidence for\nion thermal transport caused by ITG instability.", "category": "physics_plasm-ph" }, { "text": "Molecular Cloud Formation Via Thermal Instability of Finite Resistive\n Viscous Radiating Plasma with Finite Larmor Radius Corrections: The effect of radiative heat-loss function and finite ion Larmor radius (FLR)\ncorrections on the thermal instability of infinite homogeneous viscous plasma\nhas been investigated incorporating the effects of thermal conductivity and\nfinite electrical resistivity for the formation of a molecular cloud. The\ngeneral dispersion relation is derived using the normal mode analysis method\nwith the help of relevant linearized perturbation equations of the problem.\nFurthermore the wave propagation along and perpendicular to the direction of\nexternal magnetic field has been discussed. Stability of the medium is\ndiscussed by applying Routh Hurwitzs criterion and it is found that thermal\ninstability criterion determines the stability of the medium. We find that the\npresence of radiative heat-loss function and thermal conductivity modify the\nfundamental criterion of thermal instability into radiatively driven thermal\ninstability criterion. In longitudinal direction FLR corrections, viscosity,\nmagnetic field and finite resistivity have no effect on thermal instability\ncriterion. The presence of radiative heat-loss function and thermal\nconductivity modify the fundamental thermal instability criterion into\nradiatively driven thermal instability criterion. Also the FLR corrections\nmodify the growth rate of the Alfven mode. For transverse wave propagation FLR\ncorrections, radiative heat-loss function, magnetic field and thermal\nconductivity modify the thermal instability criterion. From the curves it is\nclear that heat-loss function, FLR corrections and viscosity have stabilizing\neffect, while finite resistivity has destabilizing effect on the thermal modes.\nOur results show that the FLR corrections and radiative heat-loss functions\naffect the evolution of interstellar molecular clouds and star formation.", "category": "physics_plasm-ph" }, { "text": "Nonlinear dynamics of large amplitude dust acoustic shocks and solitary\n pulses in dusty plasmas: We present a fully nonlinear theory for dust acoustic (DA) shocks and DA\nsolitary pulses in a strongly coupled dusty plasma, which have been recently\nobserved experimentally by Heinrich et al. [Phys. Rev. Lett. 103, 115002\n(2009)], Teng et al. [Phys. Rev. Lett. 103, 245005 (2009)], and Bandyopadhyay\net al. [Phys. Rev. Lett. 101, 065006 (2008)]. For this purpose, we use a\ngeneralized hydrodynamic model for the strongly coupled dust grains, accounting\nfor arbitrary large amplitude dust number density compressions and potential\ndistributions associated with fully nonlinear nonstationary DA waves.\nTime-dependent numerical solutions of our nonlinear model compare favorably\nwell with the recent experimental works (mentioned above) that have reported\nthe formation of large amplitude non-stationary DA shocks and DA solitary\npulses in low-temperature dusty plasma discharges.", "category": "physics_plasm-ph" }, { "text": "Momentum Injection via Dielectric Barrier Discharge Actuators in\n Low-Speed External Flow: Dielectric barrier discharge (DBD) plasma actuators can generate a wall jet\nwithout moving parts through interaction between ionized and neutral molecules\nin an electric field. The coupling between electro-hydrodynamic, turbulence,\nand viscous effects in the flow boundary layer remains unclear and deserves\ncareful investigation. We present an experimental investigation of momentum\ninjection by DBD actuators in a U_external = 5 m/s and U_external = 11 m/s\nco-flow and counter-flow configuration over a range of VAC = 12 kV - 19.5 kV\npeak-to-peak at a frequency of 2 kHz. In the co-flow configuration, the DBD\nactuator adds momentum to the boundary layer, similar to an electrohydrodynamic\n(EHD) jet in quiescent conditions. In the counter-flow configuration, flow\nseparation is observed at free stream velocity U_external = 5 m/s. The momentum\ndisplacement in the counter-flow configuration is ~ 6x greater than EHD jet\nmomentum in a quiescent environment. Both co-flow and counter-flow momentum\ninjections show diminishing effects with increased external flow speed. This\nwork highlights that the resulting flow pattern is not a simple superposition\nof the EHD jet and the free stream but is determined by a balance between the\ninertial, viscous and Coulombic forces of the EHD and the external flow. The\nvelocity profiles and momentum characteristics can be used to validate\nnumerical models and inform the design of DBD actuators for active flow\ncontrol.", "category": "physics_plasm-ph" }, { "text": "On certain aspects of the THERMOS toolkit for modeling experiments: The THERMOS toolkit has been developed to calculate radiative properties of\nplasmas. This article contains a brief survey of some of its key features used\nby calculation of opacities and emissivities and by analysis of specific\nexperiments. The code has recently been upgraded to account for the effect of\nionization potential lowering in dense plasmas. The functionality of the code\nis illustrated for several cases from the 10th NLTE Code Comparison Workshop,\nin particular, for the experimental spectra of chlorine [1] and for the\nmeasured transmission of a silicon plasma [2].", "category": "physics_plasm-ph" }, { "text": "Interchange instability and transport in matter-antimatter plasmas: Symmetric electron-positron plasmas in inhomogeneous magnetic fields are\nintrinsically subject to interchange instability and transport. Scaling\nrelations for the propagation velocity of density blob perturbations relevant\nto transport in isothermal magnetically confined electron-positron plasmas are\ndeduced, including damping effects when Debye lengths are large compared to\nLarmor radii. The relations are verified by nonlinear full-F gyrofluid\ncomputations. Results are in favour of sufficient magnetic confinement for\nplanned electron-positron plasma experiments. The model is generalised to other\nmatter-antimatter plasmas. Magnetised electron-positron-proton-antiproton\nplasmas are susceptible to interchange driven local matter-antimatter\nseparation, which can be expected to impede (so far unrealised) sustained\nlaboratory magnetic confinement.", "category": "physics_plasm-ph" }, { "text": "Electron fishbones: Theory and experimental evidence: We discuss the processes underlying the excitation of fishbone-like internal\nkink instabilities driven by supra-thermal electrons generated experimentally\nby different means: Electron Cyclotron Resonance Heating (ECRH) and by Lower\nHybrid (LH) power injection. The peculiarity and interest of exciting these\nelectron fishbones by ECRH only or by LH only is also analyzed. Not only the\nmode stability is explained, but also the transition between steady state\nnonlinear oscillations to bursting (almost regular) pulsations, as observed in\nFTU, is interpreted in terms of the LH power input. These results are directly\nrelevant to the investigation of trapped alpha particle interactions with\nlow-frequency MHD modes in burning plasmas: in fact, alpha particles in reactor\nrelevant conditions are characterized by small dimensionless orbits, similarly\nto electrons; the trapped particle bounce averaged dynamics, meanwhile, depends\non energy and not mass.", "category": "physics_plasm-ph" }, { "text": "A Particle-In-Cell Code Comparison for Ion Acceleration: EPOCH, LSP, and\n WarpX: There are now more Particle-in-Cell (PIC) codes than ever before that\nresearchers use to simulate intense laser-plasma interactions. To date, there\nhave been relatively few direct comparisons of these codes in the literature,\nespecially for relativistic intensity lasers interacting with thin overdense\ntargets. To address this we perform a code comparison of three PIC codes:\nEPOCH, LSP, and WarpX for the problem of laser-driven ion acceleration in a\n2D(3v) geometry for a $10^{20}$ W cm$^{-2}$ intensity laser. We examine the\nplasma density, ion energy spectra, and laser-plasma coupling of the three\ncodes and find strong agreement. We also run the same simulation 20 times with\ndifferent random seeds to explore statistical fluctuations of the outputs. We\nthen compare the execution times and memory usage of the codes (without\n\"tuning\" to improve performance) using between 1 and 48 processors on one node.\nWe provide input files to encourage larger and more frequent code comparisons\nin this field.", "category": "physics_plasm-ph" }, { "text": "Particle-in-Cell Codes for plasma-based particle acceleration: Basic principles of particle-in-cell (PIC ) codes with the main application\nfor plasma-based acceleration are discussed. The ab initio full electromagnetic\nrelativistic PIC codes provide the most reliable description of plasmas. Their\nproperties are considered in detail. Representing the most fundamental model,\nthe full PIC codes are computationally expensive. The plasma-based acceler-\nation is a multi-scale problem with very disparate scales. The smallest scale\nis the laser or plasma wavelength (from one to hundred microns) and the largest\nscale is the acceleration distance (from a few centimeters to meters or even\nkilometers). The Lorentz-boost technique allows to reduce the scale disparity\nat the costs of complicating the simulations and causing unphysical numerical\ninstabilities in the code. Another possibility is to use the quasi-static\napproxi- mation where the disparate scales are separated analytically.", "category": "physics_plasm-ph" }, { "text": "Kinetic Alfv\u00e9n wave generation by velocity shear in collisionless\n plasmas: The evolution of a linearly-polarized, long-wavelength Alfv\\'en wave\n--propagating in a collisionless magnetized plasma with a sheared\nparallel-directed velocity flow-- is here studied by means of two-dimensional\nhybrid Vlasov-Maxwell (HVM) simulations. The unperturbed sheared flow has been\nrepresented by an exact solution of the HVM set of equations (Malara {\\it et\nal.}, Phys. Rev. E 97, 053212), this avoiding spurious oscillations that would\narise from the non-stationary initial state and inevitably affect the dynamics\nof the system. We have considered the evolution of both a small and a moderate\namplitude Alfv\\'en wave, in order to separate linear wave-shear flow couplings\nfrom kinetic effects, the latter being more relevant for larger wave\namplitudes. The phase-mixing generated by the shear flow modifies the initial\nperturbation, leading to the formation of small-scale transverse fluctuations\nat scales comparable with the proton inertial length. By analyzing both the\npolarization and group velocity of perturbations in the shear regions, we\nidentify them as Kinetic Alfv\\'en Waves (KAWs). In the moderate amplitude run,\nkinetic effects distort the proton distribution function in the shear region.\nThis leads to the formation of a proton beam, at the Alfv\\'en speed and\nparallel to the magnetic field. Such a feature, due to the parallel electric\nfield associated with KAWs, positively compares with solar-wind observations of\nsuprathermal ions' populations, suggesting that it may be related to the\npresence of ion-scales KAW-like fluctuations.", "category": "physics_plasm-ph" }, { "text": "Note on quantitatively correct simulations of the kinetic beam-plasma\n instability: A large number of model particles is shown necessary for quantitatively\ncorrect simulations of the kinetic beam-plasma instability with the\nclouds-in-cells method. The required number of particles scales inversely with\nthe expected growth rate, as in the kinetic regime only a narrow interval of\nbeam velocities is resonant with the wave.", "category": "physics_plasm-ph" }, { "text": "Ground state structures of superparamagnetic 2D dusty plasma crystals: Ground state structures of finite, cylindrically confined two-dimensional\nYukawa systems composed of charged superparamagnetic dust grains in an external\nmagnetic field are investigated numerically, using molecular dynamic\nsimulations and lattice summation methods. The ground state configuration of\nthe system is identified using, as an approximation, the experimentally\nobtained shape of the horizontal confinement potential in a classical single\nlayer dusty plasma experiment with non-magnetic grains. Results are presented\nfor the dependence of the number density and lattice parameters of the dust\nlayer on (1) the ratio of the magnetic dipole-dipole force to electrostatic\nforce between the grains and (2) the orientation of the grain magnetic moment\nwith respect to the layer.", "category": "physics_plasm-ph" }, { "text": "Turbulent Dissipation Challenge -- Problem Description: The goal of this document is to present a detailed description of the goals,\nsimulation setup and diagnostics for the Turbulent Dissipation Challenge\n(arXiv:1303.0204) as discussed in the Solar Heliospheric and INterplanetary\nEnvironment (SHINE) 2013 workshop, American Geophysical Union Fall Meeting 2013\nand the accompanying antenna meeting in Berkeley.", "category": "physics_plasm-ph" }, { "text": "Wave Breaking limit in Arbitrary Mass Ratio Warm Plasmas: The maximum sustainable amplitude, so-called wave breaking limit, of a\nnonlinear plasma wave in arbitrary mass ratio warm plasmas is obtained in the\nnon-relativistic regime. Using the method of Sagdeev potential a general wave\nbreaking formula is derived by taking into account the dynamics of both the\nspecies having finite temperature. It is found, that the maximum amplitude of\nthe plasma wave decreases monotonically with the increase in temperature and\nmildly increases with increase in mass ratio.", "category": "physics_plasm-ph" }, { "text": "Shaping the edge radial electric field to create shearless transport\n barriers in tokamaks: In tokamak-confined plasmas, particle transport can be reduced by modifying\nthe radial electric field. In this paper, we investigate the influence of both\na well-like and a hill-like shaped radial electric field profile on the\ncreation of shearless transport barriers (STBs) at the plasma edge, which are a\ntype of barrier that can prevent chaotic transport and are related to the\npresence of extreme values in the rotation number profile. For that, we apply\nan ExB drift model to describe test particle orbits in large aspect-ratio\ntokamaks. We show how these barriers depend on the electrostatic fluctuation\namplitudes and on the width and depth (height) of the radial electric field\nwell-like (hill-like) profile. We find that, as the depth (height) increases,\nthe STB at the plasma edge becomes more resistant to fluctuations, enabling\naccess to an improved confinement regime that prevents chaotic transport. We\nalso present parameter spaces with the radial electric field parameters,\nindicating the STB existence for several electric field configurations at the\nplasma edge, for which we obtain a fractal structure at the barrier/non-barrier\nfrontier, typical of quasi-integrable Hamiltonian systems.", "category": "physics_plasm-ph" }, { "text": "Micro-Faraday cup matrix detector for ion beam measurements in fusion\n plasmas: Atomic Beam Probe (ABP) is an extension of the routinely used Beam Emission\nSpectroscopy (BES) diagnostic for plasma edge current fluctuation measurement\nat magnetically confined plasmas. Beam atoms ionized by the plasma are directed\nto a curved trajectory by the magnetic field and may be detected close to the\nwall of the device. The arrival location and current distribution of the ions\ncarry information about the plasma current distribution, the density profile\nand the electric potential in the plasma edge. This paper describes a\nmicro-Faraday cup matrix detector for the measurement of the few microampere\nion current distribution close to the plasma edge. The device implements a\nshallow Faraday cup matrix, produced by printed-circuit board technology.\nSecondary electrons induced by the plasma radiation and the ion bombardment are\nbasically confined into the cups by the tokamak magnetic field. Additionally, a\ndouble mask is installed in the front face to limit ion influx into the cups\nand supplement secondary electron suppression. The setup was tested in detail\nusing a Lithium ion beam in the laboratory. Switching time, cross talk and\nfluctuation sensitivity test results in the lab setup are presented, along with\nthe detector setup to be installed at the COMPASS tokamak.", "category": "physics_plasm-ph" }, { "text": "Lagrangian coherent structures and plasma transport processes: A dynamical system framework is used to describe transport processes in\nplasmas embedded in a magnetic field. For periodic systems with one degree of\nfreedom the Poincar\\'e map provides a splitting of the phase space into regions\nwhere particles have different kinds of motion: periodic, quasi-periodic or\nchaotic. The boundaries of these regions are transport barriers; i.e., a\ntrajectory cannot cross such boundaries during the whole evolution of the\nsystem. Lagrangian Coherent Structure (LCS) generalize this method to systems\nwith the most general time dependence, splitting the phase space into regions\nwith different qualitative behaviours. This leads to the definition of\nfinite-time transport barriers, i.e. trajectories cannot cross the barrier for\na finite amount of time. This methodology can be used to identify fast\nrecirculating regions in the dynamical system and to characterize the transport\nbetween them.", "category": "physics_plasm-ph" }, { "text": "Magnetic reconnection with null and X-points: Null and X-points are not themselves directly important to magnetic\nreconnection because distinguishable field lines do not approach them closely.\nEven in a collision-free plasma, magnetic field lines that approach each other\non a scale $c/\\omega_{pe}$ become indistinguishable during an evolution. What\nis important is the different regions of space that can be explored by magnetic\nfield lines that pass in the vicinity of null and X-points. Traditional\nreconnection theories made the assumption that the reconnected magnetic flux\nmust be dissipated or diffused by an electric field. This assumption is false\nin three dimensional systems because an ideal evolution can cause magnetic\nfield lines that cover a large volume to approach each other within the\nindistinguishability scale $c/\\omega_{pe}$. When the electron collision time\n$\\tau_{ei}$ is short compared to the evolution time of the magnetic field\n$\\tau_{ev}$, the importance of $c/\\omega_{pe}$ is replaced by the resistive\ntime scale $\\tau_\\eta=(\\eta/\\mu_0)L^2$ with $L$ the system scale. The magnetic\nReynolds number is $R_m\\equiv\\tau_\\eta/\\tau_{ev}$ is enormous in many\nreconnection problems of interest. Magnetic flux diffusion implies the current\ndensity required for reconnection to compete with evolution scales as $R_m$\nwhile flux mixing implies the required current density to compete scales as\n$\\ln R_m$.", "category": "physics_plasm-ph" }, { "text": "A phase-shift-periodic parallel boundary condition for\n low-magnetic-shear scenarios: We formulate a generalized periodic boundary condition as a limit of the\nstandard twist-and-shift parallel boundary condition that is suitable for\nsimulations of plasmas with low magnetic shear. This is done by applying a\nphase shift in the binormal direction when crossing the parallel boundary.\nWhile this phase shift can be set to zero without loss of generality in the\nlocal flux-tube limit when employing the twist-and-shift boundary condition, we\nshow that this is not the most general case when employing periodic parallel\nboundaries, and may not even be the most desirable. A non-zero phase shift can\nbe used to avoid the convective cells that plague simulations of the\nthree-dimensional Hasegawa-Wakatani system, and is shown to have measurable\neffects in periodic low-magnetic-shear gyrokinetic simulations. We propose a\nnumerical program where a sampling of periodic simulations at random\npseudo-irrational flux surfaces are used to determine physical observables in a\nstatistical sense. This approach can serve as an alternative to applying the\ntwist-and-shift boundary condition to low-magnetic-shear scenarios which, while\nmore straightforward, can be computationally demanding.", "category": "physics_plasm-ph" }, { "text": "Radiation and Heat Transport in Divergent Shock-Bubble Interactions: Shock-bubble interactions (SBI) are important across a wide range of physical\nsystems. In inertial confinement fusion, interactions between laser-driven\nshocks and micro-voids in both ablators and foam targets generate instabilities\nthat are a major obstacle in achieving ignition. Experiments imaging the\ncollapse of such voids at high energy densities (HED) are constrained by\nspatial and temporal resolution, making simulations a vital tool in\nunderstanding these systems. In this study, we benchmark several radiation and\nthermal transport models in the xRAGE hydrodynamic code against experimental\nimages of a collapsing mesoscale void during the passage of a 300 GPa shock. We\nalso quantitatively examine the role of transport physics in the evolution of\nthe SBI. This allows us to understand the dynamics of the interaction at\ntimescales shorter than experimental imaging framerates. We find that all\nradiation models examined reproduce empirical shock velocities within\nexperimental error. Radiation transport is found to reduce shock pressures by\nproviding an additional energy pathway in the ablation region, but this effect\nis small ($\\sim$1\\% of total shock pressure). Employing a flux-limited Spitzer\nmodel for heat conduction, we find that flux limiters between 0.03 and 0.10\nproduce agreement with experimental velocities, suggesting that the system is\nwell-within the Spitzer regime. Higher heat conduction is found to lower\ntemperatures in the ablated plasma and to prevent secondary shocks at the\nablation front, resulting in weaker primary shocks. Finally, we confirm that\nthe SBI-driven instabilities observed in the HED regime are baroclinically\ndriven, as in the low energy case.", "category": "physics_plasm-ph" }, { "text": "Manipulation of $\u03b3$ ray polarization in Compton scattering: High-brilliance high-polarization $\\gamma$ rays based on Compton scattering\nare of great significance in broad areas, such as nuclear, high-energy,\nastro-physics, etc. However, the transfer mechanism of spin angular momentum in\nthe transition from linear, through weakly into strongly nonlinear processes is\nstill unclear, which severely limits the simultaneous control of brilliance and\npolarization of high-energy $\\gamma$ rays. In this work, we investigate the\nmanipulation mechanism of high-quality polarized $\\gamma$ rays in Compton\nscattering of the ultrarelativistic electron beam colliding with an intense\nlaser pulse. We find that the contradiction lies in the simultaneous\nachievement of high-brilliance and high-polarization of $\\gamma$ rays by\nincreasing laser intensity, since the polarization is predominately contributed\nby the electron (laser photon) spin via multi-photon (single-photon) absorption\nchannel. Moreover, we confirm that the signature of $\\gamma$-ray polarization\ncan be applied for observing the nonlinear effects (multi-photon absorption) of\nCompton scattering with moderate-intensity laser facilities.", "category": "physics_plasm-ph" }, { "text": "Variational formulation of classical and quantum models for intense\n laser pulse propagation: We consider the theoretical description of intense laser pulses propagating\nthrough gases. Starting from a first-principles description of both the\nelectromagnetic field and the electron motion within the gas atoms, we derive a\nhierarchy of reduced models. We obtain a parallel set of models, where the\natomic electrons are treated classically on the one hand, and\nquantum-mechanically on the other. By working consistently in either a\nLagrangian formulation or a Hamiltonian formulation, we ensure that our reduced\nmodels preserve the variational structure of the parent models. Taking\nadvantage of the Hamiltonian formulation, we deduce a number of conserved\nquantities of the reduced models.", "category": "physics_plasm-ph" }, { "text": "New experimental findings of non-local transport in J-TEXT plasmas: In the cold pulse experiments in J-TEXT, not only are the rapid electron\ntemperature increases in core observed, but also the steep rises of inner\ndensity are found. Moreover, the core toroidal rotation is also accelerated\nduring the non-local transport process of electron temperature. These new\nfindings of cold pulse experiments in J-TEXT reveal that turbulence spreading\nis the possible mechanism for the non-local transport dynamics.", "category": "physics_plasm-ph" }, { "text": "Computation of the spectrum of spatial Lyapunov exponents for the\n spatially extended beam-plasma systems and electron-wave devices: The spectrum of Lyapunov exponents is powerful tool for the analysis of the\ncomplex system dynamics. In the general framework of nonlinear dynamical\nsystems a number of the numerical technics have been developed to obtain the\nspectrum of Lyapunov exponents for the complex temporal behavior of the systems\nwith a few degree of freedom. Unfortunately, these methods can not apply\ndirectly to analysis of complex spatio-temporal dynamics in plasma devices\nwhich are characterized by the infinite phase space, since they are the\nspatially extended active media. In the present paper we propose the method for\nthe calculation of the spectrum of the spatial Lyapunov exponents (SLEs) for\nthe spatially extended beam-plasma systems. The calculation technique is\napplied to the analysis of chaotic spatio-temporal oscillations in three\ndifferent beam-plasma model: (1) simple plasma Pierce diode, (2) coupled Pierce\ndiodes, and (3) electron-wave system with backward electromagnetic wave. We\nfind an excellent agreement between the system dynamics and the behavior of the\nspectrum of the spatial Lyapunov exponents. Along with the proposed method, the\npossible problems of SLEs calculation are also discussed. It is shown that for\nthe wide class of the spatially extended systems the set of quantities included\nin the system state for SLEs calculation can be reduced using the appropriate\nfeature of the plasma systems.", "category": "physics_plasm-ph" }, { "text": "Observation of the ion-ion instability and its suppression mechanism in\n a dusty double plasma device: Ion-ion instability is excited due to the counter streaming of ion beams in\nany plasma system. An experiment has been carried out to observe the ion-ion\ninstability in dusty plasma and in presence of different gas compositions viz.,\nH2, Ar, Xe and He. The experiment is performed in a double plasma device\nconsisting of 90 cm in diameter and 120 cm in length and separated by a mesh\ngrid of 81% optical transparency. Glass beads of 10 mm average dust\ndistributions are used as a dust grains for the whole set of experiment and are\nallowed to fall within a particular region of plasma column inside the system.\nThe growing and suppression of ion-ion instability is observed by translating\nthe probe spatially with respect to different dust density compositions and\ncompared the same also in presence of other gasses. The suppression mechanism\nof this kind of instability in plasmas was explained by RF pumps[1]. However,\nin this experimental finding the suppression phenomena are discussed in terms\nof dust density inside the system. The instability is found to be suppressed\ncompletely at a critical dust density (Ndcr). The complete set of experimental\nresults will be tried to explain with the numerical solution of Van der Pol\nEquation.", "category": "physics_plasm-ph" }, { "text": "Characteristics of X-point Lobe Structures in Single-Null Discharges on\n MAST: Lobe structures due to the application of resonant magnetic perturbations\n(RMPs) have been observed using wide-angle imaging of light from He1+ ions in\nthe vicinity of the lower X-point in MAST. The data presented are from lower\nsingle-null discharges where RMPs of toroidal mode number, n, of 4 and 6 were\napplied. It has been found that, above a threshold value, the lobe structures\nextend radially, linearly with the coil current, both in L-mode and H-mode. It\nis observed that after the application of the RMP, as the toroidal rotation in\nthe confined plasma decreases, the lobes extend radially, suggesting the plasma\nis less effectively screening the RMP field. Comparing the imaging data with\nresults from vacuum modelling shows that this technique can accurately predict\nthe number and poloidal location of the lobes, but over-estimates their radial\nextent. More accurate estimates of the extent of the lobes can be made by\naccounting for plasma screening of the RMP field. Qualitative agreement between\nsimulation and experiment is found if it is assumed that the RMP penetrates 2%\nin normalised radius from the last closed flux surface.", "category": "physics_plasm-ph" }, { "text": "Coupling of alpha-channeling to parallel wavenumber upshift in lower\n hybrid current drive: Although lower hybrid waves have been shown to be effective in driving plasma\ncurrent in present-day tokamaks, they are predicted to strongly interact with\nthe energetic alpha particles born from fusion reactions in eventual tokamak\nreactors. However, in the presence of the expected steep alpha particle birth\ngradient, this interaction can produce wave amplification rather than wave\ndamping. Here, we identify the flexibilities and constraints in achieving this\namplification effect through a consideration of symmetries in the channeling\ninteraction, in the wave propagation, and in the tokamak field configuration.\nInterestingly, for standard LH current drive that supports the poloidal\nmagnetic field, we find that wave amplification through alpha channeling is\nfundamentally coupled to the poorly understood parallel wavenumber upshift. In\nso doing, we show that wave launch from the tokamak high-field side is\nfavorable both for alpha-channeling and for achieving the parallel wavenumber\nupshift.", "category": "physics_plasm-ph" }, { "text": "Chirped standing wave acceleration of ions with intense lasers: We propose a novel mechanism for ion acceleration based on the guided motion\nof electrons from a thin target. The electron motion is locked to the moving\nnodes of a standing wave formed by a chirped laser pulse reflected from a\nmirror behind the target. This provides a stable longitudinal field of charge\nseparation, thus giving rise to chirped standing wave acceleration (CSWA) of\nthe residual ions of the layer. We demonstrate, both analytically and\nnumerically, that quasi-monoenergetic ion beams with energies of the order 100\nMeV are feasible for realistic pulse energies of 10 J. Moreover, a scaling law\nfor higher laser intensities and layer densities is presented, indicating\nstable GeV-level energy gains of dense ion bunches, for soon-to-be available\nlaser intensities.", "category": "physics_plasm-ph" }, { "text": "Modulation instability of obliquely propagating ion acoustic waves in a\n collisionless magnetized plasma consisting of nonthermal and isothermal\n electrons: We have studied the modulation instability of obliquely propagating ion\nacoustic waves in a collisionless magnetized warm plasma consisting of warm\nadiabatic ions and two different species of electrons at different\ntemperatures. We have derived a nonlinear Schr{\\\"o}dinger equation using the\nstandard reductive perturbation method to describe the nonlinear amplitude\nmodulation of ion acoustic wave satisfying the dispersion relation of ion\nacoustic wave propagating at an arbitrary angle to the direction of the\nexternal uniform static magnetic field. We have investigated the correspondence\nbetween two nonlinear Schr{\\\"o}dinger equations $-$ one describes the amplitude\nmodulation of ion acoustic waves propagating along any arbitrary direction to\nthe direction of the magnetic field and other describes the amplitude\nmodulation of ion acoustic waves propagating along the direction of the\nmagnetic field. We have derived the instability condition and the maximum\ngrowth rate of instability of the modulated ion acoustic wave. We have seen\nthat the region of existence of maximum growth rate of instability decreases\nwith increasing values of the magnetic field intensity whereas the region of\nexistence of the maximum growth rate of instability increases with increasing\n$\\cos \\theta$, where $\\theta$ is the angle of propagation of the ion acoustic\nwave with the external uniform static magnetic field. Again, the maximum growth\nrate of instability increases with increasing $\\cos \\theta$ and also this\nmaximum growth rate of instability increases with increasing $\\beta_{e}$ upto a\ncritical value of the wave number, where $\\beta_{e}$ is the parameter\nassociated with the nonthermal distribution of hotter electron species.", "category": "physics_plasm-ph" }, { "text": "Investigating overtaking collisions of solitary waves in the Schamel\n equation: This article presents a numerical investigation of overtaking collisions\nbetween two solitary waves in the context of the Schamel equation. Our study\nreveals different regimes characterized by the behavior of the wave\ninteractions. In certain regimes, the collisions maintain two well-separated\ncrests consistently over time, while in other regimes, the number of local\nmaxima undergoes variations following the patterns of $2\\rightarrow\n1\\rightarrow 2\\rightarrow 1\\rightarrow 2$ or $2\\rightarrow 1\\rightarrow 2$.\nThese findings demonstrate that the geometric Lax-categorization observed in\nthe Korteweg-de Vries equation (KdV) for two-soliton collisions remains\napplicable to the Schamel equation. However, in contrast to the KdV, we\ndemonstrate that an algebraic Lax-categorization based on the ratio of the\ninitial solitary wave amplitudes is not feasible for the Schamel equation.\nAdditionally, we show that the statistical moments for two-solitary wave\ncollisions are qualitatively similar to the KdV equation and the phase shifts\nafter soliton interactions are close to ones in integrable KdV and modified KdV\nmodels.", "category": "physics_plasm-ph" }, { "text": "The ion motion in self-modulated plasma wakefield accelerators: The effects of plasma ion motion in self-modulated plasma based accelerators\nis examined. An analytical model describing ion motion in the narrow beam limit\nis developed, and confirmed through multi-dimensional particle-in-cell\nsimulations. It is shown that the ion motion can lead to the early saturation\nof the self-modulation instability, and to the suppression of the accelerating\ngradients. This can reduce the total energy that can be transformed into\nkinetic energy of accelerated particles. For the parameters of future\nproton-driven plasma accelerator experiments, the ion dynamics can have a\nstrong impact. Possible methods to mitigate the effects of the ion motion in\nfuture experiments are demonstrated.", "category": "physics_plasm-ph" }, { "text": "Coupling Effects in Multi-Stage Laser Wake-field Acceleration of\n Electrons: Staging laser wake-field acceleration is considered as a necessary technique\nfor developing full-optical jitter-free electron accelerators. Splitting of the\nacceleration length into several technical parts with their lengths smaller\nthan the dephasing length and with independent laser drivers allows generation\nof stable, reproducible acceleration fields. Temporal and spatial coupling of\npre-accelerated electron bunches for their injection in the acceleration phase\nof a successive laser pulse wake field is the key part of the staging\nlaser-driven acceleration. Here, characterization of the coupling is performed\nwith dense, stable, a narrow energy band <3% and energy selectable electron\nbeams with charges ~1.6 pC and energy ~10 MeV generated from a laser plasma\ncathode. Cumulative focusing of electron bunches in a low density pre-plasma,\nexhibiting the Budker- Bennett effect, is shown to result in the efficient\ninjection of electrons even with a long distance between the injector and the\nbooster in the laser pulse wake. Measured characteristics of electron beams\nmodified by the booster wake field agree well with those obtained by\nmultidimensional particle-in-cell simulations.", "category": "physics_plasm-ph" }, { "text": "Relating the Phases of Magnetic Reconnection Growth to the Temporal\n Evolution of X-line Structures in a Collisionless Plasma: The efficiency of energy conversion during magnetic reconnection is related\nto the reconnection rate. While the stable reconnection rate has been studied\nextensively, its growth between the time of reconnection onset and the peak\nreconnection rate has not been thoroughly discussed. We use a 2D\nparticle-in-cell (PIC) simulation to examine how the non-ideal reconnection\nelectric field evolves during the growth process and how it relates to changes\nnear the x-line. We identify three phases of growth: 1) slow quasi-linear\ngrowth, 2) rapid exponential growth, and 3) tapered growth followed by negative\ngrowth after the reconnection rate peaks. Through analysis of the structural\nchanges of the EDR, we associate the early phases with the breaking of x-line\nsymmetry through the erosion of the pre-onset bipolar Ez and the emergence of a\ndiverging Ex pattern at the neutral line in phase 1 followed by the expansion\nof the inflow region and the enhancement of inflow Poynting flux Sz associated\nwith the out-of-plane electric field Ey in phase 2. We show how the Hall fields\nfacilitate rapid growth in phase 2 by opening up the exhaust, relieving the\nelectron-scale bottleneck and allowing large Poynting flux across the\nseparatrices. We find that the rapid inflow of electromagnetic energy\naccumulates until the downstream electromagnetic energy density in phase 3\napproaches the initial upstream asymptotic value. Finally, we examine how the\nelectron outflow and the downstream ion populations interact in phase 3 and how\neach species exchanges energy with the local field structures in the exhaust.", "category": "physics_plasm-ph" }, { "text": "Multi-ion-species effects on magnetosonic waves and energy transfer in\n thermal equilibrium plasmas: Magnetosonic waves propagating perpendicular to an external magnetic field\nare studied with attention to the effect of multiple ion species. First, power\nspectra of magnetic field fluctuations and autocorrelation functions in thermal\nequilibrium plasmas are numerically obtained. In a multi-ion-species plasma,\nbesides $\\omega \\simeq kv_{\\rm A}$ mode, numerous waves are present near many\ndifferent ion cyclotron frequencies. The autocorrelation function of the\nquasi-mode consisting of these waves is not recovered to its initial value,\nowing to the phase mixing of these waves. Next, with particle simulations,\nevolution of a macroscopic perpendicular disturbance is investigated. In a\nmulti-ion-species plasma, this disturbance is damped. The energy is transferred\nto from the magnetic field to the ions.", "category": "physics_plasm-ph" }, { "text": "Numerical equilibria with pressure anisotropy and incompressible plasma\n rotation parallel to the magnetic field: It is believed that plasma rotation can affect the transitions to the\nadvanced confinement regimes in tokamaks. In addition, in order to achieve\nfusion temperatures modern tokamaks rely on auxiliary heating methods. These\nmethods generate pressure anisotropy in the plasma. For incompressible rotation\nwith pressure anisotropy the equilibrium is governed by a Generalized\nGrad-Shafranov (GGS) equation and a decoupled Bernoulli-type equation for the\neffective pressure, $\\bar{p}=(p_\\parallel+p_\\perp)/2$, where $p_\\parallel$\n($p_\\perp$) is the pressure tensor element parallel (perpendicular) to the\nmagnetic field. In the case of plasma rotation parallel to the magnetic field\nthe GGS equation can be transformed to one equation identical in form with the\nGS equation. In this study by making use of the aforementioned property of the\nGGS equation for parallel plasma rotation we have constructed ITER-like\nnumerical equilibria by extending HELENA, an equilibrium fixed-boundary solver\nand examined the impact of rotation and anisotropy on certain equilibrium\nquantities. The main conclusions are that the addition of pressure anisotropy\nto rotation allows the profile shaping of the equilibrium quantities in much\nmore extent thus favouring the confinement and allows extension of the\nparametric space of the Mach number corresponding to higher values.\nFurthermore, the impact of pressure anisotropy in the equilibrium quantities is\nstronger than that of the rotation, for most of the quantities examined. For\nthe pressure components the impact of the pressure anisotropy is the same\nregardless of whether the power is deposited parallel or perpendicular to the\nmagnetic surfaces, thus implying that there is no preferable heating direction,\nwhile for the current density, the heating parallel to the magnetic surfaces\nseems to be beneficial for the current-gradient driven instabilities.", "category": "physics_plasm-ph" }, { "text": "Islands and current singularities in quasisymmetric toroidal plasmas: The presence of current singularities in a quasisymmetric magnetic field is\nexplored. Quasisymmetry is shown effective in isolating Pfirsch-Schl\\\"{u}ter\nsingularities, to leading order, to a single magnetic surface resonant with the\nhelicity of the symmetry. The effects of pressure driven currents are analysed,\nindicating that exclusion of this surface from the plasma volume reduces the\npotential opening of islands, but does not generally eliminate them completely\ndue to higher order asymmetric geometric effects. These three-dimensional\neffects are contained in quasisymmetry and indicate the complexity of finding\nconsistent solutions and their potential sensitivity. The $\\delta$-function\ncurrent singularities show a distinct quasisymmetric behaviour only when the\nhigher-order Fourier content of $B$ is relevant for the resistive stability\nparameter $D_R$ (not included in leading-order near-axis expansions). In such\nscenarios, quasisymmetry proves advantageous, both in simplicity and avoidance\nof amplification by low-order rational surfaces.", "category": "physics_plasm-ph" }, { "text": "Optimization of Tritium Breeding Ratio in a DT and DD Submersion Tokamak\n Fusion Reactor: The mass of stars is enough to confine a plasma to fuse light atoms, but this\nis not possible to engineer on Earth. Fortunately, nuclear engineering can rely\non the magnetic confinement of a plasma using superconducting coils so long as\nthe Tritium Breeding Ratio (TBR) is optimized. This paper will investigate some\nof the materials which can increase the rate at which Tritium is produced\nwithin the breeding blanket layer of Submersion Tokamak reactors, a design that\nuses magnetic confinement of a plasma in the shape of a torus to execute\nnuclear fusion. Using the Paramak Python module to model several geometries and\nOpenMC to run a simulation, it can be observed how neutron multipliers,\nenrichment, and the neutron energy spectrum affect TBR. This experiment will\nmainly observe different material choices that have been considered and their\nTBR based on their cross sections, dose rate, thermal properties and safety. By\naltering the neutron energy spectrum to account for DD and DT plasma, the\ndifference in these compounds' Tritium breeding efficacy is noted. Neutron\nenergy spectra are an important factor in optimising the TBR levels as the\nneutrons generated by the fusion reactions in the plasma interact with the\nbreeder material in the blanket and produce tritium through the reaction with\nLithium. Since Tritium is a rare isotope of hydrogen that is used as fuel in\nfusion reactions and has a short half-life, it is essential to produce tritium\nwithin the fusion reactor itself. Without the tritium breeding capability, it\nwould not be feasible to generate energy via fusion. A TBR greater than unity\nindicates that the reactor can generate more tritium than it consumes, ensuring\nself-sufficiency in the tritium inventory. Since Tritium is the most reliable\nand efficient fuel for these reactors, optimising the TBR is of paramount\nimportance in the long road to commercialization of nuclear fusion.", "category": "physics_plasm-ph" }, { "text": "Strongly coupled Yukawa trilayer liquid: Structure and dynamics: The equilibrium structure and the dispersion relations of collective\nexcitations in trilayer Yukawa systems in the strongly coupled liquid regime\nare examined. The equilibrium correlations reveal a variety of structures in\nthe liquid phase, reminiscent of the corresponding structures in the solid\nphase. At small layer separation substitutional disorder becomes the governing\nfeature. Theoretical dispersion relations are obtained by applying the\nQuasi-Localised Charge Approximation (QLCA) formalism, while numerical data are\ngenerated by micro-canonical molecular dynamics (MD) simulations. The\ndispersions and polarizations of the collective excitations obtained through\nboth of these methods are compared and discussed in detail. We find that the\nQLCA method is, in general, very satisfactory, but that there are phenomena not\ncovered by the QLCA. In particular, by analyzing the dynamical longitudinal and\ntransverse current fluctuation spectra we discover the existence of a novel\nstructure, not related to the collective mode spectra. This also provides a new\ninsight into the long-standing problem of the gap frequency discrepancy,\nobserved in strongly coupled layered systems in earlier studies.", "category": "physics_plasm-ph" }, { "text": "Dynamics of nanosecond laser pulse propagation and of associated\n instabilities in a magnetized underdense plasma: The propagation and energy coupling of intense laser beams in plasmas are\ncritical issues in laser-driven inertial confinement fusion. Applying magnetic\nfields to such a setup has been evoked to enhance fuel confinement and heating,\nand mitigate laser energy losses. Here we report on experimental measurements\ndemonstrating improved transmission and increased smoothing of a high-power\nlaser beam propagating in an underdense magnetized plasma. We also measure\nenhanced backscattering, which our simulations show is due to hot electrons\nconfinement, thus leading to reduced target preheating.", "category": "physics_plasm-ph" }, { "text": "The slow relaxation of non-equilibrium state in metal target excited by\n picosecond electron beam: interferometric and simulation studies: The experimental EXCITOR setup for obtaining the intense electron beams of\npicosecond duration is presented in this work. Cathodes of tungsten, graphite,\ncopper and samarium of the same shape were used in these experiments. The\ninterelectrode gap varied from 0.5 to 6 mm. Interferometric technique to detect\nanode rear side displacement was used. The time evolution of the displacement\nand speed of the rear side of copper anode of 0.1 and 3.0 mm thickness was\nexperimentally obtained. Simulation of the space-time characteristics evolution\nof the anode material is presented.", "category": "physics_plasm-ph" }, { "text": "Landau damping of electron-acoustic waves due to multi-plasmon\n resonances: The linear and nonlinear theories of electron-acoustic waves (EAWs) are\nstudied in a partially degenerate quantum plasma with two-temperature electrons\nand stationary ions. The initial equilibrium of electrons is assumed to be\ngiven by the Fermi-Dirac distribution at finite temperature. By employing the\nmulti-scale asymptotic expansion technique to the one-dimensional Wigner-Moyal\nand Poisson equations, it is shown that the effects of multi-plasmon resonances\nlead to a modified complex Korteweg-de Vries (KdV) equation with a new nonlocal\nnonlinearity. Besides giving rise to a nonlocal nonlinear term, the\nwave-particle resonance also modifies the local nonlinear coupling coefficient\nof the KdV equation. The latter is shown to conserve the number of particles,\nhowever, the wave energy decays with time. A careful analysis shows that the\ntwo-plasmon resonance is the dominant mechanism for nonlinear Landau damping of\nEAWs. An approximate soliton solution of the KdV equation is also obtained, and\nit is shown that the nonlinear Landau damping causes the wave amplitude to\ndecay slowly with time compared to the classical theory.", "category": "physics_plasm-ph" }, { "text": "Ion-acoustic rogue waves in multi-ion plasmas: The basic properties of nonlinear ion-acoustic (IA) waves (IAWs),\nparticularly finite amplitude IA rogue waves (IARWs) in a plasma medium\n(containing pair ions, iso-thermal positrons and non-thermal electrons) are\ntheoretically investigated by deriving the nonlinear Schr\\\"{o}dinger equation\n(NLSE). The criteria for the modulational instability of IAWs, and the basic\nfeatures of finite amplitude IARWs are identified. The modulationally stable\nand unstable regions are determined by the sign of the ratio of the dispersive\ncoefficient to the nonlinear coefficient of NLSE. The latter is analyzed to\nobtain the region for the existence of the IARWs, which corresponds to the\nunstable region. The shape of the profile of the rogue waves depends on the\nnon-thermal parameter $\\alpha$ and the ratio of electron temperature to\npositron temperature. It is found that the increase in the value of the\nnon-thermal parameter enhances both the amplitude and width of IARWs, and that\nthe enhancement of positron (electron) temperature reduces (enhances) the\namplitude and width of IARWs. It is worth to mention that our present\ninvestigation may be useful for understanding the salient features of IARWs in\nspace (viz., upper region of Titan's atmosphere, cometary comae, and Earth's\nionosphere, etc.) and laboratory (viz., plasma processing reactor and neutral\nbeam sources, etc.) plasmas.", "category": "physics_plasm-ph" }, { "text": "Acoustic $\u03b1$-disk: It is shown that the turbulent flow of acoustic waves propagating outward\nfrom the inner edge of the disk causes the accretion of the matter onto the\ncenter. The exponential amplification of waves takes place in the resonance\nregion, $ \\omega = (n\\pm 1)\\Omega $. Here $ \\omega $ is the frequency of the\nacoustic wave, $ n $ is its azimuthal wave number, $ \\Omega (r) $ is the\nangular frequency of rotation of the disk. The effect is similar to the inverse\nLandau damping in a collisionless plasma. Energy comes from the energy of\nrotation of the disk. That leads to decrease of the disk angular momentum and\nto accretion of the matter. The value of the accretion rate $dM/dt$ is ${\\dot\nM} = \\pi rc_s \\Sigma_0 (c_s / v_{\\phi 0})^2 W $. Here $ c_s $ is the speed of\nsound of the disk gas, $ v_{\\phi 0} $ is the Keplerian rotation velocity,\n$\\Sigma_0 $ is the surface density of the disk, $ W $ is total power of the\nacoustic turbulence, $ W \\simeq \\int_0^\\infty d \\omega \\sum_{n\\geq 0} \\Big {|}\n\\frac {\\Sigma'} {\\Sigma_0} \\Big {|}^2 (\\omega, n) $, $ |\\Sigma'|^2 (\\omega, n)\n$ is the spectral power of turbulence. The presented picture of accretion is\nconsistent with the observed variations of X-ray and optical radiation from\nobjects whose activity is associated with accretion of gas onto them.", "category": "physics_plasm-ph" }, { "text": "Testing the conservative character of particle simulations: II. Spurious\n heating of guiding centers and full orbits subject to fluctuations expressed\n in terms of ${\\bf E}$ and ${\\bf B}$: For an axisymmetric tokamak plasma, Hamiltonian theory predicts that the\norbits of charged particles must stay on invariant tori of conserved energy in\nthe moving frame of reference of a wave that propagates along the torus with a\nfixed angular phase velocity. In principle, this is true for arbitrary mode\nstructures in the poloidal plane, but only if the fluctuations are expressed in\nterms of potentials $\\Phi$ and ${\\bf A}$, which satisfy Faraday's law by\ndefinition. Here, we use the physical fields ${\\bf E}$ and ${\\bf B}$, where\nFaraday's law may be violated by errors introduced during the process of\ncomputing or designing the wave field through numerical inaccuracies,\napproximations, or gross negligence. Numerical heating caused by noise-like\nartifacts on the grid scale can to some extent be reduced via shorter time\nsteps. In contrast, coherent inconsistencies between ${\\bf E}$ and ${\\bf B}$\ncause spurious acceleration that is independent of time steps or numerical\nmethods, but can be sensitive to geometry. In particular, we show that secular\nacceleration is enhanced when one imposes nonnormal modes that possess strong\nup-down asymmetry instead of the usual in-out asymmetry of normal toroidal\n(eigen)modes. Our tests are performed for full gyroorbit and guiding center\n(GC) models, which give similar results. In addition, we show that $N$-point\ngyroaveraging is not a recommendable method to enhance the realism of GC\nsimulations. Besides breaking conservation laws, $N$-point gyroaveraging in our\nexample makes the GC results deviate further from the full orbit results,\nshowing that this method can even give the wrong trend.", "category": "physics_plasm-ph" }, { "text": "Dynamical Schwinger effect: Properties of the $e^{+}e^{-}$ plasma\n created from vacuum in strong laser fields: We study the dynamical Schwinger effect in the vacuum excitation of the\nelectron-positron plasma under action of a \"laser pulse\" of the simplest\nconfiguration: a linearly polarized, time-dependent and spatially homogeneous\nelectric field. Methodical basis of this investigation is the kinetic equation\nwhich is an exact consequence of the basic equations of motion of QED in the\nconsidered field model. In the present work we investigate some features of the\nresidual electron-positron plasma and the transient process of its formation.", "category": "physics_plasm-ph" }, { "text": "Deuterium retention and thermal conductivity in ion-beam\n displacement-damaged tungsten: Retention of plasma-implanted D is studied in W targets damaged by a Cu ion\nbeam at up to 0.2 dpa with sample temperatures between 300 K and 1200 K. At a D\nplasma ion fluence of $10^{24}/m^2$ on samples damaged to 0.2 dpa at 300 K, the\nretained D retention inventory is $4.6x10^{20} D/m^2$, about ~5.5 times higher\nthan in undamaged samples. The retained inventory drops to $9x10^{19} D/m^2$\nfor samples damaged to 0.2 dpa at 1000 K, consistent with onset of vacancy\nannealing at a rate sufficient to overcome the elevated rate of ion beam\ndamage; at a damage temperature of 1200 K retention is nearly equal to values\nseen in undamaged materials. A nano-scale technique provides thermal\nconductivity measurements from the Cu- ion beam displacement damaged region. We\nfind the thermal conductivity of W damaged to 0.2 dpa at room temperature drops\nfrom the un-irradiated value of 182 +/- 3.3 W/m-K to 53 +/- 8 W/m-K.", "category": "physics_plasm-ph" }, { "text": "Kinetic limit of N-body description of wave-particle self- consistent\n interaction: A system of N particles eN=(x1,v1,...,xN,vN) interacting self-consistently\nwith M waves Zn=An*exp(iTn) is considered. Hamiltonian dynamics transports\ninitial data (eN(0),Zn(0)) to (eN(t),Zn(t)). In the limit of an infinite number\nof particles, a Vlasov-like kinetic equation is generated for the distribution\nfunction f(x,v,t), coupled to envelope equations for the M waves. Any initial\ndata (f(0),Z(0)) with finite energy is transported to a unique (f(t),Z(t)).\nMoreover, for any time T>0, given a sequence of initial data with N particles\ndistributed so that the particle distribution fN(0)-->f(O) weakly and with\nZn(0)-->Z(O) as N tends to infinity, the states generated by the Hamiltonian\ndynamics at all time 090%) for\ndensity and temperature predictions. These results demonstrate the potential of\nmachine learning in providing rapid or real-time analysis of emission\nspectroscopy data in pulsed-power-driven plasmas.", "category": "physics_plasm-ph" }, { "text": "A mechanism for magnetic field stochastization and energy release during\n an edge pedestal collapse: On the basis of three-dimensional nonlinear magnetohydrodynamic simulations,\nwe propose a new dynamical process leading to the stochastization of magnetic\nfields during an edge pedestal collapse. Primary tearing modes are shown to\ngrow by extracting kinetic energy of unstable ballooning modes, eventually\nleading to the island overlap. Secondary tearing modes, which are generated\nthrough a coherent nonlinear interaction between adjacent ballooning modes,\nplay a key role in this process, mediating the energy transfer between primary\nballooning and tearing modes. Explicit calculations of the parallel energy loss\nthrough the stochastic field lines show that it can be a likely dominant energy\nloss mechanism during an edge pedestal collapse.", "category": "physics_plasm-ph" }, { "text": "High-power laser experiment on developing supercritical shock\n propagating in homogeneously magnetized plasma of ambient gas origin: A developing supercritical collisionless shock propagating in a homogeneously\nmagnetized plasma of ambient gas origin having higher uniformity than the\nprevious experiments is formed by using high-power laser experiment. The\nambient plasma is not contaminated by the plasma produced in the early time\nafter the laser shot. While the observed developing shock does not have\nstationary downstream structure, it possesses some characteristics of a\nmagnetized supercritical shock, which are supported by a one-dimensional full\nparticle-in-cell simulation taking the effect of finite time of laser-target\ninteraction into account.", "category": "physics_plasm-ph" }, { "text": "The generation of warm dense matter using a magnetic anvil cell: Warm dense matter is present at the heart of gas giants and large\nexo-planets. Yet, its most basic properties are unknown and limit our\nunderstanding of planetary formation and evolution. In this state, where\npressure climbs above 1 Mbar, matter is strongly coupled and quantum\ndegenerate. This combination invalidates most theories capable of predicting\nthe equation of state, the viscosity or heat conductivity of the material. When\nsuch properties are missing, understanding planetary evolution becomes an\narduous endeavor. Henceforth, research in this field is actively growing, using\nhigh power laser or heavy ion beams to produce samples dense enough to overcome\nthe 1 Mbar limit. These samples are not actively confined and tend to expand\nrapidly, precluding the existence of any thermodynamically stable equilibrium.\nHowever, a mega-ampere-class pulsed-power generator can produce confined matter\nin the Mbar range, providing two conditions are being met. First, the sample\nneeds to be compressed cylindrically, to maximize magnetic pressure (compared\nto slab compression). Second, a damper must be used to preclude the formation\nof a corona around the sample. This corona robs the main sample from valuable\ncurrent and limits the homogeneity of the compression. According to numerical\nsimulations, the setup proposed here, and called a magnetic anvil cell, can\nreach pressure on the order of 1 Mbar using a mega-ampere pulsed power driver.\nThese samples span several millimeters in length. Unlike diamond anvil cell,\nwhich pressure is limited below 1 Mbar due to materials strength, the magnetic\nanvil cell has virtually no pressure limit. Further, the current heats the\nsample to several eV, a temperature well beyond diamond anvil cell\ncapabilities.", "category": "physics_plasm-ph" }, { "text": "Nonlinear frequency shift of electrostatic waves in general\n collisionless plasma: unifying theory of fluid and kinetic nonlinearities: The nonlinear frequency shift is derived in a transparent asymptotic form for\nintense Langmuir waves in general collisionless plasma. The formula describes\nboth fluid and kinetic effects simultaneously. The fluid nonlinearity is\nexpressed, for the first time, through the plasma dielectric function, and the\nkinetic nonlinearity accounts for both smooth distributions and\ntrapped-particle beams. Various known limiting scalings are reproduced as\nspecial cases. The calculation avoids differential equations and can be\nextended straightforwardly to other nonlinear plasma waves.", "category": "physics_plasm-ph" }, { "text": "A preliminary assessment of the sensitivity of uniaxially-driven fusion\n targets to flux-limited thermal conduction modeling: The role of flux-limited thermal conduction on the fusion performance of the\nuniaxially-driven targets studied by Derentowicz et al.; Jour. Tech. Phys. 18,\n465 (1977) and Jour. Tech. Phys. 25, 135 (1977), is explored as part of a wider\neffort to understand and quantify uncertainties in ICF systems sharing\nsimilarities with First Light Fusion's projectile-driven concept. We examine\nthe role of uncertainties in plasma microphysics and different choices for the\nnumerical implementation of the conduction operator on simple metrics\nencapsulating the target performance. The results indicate that choices which\naffect the description of ionic heat flow between the heated fusion fuel and\nthe gold anvil used to contain it are the most important. The electronic\ncontribution is found to be robustly described by local diffusion. The\nsensitivities found suggest a prevalent role for quasi-nonlocal ionic\ntransport, especially in the treatment of conduction across material interfaces\nwith strong gradients in temperature and conductivity. We note that none of the\nsimulations produce neutron yields which substantiate those reported by\nDerentowicz et al. Jour. Tech. Phys. 25, 135 (1977), leaving open future\nstudies aimed at more fully understanding this class of ICF systems.", "category": "physics_plasm-ph" }, { "text": "Turbulence and structure formation in complex plasmas and fluids: The formation and evolution of nonlinear and turbulent dynamical structures\nin two-dimensional complex plasmas and fluids is explored by means of\ngeneralised (drift) fluid simulations. Recent numerical results on turbulence\nin dusty magnetised plasmas, strongly coupled fluids, semi-classical\n(\"quantum\") plasmas and in rotating quantum fluids are reviewed and discussed.", "category": "physics_plasm-ph" }, { "text": "The concept of isochoric central spark ignition and its fuel gain in\n inertial fusion: One of the best methods in inertial confinement fusion (ICF) is the concept\nof central spark ignition, consisting of two distinct regions named as hot and\ncold regions and formed by hydro-dynamical implosion of fuel micro-sphere\ncentral spark ignition method in inertial fusion and fuel pellet design\ncondition in fusion power plant has been investigated and fuel gain for\nisochoric model in this method is calculated. We have shown the effects of\ndifferent physical parameters of inertial fusion on fuel gain and optimized\nlimit for fuel density and fuel pellet radius has been calculated.", "category": "physics_plasm-ph" }, { "text": "Adaptive Multi-Dimensional Particle In Cell: Kinetic Particle In Cell (PIC) methods can extend greatly their range of\napplicability if implicit time differencing and spatial adaption are used to\naddress the wide range of time and length scales typical of plasmas. For\nimplicit differencing, we refer the reader to our recent summary of the\nimplicit moment PIC method implemented in our CELESTE3D code [G. Lapenta, Phys.\nPlasmas, 13, 055904 (2006)]. Instead, the present document deals with the issue\nof PIC spatial adaptation. Adapting a kinetic PIC code requires two tasks:\nadapting the grid description of the fields and moments and adapting the\nparticle description of the distribution function. Below we address both\nissues. First, we describe how grid adaptation can be guided by appropriate\nmeasures of the local accuracy of the solution. Based on such information, grid\nadaptation can be obtained by moving grid points from regions of lesser\ninterest to regions of higher interest or by adding and removing points. We\ndiscuss both strategies. Second, we describe how to adapt the local number of\nparticles to reach the required statistical variance in the description of the\nparticle population. Finally two typical applications of adaptive PIC are\nshown: collisionless shocks and charging of small bodies immersed in a plasma.", "category": "physics_plasm-ph" }, { "text": "Boosted High Order Harmonics from Electron Density Singularity Formed at\n the Relativistic Laser Bow Wave: We demonstrate coherent hard electromagnetic radiation generation from\nreflection by the electron density singularity formed at the relativistic bow\nwave in laser plasma via particle-in-cell simulations. Wake and bow waves\ndriven by an intense laser pulse form an electron density singularity at the\nlaser pulse front where they join. A counter-propagating laser pulse is\nreflected at the electron density modulations moving with relativistic\nvelocity. The reflected electromagnetic pulse is compressed and its frequency\nis upshifted. Its frequency spectrum contains relativistic harmonics of the\ndriver pulse frequency generated at the bow wave front, all upshifted with the\nsame factor as the fundamental mode of the incident light.", "category": "physics_plasm-ph" }, { "text": "H2+ embedded in a Debye plasma: Electronic and vibrational properties: The effect of plasma screening on the electronic and vibrational properties\nof the H2+ molecular ion was analyzed within the Born-Oppenheimer\napproximation. When a molecule is embedded in a plasma, the plasma screens the\nelectrostatic interactions. This screening is accounted for in the\nSchr\\\"odinger equation by replacing the Coulomb potentials with Yukawa\npotentials that incorporate the Debye length as a screening parameter.\nVariational expansions in confocal elliptical coordinates were used to\ncalculate energies of the 1ssg and the 2psu states over a range of Debye\nlengths and bond distances. When the Debye length is comparable to the\nequilibrium bond distance, the plasma screening reshapes the potential energy\ncurve. Expectation values, dipole polarizabilities and spectroscopic constants\nwere calculated for the 1ssg state.", "category": "physics_plasm-ph" }, { "text": "Projection-operator methods for classical transport in magnetized\n plasmas. I. Linear response, the Braginskii equations, and fluctuating\n hydrodynamics: An introduction to the use of projection-operator methods for the derivation\nof classical fluid transport equations for weakly coupled, magnetized plasmas\nis given. In the present work, linear response is addressed. In the\nSchr\\\"odinger representation, projection onto the hydrodynamic subspace leads\nto the conventional Braginskii fluid equations, while the orthogonal projection\nleads to an alternative derivation of the Braginskii correction equations for\nthe nonhydrodynamic part of the distribution function f. Although ultimately\nmathematically equivalent to Braginskii's calculations, the projection-operator\napproach provides a usefully intuitive way of discussing the derivation of\ntransport equations and interpreting the significance of the various parts of\nthe perturbed distribution function; it is also technically more concise. A\nspecial case of the Weinhold metric is used to provide a covariant\nrepresentation of the formalism; this allows a succinct demonstration of the\nOnsager symmetries for classical transport. The Heisenberg representation is\nused to derive a generalized Langevin system whose mean recovers the Braginskii\nequations but that also includes fluctuating forces. Transport coefficients are\nsimply related to the two-time correlation functions of those forces, and\nphysical pictures of the various transport processes are naturally couched in\nterms of them. A number of appendices review the traditional Chapman-Enskog\nprocedure; record results about the linearized Landau collision operator;\ndiscuss the covariant representation of the hydrodynamic projection; provide an\nexample of the calculation of some transport effects; describe the\ndecomposition of the stress tensor for magnetized plasma; introduce the linear\neigenmodes of the Braginskii equations; and, with the aid of several examples,\nmention some caveats for the use of projection operators.", "category": "physics_plasm-ph" }, { "text": "Arbitrary amplitude inertial Alfven waves in homogeneous magnetized\n electron-positron-ion plasmas: Nonlinear set of equations for inertial or slow shear Alfven wave (SSAW) in\nideal electron-positron-ion (e-p-i) plasmas are presented. The analytical\nsolution for arbitrary amplitude SSAW in such multi-component plasmas is\nobtained using Sagdeev potential approach. The numerical solutions for several\ndifferent cases have also been presented for illustrative purpose. It is found\nthat the electron density dips of SSAW are formed in the super Alfvenic region.\nThe amplitude and the width of the nonlinear shear Alfven wave reduces with the\nincrease in the concentration of positrons in electron-ion (e-i) plasmas. The\nwidth of the soliton also depends upon the direction of propagation of the\nperturbation in both e-i and e-p-i plasmas.", "category": "physics_plasm-ph" }, { "text": "Nonlinear microtearing modes in MAST and their stochastic layer\n formation: First nonlinear gyrokinetic simulations of microtearing modes in the core of\na MAST case are performed on two surfaces of the high-collisionality discharge\nused in Valovi\\v{c} et al. Nucl. Fusion 51.7 (2011) to obtain the favorable\nenergy confinement scaling with collisionality, $\\tau_E\\propto\\,\\nu_*^{-1}$. On\nthe considered surfaces microtearing modes dominate linearly at binormal length\nscales of the order of the ion Larmor radius. While the effect of electron\ncollision frequency is moderate in linear simulations, a strong dependence on\nthis parameter is found in nonlinear simulations at $r/a=0.5$, where $r$ and\n$a$ are the surface and tokamak minor radius, respectively. The dynamics of\nmagnetic islands generated by microtearing modes is analysed, showing that the\nradial extent of the stochastic region caused by islands overlapping plays an\nimportant role in determining the saturation level of the microtearing mode\ndriven heat flux. Local nonlinear gyrokinetic simulations show that the\nmicrotearing mode driven heat flux, $Q_e^\\mathrm{MTM}$, is largely dominated by\nmagnetic flutter and depends strongly on the magnetic shear, $\\hat{s}$.\nComparing two surfaces, $r/a=0.5$ and $r/a=0.6$, reveals that\n$Q_e^\\mathrm{MTM}$ is negligible at $r/a=0.5$ ($\\hat{s}=0.34$), with the\nelectron temperature gradient driven heat flux, $Q_e^\\mathrm{ETG}$, comparable\nto the experimental electron heat flux, $Q_e^\\mathrm{exp}$, while\n$Q_e^\\mathrm{MTM}$ is significantly larger and comparable to $Q_e^\\mathrm{ETG}$\nand $Q_e^\\mathrm{exp}$ at $r/a=0.6$ ($\\hat{s}=1.1$). Microtearing modes cause\nmore experimentally significant transport in higher $\\hat{s}$ regions and may\ninfluence (together with electron temperature gradient modes) the observed\nscaling of energy confinement time with collisionality (Valovi\\v{c} et al.\nNucl. Fusion 51.7 (2011)).", "category": "physics_plasm-ph" }, { "text": "Generation of longitudinal electric current by transversal\n electromagnetic field in Maxwellian plasmas: The analysis of nonlinear interaction of transversal electromagnetic field\nwith Maxwellian collisionless classical and quntum plasmas is carried out.\nFormulas for calculation electric current in Maxwellian collisionless classical\nand quntum plasmas are deduced. It has appeared, that the nonlinearity account\nleads to occurrence of the longitudinal electric current directed along a wave\nvector. This second current is orthogonal to the known transversal current,\nreceived at the classical linear analysis. Graphic comparison of density of\nelectric current for classical Maxwellian plasmas and Fermi---Dirac plasmas\n(plasmas with any degree of degeneration of electronic gas) is carried out.\nGraphic comparison of density of electric current for classical and quantum\nMaxwellian plasmas is carried out. Also comparison of dependence of density of\nelectric current of quantum Maxwellian plasmas from dimensionless wave number\nat various values of dimensionless frequency of oscillations of electromagnetic\nfield is carried out.", "category": "physics_plasm-ph" }, { "text": "Anomalous plasma acceleration in colliding high-power laser-produced\n plasmas: We developed an experimental platform for studying magnetic reconnection in\nan external magnetic field with simultaneous measurements of plasma imaging,\nflow velocity, and magnetic-field variation. Here, we investigate the\nstagnation and acceleration in counter-streaming plasmas generated by\nhigh-power laser beams. A plasma flow perpendicular to the initial flow\ndirections is measured with laser Thomson scattering. The flow is,\ninterestingly, accelerated toward the high-density region, which is opposite to\nthe direction of the acceleration by pressure gradients. This acceleration is\npossibly interpreted by the interaction of two magnetic field loops initially\ngenerated by Biermann battery effect, resulting in a magnetic reconnection\nforming a single field loop and additional acceleration by a magnetic tension\nforce.", "category": "physics_plasm-ph" }, { "text": "Zonal Flows and Turbulence in Fluids and Plasmas: In geophysical and plasma contexts, zonal flows are well known to arise out\nof turbulence. We elucidate the transition from statistically homogeneous\nturbulence without zonal flows to statistically inhomogeneous turbulence with\nsteady zonal flows. Starting from the Hasegawa--Mima equation, we employ both\nthe quasilinear approximation and a statistical average, which retains a great\ndeal of the qualitative behavior of the full system. Within the resulting\nframework known as CE2, we extend recent understanding of the symmetry-breaking\n`zonostrophic instability'. Zonostrophic instability can be understood in a\nvery general way as the instability of some turbulent background spectrum to a\nzonally symmetric coherent mode. As a special case, the background spectrum can\nconsist of only a single mode. We find that in this case the dispersion\nrelation of zonostrophic instability from the CE2 formalism reduces exactly to\nthat of the 4-mode truncation of generalized modulational instability. We then\nshow that zonal flows constitute pattern formation amid a turbulent bath.\nZonostrophic instability is an example of a Type I$_s$ instability of\npattern-forming systems. The broken symmetry is statistical homogeneity. Near\nthe bifurcation point, the slow dynamics of CE2 are governed by a well-known\namplitude equation, the real Ginzburg-Landau equation. The important features\nof this amplitude equation, and therefore of the CE2 system, are multiple.\nFirst, the zonal flow wavelength is not unique. In an idealized, infinite\nsystem, there is a continuous band of zonal flow wavelengths that allow a\nnonlinear equilibrium. Second, of these wavelengths, only those within a\nsmaller subband are stable. Unstable wavelengths must evolve to reach a stable\nwavelength; this process manifests as merging jets. These behaviors ...", "category": "physics_plasm-ph" }, { "text": "Parametric Resonance in the Drift Motion of an Ionic Bubble in near\n critical Ar Gas: The drift mobility of the negative oxygen ion in dense Argon gas near the\nliquid-vapor critical point has been measured as a function of the density.\nNear the critical temperature the zero-field density-normalized ion mobility\nshows a deep minimum at a density smaller than the critical one. This\nphenomenon was previously intepreted as the result of the formation of a\ncorrelated cluster of Argon atoms around the ion because of the strong\nelectrostriction exerted by the ion on the highly polarizable and compressible\ngas. We now suggest that a possible alternative explanation is related to the\nonset of a parametric resonance of a bubble surrounding the ion. At resonance a\nlarge amount of energy is dissipated by sound waves in addition to viscous\ndissipation processes, resulting in the large mobility drop observed.", "category": "physics_plasm-ph" }, { "text": "Numerical convergence of the branching time of negative streamers: In sufficiently large gaps and electric fields, discharge streamers do\nbranch. In [Arrayas et al., PRL 88, 174502 (2002)], we observed streamer\nbranching numerically within a deterministic particle density model and\nexplained it as a Laplacian instability of a thin space charge layer. Our\nnumerical results were criticized in [Kulikovsky, PRL 89, 229401 (2002)]. We\nhere present an adaptive grid refinement method for streamer simulations, and\nwe carry out the first conclusive investigation on the effect of the numerical\ngrid on streamer branching in different fields. On stepwise finer grids the\nbranching time converges, hence streamer branching is for the first time\npredicted quantitatively.", "category": "physics_plasm-ph" }, { "text": "Benchmarking Exchange-Correlation Functionals in the Spin-Polarized\n Inhomogeneous Electron Gas under Warm Dense Conditions: Warm dense matter is a highly active research area both at the frontier and\ninterface of material science and plasma physics. We assess the performance of\ncommonly used exchange-correlation (XC) approximation (LDA, PBE, PBEsol, and\nAM05) in the spin-polarized inhomogeneous electron gas under warm dense\nconditions based on exact path-integral quantum Monte-Carlo calculations. This\nextends our recent analysis on the relevance of inhomogeneities in the\nspin-unpolarized warm dense electron gas [Z.~Moldabekov et al., J. Chem. Phys.\n155, 124116 (2021)]. We demonstrate that the predictive accuracy of these XC\nfunctionals deteriorates with (1) a decrease in density (corresponding to an\nincrease in the inter-electronic correlation strength) and (2) an increase of\nthe characteristic wave number of the density perturbation. We provide\nrecommendations for the applicability of the considered XC functionals at\nconditions typical for warm dense matter. Furthermore, we hint at future\npossibilities for constructing more accurate XC functionals under these\nconditions.", "category": "physics_plasm-ph" }, { "text": "Self-consistent theory for the linear and nonlinear propagation of a\n sinusoidal electron plasma wave. Application to stimulated Raman scattering\n in a non-uniform and non-stationary plasma: In this paper, we address the theoretical resolution of the Vlasov-Gauss\nsystem from the linear regime to the strongly nonlinear one, when significant\ntrapping has occurred. The electric field is that of a sinusoidal electron\nplasma wave (EPW) which is assumed to grow from the noise level, and to keep\ngrowing at least up to the amplitude when linear theory in no longer valid\n(while the wave evolution in the nonlinear regime may be arbitrary). The ions\nare considered as a neutralizing fluid, while the electron response to the wave\nis derived by matching two different techniques. We make use of a perturbation\nanalysis similar to that introduced to prove the Kolmogorov-Arnold- Moser\ntheorem, up to amplitudes large enough for neo-adiabatic results to be valid.\nOur theory is applied to the growth and saturation of the beam-plasma\ninstability, and to the three-dimensional propagation of a driven EPW in a\nnon-uniform and non-stationary plasma. For the latter example, we lay a special\nemphasis on nonlinear collisionless dissipation. We provide an explicit\ntheoretical expression for the nonlinear Landau-like damping rate which, in\nsome instances, is amenable to a simple analytic formula. We also insist on the\nirreversible evolution of the electron distribution function, which is nonlocal\nin the wave amplitude and phase velocity. This makes trapping an effective\nmeans of dissipation for the electrostatic energy, and also makes the wave\ndispersion relation nonlocal. Our theory is generalized to allow for stimulated\nRaman scattering, which we address up to saturation by accounting for plasma\ninhomogeneity and non-stationarity, nonlinear kinetic effects, and interspeckle\ncoupling.", "category": "physics_plasm-ph" }, { "text": "Nonlinear kinetic modeling of stimulated Raman scattering in a\n multidimensional geometry: In this paper, we derive coupled envelope equations modeling the growth of\nstimulated Raman scattering (SRS) in a multi-dimensional geometry, and\naccounting for nonlinear kinetic effects. In particular, our envelope equations\nallow for the nonlinear reduction of the Landau damping rate, whose decrease\nwith the plasma wave amplitude depends on the rate of side-loss. Account is\nalso made of the variations in the extent of the plasma wave packet entailed by\nthe collisionless dissipation due to trapping. The dephasing between the\nelectron plasma wave (EPW) and the laser drive, as well as the self-focussing\nof the plasma wave, both induced by the EPW nonlinear frequency shift, are also\nincluded in our envelope equations. These equations are solved in a\nmulti-dimensional geometry using our code dubbed BRAMA, whose predictions\nregarding the evolution of Raman reflectivity as a function of the laser\nintensity are compared against previously published PIC results, thus\nillustrating the ability of BRAMA simulations to provide the correct laser\nthreshold intensity for SRS, as well as the right order of magnitude of Raman\nreflectivity above threshold.", "category": "physics_plasm-ph" }, { "text": "K$_\u03b1$ Emission Profiles of Warm Dense Argon Plasmas: K-line profiles emitted from a warm dense plasma environment are used for\ndiagnostics of Ar droplet plasmas created by high energy laser pulses.\nAnalyzing the temporally and spacially integrated spectra, we infer temperature\ngradients within the Ar droplet from cold temperatures of the order of some 10\neV up to higher temperatures of 250 eV and beyond. To characterize the\ninfluence of the warm and dense environment on the emitters, plasma screening\nis considered within a perturbative approach to the Hamiltonian. The plasma\naffects the many-particle system resulting in energy shifts of emission as well\nas ionization energies due to electron-ion and electron-electron interaction.\nWith this approach we get a good reproduction of spectral features that are\nstrongly influenced by ionization and excitation processes within the plasma.\nComparing with the widely known FLYCHK code and the temperature distribution\ngiven in the original paper, counting for internal degrees of freedom (bound\nstates) and treating pressure ionization within our quantum statistical\napproach results in a more detailed temperature-density-relation and leads to\ndifferent results for the inferred temperature distribution.", "category": "physics_plasm-ph" }, { "text": "Consistency in Drift-ordered Fluid Equations: We address several concerns related to the derivation of drift-ordered fluid\nequations. Starting from a fully Galilean invariant fluid system, we show how\nconsistent sets of perturbative drift-fluid equations in the case of a\nisothermal collisionless fluid can be obtained. Treating all the dynamical\nfields on equal footing in the singular-drift expansion, we show under what\nconditions a set of perturbative equations can have a non-trivial quasi-neutral\nlimit. We give a suitable perturbative setup where we provide the full set of\nperturbative equations for obtaining the first-order corrected fields and show\nthat all the constants of motion are preserved at each order. With the\ndynamical field variables under perturbative control, we subsequently provide a\nquantitative analysis by means of numerical simulations. With direct access to\nfirst-order corrections the convergence properties are addressed for different\nregimes of parameter space and the validity of the first-order approximation is\ndiscussed in the three settings: cold ions, hot ions and finite charge density.", "category": "physics_plasm-ph" }, { "text": "Influence of plasma turbulence on microwave propagation: It is not fully understood how electromagnetic waves propagate through plasma\ndensity fluctuations when the size of the fluctuations is comparable with the\nwavelength of the incident radiation. In this paper, the perturbing effect of a\nturbulent plasma density layer on a traversing microwave beam is simulated with\nfull-wave simulations. The deterioration of the microwave beam is calculated as\na function of the characteristic turbulence structure size, the turbulence\namplitude, the depth of the interaction zone and the size of the waist of the\nincident beam. The maximum scattering is observed for a structure size on the\norder of half the vacuum wavelength. The scattering and beam broadening was\nfound to increase linearly with the depth of the turbulence layer and\nquadratically with the fluctuation strength. Consequences for experiments and\n3D effects are considered.", "category": "physics_plasm-ph" }, { "text": "Global effects on neoclassical transport in the pedestal with impurities: We present a numerical study of collisional transport in a tokamak pedestal\nin the presence of non-trace impurities, using the radially global $\\delta f$\nneoclassical solver PERFECT [M. Landreman et al. 2014 Plasma Phys. Control.\nFusion 56 045005]. It is known that in a tokamak core with non-trace impurities\npresent the radial impurity flux opposes the bulk ion flux to provide an\nambipolar particle transport, with the electron transport being negligibly\nsmall. However, in a sharp density pedestal with sub-sonic ion flows the\nelectron transport can be comparable to the ion and impurity flows.\nFurthermore, the neoclassical particle transport is not intrinsically\nambipolar, and the non-ambipolarity of the fluxes extends outside the pedestal\nregion by the radial coupling of the perturbations. The neoclassical momentum\ntransport, which is finite in the presence of ion orbit-width scale profile\nvariations, is significantly enhanced when impurities are present in non-trace\nquantities, even if the total parallel mass flow is dominated by the bulk ions.", "category": "physics_plasm-ph" }, { "text": "Two-dimensional simulations of nonlinear beam-plasma interaction in\n isotropic and magnetized plasmas: Nonlinear interaction of a low density electron beam with a uniform plasma is\nstudied using two-dimensional particle-in-cell (PIC) simulations. We focus on\nformation of coherent phase space structures in the case, when a wide\ntwo-dimensional wave spectrum is driven unstable, and we also study how\nnonlinear evolution of these structures is affected by the external magnetic\nfield. In the case of isotropic plasma, nonlinear buildup of filamentation\nmodes due to the combined effects of two-stream and oblique instabilities is\nfound to exist and growth mechanisms of secondary instabilities destroying the\nBGK--type nonlinear wave are identified. In the weak magnetic field, the energy\nof beam-excited plasma waves at the nonlinear stage of beam-plasma interaction\ngoes predominantly to the short-wavelength upper-hybrid waves propagating\nparallel to the magnetic field, whereas in the strong magnetic field the\nspectral energy is transferred to the electrostatic whistlers with oblique\npropagation.", "category": "physics_plasm-ph" }, { "text": "Solitary magnetic perturbations at the ELM onset: Edge localised modes (ELMs) allow maintaining sufficient purity of tokamak\nH-mode plasmas and thus enable stationary H-mode. On the other hand in a future\ndevice ELMs may cause divertor power flux densities far in excess of tolerable\nmaterial limits. The size of the energy loss per ELM is determined by\nsaturation effects in the non-linear phase of the ELM, which at present is\nhardly understood. Solitary magnetic perturbations (SMPs) are identified as\ndominant features in the radial magnetic fluctuations below 100kHz. They are\ntypically observed close (+-0.1ms) to the onset of pedestal erosion. SMPs are\nfield aligned structures rotating in the electron diamagnetic drift direction\nwith perpendicular velocities of about 10km/s. A comparison of perpendicular\nvelocities suggests that the perturbation evoking SMPs is located at or inside\nthe separatrix. Analysis of very pronounced examples showed that the number of\npeaks per toroidal turn is 1 or 2, which is clearly lower than corresponding\nnumbers in linear stability calculations. In combination with strong peaking of\nthe magnetic signals this results in a solitary appearance resembling modes\nlike palm tree modes, edge snakes or outer modes. This behavior has been\nquantified as solitariness and correlated to main plasma parameters. SMPs may\nbe considered as a signature of the non-linear ELM-phase originating at the\nseparatrix or further inside. Thus they provide a handle to investigate the\ntransition from linear to non-linear ELM phase. By comparison with data from\ngas puff imaging processes in the non-linear phase at or inside the separatrix\nand in the scrape-off-layer (SOL) can be correlated. A connection between the\npassing of an SMP and the onset of radial filament propagation has been found.\nEventually the findings related to SMPs may contribute to a future quantitative\nunderstanding of the non-linear ELM evolution.", "category": "physics_plasm-ph" }, { "text": "Thermal balance of tungsten monocrystalline nanoparticles in high\n pressure magnetron discharges: Nanoparticles are produced in sputtering magnetron discharges operating with\na tungsten cathode at 30 Pa argon pressure. Structure analyses show that they\nare of core-shell type. The core is a monocrystal mainly in the metastable\nbeta-tungsten phase and the shell is made of tungsten oxide. The origin of the\nmetastable phase is attributed to the presence of residual oxygen in the\ndevice. Since this phase transforms into the stable alpha-tungsten phase by\nannealing, a standard model on the thermal balance of nanoparticles was used to\nfind the temperature that they can reach under the considered experimental\nconditions. It is shown that this temperature is significantly higher than the\ngas one but not high enough to transform the monocrystalline metastable\nbeta-phase during the plasma process.", "category": "physics_plasm-ph" }, { "text": "Solitary zonal structures in subcritical drift waves: a minimum model: Solitary zonal structures have recently been identified in gyrokinetic\nsimulations of subcritical drift-wave (DW) turbulence with background shear\nflows. However, the nature of these structures has not been fully understood\nyet. Here, we show that similar structures can be obtained within a reduced\nmodel, which complements the modified Hasegawa-Mima equation with a generic\nprimary instability and a background shear flow. We also find that these\nstructures can be qualitatively reproduced in the modified Hasegawa-Wakatani\nequation, which subsumes the reduced model as a limit. In particular, we\nillustrate that in both cases, the solitary zonal structures approximately\nsatisfy the same ''equation of state'', which is a local relation connecting\nthe DW envelope with the zonal-flow velocity. Due to this generality, our\nreduced model can be considered as a minimum model for solitary zonal\nstructures in subcritical DWs.", "category": "physics_plasm-ph" }, { "text": "Nonlinear propagation of broadband intense electromagnetic waves in an\n electron-positron plasma: A kinetic equation describing the nonlinear evolution of intense\nelectromagnetic pulses in electron-positron (e-p) plasmas is presented. The\nmodulational instability is analyzed for a relativistically intense partially\ncoherent pulse, and it is found that the modulational instability is inhibited\nby the spectral pulse broadening. A numerical study for the one-dimensional\nkinetic photon equation is presented. Computer simulations reveal a\nFermi-Pasta-Ulam-like recurrence phenomena for localized broadband pulses. The\nresults should be of importance in understanding the nonlinear propagation of\nbroadband intense electromagnetic pulses in e-p plasmas in laser-plasma systems\nas well as in astrophysical plasma settings.", "category": "physics_plasm-ph" }, { "text": "Experimental and numerical study of error fields in the CNT stellarator: Sources of error fields were indirectly inferred in a stellarator by\nreconciling computed and numerical flux surfaces. Sources considered so far\ninclude the displacements and tilts (but not the deformations, yet) of the four\ncircular coils featured in the simple CNT stellarator. The flux surfaces were\nmeasured by means of an electron beam and phosphor rod, and were computed by\nmeans of a Biot-Savart field-line tracing code. If the ideal coil locations and\norientations are used in the computation, agreement with measurements is poor.\nDiscrepancies are ascribed to errors in the positioning and orientation of the\nin-vessel interlocked coils. To that end, an iterative numerical method was\ndeveloped. A Newton-Raphson algorithm searches for the coils' displacements and\ntilts that minimize the discrepancy between the measured and computed flux\nsurfaces. This method was verified by misplacing and tilting the coils in a\nnumerical model of CNT, calculating the flux surfaces that they generated, and\ntesting the algorithm's ability to deduce the coils' displacements and tilts.\nSubsequently, the numerical method was applied to the experimental data,\narriving at a set of coil displacements whose resulting field errors exhibited\nsignificantly improved quantitative and qualitative agreement with experimental\nresults.", "category": "physics_plasm-ph" }, { "text": "A Simple Method to Measure the Interaction Potential of Dielectric\n Grains in a Dusty Plasma: A simple minimally perturbative method is introduced which provides the\nability to experimentally measure both the radial confining potential and the\ninteraction potential between two individual dust particles, levitated in the\nsheath of a radio-frequency (RF) argon discharge. In this technique, a single\ndust particle is dropped into the plasma sheath to interact with a second\nindividual dust particle already situated at the system's equilibrium point,\nwithout introducing any external perturbation. The resulting data is analyzed\nusing a method employing a polynomial fit to the particle displacement(s),\nX(t), to reduce uncertainty in calculation. Employing this technique, the\nhorizontal confinement is shown to be parabolic over a wide range of pressures\nand displacements from the equilibrium point. The interaction potential is also\nmeasured and shown to be well-described by a screened Coulomb potential and to\ndecrease with increasing pressure. Finally, the charge on the particle and the\neffective dust screening distance are calculated. It is shown for the first\ntime experimentally that the charge on a particle in the sheath of an RF plasma\ndecreases with increasing pressure, in agreement with theoretical predictions.\nThe screening distance also decreases with increasing pressure as expected.\nThis technique can be used for rapid determination of particle parameters in\ndusty plasma.", "category": "physics_plasm-ph" }, { "text": "Nonlinear magnetohydrodynamic modeling of current-drive-induced\n sawtooth-like crashes in the W7-X stellarator: Sawtooth-like core electron temperature crashes have been observed in W7-X\nexperiments with electron cyclotron current drive. We present nonlinear\nsingle-fluid magnetohydrodynamic simulations of this phenomenon using the newly\ndeveloped stellarator modeling capability of the M3D-$C^1$ code. The near-axis\ncurrent drive gives rise to two $\\iota=1$ resonances in the equilibrium\nrotational transform profile so that two consecutive $(1,1)$ internal kink\nmodes are seen in the simulations. A small-amplitude crash at the inner\nresonance occurs first, which may correspond to the sawtooth precursors\nobserved in the experiments. A bigger crash at the outer resonance then\nflattens the core temperature profile, which shows semi-quantitative agreements\nwith experimental measurements on certain metrics such as the crash amplitude\nand the inversion radius of the temperature change. These results illustrate a\nlikely mechanism of the current-drive-induced sawtooth-like crashes in W7-X\nand, to some extent, validate the stellarator modeling capability of M3D-$C^1$.", "category": "physics_plasm-ph" }, { "text": "Generation of forerunner electron beam during interaction of ion beam\n pulse with plasma: The long-time evolution of the two-stream instability of a cold ion beam\npulse propagating though the background plasma is investigated using a\nlarge-scale one-dimensional electrostatic kinetic simulation. The three stages\nof the instability are identified and investigated in detail. After the initial\nlinear growth and saturation by the electron trapping, a portion of the\ninitially trapped electrons becomes detrapped and moves ahead of the ion beam\npulse forming a {forerunner} electron beam, which causes a secondary two-stream\ninstability that preheats the upstream plasma electrons. Consequently, the\nself-consistent nonlinear-driven turbulent state is set up at the head of the\nion beam pulse with the saturated plasma wave sustained by the influx of the\ncold electrons from the upstream of the beam that lasts until the final stage\nwhen the beam ions become trapped by the plasma wave. The beam ion trapping\nleads to the nonlinear heating of the beam ions that eventually extinguishes\nthe instability.", "category": "physics_plasm-ph" }, { "text": "Radiation reaction in electron-beam interactions with high-intensity\n lasers: Charged particles accelerated by electromagnetic fields emit radiation, which\nmust, by the conservation of momentum, exert a recoil on the emitting particle.\nThe force of this recoil, known as radiation reaction, strongly affects the\ndynamics of ultrarelativistic electrons in intense electromagnetic fields. Such\nenvironments are found astrophysically, e.g. in neutron star magnetospheres,\nand will be created in laser-matter experiments in the next generation of\nhigh-intensity laser facilities. In many of these scenarios, the energy of an\nindividual photon of the radiation can be comparable to the energy of the\nemitting particle, which necessitates modelling not only of radiation reaction,\nbut quantum radiation reaction. The worldwide development of multi-petawatt\nlaser systems in large-scale facilities, and the expectation that they will\ncreate focussed electromagnetic fields with unprecedented intensities $>\n10^{23}~\\mathrm{W}\\text{cm}^{-2}$, has motivated renewed interest in these\neffects. In this paper I review theoretical and experimental progress towards\nunderstanding radiation reaction, and quantum effects on the same, in\nhigh-intensity laser fields that are probed with ultrarelativistic electron\nbeams. In particular, we will discuss how analytical and numerical methods give\ninsight into new kinds of radiation-reaction-induced dynamics, as well as how\nthe same physics can be explored in experiments at currently existing laser\nfacilities.", "category": "physics_plasm-ph" }, { "text": "MHD equilibria with incompressible flows: symmetry approach: We identify and discuss a family of azimuthally symmetric, incompressible,\nmagnetohydrodynamic plasma equilibria with poloidal and toroidal flows in terms\nof solutions of the Generalized Grad Shafranov (GGS) equation. These solutions\nare derived by exploiting the incompressibility assumption, in order to rewrite\nthe GGS equation in terms of a different dependent variable, and the continuous\nLie symmetry properties of the resulting equation and in particular a special\ntype of \"weak\" symmetries.", "category": "physics_plasm-ph" }, { "text": "Large-scale jets in the magnetosheath and plasma penetration across the\n magnetopause: THEMIS observations: THEMIS multi-point observation of the plasma and magnetic fields, conducted\nsimultaneously in the dayside magnetosheath and magnetosphere, were used to\ncollect 646 large-scale magnetosheath plasma jets interacting with the\nmagnetopause. The jets were identified as dense and fast streams of the\nmagnetosheath plasma whose energy density is higher than that of the upstream\nsolar wind. The jet interaction with the magnetopause was revealed from sudden\ninward motion of the magnetopause and an enhancement in the geomagnetic field.\nThe penetration was determined as appearance of the magnetosheath plasma\nagainst the background of the hot magnetospheric particle population. We found\nthat almost 60% of the jets penetrated through the magnetopause. Vast majority\nof the penetrating jets was characterized by high velocities V > 220 km/s and\nkinetic bk > 1 that corresponded to a combination of finite Larmor radius\neffect with a mechanisms of impulsive penetration. The average plasma flux in\nthe penetrating jets was found to be 1.5 times larger than the average plasma\nflux of the solar wind. The average rate of jet-related penetration of the\nmagnetosheath plasma into the dayside magnetosphere was estimated to be ~10^29\nparticles per day. The rate varies highly with time and can achieve values of\n1.5*10^29 particles per hour that is comparable with estimates of the total\namount of plasma entering the dayside magnetosphere.", "category": "physics_plasm-ph" }, { "text": "Biermann battery effects on the turbulent dynamo in a colliding plasma\n jets produced by high-power lasers: The implication of the Biermann battery (BB) on turbulent magnetic field\namplification in colliding plasma jets produced by high-power lasers is studied\nby using the FLASH code. It is found that the BB can play a significant role in\nturbulent field amplification. The small scale fluid structures introduced by\nturbulence can allow the BB to effectively amplify the magnetic field. When the\nflow is perpendicular to the magnetic field, the magnetic field amplification\nis shown to be greater than the case where the flow is parallel.", "category": "physics_plasm-ph" }, { "text": "Robust dynamic mitigation of instabilities: A dynamic mitigation mechanism for instability growth was proposed and\ndiscussed in the paper [Phys. Plasmas 19, 024503 (2012)]. In the present paper\nthe robustness of the dynamic instability mitigation mechanism is discussed\nfurther. The results presented here show that the mechanism of the dynamic\ninstability mitigation is rather robust against changes in the phase, the\namplitude and the wavelength of the wobbling perturbation applied. Generally\ninstability would emerge from the perturbation of the physical quantity.\nNormally the perturbation phase is unknown so that the instability growth rate\nis discussed. However, if the perturbation phase is known, the instability\ngrowth can be controlled by a superposition of perturbations imposed actively:\nif the perturbation is induced by, for example, a driving beam axis oscillation\nor wobbling, the perturbation phase could be controlled and the instability\ngrowth is mitigated by the superposition of the growing perturbations.", "category": "physics_plasm-ph" }, { "text": "Fluid Simulations of Three-Dimensional Reconnection that Capture the\n Lower-Hybrid Drift Instability: Fluid models that approximate kinetic effects have received attention\nrecently in the modelling of large scale plasmas such as planetary\nmagnetospheres. In three-dimensional reconnection, both reconnection itself and\ncurrent sheet instabilities need to be represented appropriately. We show that\na heat flux closure based on pressure gradients enables a ten moment fluid\nmodel to capture key properties of the lower-hybrid drift instability (LHDI)\nwithin a reconnection simulation. Characteristics of the instability are\nexamined with kinetic and fluid continuum models, and its role in the\nthree-dimensional reconnection simulation is analysed. The saturation level of\nthe electromagnetic LHDI is higher than expected which leads to strong kinking\nof the current sheet. Therefore, the magnitude of the initial perturbation has\nsignificant impact on the resulting turbulence.", "category": "physics_plasm-ph" }, { "text": "Plasma dynamics of a laser filamentation-guided spark: We investigate experimentally the plasma dynamics of a centimeter-scale,\nlaser filamentation-guided spark discharge. Using electrical and optical\ndiagnostics to study monopolar discharges with varying current pulses we show\nthat plasma decay is dominated by free electron recombination if the current\ndecay time is shorter than the recombination characteristic time. In the\nopposite case, the plasma electron density closely follows the current\nevolution. We demonstrate that this criterion holds true in the case of damped\nAC sparks, and that alternative current is the best option to achieve a long\nplasma lifetime for a given peak current.", "category": "physics_plasm-ph" }, { "text": "Free Expansion of Yukawa Gas in the Constant Plasma Background: We discuss the irreversible free expansion phenomenon for Yukawa gas and\nobtain the corresponding change in temperature of the gas. In our expansion\nscheme, the system is not allowed to exchange heat with surroundings during\nfree expansion, therefore, present expansion scheme refers to adiabatic free\nexpansion. Using first principle classical Molecular Dynamics (MD) simulation\nwith reflecting boundary conditions, we show that the Yukawa gases exhibits\nheating effect during the process of adiabatic free expansion. We also obtain\nthe scaling for change in temperature and establish that the change in\ntemperature is directly proportional to the change in number density of gas.\nThe scaling for temperature is also obtained analytically taking the mean field\nlimit of thermodynamic model proposed by Avinash. The simulation results are\nconsistent with the analytical results.", "category": "physics_plasm-ph" }, { "text": "Self-similar analytical model of the plasma expansion in a magnetic\n field: The study of hot plasma expansion in a magnetic field is of interest for many\nastrophysical applications. In order to observe this process in laboratory, an\nexperiment is proposed in which an ultrashort laser pulse produces a\nhigh-temperature plasma by irradiation of a small target. In this paper an\nanalytical model is proposed for an expanding plasma cloud in an external\ndipole or homogeneous magnetic field. The model is based on the self-similar\nsolution of a similar problem which deals with sudden expansion of spherical\nplasma into a vacuum without ambient magnetic field. The expansion\ncharacteristics of the plasma and deceleration caused by the magnetic field are\nexamined analytically. The results obtained can be used in treating\nexperimental and simulation data, and many phenomena of astrophysical and\nlaboratory significance.", "category": "physics_plasm-ph" }, { "text": "Quantum effects in beam-plasma instabilities: Among the numerous works on quantum effects that have been published in\nrecent years, streaming instabilities in plasma have also been revisited. Both\nthe fluid quantum and the kinetic Wigner-Maxwell models have been used to\nexplore quantum effects on the Weibel, Filamentation and Two-Stream\ninstabilities. While quantum effects usually tend to reduce the instabilities,\nthey can also spur new unstable branches. A number of theoretical results will\nbe reviewed together with the implications to one physical setting, namely the\nelectron driven fast ignition scenario.", "category": "physics_plasm-ph" }, { "text": "Training and Upgrading Tokamak Power Plants with Remountable\n Superconducting Magnets: All high field superconductors producing magnetic fields above 12 T are\nbrittle. Nevertheless, they will probably be the materials of choice in\ncommercial tokamaks because the fusion power density in a tokamak scales as the\nfourth power of magnetic field. Here we propose using robust, ductile\nsuperconductors during the reactor commissioning phase in order to avoid\nbrittle magnet failure while operational safety margins are being established.\nHere we use the PROCESS systems code to inform development strategy and to\nprovide detailed capital-cost-minimised tokamak power plant designs. We propose\nbuilding a 'demonstrator' tokamak with an electric power output of 100 MWe, a\nplasma fusion gain Qplasma = 17, a net gain Qnet = 1.3, a cost of electricity\n(COE) of \\$ 1148 (2021 US) per MWh (at 75 % availability) and high temperature\nsuperconducting operational TF magnets producing 5.4 T on-axis and 12.5 T\npeak-field. It uses Nb-Ti training magnets and will cost about \\$ 9.75 Bn (2021\nUS). An equivalent 500 MWe plant has a COE of \\$ 608 per MW suggesting that\nlarge tokamaks may eventually dominate the commercial market. We consider a\nrange of designs optimised for capital cost (as the reactors considered are\npilot plants) consisting of both 100 MWe and 500 MWe plants with each of two\napproaches for the magnets: training and upgrading. With training magnets, the\nplant is cost-optimised for REBCO TF magnets. For a 100 MWe plant, the Nb-Ti\ntraining magnets typically produce 70 % peak field on the toroidal field coils\ncompared to REBCO magnets, 65 % peak field on the central solenoid and cost\napprox. 10 % of the total machine cost. Training magnets could in principle be\nreused for each of say 10 subsequent (commercial) machines and hence at 1 %\nbring only marginal additional cost.", "category": "physics_plasm-ph" }, { "text": "Local and nonlocal parallel heat transport in general magnetic fields: A novel approach that enables the study of parallel transport in magnetized\nplasmas is presented. The method applies to general magnetic fields with local\nor nonlocal parallel closures. Temperature flattening in magnetic islands is\naccurately computed. For a wave number $k$, the fattening time scales as\n$\\chi_{\\parallel} \\tau \\sim k^{-\\alpha}$ where $\\chi$ is the parallel\ndiffusivity, and $\\alpha=1$ ($\\alpha=2$) for non-local (local) transport. The\nfractal structure of the devil staircase temperature radial profile in weakly\nchaotic fields is resolved. In fully chaotic fields, the temperature exhibits\nself-similar evolution of the form $T=(\\chi_{\\parallel} t)^{-\\gamma/2} L \\left[\n(\\chi_{\\parallel} t)^{-\\gamma/2} \\delta \\psi \\right]$, where $\\delta \\psi$ is a\nradial coordinate. In the local case, $f$ is Gaussian and the scaling is\nsub-diffusive, $\\gamma=1/2$. In the non-local case, $f$ decays algebraically,\n$L (\\eta) \\sim \\eta^{-3}$, and the scaling is diffusive, $\\gamma=1$.", "category": "physics_plasm-ph" }, { "text": "Laser-driven shock acceleration of monoenergetic ion beams: We show that monoenergetic ion beams can be accelerated by moderate Mach\nnumber collisionless, electrostatic shocks propagating in a long scale-length\nexponentially decaying plasma profile. Strong plasma heating and density\nsteepening produced by an intense laser pulse near the critical density can\nlaunch such shocks that propagate in the extended plasma at high velocities.\nThe generation of a monoenergetic ion beam is possible due to the small and\nconstant sheath electric field associated with the slowly decreasing density\nprofile. The conditions for the acceleration of high-quality, energetic ion\nbeams are identified through theory and multidimensional particle-in-cell\nsimulations. The scaling of the ion energy with laser intensity shows that it\nis possible to generate $\\sim 200$ MeV proton beams with state-of-the-art 100\nTW class laser systems.", "category": "physics_plasm-ph" }, { "text": "Study of the likelihood of Alfv\u00e9nic mode bifurcation in NSTX and\n predictions for ITER baseline scenarios: Rare Alfv\\'enic wave transitions between fixed-frequency and chirping phases\nare identified in NSTX, where Alfv\\'enic waves are normally observed to exhibit\neither chirping or avalanching responses. For those transitions, we apply a\ncriterion [Duarte et al, Nucl. Fusion 57, 054001 (2017)] to predict the nature\nof fast ion redistribution in tokamaks to be in the convective or diffusive\nnonlinear regimes. For NSTX discharges in which the transition is not\naccompanied by changes in the beam deposited power or modifications in the\ninjected radiofrequency power, it has been found that the anomalous fast ion\ntransport is a likely mediator of the bifurcation between the fixed-frequency\nmode behavior and rapid chirping. For a quantitative assessment, global\ngyrokinetic simulations of the effects of electrostatic ion temperature\ngradient turbulence and trapped electron mode turbulence on chirping were\npursued using the GTS code. The investigation is extended by means of\npredictive studies of the probable spectral behavior of Alfv\\'enic eigenmodes\nfor baseline ITER cases consisting of elmy, advanced and hybrid scenarios. It\nhas been observed that most modes are found to be borderline between the steady\nand the chirping phases.", "category": "physics_plasm-ph" }, { "text": "The Reduction of Magnetic Reconnection Outflow Jets to Sub-Alfv\u00e9nic\n Speeds: The outflow velocity of jets produced by collisionless magnetic reconnection\nis shown to be reduced by the ion exhaust temperature in simulations and\nobservations. We derive a scaling relationship for the outflow velocity based\non the upstream Alfv\\'en speed and the parallel ion exhaust temperature, which\nis verified in kinetic simulations and observations. The outflow speed\nreduction is shown to be due to the firehose instability criterion, and so for\nlarge enough guide fields this effect is suppressed and the outflow speed\nreaches the upstream Alfv\\'en speed based on the reconnecting component of the\nmagnetic field.", "category": "physics_plasm-ph" }, { "text": "Temperature Dependent Functions of the Electron Neutral Momentum\n Transfer Collision Cross Sections of Selected Combustion Plasma Species: The collision cross sections (CCS), momentum transfer cross sections (MTCS),\nor scattering cross sections (SCS) of an electron neutral pair are important\ncomponents for computing the electric conductivity of a plasma gas. Larger\ncollision cross sections for electrons moving freely within neutral particles\n(molecules or atoms) cause more scattering of these electrons by the neutral\nparticles, which leads to degraded electron mobility, and thus reduced electric\nconductivity of the plasma gas that consists of electrons, neutral particles,\nand ions. The present work aimed to identify the level of disagreement between\nfour different methods for describing how electron neutral collision cross\nsections vary when they are treated as a function of electron temperature\nalone. These four methods are based on data or models previously reported in\nthe literature. The analysis covered six selected gaseous species that are\nrelevant to combustion plasma, which are as follows: carbon monoxide (CO),\ncarbon dioxide (CO2), molecular hydrogen (H2), water vapor (H2O), potassium\nvapor (K), and molecular oxygen (O2). The temperature dependence of the\ncollision cross sections for these species was investigated in the range from\n2000 K to 3000 K, which is suitable for both conventional air fuel combustion\nand elevated temperature oxygen fuel (oxy-fuel) combustion. The findings of the\npresent study suggest that linear functions are enough to describe the\nvariations in the collision cross sections of the considered species in the\ntemperature range of interest for combustion plasma. Also, the values of the\ncoefficient of variation (defined as the sample standard deviation divided by\nthe mean) in the collision cross sections using the four methods were\napproximately 27% for CO, 42% for CO2, 13% for H2, 39% for H2O, 44% for K, and\n19% for O2.", "category": "physics_plasm-ph" }, { "text": "Ionization potential depression and optical spectra in a Debye plasma\n model: We show how optical spectra in dense plasmas are determined by the shift of\nenergy levels as well as the broadening owing to collisions with the plasma\nparticles. In lowest approximation, the interaction with the plasma particles\nis described by the RPA dielectric function, leading to the Debye shift of the\ncontinuum edge. The bound states remain nearly un-shifted, their broadening is\ncalculated in Born approximation. The role of ionization potential depression\nas well as the Inglis-Teller effect are shown. The model calculations have to\nbe improved going beyond the lowest (RPA) approximation when applying to WDM\nspectra.", "category": "physics_plasm-ph" }, { "text": "Simulation Study of the Influence of Experimental Variations on the\n Structure and Quality of Plasma Liners: Simulation studies of a section of a spherically imploding plasma liner,\nformed by the merger of six hypersonic plasma jets, have been performed at\nconditions relevant to the Plasma Liner Experiment (PLX) [S. C. Hsu et al.,\nIEEE Trans. Plasma Sci.~{\\bf 46}, 1951 (2018)]. The main aim of simulations was\nthe sensitivity study of the detailed structure of plasma liners and their\nglobal properties to experimental mass variations and timing jitter across the\nsix plasma jets. Experimentally observable synthetic quantities have been\ncomputed using simulation data and compared with the available experimental\ndata. Simulations predicted that the primary oblique shock wave structure is\npreserved at small experimental variations. At later phases of the liner\nimplosion, primary shocks and, especially, secondary shocks are more sensitive\nto experimental variations. These conclusions follow from the simulation data\nas well as comparisons between synthetic and experimental interferometry and\nvisible images. Small displacements of the shock wave structures may cause\nsignificant changes in the synthetic interferometer data at early time. Our\nstudies also showed that the global properties of the plasma liners (averaged\nMach number and averaged ram pressure along leading edges of plasma liners) are\nnot very sensitive to experimental variations. Simulation data of the liner\nstructure were largely confirmed by the PLX experimental data.", "category": "physics_plasm-ph" }, { "text": "Acceleration and focusing of multispecies ion beam using a converging\n laser-driven shock: We demonstrate an ion acceleration scheme capable of simultaneously focusing\nand accelerating a multispecies ion beam with monoenergetic spectra to a few\nmicron radius. The focal length and ion mean energy can be independently\ncontrolled: the former by using a different front-surface shape and the latter\nby tuning the laser-plasma parameters. We interpret the results using simple\nmodels and validate the results using first-principles simulations. The scheme\nis applicable to different laser transverse profiles and multi-ion species\ntarget, and limiting factors for the ion focusing are delineated. The generated\nion bean exhibits high charge, low emittance , and high energy flux and is of\ninterest to various applications including inertial Confinement Fusion (ICF),\nhigh flux neutron generation, and biomedical applications.", "category": "physics_plasm-ph" }, { "text": "Oscillating Ponomarenko dynamo in the highly conducting limit: This paper considers dynamo action in smooth helical flows in cylindrical\ngeometry, otherwise known as Ponomarenko dynamos, with periodic time\ndependence. An asymptotic framework is developed that gives growth rates and\nfrequencies in the highly conducting limit of large magnetic Reynolds number,\nwhen modes tend to be localized on resonant stream surfaces. This theory is\nvalidated by means of numerical simulations.", "category": "physics_plasm-ph" }, { "text": "Capacitively-coupled rf discharge with a large amount of microparticles:\n spatiotemporal emission pattern and microparticle arrangement: The effect of micron-sized particles on a low-pressure capacitively-coupled\nrf discharge is studied both experimentally and using numerical simulations. In\nthe laboratory experiments, microparticle clouds occupying a considerable\nfraction of the discharge volume are supported against gravity with the help of\nthe thermophoretic force. The spatiotemporally resolved optical emission\nmeasurements are performed with different arrangements of microparticles. The\nnumerical simulations are carried out on the basis of a one-dimensional hybrid\n(fluid-kinetic) discharge model describing the interaction between plasma and\nmicroparticles in a self-consistent way. The study is focused on the role of\nmicroparticle arrangement in interpreting the spatiotemporal emission\nmeasurements. We show that it is not possible to reproduce simultaneously the\nobserved microparticle arrangement and emission pattern in the framework of the\nconsidered one-dimensional model. This disagreement is discussed and attributed\nto two-dimensional effects, e.g., radial diffusion of the plasma components.", "category": "physics_plasm-ph" }, { "text": "Ion versus electron heating in compressively driven astrophysical\n gyrokinetic turbulence: The partition of irreversible heating between ions and electrons in\ncompressively driven (but subsonic) collisionless turbulence is investigated by\nmeans of nonlinear hybrid gyrokinetic simulations. We derive a prescription for\nthe ion-to-electron heating ratio $Q_\\rmi/Q_\\rme$ as a function of the\ncompressive-to-Alfv\\'enic driving power ratio $P_\\compr/P_\\AW$, of the ratio of\nion thermal pressure to magnetic pressure $\\beta_\\rmi$, and of the ratio of\nion-to-electron background temperatures $T_\\rmi/T_\\rme$. It is shown that\n$Q_\\rmi/Q_\\rme$ is an increasing function of $P_\\compr/P_\\AW$. When the\ncompressive driving is sufficiently large, $Q_\\rmi/Q_\\rme$ approaches $\\simeq\nP_\\compr/P_\\AW$. This indicates that, in turbulence with large compressive\nfluctuations, the partition of heating is decided at the injection scales,\nrather than at kinetic scales. Analysis of phase-space spectra shows that the\nenergy transfer from inertial-range compressive fluctuations to\nsub-Larmor-scale kinetic Alfv\\'en waves is absent for both low and high\n$\\beta_\\rmi$, meaning that the compressive driving is directly connected to the\nion entropy fluctuations, which are converted into ion thermal energy. This\nresult suggests that preferential electron heating is a very special case\nrequiring low $\\beta_\\rmi$ and no, or weak, compressive driving. Our heating\nprescription has wide-ranging applications, including to the solar wind and to\nhot accretion disks such as M87 and Sgr A*.", "category": "physics_plasm-ph" }, { "text": "Controlling ion kinetic energy distributions in laser produced plasma\n sources by means of a picosecond pulse pair: The next generation of lithography machines uses extreme ultraviolet (EUV)\nlight originating from laser-produced plasma (LPP) sources, where a small tin\ndroplet is ionized by an intense laser pulse to emit the requested light at\n13.5 nm. Numerous irradiation schemes have been explored to increase conversion\nefficiency (CE), out of which a double-pulse approach comprising a weak\npicosecond Nd:YAG pre-pulse followed by a powerful pulse is considered to be\nvery promising [1]. Nevertheless, even for such CE-optimized schemes, ion\ndebris ejected from the plasma with kinetic energies up to several keV remain a\nfactor that hampers the maximum performance of LPP sources. In this letter we\npropose a novel pre-pulse scheme consisting of a picosecond pulse pair at 1064\nnm, which decreases the amount of undesirable fast ions, avoids\nback-reflections to the lasers and enables one to tailor the target shape.", "category": "physics_plasm-ph" }, { "text": "Magnetic Shield Approach Toward Non-adiabatic Low to High Field Positron\n Beam Transition for Microtraps: This simulation study sheds light on effectiveness of a magnetic shield on\nnon-adiabatic transport of a positron beam from 3700G to 100G where we were\nable to maximize the low to high field transmission efficiency for 6.5KeV\npositrons. An initially brighter beam can be transported even more efficiently.\nCross-sectional uniformity of the beam is also studied for different beam\nenergies. This is important for filling microtraps consist of thousands tubes\nwith diameters of tens of microns each. Transverse momentum of the particles is\nalso studied since it causes mirroring in the high field and compromises\nefficiency. Beam remoderation is required as ~eV energy positrons will be\ntrapped in the Microtrap. The observed magnetic shield assisted low to high\nfiled transition opens new avenue for high efficiency manipulating of\nnon-neutral beams.", "category": "physics_plasm-ph" }, { "text": "3-D exact analytical solutions of two fluid plasma, MHD and neutral\n fluid equations for the creation of ordered structures as well as jet-like\n flows: The 3-D exact analytical solutions of ideal two fluid plasma, single fluid\nplasma (MHD) and neutral fluid equations have been found using physically\njustifiable assumptions. Surprisingly these solutions satisfy all\nnon-linearities in the systems. It is pointed out that these solutions explain\nthe fundamental mechanism behind the creation of vast variety of ordered\nstructures in plasmas and fluids. In the limiting case of two dimensional (2-D)\ndependence of fields, the theoretical model for plasma is applied to explain\nthe formation of spicules in solar chromosphere. It is pointed out that the\nmain contribution of electron (ion) baro clinic vectors is to produce vorticity\nin the plasma and that magnetic field generation is coupled with the flow of\nboth electrons and ions.", "category": "physics_plasm-ph" }, { "text": "Flux surface shaping effects on tokamak edge turbulence and flows: Shaping of magnetic flux surfaces is found to have a strong impact on\nturbulence and transport in tokamak edge plasmas. A series of axisymmetric\nequilibria with varying elongation and triangularity, and a divertor\nconfiguration are implemented into a computational gyrofluid turbulence model.\nThe mechanisms of shaping effects on turbulence and flows are identified.\nTransport is mainly reduced by local magnetic shearing and an enhancement of\nzonal shear flows induced by elongation and X-point shaping.", "category": "physics_plasm-ph" }, { "text": "Two-fluid and magnetohydrodynamic modelling of magnetic reconnection in\n the MAST spherical tokamak and the solar corona: Twisted magnetic flux ropes are ubiquitous in space and laboratory plasmas,\nand the merging of such flux ropes through magnetic reconnection is an\nimportant mechanism for restructuring magnetic fields and releasing free\nmagnetic energy. The merging-compression scenario is one possible start up\nscheme for spherical tokamaks, which has been used on the Mega Amp Spherical\nTokamak MAST. Two current-carrying plasma rings, or flux ropes, approach each\nother through the mutual attraction of their like currents, and merge, through\nmagnetic reconnection, into a single plasma torus, with substantial plasma\nheating. 2D resistive MHD and Hall MHD simulations of this process are\nreported, and new results for the temperature distribution of ions and\nelectrons are presented. A model of the based on relaxation theory is also\ndescribed, which is now extended to tight aspect ratio geometry. This model\nallows prediction of the final merged state and the heating. The implications\nof the relaxation model for heating of the solar corona are also discussed, and\na model of the merger of two or more twisted coronal flux ropes is presented,\nallowing for different senses of twist.", "category": "physics_plasm-ph" }, { "text": "Direct Microstability Optimization of Stellarator Devices: Turbulent transport is regarded as one of the key issues in magnetic\nconfinement nuclear fusion, both for tokamaks in stellarators. In this letter,\nwe show that a significant decrease in the turbulent heat flux can be obtained\nin an efficient manner by coupling stellarator optimization with linear\ngyrokinetic simulations. This is accomplished by computing the quasi-linear\nheat flux at each step of the optimization process, as well as the deviation\nfrom quasisymmetry, and minimizing their sum, leading to a balance between\nneoclassical and turbulent transport.", "category": "physics_plasm-ph" }, { "text": "Current Sheets and Collisionless Damping in Kinetic Plasma Turbulence: We present the first study of the formation and dissipation of current sheets\nat electron scales in a wave-driven, weakly collisional, 3D kinetic turbulence\nsimulation. We investigate the relative importance of dissipation associated\nwith collisionless damping via resonant wave-particle interactions versus\ndissipation in small-scale current sheets in weakly collisional plasma\nturbulence. Current sheets form self-consistently from the wave-driven\nturbulence, and their filling fraction is well correlated to the electron\nheating rate. However, the weakly collisional nature of the simulation\nnecessarily implies that the current sheets are not significantly dissipated\nvia Ohmic dissipation. Rather, collisionless damping via the Landau resonance\nwith the electrons is sufficient to account for the measured heating as a\nfunction of scale in the simulation, without the need for significant Ohmic\ndissipation. This finding suggests the possibility that the dissipation of the\ncurrent sheets is governed by resonant wave-particle interactions and that the\nlocations of current sheets correspond spatially to regions of enhanced\nheating.", "category": "physics_plasm-ph" }, { "text": "Ponderomotive forces in unmagnetized plasmas described by Kappa\n distribution functions: The Washimi and Karpman ponderomotive interaction due to electromagnetic\nwaves propagation is investigated for unmagnetized plasmas described by a\nisotropic Kappa distribution. We performed a brief analysis of the influence of\nthe Kappa distribution in the dispersion relations for a low temperature plasma\nexpansion at the lowest order in which the thermal effects are appreciated. The\nspatial and temporal factor of the ponderomotive force is obtained as a\nfunction of the wavenumber, the spectral index $\\kappa$ and the ratio between\nthe plasma thermal velocity and the speed of light. Our results show that for\nunmagnetized plasmas non-thermal effects are negligible for the spatial\nponderomotive force when non-relativistic thermal velocities are considered.\nHowever, for unmagnetized plasmas the temporal factor of the ponderomotive\nforce appears only due to the presence of suprathermal particles, with a clear\ndependence on the $\\kappa$ index. We have also analysed the role of the\nnon-thermal effect in the induced Washimi and Karpman ponderomotive\nmagnetization and the total power radiated associated with it. We have also\nshown that the slowly varying induced ponderomotive magnetic field magnitude\nincreases as the plasma moves away from thermal equilibrium.", "category": "physics_plasm-ph" }, { "text": "Studies of a hollow cathode discharge using mass spectrometry and\n electrostatic probe techniques: Hollow cathode discharges (HCD) are capable of generating dense plasmas and\nhave been used for development of high-rate, low-pressure, high-efficiency\nprocessing machines. The geometric feature of a HCD promotes oscillations of\nhot electrons inside the cathode, thereby enhancing ionization, ion bombardment\nof inner walls and other subsequent processes. At the same power the hollow\ncathode exhibits plasma density one to two orders of magnitude higher than that\nof conventional planar electrodes. The aim of the present studies was to obtain\nexperimental observations about the main features of a dc hollow cathode\ndischarge in order to evaluate its capability of generating compounds in the\nplasma medium, by reaction between sputtered species from the cathode and\nradicals from the gas discharge. Especial interest is focused on aluminum\nnitride (AlN) formation which is very desirable if a deposition of thin film of\nthis material is concerned. Plasma properties were inferred from the\ncurrent-voltage characteristics of a single Langmuir probe positioned at the\ninter-cathode space. Through mass spectrometry technique some species in the\nplasma gas phase could be monitored for various operating conditions of the\ndischarge. This mass analysis together with the probe measurements gives\nguidance for optimization of AlN generation in the discharge consequently for\ndeposition of thin films of this material.", "category": "physics_plasm-ph" }, { "text": "Estimation of Total Fusion Reactivity and Contribution from Suprathermal\n Tail using 3-parameter Dagum Ion Speed Distribution: Thermonuclear fusion reactivity is a pivotal quantity in the studies\npertaining to fusion energy production, fusion ignition and energy break-even\nanalysis in both inertially and magnetically confined systems. Although nuclear\nfusion reactivity and thereafter the power density of a magnetic confinement\nfusion reactor and the fulfillment of the ignition criterion are quantitatively\ndetermined by assuming the ion speed distribution to be Maxwellian, a\nsignificant population of suprathermal ions,with energy greater than the\nquasi-Maxwellian background plasma temperature, is generated by the fusion\nreactions and auxiliary heating in the fusion devices. In the current work\n3-parameter Dagum speed distribution has been introduced to include the effect\nof suprathermal ion population in the calculation of total fusion reactivity.\nThe extent of enhancement in the fusion reactivity, at different back-ground\ntemperatures of the fusion fuel plasma, due to the suprathermal ion population\nhas also been discussed.", "category": "physics_plasm-ph" }, { "text": "The Darrieus-Landau instability in fast deflagration and laser ablation: The problem of the Darrieus-Landau instability at a discontinuous\ndeflagration front in a compressible flow is solved. Numerous previous attempts\nto solve this problem suffered from the deficit of boundary conditions. Here,\nthe required additional boundary condition is derived rigorously taking into\naccount the internal structure of the front. The derived condition implies a\nconstant mass flux at the front; it reduces to the classical Darrieus-Landau\ncondition in the limit of an incompressible flow. It is demonstrated that in\ngeneral the solution to the problem depends on the type of energy source\npresent in the system. In the common case of a strongly localized source,\ncompression effects make the Darrieus-Landau instability considerably weaker.\nIn particular, the Darrieus-Landau instability growth rate is reduced for laser\nablation in comparison with the classical incompressible case. The instability\ndisappears completely in the Chapman-Jouguet regime of ultimately fast\ndeflagration.", "category": "physics_plasm-ph" }, { "text": "Ultracold Plasma Expansion in a Magnetic Field: We measure the expansion of an ultracold plasma across the field lines of a\nuniform magnetic field. We image the ion distribution by extracting the ions\nwith a high voltage pulse onto a position-sensitive detector. Early in the\nlifetime of the plasma ($< 20$ $\\mu$s), the size of the image is dominated by\nthe time-of-flight Coulomb explosion of the dense ion cloud. For later times,\nwe measure the 2-D Gaussian width of the ion image, obtaining the transverse\nexpansion velocity as a function of magnetic field (up to 70 G). We observe\nthat the expansion velocity scales as B$^{-1/2}$, explained by a nonlinear\nambipolar diffusion model with anisotropic diffusion in two different\ndirections.", "category": "physics_plasm-ph" }, { "text": "Using Dust as Probes to Determine Sheath Extent and Structure: Two in-situ experimental methods are presented in which dust particles are\nused to determine the extent of the sheath and gain information about the\ntime-averaged electric force profile within a RF plasma sheath. These methods\nare advantageous because they are not only simple and quick to carry out, but\nthey also can be performed using standard dusty plasma experimental equipment.\nIn the first method, dust particles are tracked as they fall through the plasma\ntoward the lower electrode. These trajectories are then used to determine the\nelectric force on the particle as a function of height as well as the extent of\nthe sheath. In the second method, dust particle levitation height is measured\nacross a wide range of RF voltages. Similarities were observed between the two\nexperiments, but in order to understand the underlying physics behind these\nobservations, the same conditions were replicated using a self-consistent fluid\nmodel. Through comparison of the fluid model and experimental results, it is\nshown that the particles exhibiting a levitation height that is independent of\nRF voltage indicate the sheath edge - the boundary between the quasineutral\nbulk plasma and the sheath. Therefore, both of these simple and inexpensive,\nyet effective, methods can be applied across a wide range of experimental\nparameters in any ground-based RF plasma chamber to gain useful information\nregarding the sheath, which is needed for interpretation of dusty plasma\nexperiments.", "category": "physics_plasm-ph" }, { "text": "Spherical dust acoustic solitary waves with two temperature ions: The nonlinear dust acoustic waves in unmagnetized dusty plasma which consists\nof two temperature Boltzmann distributed ions and Boltzmann distributed\nelectrons in spherical dimension investigated and obtained spherical Kadomtsev\nPetviashvili (SKP) equation and shown that the dust acoustic solitary wave can\nexist in the SKP equation.", "category": "physics_plasm-ph" }, { "text": "Hard X-rays from laser-wakefield accelerators in density tailored\n plasmas: Betatron x-ray sources from laser-plasma accelerators combine compactness,\nhigh peak brightness, femtosecond pulse duration and broadband spectrum.\nHowever, when produced with Terawatt class lasers, their energy was so far\nrestricted to a few kilo-electronvolt (keV), limiting the range of possible\napplications. Here we present a simple method to increase the energy and the\nflux by an order of magnitude without increasing the laser energy. The orbits\nof the relativistic electrons emitting the radiation were controlled using\ndensity tailored plasmas so that the efficiency of the Betatron source is\nsignificantly improved.", "category": "physics_plasm-ph" }, { "text": "Magnetic reconnection and thermal equilibration: When a magnetic field is forced to evolve on a time scale $\\tau_{ev}$, as by\nfootpoint motions driving the solar corona or non-axisymmetric instabilities in\ntokamaks, the magnetic field lines undergo large-scale changes in topology on a\ntime scale approximately an order of magnitude longer than $\\tau_{ev}$. But,\nthe physics that allows such changes operates on a time scale eight or more\norders of magnitude slower. An analogous phenomenon occurs in air. Temperature\nequilibration occurs on a time scale approximately an order of magnitude longer\nthan it takes air to cross a room, $\\tau_{ev}$, although the physical mechanism\nthat allows temperature equilibration is approximately four orders of magnitude\nslower than $\\tau_{ev}$. The use of Lagrangian coordinates allows the\nfundamental equations to be solved and both phenomena explained. The paradigms\nand presumptions of traditional theories of magnetic reconnection are so\ningrained that the understanding gained from analyses using Lagrangian\ncoordinates has been largely ignored. The theories of thermal equilibration and\nmagnetic reconnection are developed in parallel to help readers obtain an\nunderstanding of the importance and implications of analyses using Lagrangian\ncoordinates.", "category": "physics_plasm-ph" }, { "text": "Inhomogeneity of dusty crystals and plasma diagnostics: Real dusty crystals are inhomogeneous due to the presence of external forces.\nWe suggest approximations for calculations of different types of inhomogeneous\nDC (chain and DC with a few slabs) in the equilibrium state. The results are in\na good agreement with experimental results and can be used as an effective\ndiagnostic method for many dusty systems.", "category": "physics_plasm-ph" }, { "text": "On the Distribution of Plasmoids In High-Lundquist-Number Magnetic\n Reconnection: The distribution function $f(\\psi)$ of magnetic flux $\\psi$ in plasmoids\nformed in high-Lundquist-number current sheets is studied by means of an\nanalytic phenomenological model and direct numerical simulations. The\ndistribution function is shown to follow a power law $f(\\psi)\\sim\\psi^{-1}$,\nwhich differs from other recent theoretical predictions. Physical explanations\nare given for the discrepant predictions of other theoretical models.", "category": "physics_plasm-ph" }, { "text": "Transition from no-ELM response to pellet ELM triggering during pedestal\n build-up -- insights from extended MHD simulations: Pellet ELM triggering is a well established scheme for decreasing the time\nbetween two successive ELM crashes below its natural value. Reliable ELM pacing\nhas been demonstrated experimentally in several devices increasing the ELM\nfrequency considerably. However, it was also shown that the frequency cannot be\nincreased arbitrarily due to a so-called lag-time. During this time after a\npreceding natural or triggered ELM crash, neither a natural ELM crash occurs\nnor the triggering of an ELM crash by pellet injection is possible. For this\narticle, pellet ELM triggering simulations are advanced beyond previous studies\nin two ways. Firstly, realistic ExB and diamagnetic background flows are\nincluded. And secondly, the pellet is injected at different stages of the\npedestal build-up. This allows to recover the lag-time for the first time in\nsimulations and investigate it in detail. A series of non-linear extended MHD\nsimulations is performed to investigate the plasma dynamics resulting from an\ninjection at different time points during the pedestal build-up. The\nexperimentally observed lag-time is qualitatively reproduced well. In\nparticular, a sharp transition is observed between the regime where no ELMs can\nbe triggered and the regime where pellet injection causes an ELM crash. Via\nvariations of pellet parameters and injection time, the two regimes are studied\nand compared in detail revealing pronounced differences in the non-linear\ndynamics. The toroidal mode spectrum is significantly broader when an ELM crash\nis triggered enhancing the stochasticity and therefore also the losses of\nthermal energy along magnetic field lines. In the heat fluxes to the divertor\ntargets, pronounced toroidal asymmetries are observed. In case of high\ninjection velocities leading to deep penetration, also the excitation of core\nmodes like the $2/1$ neoclassical tearing mode is observed.", "category": "physics_plasm-ph" }, { "text": "Prompt electrons driving ion acceleration and formation of a two\n temperatures plasma in nanosecond laser-ablation domain: We present the results of an experiment on plasma generation via laser\nablation at 10^12 W/cm^2 of power intensity and in a nanosecond domain. Prompt\nelectrons emission and complex plasma plume fragmentation were simultaneously\nobserved for the first time in this laser intensity regime, along with a double\nelectron temperature inside the plasma bulk surviving for a long time to the\nplume expansion. 1D PIC simulations are in agreement with experimental data as\nlong as the emission of initial prompt electrons is considered. This assumption\nresults to be the key to explain all the other experimental evidences.", "category": "physics_plasm-ph" }, { "text": "Kinetic Interpretation of Resonance Phenomena in Low Pressure\n Capacitively Coupled Radio Frequency Plasmas: The kinetic origin of resonance phenomena in capacitively coupled radio\nfrequency plasmas is discovered based on particle-based numerical simulations.\nThe analysis of the spatio-temporal distributions of plasma parameters such as\nthe densities of hot and cold electrons, as well as the conduction and\ndisplacement currents reveals the mechanism of the formation of multiple\nelectron beams during sheath expansion. The interplay between highly energetic\nbeam electrons and low energetic bulk electrons is identified as the physical\norigin of the excitation of harmonics in the current.", "category": "physics_plasm-ph" }, { "text": "Adjoint methods for stellarator shape optimization and sensitivity\n analysis: The design of a stellarator with acceptable confinement properties requires\noptimization of the magnetic field in the non-convex, high-dimensional spaces\ndescribing their geometry. Another major challenge facing the stellarator\nprogram is the sensitive dependence of confinement properties on\nelectro-magnetic coil shapes, necessitating the construction of the coils under\ntight tolerances. In this Thesis, we address these challenges with the\napplication of adjoint methods and shape sensitivity analysis. Adjoint methods\nenable the efficient computation of the gradient of a function that depends on\nthe solution to a system of equations, such as linear or nonlinear PDEs. This\nenables gradient-based optimization in high-dimensional spaces and efficient\nsensitivity analysis. We present the first applications of adjoint methods for\nstellarator shape optimization. The first example we discuss is the\noptimization of coil shapes based on the generalization of a continuous current\npotential model. Understanding the sensitivity of coil metrics to perturbations\nof the winding surface allows us to understand features of configurations that\nenable simpler coils. We next consider solutions of the drift-kinetic equation.\nAn adjoint drift-kinetic equation is derived based on the self-adjointness\nproperty of the Fokker-Planck collision operator, allowing us to compute the\nsensitivity of neoclassical quantities to perturbations of the magnetic field\nstrength. Finally, we consider functions that depend on solutions of the MHD\nequilibrium equations. We generalize the self-adjointness property of the MHD\nforce operator to include perturbations of the rotational transform and the\ncurrents outside the confinement region. This self-adjointness property is\napplied to develop an adjoint method for computing the derivatives of such\nfunctions with respect to perturbations of coil shapes or the plasma boundary.", "category": "physics_plasm-ph" }, { "text": "Transition to super-diffusive transport in turbulent plasmas: We investigate the motion of charged particles in a turbulent electrostatic\npotential using guiding-center theory. By increasing the Larmor radius, the\ndynamics exhibit close-to-ballistic transport properties. The transition from\ndiffusive to ballistic transport is analyzed using nonlinear dynamics. It is\nfound that twistless invariant tori in the guiding-center dynamics are\nresponsible for this transition, drastically affecting transport properties of\ncharged particles.", "category": "physics_plasm-ph" }, { "text": "Ion acceleration by superintense laser-plasma interaction: Ion acceleration driven by superintense laser pulses is attracting an\nimpressive and steadily increasing effort. Motivations can be found in the\npotential for a number of foreseen applications and in the perspective to\ninvestigate novel regimes as far as available laser intensities will be\nincreasing. Experiments have demonstrated in a wide range of laser and target\nparameters the generation of multi-MeV proton and ion beams with unique\nproperties such as ultrashort duration, high brilliance and low emittance. In\nthis paper we give an overview of the state-of-the art of ion acceleration by\nlaser pulses as well as an outlook on its future development and perspectives.\nWe describe the main features observed in the experiments, the observed scaling\nwith laser and plasma parameters and the main models used both to interpret\nexperimental data and to suggest new research directions.", "category": "physics_plasm-ph" }, { "text": "Band Structure of the Growth Rate of the Two-Stream Instability of an\n Electron Beam Propagating in a Bounded Plasma: This paper presents a study of the two-stream instability of an electron beam\npropagating in a finite-size plasma placed between two electrodes. It is shown\nthat the growth rate in such a system is much smaller than that of an infinite\nplasma or a finite size plasma with periodic boundary conditions. Even if the\nwidth of the plasma matches the resonance condition for a standing wave, a\nspatially growing wave is excited instead with the growth rate small compared\nto that of the standing wave in a periodic system. The approximate expression\nfor this growth rate is $\\gamma \\approx\n(1/13)\\omega_{pe}(n_{b}/n_{p})(L\\omega_{pe}/v_{b})\\ln (L\\omega_{pe}/v_{b})[\n1-0.18\\cos ( L\\omega_{pe}/v_{b}+{\\pi }/{2}) ]$, where $\\omega_{pe}$ is the\nelectron plasma frequency, $n_{b}$ and $n_{p}$ are the beam and the plasma\ndensities, respectively, $v_{b}$ is the beam velocity, and $L$ is the plasma\nwidth. The frequency, wave number and the spatial and temporal growth rates as\nfunctions of the plasma size exhibit band structure. The amplitude of\nsaturation of the instability depends on the system length, not on the beam\ncurrent. For short systems, the amplitude may exceed values predicted for\ninfinite plasmas by more than an order of magnitude.", "category": "physics_plasm-ph" }, { "text": "Three-dimensional structure and stability of discontinuities between\n unmagnetized pair plasma and magnetized electron-proton plasma: We study with a 3D PIC simulation discontinuities between an\nelectron-positron pair plasma and magnetized electrons and protons. A pair\nplasma is injected at one simulation boundary with a speed 0.6$c$ along its\nnormal. It expands into an electron-proton plasma and a magnetic field that\npoints orthogonally to the injection direction. Diamagnetic currents expel the\nmagnetic field from within the pair plasma and pile it up in front of it. It\npushes electrons, which induces an electric field pulse ahead of the magnetic\none. This initial electromagnetic pulse (EMP) confines the pair plasma\nmagnetically and accelerates protons electrically. The fast flow of the\ninjected pair plasma across the protons behind the initial EMP triggers the\nfilamentation instability. Some electrons and positrons cross the injection\nboundary and build up a second EMP. Electron-cyclotron drift instabilities\nperturb the plasma ahead of both EMPs seeding a Rayleigh-Taylor-type\ninstability. Despite equally strong perturbations ahead of both EMPs, the\nsecond EMP is much more stable than the initial one. We attribute the rapid\ncollapse of the initial EMP to the filamentation instability, which perturbed\nthe plasma behind it. The Rayleigh-Taylor-type instability transforms the\nplanar EMPs into transition layers, in which magnetic flux ropes and\nelectrostatic forces due to uneven numbers of electrons and positrons slow down\nand compress the pair plasma and accelerate protons. In our simulation, the\nexpansion speed of the pair cloud decreased by about an order of magnitude and\nits density increased by the same factor. Its small thickness implies that it\nis capable of separating a relativistic pair outflow from an electron-proton\nplasma, which is essential for collimating relativistic jets of pair plasma in\ncollisionless astrophysical plasma.", "category": "physics_plasm-ph" }, { "text": "Three-dimensional particle-in-cell modeling of parametric instabilities\n near the quarter-critical density in plasmas: The nonlinear regime of laser-plasma interaction including both\ntwo-plasmon--decay (TPD) and stimulated Raman scattering (SRS) instabilities\nhas been studied in three-dimensional (3-D) particle-in-cell simulations with\nparameters relevant to the inertial confinement fusion (ICF) experiments. SRS\nand TPD develop in the same region in plasmas, and the generation of fast\nelectrons can be described accurately with only the full model including both\nSRS and TPD. The growth of instabilities in the linear stage is found to be in\ngood agreement with analytical theories. In the saturation stage the enhanced\nlow-frequency density perturbations driven by the daughter waves of the SRS\nsidescattering can saturate the TPD and consequently inhibit the fast-electron\ngeneration. The fast-electron flux in 3-D modeling is up to an order of\nmagnitude smaller than previously reported in 2-D TPD simulations, bringing it\nclose to the results of ICF experiments.", "category": "physics_plasm-ph" }, { "text": "Direct-drive measurements of laser-imprint-induced shock velocity\n nonuniformities: Perturbations in the velocity profile of a laser-ablation-driven shock wave\nseeded by speckle in the spatial beam intensity (i.e., laser imprint) have been\nmeasured. Direct measurements of these velocity perturbations were recorded\nusing a two-dimensional high-resolution velocimeter probing plastic material\nshocked by a 100-ps picket laser pulse from the OMEGA laser system. The\nmeasured results for experiments with one, two, and five overlapping beams\nincident on the target clearly demonstrate a reduction in long-wavelength\n($>$25 um) perturbations with an increasing number of overlapping laser beams,\nconsistent with theoretical expectations. These experimental measurements are\ncrucial to validate radiation-hydrodynamics simulations of laser imprint for\nlaser direct drive inertial confinement fusion research since they highlight\nthe significant (factor of 3) underestimation of the level of seeded\nperturbation when the microphysics processes for initial plasma formation, such\nas multiphoton ionization are neglected.", "category": "physics_plasm-ph" }, { "text": "Behavior of ZnO-coated alumina dielectric barrier discharge in\n atmospheric pressure air: A complete investigation of the discharge behavior of dielectric barrier\ndischarge device using ZnO-coated dielectric layer in atmospheric pressure is\nmade. Highly conductive ZnO film was deposited on the dielectric surface.\nDischarge characteristic of the dielectric barrier discharge are examined in\ndifferent aspects. Experimental result shows that discharge uniformity is\nimproved definitely in the case of ZnO-coated dielectric barrier discharge. And\nrelevant theoretical models and explanation are presented to describing its\ndischarge physics.", "category": "physics_plasm-ph" }, { "text": "Absolute instability modes due to rescattering of stimulated Raman\n scattering in a large nonuniform plasma: Absolute instability modes due to rescattering of SRS in a large nonuniform\nplasma are studied theoretically and numerically. The backscattered light of\nconvective SRS can be considered as a pump light with a finite bandwidth. The\ndifferent frequency components of the backscattered light can be coupled to\ndevelop absolute stimulated Raman scattering (SRS) and two plasmon decay (TPD)\ninstability near their quarter-critical densities via rescattering process. The\nabsolute SRS mode develops a Langmuir wave with a high phase velocity about\n$c/\\sqrt{3}$ with $c$ the light speed in vacuum. Given that most electrons are\nat low velocities in the linear stage, the absolute SRS mode grows with much\nweak Landau damping. When the interaction evolves into the nonlinear regime,\nthe Langmuir wave can heat abundant electrons up to a few hundred keV. Our\ntheoretical model is validated by particle-in-cell simulations. The absolute\ninstabilities may play a considerable role in the experiments of inertial\nconfined fusion.", "category": "physics_plasm-ph" }, { "text": "Fast acceleration of transrelativistic electrons in astrophysical\n turbulence: Highly energetic, relativistic electrons are commonly present in many\nastrophysical systems, from solar flares to the intra-cluster medium, as\nindicated by observed electromagnetic radiation. However, open questions remain\nabout the mechanisms responsible for their acceleration, and possible\nre-acceleration. Ubiquitous plasma turbulence is one of the possible universal\nmechanisms. We study the energization of transrelativistic electrons in\nturbulence using hybrid particle-in-cell, which provide a realistic model of\nAlfv\\'{e}nic turbulence from MHD to sub-ion scales, and test particle\nsimulations for electrons. We find that, depending on the electron initial\nenergy and turbulence strength, electrons may undergo a fast and efficient\nphase of energization due to the magnetic curvature drift during the time they\nare trapped in dynamic magnetic structures. In addition, electrons are\naccelerated stochastically which is a slower process that yields lower maximum\nenergies. The combined effect of these two processes determines the overall\nelectron acceleration. With appropriate turbulence parameters, we find that\nsuperthermal electrons can be accelerated up to relativistic energies. For\nexample, with heliospheric parameters and a relatively high turbulence level,\nrapid energization to MeV energies is possible.", "category": "physics_plasm-ph" }, { "text": "Comment: Anomalous Anisotropic Light Scattering in Ge-Doped Silica Glass: This is a comment on a paper by Kazansky et al. appeared in Phys. Rev. Lett.,\n82, 2199 (1999). We demonstrate that the bremsstrahlung of photoelectrons,\nwhich oscillate in the light field of an ultrashort laser pulse, accounts for\nthe anomalous anisotropic light scattering, the \"propeller effect\", described\nin the paper of Kazansky et al. This mechanism of light emission could explain\nsome results of white-light continuum generation and dielectric breakdown\nemission.", "category": "physics_plasm-ph" }, { "text": "Study of diamagnetism in laser-produced plasma using B-dot probe: Time-varying diamagnetism in laser-produced plasma moving across the\ntransverse magnetic field with different field strengths has been studied using\nfast imaging and magnetic probe. The emphasis of the present work is on the\ndevelopment of suitable B-dot probe and quantitative analysis of induced\ndiamagnetic field in an expanding plasma plume. A Helmholtz coil associated\nwith pulsed power system is used to produce uniform magnetic field of different\nstrengths. Helmholtz coil allowed the plume imaging along the magnetic field\nlines, which gives the direct structural information of the induced diamagnetic\ncavity. A high frequency three-axis B-dot probe has been developed to measure\nthe transient magnetic field. Different experimental approaches have been used\nto test the response, sensitivity and calibration of the developed probe.\nFindings indicate that plasma plume shows the perfect diamagnetic behaviour for\nthe external field varying from 0.13 T to 0.57 T where the induced magnetic\nfield almost completely displaced the external magnetic field.", "category": "physics_plasm-ph" }, { "text": "Multi-GeV Wakefield Acceleration in a Plasma-Modulated Plasma\n Accelerator: We investigate the accelerator stage of a Plasma-Modulated Plasma Accelerator\n(P-MoPA) [Phys. Rev. Lett. 127, 184801 (2021)] using both the paraxial wave\nequation and particle-in-cell (PIC) simulations. We show that adjusting the\nlaser and plasma parameters of the modulator stage of a P-MoPA allows the\ntemporal profile of pulses within the pulse train to be controlled, which in\nturn allows the wake amplitude in the accelerator stage to be as much as 72%\nlarger than that generated by a plasma beat-wave accelerator with the same\ntotal drive laser energy. Our analysis shows that Rosenbluth-Liu detuning is\nunimportant in a P-MoPA if the number of pulses in the train is less than\n$\\sim$ 30, and that this detuning is also partially counteracted by increased\nred-shifting, and hence increased pulse spacing, towards the back of the train.\nAn analysis of transverse mode oscillations of the driving pulse train is found\nto be in good agreement with 2D PIC simulations. PIC simulations demonstrating\nenergy gains of $\\sim$ 1.5 GeV ($\\sim$ 2.5 GeV) for drive pulse energies of 2.4\nJ (5.0 J) are presented. Our results suggest that P-MoPAs driven by few-joule,\npicosecond pulses, such as those provided by high-repetition-rate thin-disk\nlasers, could accelerate electron bunches to multi-GeV energies at pulse\nrepetition rates in the kilohertz range.", "category": "physics_plasm-ph" }, { "text": "Geodesic acoustic modes with poloidal mode couplings ad infinitum: Geodesic acoustic modes (GAMs) are studied, for the first time, including all\npoloidal mode $(m)$ couplings using drift reduced fluid equations. The nearest\nneighbor coupling pattern, due to geodesic curvature, leads to a semi-infinite\nchain model of the GAM with the mode-mode coupling matrix elements proportional\nto the radial wave number $k_{r}$. The infinite chain can be reduced to a\nrenormalized bi-nodal chain with a matrix continued fractions. Convergence\nstudy of linear GAM dispersion with respect to $k_{r}$ and the $m$-spectra\nconfirms that high m couplings become increasingly important with $k_{r}$. The\nradially sorted roots overlap with experimentally measured GAM frequency\nprofile in low collisionality shots in Tore Supra thus explaining the reduced\nfrequency of GAM in Tore Supra.", "category": "physics_plasm-ph" }, { "text": "Sub-laser-cycle control of relativistic plasma mirrors: We present measurements of high-order harmonics and relativistic electrons\nemitted into the vacuum from a plasma mirror driven by temporally-shaped\nultra-intense laser waveforms, produced by collinearly combining the main laser\nfield with its second harmonic. We experimentally show how these observables\nare influenced by the phase delay between these two frequencies at the\nattosecond timescale, and relate these observations to the underlying physics\nthrough an advanced analysis of 1D/2D Particle-In-Cell simulations. These\nresults demonstrate that sub-cycle shaping of the driving laser field provides\nfine control on the properties of the relativistic electron bunches responsible\nfor harmonic and particle emission from plasma mirrors.", "category": "physics_plasm-ph" }, { "text": "Observation of the Korteweg-de Vries soliton in molecular dynamics\n simulations of a dusty plasma medium: The excitation and evolution of Korteweg-de Vries (KdV) solitons in a dusty\nplasma medium are studied using Molecular Dynamics (MD) simulations. The dusty\nplasma medium is modelled as a collection of dust particles interacting through\nYukawa potential, which takes into account dust charge screening due to the\nlighter electron and ion species. The collective response of such screened dust\nparticles to an applied electric field impulse is studied here. An excitation\nof a perturbed positive density pulse propagating in one direction along with a\ntrain of negative perturbed rarefactive density oscillations in the opposite\ndirection is observed. These observations are in accordance with evolution\ngoverned by the KdV equation. Detailed studies of (a) amplitude vs. width\nvariation of the observed pulse, (b) the emergence of intact separate pulses\nwith an associated phase shift after collisional interaction amidst them, etc.,\nconclusively qualify the positive pulses observed in the simulations as KdV\nsolitons. It is also observed that by increasing the strength of the electric\nfield impulse, multiple solitonic structures get excited. The excitations of\nthe multiple solitons are similar to the experimental observations reported\nrecently by Boruah et al. [Phys. Plasmas 23, 093704 (2016)] for dusty plasmas.\nThe role of coupling parameter has also been investigated here, which shows\nthat with increasing coupling parameter, the amplitude of the solitonic pulse\nincreases whereas its width decreases.", "category": "physics_plasm-ph" }, { "text": "The Ehrenfest's Paradox and Radial Electric Field in Quasi-Neutral\n Tokamak Plasma: A relation between physical consequences of the so-called Ehrenfest's Paradox\nand the radial electric field in the classical quasi-neutral tokamak plasma is\nshown. Basic author's approach to the relativistic nature of the tokamak has\nbeen described in [1]. The experiment which can resolve the Ehrenfest's Paradox\nis presented.", "category": "physics_plasm-ph" }, { "text": "Instability of Shear Waves in an Inhomogeneous Strongly Coupled Dusty\n Plasma: It is demonstrated that low frequency shear modes in a strongly coupled,\ninhomogeneous, dusty plasma can grow on account of an instability involving the\ndynamical charge fluctuations of the dust grains. The instability is driven by\nthe gradient of the equilibrium dust charge density and is associated with the\nfinite charging time of the dust grains. The present calculations, carried out\nin the generalized hydrodynamic viscoelastic formalism, also bring out\nimportant modifications in the threshold and growth rate of the instability due\nto collective effects associated with coupling to the compressional mode.", "category": "physics_plasm-ph" }, { "text": "Spatial and Wavenumber Resolution of Doppler Reflectometry: Doppler reflectometry spatial and wavenumber resolution is analyzed within\nthe framework of the linear Born approximation in slab plasma model. Explicit\nexpression for its signal backscattering spectrum is obtained in terms of\nwavenumber and frequency spectra of turbulence which is assumed to be radially\nstatistically inhomogeneous. Scattering efficiency for both back and forward\nscattering (in radial direction) is introduced and shown to be inverse\nproportional to the square of radial wavenumber of the probing wave at the\nfluctuation location thus making the spatial resolution of diagnostics\nsensitive to density profile. It is shown that in case of forward scattering\nadditional localization can be provided by the antenna diagram. It is\ndemonstrated that in case of backscattering the spatial resolution can be\nbetter if the turbulence spectrum at high radial wavenumbers is suppressed. The\nimprovement of Doppler reflectometry data localization by probing beam focusing\nonto the cut-off is proposed and described. The possibility of Doppler\nreflectometry data interpretation based on the obtained expressions is shown.", "category": "physics_plasm-ph" }, { "text": "Modulational instability of dust-ion-acoustic waves and associated\n envelope solitons in a non-thermal plasma: A theoretical investigation has been made to understand the mechanism of the\nformation of both bright and dark envelope soltions associated with\ndust-ion-acoustic waves (DIAWs) propagating in an unmagnetized three component\ndusty plasma medium having inertial warm positive ions and negative dust\ngrains, and inertialess non-thermal Cairns' distributed electrons. A nonlinear\nSchr\\\"{o}dinger equation (NLSE) is derived by employing reductive perturbation\nmethod. The effects of plasma parameters, viz., $\\gamma_2$ (the ratio of the\npositive ion temperature to electron temperature times the charge state of ion)\nand $\\nu$ (the ratio of the charge state of negative dust grain to positive\nion) on the modulational instability of DIAW which is governed by NLSE, are\nextensively studied. It is found that increasing the value of the ion\n(electron) temperature reduces (enhances) the critical wave number ($k_c$). The\nresults of our present theoretical work may be used to interpret the nonlinear\nelectrostatic structures which can exist in many astrophysical environments and\nlaboratory plasmas.", "category": "physics_plasm-ph" }, { "text": "A Nuclear-Based Diagnostic Scheme for Nonthermal Ion Spectrum in High\n Energy Density Plasmas Experiments: The nuclear reactions in a plasma system with energy distribution deviated\nfrom Maxwellian are proved to have some unique characteristics including those\nin their product energy spectrum. Based on this, a new nuclear diagnostic\nscheme for measuring the nonthermal ion energy spectrum in high-energy-density\nplasmas is proposed, where the effective temperature and spectral peak of the\nnonthermal ion energy spectrum are uniquely determined by the spectral width\nand spectral peak of the product energy spectrum. Then, taking the laser-driven\nmagnetic reconnection experiment as an example, according to the\nparticle-in-cell (PIC) simulation results coupled with the proton-boron (pB)\nfusion reaction, the pB reaction rate is increased by 4 orders of magnitude;\nmeanwhile, the product spectral peak and spectral width are shifted towards\nhigher energy, deviating from the predictions of the thermal equilibrium\nassumption, both of which are on account of the accelerated and heated\nsuprathermal protons in magnetic reconnection. Furthermore, analyzing a series\nof simulation results, the new product spectrum analysis approach suitable for\nthe parameter range of laser-driven magnetic reconnection is found.", "category": "physics_plasm-ph" }, { "text": "Nonlocal adiabatic theory. II. Nonlinear frequency shift of an electron\n plasma wave in a multidimensional inhomogeneous plasma: In this article, we provide a general derivation of the nonlinear frequency\nshift, $\\delta \\omega$, for a sinusoidal electron plasma wave (EPW) that varies\nslowly enough for the results derived in the companion paper, on the action\ndistribution function, to apply. We first consider the situation when the EPW\nmonotonously grows and then monotonously decays in a homogeneous plasma. In\nthis situation, we show a hysteresis in the wave frequency, which does not\nconverge back to its linear value as the wave decays to small amplitudes. We\nthen address the derivation of $\\delta \\omega$ for an EPW that keeps growing in\na one-dimensional (1-D) inhomogeneous plasma. We show that, usually, the\nfrequency shift does not only depend on the local EPW amplitude and wavenumber.\nIt also depends on the whole history of the density variations, as experienced\nby the wave. In a multidimensional inhomogeneous plasma, the values assumed by\n$\\delta \\omega$ are usually different from those derived in 1-D because, due to\nthe transverse electron motion, one must account for the hysteresis in $\\delta\n\\omega$ in addition to plasma inhomogeneity. Hence, unless the EPW keeps\ngrowing in a homogeneous one-dimensional plasma, one cannot derive $\\delta\n\\omega$ \\textit{a priori} as a function of the local wave amplitude and\nwavenumber. Due to the nonlocality in the action distribution function, $\\delta\n\\omega$ depends on the whole history of the variations of the EPW amplitude and\nplasma density.", "category": "physics_plasm-ph" }, { "text": "Unconventional Ballooning Structures for Toroidal Drift Waves: With strong gradients in the pedestal of high confinement mode (H-mode)\nfusion plasmas, gyrokinetic simulations are carried out for the trapped\nelectron and ion temperature gradient modes. A broad class of unconventional\nmode structures is found to localize at arbitrary poloidal positions or with\nmultiple peaks. It is found that these unconventional ballooning structures are\nassociated with different eigen states for the most unstable mode. At weak\ngradient (low confinement mode or L-mode), the most unstable mode is usually in\nthe ground eigen state, which corresponds to a conventional ballooning mode\nstructure peaking in the outboard mid-plane of tokamaks. However, at strong\ngradient (H-mode), the most unstable mode is usually not the ground eigen state\nand the ballooning mode structure becomes unconventional. This result implies\nthat the pedestal of H-mode could have better confinement than L-mode.", "category": "physics_plasm-ph" }, { "text": "Ideal evolution of MHD turbulence when imposing Taylor-Green symmetries: We investigate the ideal and incompressible magnetohydrodynamic (MHD)\nequations in three space dimensions for the development of potentially singular\nstructures. The methodology consists in implementing the four-fold symmetries\nof the Taylor-Green vortex generalized to MHD, leading to substantial computer\ntime and memory savings at a given resolution; we also use a re-gridding method\nthat allows for lower-resolution runs at early times, with no loss of spectral\naccuracy. One magnetic configuration is examined at an equivalent resolution of\n$6144^3$ points, and three different configurations on grids of $4096^3$\npoints. At the highest resolution, two different current and vorticity sheet\nsystems are found to collide, producing two successive accelerations in the\ndevelopment of small scales. At the latest time, a convergence of magnetic\nfield lines to the location of maximum current is probably leading locally to a\nstrong bending and directional variability of such lines. A novel analytical\nmethod, based on sharp analysis inequalities, is used to assess the validity of\nthe finite-time singularity scenario. This method allows one to rule out\nspurious singularities by evaluating the rate at which the logarithmic\ndecrement of the analyticity-strip method goes to zero. The result is that the\nfinite-time singularity scenario cannot be ruled out, and the singularity time\ncould be somewhere between $t=2.33$ and $t=2.70.$ More robust conclusions will\nrequire higher resolution runs and grid-point interpolation measurements of\nmaximum current and vorticity.", "category": "physics_plasm-ph" }, { "text": "On the application of Maxwell's theory to many-body systems, or why the\n resistive magnetohydrodynamic equations are not closed: The resistive magnetohydrodynamic (MHD) equations as usually defined in the\nquasineutral approximation refer to a system of 14 scalar equations in 14\nscalar variables, hence are determined to be complete and soluble. These\nequations are a combination of Navier-Stokes and a subset of Maxwell's.\nHowever, one of the vector equations is actually an identity when viewed from\nthe potential formulation of electrodynamics, hence does not determine any\ndegrees of freedom. Only by reinstating Gauss's law does the system of\nequations become closed, allowing for the determination of both the current and\nmass flow velocity from the equations of motion. Results of a typical analysis\nof the proposed electromagnetic hydrodynamic model including the magnetization\nforce are presented.", "category": "physics_plasm-ph" }, { "text": "Bernstein Waves in Symmetric and Asymmetric Pair Ions Plasma: Positive and negative ions forming so-called pair plasma differing in sign of\ntheir charge and asymmetric in mass and temperature support a new electrostatic\nmode. Bernstein mode for a pair ions and pair ions with contribution of\nelectrons in pair plasma both cases are investigated. By solving the linearized\nVlasov equation along Maxwell equations, a generalized expression for the\nBernstein waves is derived by employing the Maxwell distribution function. In\npaper we discuss the different types of ions Bernstein waves and comparison of\nthe symmetry and asymmetry on these ions Bernstein waves. We also apply the\nfluid limit on these Bernstein waves and we different fluid results from\nkinetic theory.", "category": "physics_plasm-ph" }, { "text": "The criterion for infinite positron feedback in dynamics of relativistic\n runaway electron avalanches: Relativistic runaway electron avalanches (RREA) accelerated by thunderstorm\nlarge-scale electric fields are one of the sources of atmospheric gamma\nradiation. In strong electric fields, RREAs can multiply by the relativistic\nfeedback. Infinite relativistic feedback makes avalanches self-sustainable and\nhypothetically can cause a terrestrial gamma-ray flash (TGF). This paper\nintroduces a kinetic approach to study the relativistic feedback caused by\npositrons since positron feedback dominates for the directly observed electric\nfield strengths. With this approach, the criterion for infinite positron\nfeedback within thunderstorms is derived. Discovered criterion allows obtaining\nthe thunderstorm electric field parameters required for infinite positron\nfeedback for any altitude. The possibility of derived thunderstorm conditions\nis discussed.", "category": "physics_plasm-ph" }, { "text": "R&D around a photoneutralizer-based NBI system (Siphore) in view of a\n DEMO Tokamak steady state fusion reactor: Since the signature of the ITER treaty in 2006, a new research programme\ntargeting the emergence of a new generation of Neutral Beam (NB) system for the\nfuture fusion reactor (DEMO Tokamak) has been underway between several\nlaboratories in Europe. The specifications required to operate a NB system on\nDEMO are very demanding: the system has to provide plasma heating, current\ndrive and plasma control at a very high level of power (up to 150 MW) and\nenergy (1 or 2 MeV), including high performances in term of wall-plug\nefficiency ($\\eta$ > 60%), high availability and reliability. To this aim, a\nnovel NB concept based on the photodetachment of the energetic negative ion\nbeam is under study. The keystone of this new concept is the achievement of a\nphotoneutralizer where a high power photon flux (~3 MW) generated within a\nFabry Perot cavity will overlap, cross and partially photodetach the intense\nnegative ion beam accelerated at high energy (1 or 2 MeV). The aspect ratio of\nthe beam-line (source, accelerator, etc.) is specifically designed to maximize\nthe overlap of the photon beam with the ion beam. It is shown that such a\nphotoneutralized based NB system would have the capability to provide several\ntens of MW of D 0 per beam line with a wall-plug efficiency higher than 60%. A\nfeasibility study of the concept has been launched between different\nlaboratories to address the different physics aspects, i.e., negative ion\nsource, plasma modelling, ion accelerator simulation, photoneutralization and\nhigh voltage holding under vacuum. The paper describes the present status of\nthe project and the main achievements of the developments in laboratories.", "category": "physics_plasm-ph" }, { "text": "On shear flow stabilization concepts for the dense z pinch: Different ways to achieve the stabilization of a linear z-pinch by a\nsuperimposed shear flow are analyzed. They are: 1) Axial shear flow proposed by\nArber and Howell with the pinch discharge in its center, and experimentally\ntested by Shumlak et al. 2) Spiral flow of a dense low temperature plasma\nsurrounding a dense pinch discharge. 3) A thin metallic projectile shot at a\nhigh velocity through the center of the pinch discharge. 4) The replacement of\nthe high velocity projectile by the shape charge effect jet in a conical\nimplosion. 5) The replacement of the jet by a stationary wire inside the\nconical implosion.", "category": "physics_plasm-ph" }, { "text": "Role of wave-particle resonance in turbulent transport in toroidal\n plasmas: Wave-particle interaction in toroidal plasmas is an essential transport\nmechanism in drift wave instability-driven microturbulence. In tokamkas,\ndifferent wave-particle resonance conditions have been found important for the\nenergy and particle transport of multiple species in various drift wave\nturbulences. To confirm the transport mechanism for electrons and ions in\ntokamak drift-wave instabilities, the effect of wave-particle resonance on\nturbulent transport is studied using global gyrokinetic particle simulations of\nthe plasma core ion temperature gradient (ITG) and collisionless trapped\nelectron mode (CTEM) turbulence. Simulation results show that in CTEM and ITG\nturbulence, electron transport is primarily regulated by wave-particle linear\nresonance, and the ion transport is regulated by nonlinear wave-particle\ndecorrelation.", "category": "physics_plasm-ph" }, { "text": "First demonstration of multi-MeV proton acceleration from a cryogenic\n hydrogen ribbon target: We show efficient laser driven proton acceleration up to 14MeV from a 50\n$\\mu$m thick cryogenic hydrogen ribbon. Pulses of the short pulse laser ELFIE\nat LULI with a pulse length of $\\approx$ 350 fs at an energy of 8 J per pulse\nare directed onto the target. The results are compared to proton spectra from\nmetal and plastic foils with different thicknesses and show a similar good\nperformance both in maximum energy as well as in proton number. Thus, this\ntarget type is a promising candidate for experiments with high repetition rate\nlaser systems.", "category": "physics_plasm-ph" }, { "text": "Evaluation of the Dreicer runaway generation rate in the presence of\n high-Z impurities using a neural network: Integrated modelling of electron runaway requires computationally expensive\nkinetic models that are self-consistently coupled to the evolution of the\nbackground plasma parameters. The computational expense can be reduced by using\nparameterized runaway generation rates rather than solving the full kinetic\nproblem. However, currently available generation rates neglect several\nimportant effects; in particular, they are not valid in the presence of\npartially ionized impurities. In this work, we construct a multilayer neural\nnetwork for the Dreicer runaway generation rate which is trained on data\nobtained from kinetic simulations performed for a wide range of plasma\nparameters and impurities. The neural network accurately reproduces the Dreicer\nrunaway generation rate obtained by the kinetic solver. By implementing it in a\nfluid runaway electron modelling tool, we show that the improved generation\nrates lead to significant differences in the self-consistent runaway dynamics\nas compared to the results using the previously available formulas for the\nrunaway generation rate.", "category": "physics_plasm-ph" }, { "text": "Fast particle-driven ion cyclotron emission (ICE) in tokamak plasmas and\n the case for an ICE diagnostic in ITER: Fast particle-driven waves in the ion cyclotron frequency range (ion\ncyclotron emission or ICE) have provided a valuable diagnostic of confined and\nescaping fast ions in many tokamaks. This is a passive, non-invasive diagnostic\nthat would be compatible with the high radiation environment of\ndeuterium-tritium plasmas in ITER, and could provide important information on\nfusion {\\alpha}-particles and beam ions in that device. In JET, ICE from\nconfined fusion products scaled linearly with fusion reaction rate over six\norders of magnitude and provided evidence that {\\alpha}-particle confinement\nwas close to classical. In TFTR, ICE was observed from super-Alfv\\'enic\n{\\alpha}-particles in the plasma edge. The intensity of beam-driven ICE in\nDIII-D is more strongly correlated with drops in neutron rate during fishbone\nexcitation than signals from more direct beam ion loss diagnostics. In ASDEX\nUpgrade ICE is produced by both super-Alfv\\'enic DD fusion products and\nsub-Alfv\\'enic deuterium beam ions.", "category": "physics_plasm-ph" }, { "text": "The interaction between shocks and plasma turbulence: phase space\n transport: The interaction of collisionless shocks with fully developed plasma\nturbulence is numerically investigated. Hybrid kinetic simulations, where a\nturbulent jet is slammed against an oblique shock, are employed to address the\nrole of upstream turbulence on plasma transport. A novel technique, using\ncoarse-graining of the Vlasov equation, is proposed, showing that the transport\nproperties strongly depend on upstream turbulence strength, with turbulent\nstructures-modulated patterns. These results might be relevant for the\nunderstanding of acceleration and heating processes in space plasmas.", "category": "physics_plasm-ph" }, { "text": "Dynamic Mode Decomposition for Plasma Diagnostics and Validation: We demonstrate the application of the Dynamic Mode Decomposition (DMD) for\nthe diagnostic analysis of the nonlinear dynamics of a magnetized plasma in\nresistive magnetohydrodynamics. The DMD method is an ideal spatio-temporal\nmatrix decomposition that correlates spatial features of computational or\nexperimental data while simultaneously associating the spatial activity with\nperiodic temporal behavior. DMD can produce low-rank, reduced order surrogate\nmodels that can be used to reconstruct the state of the system and produce\nhigh-fidelity future state predictions. This allows for a reduction in the\ncomputational cost, and, at the same time, accurate approximations of the\nproblem, even if the data are sparsely sampled. We demonstrate the use of the\nmethod on both numerical and experimental data, showing that it is a successful\nmathematical architecture for characterizing the HIT-IS magnetohydrodynamics.\nImportantly, the DMD decomposition produces interpretable, dominant mode\nstructures, including the spheromak mode and a pair of injector-driven modes.\nIn combination, the 3-mode DMD model produces excellent dynamic reconstructions\nthat can be used for future state prediction and/or control strategies.", "category": "physics_plasm-ph" }, { "text": "Magnetic Fluctuations in Gyrokinetic Simulations of Tokamak Scrape-Off\n Layer Turbulence: Understanding turbulent transport physics in the tokamak edge and scrape-off\nlayer (SOL) is critical to developing a successful fusion reactor. The dynamics\nin these regions plays a key role in achieving high fusion performance by\ndetermining the edge pedestal that suppresses turbulence in H-mode.\nAdditionally, the survivability of a reactor is set by the heat load to the\nvessel walls, making it important to understand turbulent spreading of heat as\nit flows along open magnetic field lines in the SOL. Large-amplitude\nfluctuations, magnetic X-point geometry, and plasma interactions with material\nwalls make simulating turbulence in the edge/SOL more challenging than in the\ncore region, necessitating specialized gyrokinetic codes. Further, the\ninclusion of electromagnetic effects in gyrokinetic simulations that can handle\nthe unique challenges of the boundary plasma is critical to the understanding\nof phenomena such as the pedestal and ELMs, for which electromagnetic dynamics\nare expected to be important.\n In this thesis, we develop the first capability to simulate electromagnetic\ngyrokinetic turbulence on open magnetic field lines. This is an important step\ntowards comprehensive electromagnetic gyrokinetic simulations of the coupled\nedge/SOL system. By using a continuum full-f approach via an energy-conserving\ndiscontinuous Galerkin (DG) discretization scheme that avoids the Ampere\ncancellation problem, we show that electromagnetic fluctuations can be handled\nin a robust, stable, and efficient manner in the gyrokinetic module of the\nGkeyll code. We then present results which roughly model the scrape-off layer\nof the National Spherical Torus Experiment (NSTX), and show that\nelectromagnetic effects can affect blob dynamics and transport. We also\nformulate the gyrokinetic system in field-aligned coordinates for modeling\nrealistic edge and scrape-off layer geometries in experiments.", "category": "physics_plasm-ph" }, { "text": "Sandpile modelling of pellet pacing in fusion plasmas: Sandpile models have been used to provide simple phenomenological models\nwithout incorporating the detailed features of a fully featured model. The\nChapman sandpile model Chapman et al Physical Review Letters 86, 2814 (2001)\nhas been used as an analogue for the behaviour of a plasma edge, with mass loss\nevents being used as analogues for ELMs. In this work we modify the Chapman\nsandpile model by providing for both increased and intermittent driving. We\nshow that the behaviour of the sandpile, when continuously fuelled at very high\ndriving, can be determined analytically by a simple algorithm. We observe that\nthe size of the largest avalanches is better reduced by increasing constant\ndriving than by the intermittent introduction of `pellets' of sand. Using the\nsandpile model as a reduced model of ELMing behaviour, we conject that ELM\ncontrol in a fusion plasma may similarly prove more effective with increased\ntotal fuelling than with pellet addition.", "category": "physics_plasm-ph" }, { "text": "Characterization of a plasma window as a membrane free transition\n between vacuum and high pressure: A plasma window (PW) is a device for separating two areas of different\npressures while letting particle beams pass with little to no loss. It has been\nintroduced by A. Hershcovitch. In the course of this publication, the\nproperties of a PW with apertures of 3.3 mm and 5.0 mm are presented.\nEspecially the link between the pressure properties relevant for applications\nin accelerator systems and the underlying plasma properties depending on\nexternal parameters are presented. At the low pressure side around some mbar,\nhigh-pressure values reached up to 750 mbar while operating with volume flows\nbetween 1 slm and 4 slm (standard liter per minute) and discharge currents\nranging from 45 A to 60 A. Unique features of the presented PW include\nsimultaneous plasma parameter determination and the absence of ceramic\ninsulators between the cooling plates. Optical analysis reveals no significant\ndamage or wear to the components after an operation time well over 10 h,\nwhereas the cathode needle needs replacement after 5 h.", "category": "physics_plasm-ph" }, { "text": "Numerical relaxation of a 3D MHD Taylor - Woltjer state subject to\n abrupt expansion: Since the advent of Taylor-Woltjer theory [J B Taylor, PRL, 33, 1139 (1974),\nL Woltjer, PNAS, 44, 489 (1958)], it has been widely believed that situations\nwith perfectly conducting boundaries and near ideal conditions, the final state\nof MHD system would be force-free Taylor-Woltjer states defined as\n$\\vec{\\nabla} \\times \\vec{B} = \\alpha \\vec{B}$ with $\\alpha$ as a constant and\n$\\vec{B}$ is the magnetic field defined over a volume $V$. These states are of\nfundamental importance in fusion plasmas [J B Taylor, RMP 58, 741 (1986)]. More\nrecently, several new MHD models have been proposed - for example Reduced\nMulti-region relaxed MHD [S R Hudson {\\it et al}, Phys. Plasmas, 19, 112502\n(2012)] and arbitrary scale relaxation model to Taylor-Woltjer state [H Qin\n{\\it et al}, PRL, 109, 235001 (2012)] to mention a few.\n In the present work, we use a 3D compressible MHD solver in cartesian\ngeometry which can handle conducting or periodic as well has mixed boundary\nconditions to investigate numerically the arbitrary scale relaxation model\nproposed by Qin et al [H Qin {\\it et al}, PRL, 109, 235001 (2012)]. For this\npurpose, we consider two volumes $V_{init}$ and $V_{final}$. We load the 3D MHD\nsolver in the limit of zero compressibility with a Taylor-Woltjer state\n$B_{init}(x,y,z,t=0)$ and let it again a numerical evolve with conducting\nboundaries at $V_{init}$ to make sure that we have obtained a numerically\nsteady Taylor - Woltjer state for volume $V_{init}$. Followed by this\nprocedure, we \"suddenly\" relax the boundaries to a new volume $V_{final}$, such\nthat $V_{init}$ $<$ $V_{final}$ and evaluate whether or not the system attains\nquasi-steady state. Details of the numerical method used, the protocol\nfollowed, the expansion technique and the novelty of this numerical experiment\nand details of our results have been presented in this paper.", "category": "physics_plasm-ph" }, { "text": "Dynamics of Ion Temperature Gradient Turbulence and Transport with a\n Static Magnetic Island: Understanding the interaction mechanisms between large-scale\nmagnetohydrodynamic instabilities and small-scale drift-wave microturbulence is\nessential for predicting and optimizing the performance of magnetic confinement\nbased fusion energy experiments. We report progress on understanding these\ninteractions using both analytic theory and numerical simulations performed\nwith the BOUT++ [B. Dudson et al., Comput. Phys. Comm. 180, 1467 (2009)]\nframework. This work focuses upon the dynamics of the ion temperature gradient\ninstability in the presence of a background static magnetic island, using a\nweakly electromagnetic two-dimensional five-field fluid model. It is found that\nthe island width must exceed a threshold size (comparable to the turbulent\ncorrelation length in the no-island limit) to significantly impact the\nturbulence dynamics, with the primary impact being an increase in turbulent\nfluctuation and heat flux amplitudes. The turbulent radial ion energy flux is\nshown to localize near the X-point, but does so asymmetrically in the poloidal\ndimension. An effective turbulent resistivity which acts upon the island outer\nlayer is also calculated, and shown to always be significantly (10x - 100x)\ngreater than the collisional resistivity used in the simulations.", "category": "physics_plasm-ph" }, { "text": "Mapping the X-Ray Emission Region in a Laser-Plasma Accelerator: The x-ray emission in laser-plasma accelerators can be a powerful tool to\nunderstand the physics of relativistic laser-plasma interaction. It is shown\nhere that the mapping of betatron x-ray radiation can be obtained from the\nx-ray beam profile when an aperture mask is positioned just beyond the end of\nthe emission region. The influence of the plasma density on the position and\nthe longitudinal profile of the x-ray emission is investigated and compared to\nparticle-in-cell simulations. The measurement of the x-ray emission position\nand length provides insight on the dynamics of the interaction, including the\nelectron self-injection region, possible multiple injection, and the role of\nthe electron beam driven wakefield.", "category": "physics_plasm-ph" }, { "text": "Improved Confinement in JET High {beta} Plasmas with an ITER-Like Wall: The replacement of the JET carbon wall (C-wall) by a Be/W ITER-like wall\n(ILW) has affected the plasma energy confinement. To investigate this,\nexperiments have been performed with both the C-wall and ILW to vary the\nheating power over a wide range for plasmas with different shapes.", "category": "physics_plasm-ph" }, { "text": "Deposition of SiOx films by means of atmospheric pressure microplasma\n jets: Atmospheric pressure plasma jet sources are currently in the focus of many\nresearchers for their promising applications in medical industry (e.g.\ntreatment of living tissues), surface modification or material etching or\nsynthesis. Here we report on the study of fundamental principles of deposition\nof SiOx films from microplasma jets with admixture of hexamethyldisiloxane\n[(CH3)3SiOSi(CH3)3, HMDSO] molecules and oxygen. The properties of the\ndeposited films, the composition of the plasma as measured by molecular beam\nmass spectrometry and the effect of additional treatment of grown film by\noxygen or hydrogen atoms will be presented.", "category": "physics_plasm-ph" }, { "text": "Space-resolved characterization of high frequency atmospheric-pressure\n plasma in nitrogen applying optical emission spectroscopy and numerical\n simulation: Averaged plasma parameters such as electron distribution function and\nelectron density are determined by characterization of high frequency (2.4 GHz)\nnitrogen-plasma using both experimental methods, namely optical emission\nspectroscopy (OES) and microphotography, and numerical simulation. Both direct\nand stepwise electron-impact excitation of nitrogen emissions are considered.\nThe determination of space-resolved electron distribution function, electron\ndensity, rate constant for electron-impact dissociation of nitrogen molecule\nand the production of nitrogen atoms, applying the same methods, is discussed.\nSpatial distribution of intensities of neutral nitrogen molecule and nitrogen\nmolecular ion from the microplasma is imaged by a CCD camera. The CCD images\nare calibrated using the corresponding emissions measured by\nabsolutely-calibrated OES, and are then subjected to inverse Abel\ntransformation to determine space-resolved intensities and other parameters.\nThe space-resolved parameters are compared, respectively, with the averaged\nparameters, and an agreement between them is established.", "category": "physics_plasm-ph" }, { "text": "Generation of ultra-relativistic monoenergetic electron bunches via a\n synergistic interaction of longitudinal electric and magnetic fields of a\n twisted laser: We use 3D simulations to demonstrate that high-quality ultra-relativistic\nelectron bunches can be generated upon reflection of a twisted laser beam off a\nplasma mirror. The unique topology of the beam with a twist index $|l| = 1$\ncreates an accelerating structure dominated by longitudinal laser electric and\nmagnetic fields in the near-axis region. We show that the magnetic field is\nessential for creating a train of dense mono-energetic bunches. For a 6.8~PW\nlaser, the energy reaches 1.6~GeV with a spread of 5.5\\%. The bunch duration is\n320 as, its charge is 60~pC and density is $\\sim 10^{27}$~m$^{-3}$. The results\nare confirmed by an analytical model for the electron energy gain. These\nresults enable development of novel laser-driven accelerators at multi-PW laser\nfacilities.", "category": "physics_plasm-ph" }, { "text": "Analytic Hall Magnetohydrodynamics toroidal equilibria via the\n energy-Casimir variational principle: Equilibrium equations for magnetically confined, axisymmetric plasmas are\nderived by means of the energy-Casimir variational principle in the context of\nHall magnetohydrodynamics (MHD). This approach stems from the noncanonical\nHamiltonian structure of Hall MHD, the simplest, quasineutral two-fluid model\nthat incorporates contributions due to ion Hall drifts. The axisymmetric\nCasimir invariants are used, along with the Hamiltonian functional to apply the\nenergy-Casimir variational principle for axisymmetric two-fluid plasmas with\nincompressible ion flows. This results in a system of equations of the\nGrad-Shafranov-Bernoulli (GSB) type with four free functions. Two families of\nanalytic solutions to the GSB system are then calculated, based on specific\nchoices for the free functions. These solutions are subsequently applied to\nTokamak-relevant configurations using proper boundary shaping methods. The Hall\nMHD model predicts a departure of the ion velocity surfaces from the magnetic\nsurfaces which are frozen in the electron fluid. This separation of the\ncharacteristic surfaces is corroborated by the analytic solutions calculated in\nthis study. The equilibria constructed by these solutions exhibit favorable\ncharacteristics for plasma confinement, for example they possess closed and\nnested magnetic and flow surfaces with pressure profiles peaked at the plasma\ncore. The relevance of these solutions to laboratory and astrophysical plasmas\nis finally discussed, with particular focus on systems that involve length\nscales on the order of the ion skin depth.", "category": "physics_plasm-ph" }, { "text": "Nonlinear electromagnetic wave equations for superdense magnetized\n plasmas: By using the quantum hydrodynamic and Maxwell equations, we derive nonlinear\nelectron-magnetohydrodynamic (MHD), Hall-MHD, and dust Hall-MHD equations for\ndense quantum magnetoplasmas. The nonlinear equations include the\nelectromagnetic, the electron pressure gradient, as well as the quantum\nelectron tunneling and electron spin forces. They are useful for investigating\na number of wave phenomena including linear and nonlinear electromagnetic\nwaves, as well as three-dimensional electromagnetic wave turbulence spectra\narising from the mode coupling processes in dense magnetoplasmas.", "category": "physics_plasm-ph" }, { "text": "Effect of resonant magnetic perturbations on ELMs in connected double\n null plasmas in MAST: The application of resonant magnetic perturbations (RMPs) with a toroidal\nmode number of n=3 to connected double null plasmas in the MAST tokamak\nproduces up to a factor of 9 increase in Edge Localized Mode (ELM) frequency\nand reduction in plasma energy loss associated with type-I ELMs. A threshold\ncurrent for ELM mitigation is observed above which the ELM frequency increases\napproximately linearly with current in the coils. The effect of the RMPs is\nfound to be scenario dependent. In one scenario the mitigation is only due to a\nlarge density pump out event and if the density is recovered by gas puffing a\nreturn to type I ELMs is observed. In another scenario sustained ELM mitigation\ncan be achieved irrespective of the amount of fuelling. Despite a large scan of\nparameters complete ELM suppression has not been achieved. The results have\nbeen compared to modelling performed using either the vacuum approximation or\nincluding the plasma response. The requirement for a resonant condition, that\nis an optimum alignment of the perturbation with the plasma, has been confirmed\nby performing a scan in the pitch angle of the applied field.", "category": "physics_plasm-ph" }, { "text": "Evolution of positive streamers in air over non-planar dielectrics:\n Experiments and simulations: We study positive streamers in air propagating along polycarbonate dielectric\nplates with and without small-scale surface profiles.\n The streamer development was documented using light-sensitive high-speed\ncameras and a photo-multiplier tube, and the experimental results were compared\nwith 2D fluid streamer simulations.\n Two profiles were tested, one with 0.5 mm deep semi-circular corrugations and\none with 0.5 mm deep rectangular corrugations.\n A non-profiled surface was used as a reference.\n Both experiments and simulations show that the surface profiles lead to\nsignificantly slower surface streamers, and also reduce their length.\n The rectangular-cut profile obstructs the surface streamer more than the\nsemi-circular profile.\n We find quantitative agreement between simulations and experiments.\n For the surface with rectangular grooves, the simulations also reveal a\ncomplex propagation mechanism where new positive streamers re-ignite inside the\nsurface profile corrugations.\n The results are of importance for technological applications involving\nstreamers and solid dielectrics.", "category": "physics_plasm-ph" }, { "text": "Magnetohydrodynamic Turbulence: Generalized Energy Spectra: A general framework that incorporates the Iroshnikov-Kraichnan (IK) and\nGoldreich-Sridhar (GS) phenomenalogies of magnetohydrodynamic (MHD) turbulence\nis developed. This affords a clarification of the regime of validity of the IK\nmodel and hence help resolve some controversies in this problem.", "category": "physics_plasm-ph" }, { "text": "Excitation of wakefields in a plasma channel by a laser pulse: The excitation of wakefields by a triangular temporal profile laser pulse\nwith an abrupt fall in a step density profile plasma channel is investigated\nanalytically and an exact solution is obtained. The excitation is more\nefficient for t~tp, where t is the rise time of the pulse and tp is the plasma\nperiod, though it is still significant for t~20 tp. The wake potential in the\nformer case is 1.7 times the ponderomotive potential, whereas it falls to 1.4\ntimes the ponderomotive potential for the latter.", "category": "physics_plasm-ph" }, { "text": "Formation of Cooper pairs in quantum oscillations of electrons in plasma: We study low energy quantum oscillations of electron gas in plasma. It is\nshown that two electrons participating in these oscillations acquire additional\nnegative energy when they interact by means of a virtual plasmon. The\nadditional energy leads to the formation a Cooper pair and possible existence\nof the superconducting phase in the system. We suggest that this mechanism\nsupports slowly damping oscillations of electrons without any energy supply.\nBasing on our model we put forward the hypothesis the superconductivity can\noccur in a low energy ball lightning.", "category": "physics_plasm-ph" }, { "text": "Available energy of magnetically confined plasmas: The concept of available energy of a collisionless plasma is discussed in the\ncontext of magnetic confinement. The available energy quantifies how much of\nthe plasma energy can be converted into fluctuations (including nonlinear ones)\nand is thus a measure of plasma stability, which can be used to derive linear\nand nonlinear stability criteria without solving an eigenvalue problem. In a\nmagnetically confined plasma, the available energy is determined by the density\nand temperature profiles as well as the magnetic geometry. It also depends on\nwhat constraints limit the possible forms of plasma motion, such as the\nconservation of adiabatic invariants and the requirement that the transport be\nambipolar. A general method based on Lagrange multipliers is devised to\nincorporate such constraints in the calculation of the available energy, and\nseveral particular cases are discussed. In particular, it is shown that it is\nimpossible to confine a plasma in a Maxwellian ground state relative to\nperturbations with frequencies exceeding the ion bounce frequency.", "category": "physics_plasm-ph" }, { "text": "Electron dynamics in planar radio frequency magnetron plasmas: I. The\n mechanism of Hall heating and the \u03bc-mode: The electron dynamics and the mechanisms of power absorption in\nradio-frequency (RF) driven, magnetically enhanced capacitively coupled plasmas\n(MECCPs) at low pressure are investigated. The device in focus is a\ngeometrically asymmetric cylindrical magnetron with a radially nonuniform\nmagnetic field in axial direction and an electric field in radial direction.\nThe dynamics is studied analytically using the cold plasma model and a\nsingle-particle formalism, and numerically with the inhouse energy and charge\nconserving particle-in-cell/Monte Carlo collisions code ECCOPIC1S-M. It is\nfound that the dynamics differs significantly from that of an unmagnetized\nreference discharge. In the magnetized region in front of the powered\nelectrode, an enhanced electric field arises during sheath expansion and a\nreversed electric field during sheath collapse. Both fields are needed to\nensure discharge sustaining electron transport against the confining effect of\nthe magnetic field. The corresponding azimuthal ExB-drift can accelerate\nelectrons into the inelastic energy range which gives rise to a new mechanism\nof RF power dissipation. It is related to the Hall current and is different in\nnature from Ohmic heating, as which it has been classified in previous\nliterature. The new heating is expected to be dominant in many magnetized\ncapacitively coupled discharges. It is proposed to term it the \"{\\mu}-mode\" to\nseparate it from other heating modes.", "category": "physics_plasm-ph" }, { "text": "Ion friction at small values of the Coulomb logarithm: Transport properties of high-energy-density plasmas are influenced by the ion\ncollision rate. Traditionally, this rate involves the Coulomb logarithm,\n$\\ln\\Lambda$. Typical values of $\\ln\\Lambda$ are $\\approx 10~\\mbox{to}~20$ in\nkinetic theories where transport properties are dominated by weak-scattering\nevents caused by long-range forces. The validity of these theories breaks down\nfor strongly-coupled plasmas, when $\\ln\\Lambda$ is of order one. We present\nmeasurements and simulations of collision data in strongly-coupled plasmas when\n$\\ln\\Lambda$ is small. Experiments are carried out in the first dual-species\nultracold neutral plasma (UNP), using Ca$^+$ and Yb$^+$ ions. We find strong\ncollisional coupling between the different ion species in the bulk of the\nplasma. We simulate the plasma using a two-species fluid code that includes\nCoulomb logarithms derived from either a screened Coulomb potential or a the\npotential of mean force. We find generally good agreement between the\nexperimental measurements and the simulations. With some improvements, the\nmixed Ca$^+$ and Yb$^+$ dual-species UNP will be a promising platform for\ntesting theoretical expressions for $\\ln\\Lambda$ and collision cross-sections\nfrom kinetic theories through measurements of energy relaxation, stopping\npower, two-stream instabilities, and the evolution of sculpted distribution\nfunctions in an idealized environment in which the initial temperatures,\ndensities, and charge states are accurately known.", "category": "physics_plasm-ph" }, { "text": "Synergistic Longitudinal Acceleration and Transverse Oscillation in\n High-order Harmonic Generation: We propose and demonstrate that relativistic harmonics with a slowly decaying\npower law are generated from a femtosecond lase pulse incident parallel to a\nmicro-scale overdense plasma. It is shown that due to the excitation of a\nstrong surface wave, dense electron nanobunches are continuously accelerated\nforward while oscillating in the transverse laser field. Even around the\nstationary phase point, relativistic gamma factors of the nanobunches increase\nconsiderably, leading to a much stronger attosecond burst, compared to the case\nwith constant gamma. Our two-dimensional particle-in-cell simulations and\nanalytical theory show that this synergistic function promises a power-law\nharmonic spectrum $I_n/I_0 = n^{-1}$. This is much flatter than the other\nwell-known radiation mechanisms and paves the way to unprecedentedly large\nenergy attosecond pulses.", "category": "physics_plasm-ph" }, { "text": "Predicting PDF tails of flux in plasma sheath region: This letter provides the first prediction of the probability density function\n(PDF) of flux $R$ in plasma sheath sheath region in magnetic fusion devices\nwhich is characterized by dynamical equations with exponential non-linearities.\nBy using a non-perturbative statistical theory (instantons), the PDF tails of\nfirst moment are shown to be modified Gumbel distribution which represents a\nfrequency distribution of the extreme values of the ensemble. The non-Gaussian\nPDF tails that are enhanced over Gaussian predictions are the result of\nintermittency caused by short lived coherent structures (instantons).", "category": "physics_plasm-ph" }, { "text": "Analytical Approximate Solution of a Coupled Two Frequency Hill's\n Equation: A coupled two frequency Hill's equation is solved. Analytically approximate\nsolution correct up-to first order is derived using modified Lindstedt Poincare\nperturbation method. For a wide range of controlling parameters we compare the\nnumerical and analytical solutions. The solution is the first step towards\ndeveloping a comprehensive understanding of the electrodynamics of charged\nparticles in a combinational ion trap utilizing both electrostatic DC and RF\nfields along with a constant static magnetic field with prospects of confining\nantimatter such as anti hydrogen for a reasonably long durations of time.", "category": "physics_plasm-ph" }, { "text": "Neutrino magnetohydrodynamic instabilities in presence of two-flavor\n oscillations: The influence of neutrino flavor oscillations on the propagation of\nmagnetohydrodynamic (MHD) waves and instabilities is studied in neutrino-beam\ndriven magnetoplasmas. Using the neutrino MHD model, a general dispersion\nrelation is derived which manifests the resonant interactions of MHD waves, not\nonly with the neutrino beam, but also with the neutrino flavor oscillations. It\nis found that the latter contribute to the wave dispersion and enhance the\nmagnitude of the instability of oblique magnetosonic waves. However, the\nshear-Alfv{\\'e}n wave remains unaffected by the neutrino beam and neutrino\nflavor oscillations. Such an enhancement of the magnitude of the instability of\nmagnetosonic waves can be significant for relatively long-wavelength\nperturbations in the regimes of high neutrino number density and/or strong\nmagnetic field, giving a convincing mechanism for type-II core-collapse\nsupernova explosion.", "category": "physics_plasm-ph" }, { "text": "Poloidal rotation driven by nonlinear momentum transport in strong\n electrostatic turbulence: Virtually, all existing theoretical works on turbulent poloidal momentum\ntransport are based on quasilinear theory. Nonlinear poloidal momentum flux -\n$\\langle \\tilde{v}_r \\tilde{n} \\tilde{v}_{\\theta} \\rangle$ is universally\nneglected. However, in the strong turbulence regime where relative fluctuation\namplitude is no longer small, quasilinear theory is invalid. This is true at\nthe all-important plasma edge. In this work, nonlinear poloidal momentum flux $\n\\langle \\tilde{v}_r \\tilde{n} \\tilde{v}_{\\theta} \\rangle $ in strong\nelectrostatic turbulence is calculated using Hasegawa-Mima equation, and is\ncompared with quasilinear poloidal Reynolds stress. A novel property is that\nsymmetry breaking in fluctuation spectrum is not necessary for a nonlinear\npoloidal momentum flux. This is fundamentally different from the quasilinear\nReynold stress. Furthermore, the comparison implies that the poloidal rotation\ndrive from the radial gradient of nonlinear momentum flux is comparable to that\nfrom the quasilinear Reynolds force. Nonlinear poloidal momentum transport in\nstrong electrostatic turbulence is thus not negligible for poloidal rotation\ndrive, and so may be significant to transport barrier formation.", "category": "physics_plasm-ph" }, { "text": "Nonlinear electromagnetic formulation for particle simulation of lower\n hybrid waves in toroidal geometry: Electromagnetic particle simulation model has been formulated and verified\nfor nonlinear processes of lower hybrid (LH) waves in fusion plasmas. Electron\ndynamics is described by the drift kinetic equation using either kinetic\nmomentum or canonical momentum. Ion dynamics is treated as the fluid system or\nby the Vlasov equation. Compressible magnetic perturbation is retained to\nsimulate both the fast and slow LH waves. Numerical properties are greatly\nimproved by using electron continuity equation to enforce consistency between\nelectrostatic potential and vector potential, and by using the importance\nsampling technique. The simulation model has been implemented in the\ngyrokinetic toroidal code (GTC), and verified for the dispersion relation and\nnonlinear particle trapping of the electromagnetic LH waves.", "category": "physics_plasm-ph" }, { "text": "Electron heating in a current-driven turbulence as a result of nonlinear\n interaction of electron- and ion-acoustic waves: We study electron heating in collisionless current-driven turbulence due to\nthe nonlinear interactions between electron- and ion-acoustic waves. PIC\nsimulation results show that due to a large difference between the electron and\nion mean velocities the Buneman instability excites large-amplitude\nion-acoustic waves, which strongly modifies the electron velocity distribution\nfunction, leading to a secondary instability that generates fast\nelectron-acoustic waves; and in this process, a giant electron hole is\nultimately created. This giant electron hole is responsible for strong electron\nheating due to phase mixing. The numerical simulation results are consistent\nwith the previous observations and provide insight into the key processes\nresponsible for electron heating and the generation of nonlinear waves in a\ncollisionless current-driven instability.", "category": "physics_plasm-ph" }, { "text": "Self-Synchronized Trichel Pulse Trains in Multi-Point Corona Discharge\n Systems: Evidence of self-synchronization has been observed in multi-electrode corona\ndischarge systems, where the application of high negative DC voltages induces a\nself-sustained mode of current pulse trains. These pulses, historically\nreferred to as Trichel pulses, characterize the operation of a two-electrode\nsystem where the discharge electrode is subjected to a high negative DC\nvoltage. The numerical algorithm indicates that in a multi-electrode discharge\nsystem, comprising multiple two-electrode discharges, each two-electrode system\nindependently produces pulse trains. These systems, each comprising a pair of\nelectrodes, operate in a pulsed mode, with synchronization emerging from weak\nyet significant interactions among them. These interactions arise from the\nmutual influence of electric fields and space charges generated by each\ndischarge pair. This influence extends beyond individual systems, leading to a\nsynchronization between both pairs, both in a pulsed mode. A three-species\nmodel of discharge was employed to simulate this process and it was based on\nthe finite element method formulation. Two different numerical models were\ninvestigated, a 2D model, consisting of two discharge electrodes and a third\ngrounded electrode, and two 1D-axisymmetric models, consisting dual and triple\npairs of discharge systems. Experiments show a multi-stable nature of the\ncoupled pulsed discharge systems, indicating that under appropriate conditions\nthe pulse trains exhibit two distinct modes of synchronization: in-phase\nsynchronization and anti-phase synchronization. The occurrence of each mode\ndepends on factors such as interaction strength, applied voltage level, and\nvarious system parameters. Furthermore, variations in these factors can lead to\nadditional outcomes, including out of phase synchronization, as well as\nscenarios involving near-harmonic oscillations and quenching.", "category": "physics_plasm-ph" }, { "text": "On the Thomas-Fermi model: Gabor J. Kalman's contribution and numerical\n approximations: In this article, we would like to pay tribute to Gabor Kalman, outlining his\ncontribution to a model widely used in dense plasma physics: the\nhigh-temperature Thomas-Fermi model. The approach of Ruoxian Ying and Kalman\nrelies on the separation of the bound and free electrons, a physically\nreasonable definition of the bound electrons, a description of the source\ndensity in the Poisson equation through the electron-ion and ion-ion pair\ncorrelation functions and a determination of the degree of ionization from the\nminimization of the total free energy. We also report on different\napproximations of the function $\\Phi$, which is a cornerstone of the original\nThomas-Femi model.", "category": "physics_plasm-ph" }, { "text": "Excitation of the transition radiation by a relativistic electron bunch\n in the plasma half-space: The process of transition excitation of an electromagnetic field by a\nrelativistic electron bunch in a plasma half-space is investigated. The\ninfluence of the plasma boundary on the spatial structure of plasma wake\noscillations is studied. The shape and intensity of the transition\nelectromagnetic pulse propagating in the plasma are determined.", "category": "physics_plasm-ph" }, { "text": "Scale-free vortex cascade emerging from random forcing in a strongly\n coupled system: The notions of self-organised criticality (SOC) and turbulence are\ntraditionally considered to be applicable to disjoint classes of phenomena.\nNevertheless, scale-free burst statistics is a feature shared by turbulent as\nwell as self-organised critical dynamics. It has also been suggested that\nanother shared feature is universal non-gaussian probability density functions\n(PDFs) of global fluctuations. Here, we elucidate the unifying aspects through\nanalysis of data from a laboratory dusty plasma monolayer. We compare analysis\nof experimental data with simulations of a two-dimensional (2D) many-body\nsystem, of 2D fluid turbulence, and a 2D SOC model, all subject to random\nforcing at small scales. The scale-free vortex cascade is apparent from\nstructure functions as well as spatio-temporal avalanche analysis, the latter\ngiving similar results for the experimental and all model systems studied. The\nexperiment exhibits global fluctuation statistics consistent with a\nnon-gaussian universal PDF, but the model systems yield this result only in a\nrestricted range of forcing conditions.", "category": "physics_plasm-ph" }, { "text": "Prevention of core particle depletion in stellarators by turbulence: In reactor-relevant plasmas, neoclassical transport drives an outward\nparticle flux in the core of large stellarators and predicts strongly hollow\ndensity profiles. However, this theoretical prediction is contradicted by\nexperiments. In particular, in Wendelstein 7-X, the first large optimized\nstellarator, flat or weakly peaked density profiles are generally measured,\nindicating that neoclassical theory is not sufficient and that an inward\ncontribution to the particle flux is missing in the core. In this Research\nLetter, it is shown that the turbulent contribution to the particle flux can\nexplain the difference between experimental measurements and neoclassical\npredictions. The results of this Research Letter also prove that theoretical\nand numerical tools are approaching the level of maturity needed for the\nprediction of equilibrium density profiles in stellarator plasmas, which is a\nfundamental requirement for the design of operation scenarios of present\ndevices and future reactors.", "category": "physics_plasm-ph" }, { "text": "Simple model of the slingshot effect: We present a detailed quantitative description of the recently proposed\n\"slingshot effect\" [Fiore, Fedele, De angelis 2014]. Namely, we determine a\nbroad range of conditions under which the impact of a very short and intense\nlaser pulse normally onto a low-density plasma (or matter to be locally\ncompletely ionized into a plasma by the pulse) causes the expulsion of a bunch\nof surface electrons in the direction opposite to the one of propagation of the\npulse, and the detailed, ready-for-experiments features of the expelled\nelectrons (energy spectrum, collimation, etc). The effect is due to the\ncombined actions of the ponderomotive force and the huge longitudinal field\narising from charge separation. Our predictions are based on estimating 3D\ncorrections to a simple, yet powerful plane magnetohydrodynamic (MHD) model\nwhere the equations to be solved are reduced to a system of Hamilton equations\nin one dimension (or a collection of) which become autonomous after the pulse\nhas overcome the electrons. Experimental tests seem to be at hand. If confirmed\nby the latter, the effect would provide a new extraction and acceleration\nmechanism for electrons, alternative to traditional radio-frequency-based or\nLaser-Wake-Field ones.", "category": "physics_plasm-ph" }, { "text": "Energy dissipation in astrophysical simulations: results of the\n Orszag-Tang test problem: The magnetic field through the magnetic reconnection process affects the\ndynamics and structure of astrophysical systems. Numerical simulations are the\ntools to study the evolution of these systems. However, the resolution,\ndimensions, resistivity, and turbulence of the system are some important\nparameters to take into account in the simulations. In this paper, we\ninvestigate the evolution of magnetic energy in astrophysical simulations by\nperforming a standard test problem for MHD codes, Orszag-Tang. We estimate the\nnumerical dissipation in the simulations using state-of-the-art numerical\nsimulation code in astrophysics, PLUTO. The estimated numerical resistivity in\n2D simulations corresponds to the Lundquist number $\\approx 10^{4}$ in the\nresolution of $512\\times512$ grid cells. It is also shown that the plasmoid\nunstable reconnection layer can be resolved with sufficient resolutions. Our\nanalysis demonstrates that in non-relativistic magnetohydrodynamics\nsimulations, magnetic and kinetic energies undergo conversion into internal\nenergy, resulting in plasma heating.", "category": "physics_plasm-ph" }, { "text": "Finite orbit width effects in large aspect ratio stellarators: New orbit averaged equations for low collisionality neoclassical fluxes in\nlarge aspect ratio stellarators with mirror ratios close to unity are derived.\nThe equations retain finite orbit width effects by employing the second\nadiabatic invariant $J$ as a velocity space coordinate and they have been\nimplemented in the orbit-averaged neoclassical code KNOSOS. The equations are\nused to study the $1/\\nu$ regime and the lower collisionality regimes. For\ngeneric large aspect ratio stellarators with mirror ratios close to unity, as\nthe collision frequency decreases, the $1/\\nu$ regime transitions directly into\nthe $\\nu$ regime, without passing through a $\\sqrt{\\nu}$ regime. An explicit\nformula for the neoclassical fluxes in the $\\nu$ regime is obtained. The\nformula includes the effect of particles that transition between different\ntypes of wells. While these transitions produce stochastic scattering\nindependent of the value of the collision frequency in velocity space, the\ndiffusion in real space remains proportional to the collision frequency. The\n$\\sqrt{\\nu}$ regime is only recovered in large aspect ratio stellarators close\nto omnigeneity: large aspect ratio stellarators with large mirror ratios and\noptimized large aspect ratio stellarators with mirror ratios close to unity.\nNeoclassical transport in large aspect ratio stellarators with large mirror\nratios can be calculated with the orbit-averaged equations derived by\n\\cite{calvo17}. In these stellarators, the $\\sqrt{\\nu}$ regime exists in the\ncollisionality interval $\\epsilon |\\ln \\epsilon| \\ll \\nu_{ii} R a/\\rho_i v_{ti}\n\\ll 1/\\epsilon$. In optimized large aspect ratio stellarators with mirror\nratios close to unity, the $\\sqrt{\\nu}$ regime occurs in an interval of\ncollisionality that depends on the deviation from omnigeneity $\\delta$:\n$\\delta^2 |\\ln \\delta | \\ll \\nu_{ii} R a/\\rho_i v_{ti} \\ll 1$.", "category": "physics_plasm-ph" }, { "text": "Magnetic Field Line Stickiness in Tokamaks: We present simulated figures of the diverted magnetic field lines of the\ntokamak ITER, obtained by numerically integrating a Hamiltonian model with\nelectrical currents in five wire loops and control coils. We show evidences of\na sticky island embedded in the chaotic region near the divertor plates, which\ntraps magnetic field lines for many toroidal turns increasing their connection\nlengths to these plates.", "category": "physics_plasm-ph" }, { "text": "The self-injection threshold in self-guided laser wakefield accelerators: A laser pulse traveling through a plasma can excite large amplitude plasma\nwaves that can be used to accelerate relativistic electron beams in a very\nshort distance---a technique called laser wakefield acceleration. Many\nwakefield acceleration experiments rely on the process of wavebreaking, or\nself-injection, to inject electrons into the wave, while other injection\ntechniques rely on operation without self-injection. We present an experimental\nstudy into the parameters, including the pulse energy, focal spot quality and\npulse power, that determine whether or not a wakefield accelerator will\nself-inject. By taking into account the processes of self-focusing and pulse\ncompression we are able to extend a previously described theoretical model,\nwhere the minimum bubble size required for trapping is not constant but varies\nslowly with density and find excellent agreement with this model.", "category": "physics_plasm-ph" }, { "text": "Current in Wave Driven Plasmas: A theory for the generation of current in a toroidal plasma by\nradio-frequency waves is presented. The effect of an opposing electric field is\nincluded, allowing the case of time varying currents to be studied. The key\nquantities that characterize this regime are identified and numerically\ncalculated. Circuit equations suitable for use in ray-tracing and transport\ncodes are given.", "category": "physics_plasm-ph" }, { "text": "Recoil Effects on Reflection from Relativistic Mirrors in Laser Plasmas: Relativistic mirrors can be realized with strongly nonlinear Langmuir waves\nexcited by intense laser pulses in underdense plasma. On reflection from the\nrelativistic mirror the incident light affects the mirror motion. The\ncorresponding recoil effects are investigated analytically and with\nparticle-in-cell simulations. It is found that if the fluence of the incident\nelectromagnetic wave exceeds a certain threshold, the relativistic mirror\nundergoes a significant back reaction and splits into multiple electron layers.\nThe reflection coefficient of the relativistic mirror as well as the factors of\nelectric field amplification and frequency upshift of the electromagnetic wave\nare obtained.", "category": "physics_plasm-ph" }, { "text": "Solving the Grad-Shafranov equation using spectral elements for tokamak\n equilibrium with toroidal rotation: The Grad-Shafranov equation is solved using spectral elements for tokamak\nequilibrium with toroidal rotation. The Grad-Shafranov solver builds upon and\nextends the NIMEQ code [Howell and Sovinec, Comput. Phys. Commun. 185 (2014)\n1415] previously developed for static tokamak equilibria. Both geometric and\nalgebraic convergence are achieved as the polynomial degree of the\nspectral-element basis increases. A new analytical solution to the\nGrad-Shafranov equation is obtained for Solov'ev equilibrium in presence of\nrigid toroidal rotation, in addition to a previously obtained analytical\nsolution for a defferent set of equilibrium and rotation profiles. The\nnumerical solutions from the extended NIMEQ are benchmarked with the analytical\nsolutions, with good agreements. Besides, the extended NIMEQ code is\nbenchmarked with the FLOW code [L. Guazzotto, R. Betti, et al., Phys. Plasma\n11(2004)604].", "category": "physics_plasm-ph" }, { "text": "Towards temporal characterization of intense isolated attosecond pulses\n from relativistic surface high harmonics: Relativistic surface high harmonics have been considered a unique source for\nthe generation of intense isolated attosecond pulses in the extreme\nultra-violet (XUV) and X-ray spectral range. However, its experimental\nrealization is still a challenging task requiring identification of the optimum\nconditions for the generation of isolated attosecond pulses as well as their\ntemporal characterization. Here, we demonstrate measurements in both\ndirections. Particularly, we have made a first step towards the temporal\ncharacterization of the emitted XUV radiation by adapting the attosecond streak\ncamera concept to identify the time domain characteristics of relativistic\nsurface high harmonics. The results, supported by PIC simulations, set the\nupper limit for the averaged (over many shots) XUV duration to <6 fs, even when\ndriven by not CEP controlled relativistic few-cycle optical pulses. Moreover,\nby measuring the dependence of the spectrum of the relativistic surface high\nharmonics on the carrier envelope phase (CEP) of the driving infrared laser\nfield, we experimentally determined the optimum conditions for the generation\nof intense isolated attosecond pulses.", "category": "physics_plasm-ph" }, { "text": "Effect of non-local transport of hot electrons on the laser-target\n ablation: The non-local heat transport of hot electrons during high-intensity lasers\ninteraction with plasmas can preheat the fuel and limit the heat flow in\ninertial confinement fusion. It increases the entropy of the fuel and decreases\nthe final compression. In this paper, the non-local electron transport model\nthat is based on the improved SNB algorithm has been embedded into the\nradiation hydrodynamic code and is benchmarked with two classical non-local\ntransport cases. Then we studied a 2$\\omega$ laser ablating a CH target by\nusing the non-local module. It is found that the non-local effect becomes\nsignificant when the laser intensity is above $1\\times 10^{14}\n\\mathrm{W/cm^{2}} $. The mass ablation rate from the SNB model is increased\ncompared to that of the flux-limited model due to the lower coronal plasma\ntemperature. This non-local model has a better agreement with the experimental\nresults compared to that of the flux-limited model. The non-local transport is\nstrongly dependent on the laser frequency, and the thresholds that the\nnon-local transport should be considered are obtained for lasers of different\nfrequencies. The appropriate flux-limiters that should be employed in the\nflux-limited model for different lasers are also presented. The results here\nshould have a good reference for the laser-target ablation applications.", "category": "physics_plasm-ph" }, { "text": "Fluid and hybrid simulations of the ionization instabilities in Hall\n thruster: Low-frequency axial oscillations (5-50 kHz) stand out as a pervasive feature\nobserved in many types of Hall thrusters. While it is widely recognized that\nthe ionization effects play the central role in this mode, as manifested via\nthe large scale oscillations of neutral and plasma density, the exact\nmechanism(s) of the instabilities remain unclear. To gain further insights into\nthe physics of the breathing mode and evaluate the role of kinetic effects, a\none-dimensional time-dependent full nonlinear low-frequency model describing\nneutral atoms, ions, and electrons, is developed in full fluid formulation and\ncompared to the hybrid model in which the ions and neutrals are kinetic. Both\nmodels are quasineutral and share the same electron fluid equations that\ninclude the electron diffusion, mobility across the magnetic field, and the\nelectron energy evolution. The ionization models are also similar in both\napproaches. The predictions of fluid and hybrid simulations are compared for\ndifferent test cases. Two main regimes are identified in both models: one with\npure low-frequency behaviour and the other one, where the low-frequency\noscillations coexist with higher frequency oscillations (with the\ncharacteristic time scale of the ion channel flyby time, 100-200 kHz). The\nother test case demonstrate the effect of a finite temperature of injected\natoms which is shown to have a substantial effect on the oscillation amplitude.", "category": "physics_plasm-ph" }, { "text": "Viscosity of the magnetized strongly coupled one-component plasma: The viscosity tensor of the magnetized one-component plasma, consisting of\nfive independent shear viscosity coefficients, a bulk viscosity coefficient,\nand a cross coefficient, is computed using equilibrium molecular dynamics\nsimulations and the Green-Kubo relations. A broad range of Coulomb coupling and\nmagnetization strength conditions are studied. Magnetization is found to\nstrongly influence the shear viscosity coefficients when the gyrofrequency\nexceeds the Coulomb collision frequency. Three regimes are identified as the\nCoulomb coupling strength and magnetization strength are varied. The Green-Kubo\nrelations are used to separate kinetic and potential energy contributions to\neach viscosity coefficient, showing how each contribution depends upon the\nmagnetization strength. The shear viscosity coefficient associated with the\ncomponent of the stress tensor parallel to the magnetic field, and the two\ncoefficients associated with the component perpendicular to the magnetic field,\nare all found to merge to a common value at strong Coulomb coupling.", "category": "physics_plasm-ph" }, { "text": "Single-Stage Stellarator Optimization: Combining Coils with Fixed\n Boundary Equilibria: We introduce a novel approach for the simultaneous optimization of plasma\nphysics and coil engineering objectives using fixed-boundary equilibria that is\ncomputationally efficient and applicable to a broad range of vacuum and finite\nplasma pressure scenarios. Our approach treats the plasma boundary and coil\nshapes as independently optimized variables, penalizing the mismatch between\nthe two using a quadratic flux term in the objective function. Four use cases\nare presented to demonstrate the effectiveness of the approach, including\nsimple and complex stellarator geometries. As shown here, this method\noutperforms previous 2-stage approaches, achieving smaller plasma objective\nfunction values when coils are taken into account.", "category": "physics_plasm-ph" }, { "text": "A reanalysis of a strong-flow gyrokinetic formalism: We reanalyse an arbitrary-wavelength gyrokinetic formalism [A. M. Dimits,\nPhys. Plasmas $\\bf17$, 055901 (2010)], which orders only the vorticity to be\nsmall and allows strong, time-varying flows on medium and long wavelengths. We\nobtain a simpler gyrocentre Lagrangian up to second order. In addition, the\ngyrokinetic Poisson equation, derived either via variation of the system\nLagrangian or explicit density calculation, is consistent with that of the\nweak-flow gyrokinetic formalism [T. S. Hahm, Phys. Fluids $\\bf31$, 2670 (1988)]\nat all wavelengths in the weak flow limit. The reanalysed formalism has been\nnumerically implemented as a particle-in-cell code. An iterative scheme is\ndescribed which allows for numerical solution of this system of equations,\ngiven the implicit dependence of the Euler-Lagrange equations on the time\nderivative of the potential.", "category": "physics_plasm-ph" }, { "text": "V-Langevin Equations, Continuous Time Random Walks and Fractional\n Diffusion: The following question is addressed: under what conditions can a strange\ndiffusive process, defined by a semi-dynamical V-Langevin equation or its\nassociated Hybrid kinetic equation (HKE), be described by an equivalent purely\nstochastic process, defined by a Continuous Time Random Walk (CTRW) or by a\nFractional Differential Equation (FDE)? More specifically, does there exist a\nclass of V-Langevin equations with long-range (algebraic) velocity temporal\ncorrelation, that leads to a time-fractional superdiffusive process? The answer\nis always affirmative in one dimension. It is always negative in two\ndimensions: any algebraically decaying temporal velocity correlation (with a\nGaussian spatial correlation) produces a normal diffusive process. General\nconditions relating the diffusive nature of the process to the temporal\nexponent of the Lagrangian velocity correlation (in Corrsin approximation) are\nderived.", "category": "physics_plasm-ph" }, { "text": "Rotational properties of annulus dusty plasma in a strong magnetic field: The collective dynamics of annulus dusty plasma formed between a co-centric\nconducting (non-conducting) disk and ring configuration is studied in a\nstrongly magnetized radio-frequency (rf) discharge. A superconducting\nelectromagnet is used to introduce a homogeneous magnetic field to the dusty\nplasma medium. In absence of the magnetic field, dust grains exhibit thermal\nmotion around their equilibrium position. The dust grains start to rotate in\nanticlockwise direction with increasing magnetic field (B $>$ 0.02 T), and the\nconstant value of the angular frequency at various strengths of magnetic field\nconfirms the rigid body rotation. The angular frequency of dust grains linearly\nincreases up to a threshold magnetic field (B $>$ 0.6 T) and after that its\nvalue remains nearly constant in a certain range of magnetic field. Further\nincrease in magnetic field (B $>$ 1 T) lowers the angular frequency. Low value\nof angular frequency is expected by reducing the width of annulus dusty plasma\nor the input rf power. The azimuthal ion drag force due to the magnetic field\nis assumed to be the energy source which drives the rotational motion. The\nresultant radial electric field in the presence of magnetic field determines\nthe direction of rotation. The variation of floating (plasma) potential across\nthe annular region at given magnetic field explains the rotational properties\nof the annulus dusty plasma in the presence of magnetic field.", "category": "physics_plasm-ph" }, { "text": "The dynamics of electron holes in current sheets: We present 1.5D Vlasov code simulations of the dynamics of electron holes in\nnon-uniform magnetic and electric fields typical of current sheets and,\nparticularly, of the Earth's magnetotail current sheet. The simulations show\nthat spatial width and amplitude of electron holes do not substantially vary in\nthe course of propagation, but there arises a double layer localized around the\nelectron hole and manifested as a drop of the electrostatic potential along the\nelectron hole. We demonstrate that electron holes produced around the neutral\nplane of a current sheet slow down in the course of propagation toward the\ncurrent sheet boundaries. The leading contribution to electron hole braking is\nprovided by the non-uniform magnetic field, though electrostatic fields typical\nof the current sheets do provide a noticeable contribution. The simulations\nalso show that electron holes with larger amplitudes are slowed faster. The\nsimulation results suggest that some of slow electron holes recently reported\nin the Earth's plasma sheet boundary layer may appear due to braking of\ninitially fast electron holes in the course of propagation in the current\nsheet.", "category": "physics_plasm-ph" }, { "text": "Anomalous Diffusion at Edge and Core of a Magnetized Cold Plasma: Progress in the theory of anomalous diffusion in weakly turbulent cold\nmagnetized plasmas is explained. Several proposed models advanced in the\nliterature are discussed. Emphasis is put on a new proposed mechanism for\nanomalous diffusion transport mechanism based on the coupled action of\nconductive walls (excluding electrodes) bounding the plasma drain current (edge\ndiffusion) together with the magnetic field flux \"cutting\" the area traced by\nthe charged particles in their orbital motion. The same reasoning is shown to\napply to the plasma core anomalous diffusion. The proposed mechanism is\nexpected to be valid in regimes when plasma diffusion scales as Bohm diffusion\nand at high $B/N$, when collisions are of secondary importance.", "category": "physics_plasm-ph" }, { "text": "Equilibria with incompressible flows from symmetry analysis: We identify and study new nonlinear axisymmetric equilibria with\nincompressible flow of arbitrary direction satisfying a generalized Grad\nShafranov equation by extending the symmetry analysis presented in [G. Cicogna\nand F. Pegoraro, Phys. Plasmas Vol. 22, 022520 (2015)]. In particular, we\nconstruct a typical tokamak D-shaped equilibrium with peaked toroidal current\ndensity, monotonically varying safety factor and sheared electric field.", "category": "physics_plasm-ph" }, { "text": "Disperson relation of finite amplitude Alfven wave in a relativistic\n electron- positron plasma: The linear dispersion relation of a finite amplitude, parallel, circularly\npolarized Alfv\\'en wave in a relativistic electron-positron plasma is derived.\nIn the nonrelativistic regime, the dispersion relation has two branches, one\nelectromagnetic wave, with a low frequency cutoff at\n$\\sqrt{1+2\\omega_p^2/\\Omega_p^2}$ (where $\\omega_p=(4\\pi n e^2/m)^{1/2}$ is the\nelectron/positron plasma frequency), and an Alfv\\'en wave, with high frequency\ncutoff at the positron gyrofrequency $\\Omega_p$. There is only one forward\npropagating mode for a given frequency. However, due to relativistic effects,\nthere is no low frequency cutoff for the electromagnetic branch, and there\nappears a critical wave number above which the Alfv\\'en wave ceases to exist.\nThis critical wave number is given by $ck_c/\\Omega_p=a/\\eta$, where\n$a=\\omega_p^2/\\Omega_p^2$ and $\\eta$ is the ratio between the Alfv\\'en wave\nmagnetic field amplitude and the background magnetic field. In this case, for\neach frequency in the Alfv\\'en branch, two additional forward propagating modes\nexist with equal frequency. A simple numerical example is studied: by\nnumerically solving the coupled system of fluid and Maxwell equations, normal\nincidence of a finite amplitude Alfv\\'en wave on an interface between two\nelectron-positron plasmas of different densities is considered.", "category": "physics_plasm-ph" }, { "text": "Exploring Born-Infeld electrodynamics using plasmas: The behaviour of large amplitude electrostatic waves in cold plasma is\ninvestigated in the context of Born-Infeld electrodynamics. The equations of\nmotion for the plasma are established using an unconstrained action principle.\nThe maximum amplitude and frequency of a large amplitude electrostatic wave are\ndetermined, and a lower bound on the wavelength is established. The maximum\nelectric field is found to be the same as that on a point electron at rest.", "category": "physics_plasm-ph" }, { "text": "Fundamental form of the electrostatic $\u03b4f$-PIC algorithm and\n discovery of a converged numerical instability: The $\\delta f$ particle-in-cell algorithm has been a useful tool in studying\nthe physics of plasmas, particularly turbulent magnetized plasmas in the\ncontext of gyrokinetics. The reduction in noise due to not having to resolve\nthe full distribution function indicates an efficiency advantage over standard\n(\"full-$f$\") particle-in-cell. Despite its successes, the algorithm behaves\nstrangely in some circumstances. In this work, we document a fully-resolved\nnumerical instability that occurs in the simplest of multiple-species test\ncases: the electrostatic $\\Omega_H$ mode. There is also a poorly-understood\nnumerical instability that occurs when one is under-resolved in particle\nnumber, which may require a prohibitively large number of particles to\nstabilize. Both of these are independent of the time-stepping scheme, and we\nconclude that they exist if the time advancement were exact. The exact analytic\nform of the algorithm is presented, and several schemes for mitigating these\ninstabilities are presented.", "category": "physics_plasm-ph" }, { "text": "Anomalous hot electron generation from two-plasmon decay instability\n driven by broadband laser pulses with intensity modulations: We investigate the hot electrons generated from two-plasmon decay (TPD)\ninstability driven by laser pulses with intensity modulated by a frequency\n$\\Delta \\omega_m$. Our primary focus lies on scenarios where $\\Delta \\omega_m$\nis on the same order of the TPD growth rate $ \\gamma_0$ ( $\\Delta \\omega_m \\sim\n\\gamma_0$), corresponding to moderate laser frequency bandwidths for TPD\nmitigation. With $\\Delta \\omega_m$ conveniently modeled by a basic two-color\nscheme of the laser wave fields in fully-kinetic particle-in-cell simulations,\nwe demonstrate that the energies of TPD modes and hot electrons exhibit\nintermittent evolution at the frequency $\\Delta \\omega_m$, particularly when\n$\\Delta \\omega_m \\sim \\gamma_0$. With the dynamic TPD behavior, the overall\nratio of hot electron energy to the incident laser energy, $f_{hot}$, changes\nsignificantly with $\\Delta \\omega_m$. While $f_{hot}$ drops notably with\nincreasing $\\Delta \\omega_m$ at large $\\Delta \\omega_m$ limit as expected, it\ngoes anomalously beyond the hot electron energy ratio for a single-frequency\nincident laser pulse with the same average intensity when $\\Delta \\omega_m$\nfalls below a specific threshold frequency $\\Delta \\omega_c$. We find this\nthreshold frequency primarily depends on $\\gamma_0$ and the collisional damping\nrate of plasma waves, with relatively lower sensitivity to the density scale\nlength. We develop a scaling model characterizing the relation of $\\Delta\n\\omega_c$ and laser plasma conditions, enabling the potential extention of our\nfindings to more complex and realistic scenarios.", "category": "physics_plasm-ph" }, { "text": "Compact Petawatt-Class Laser Wakefield Acceleration with Plasma\n Telescope: The compactness of laser wakefield acceleration (LWFA) is limited by its long\nfocal length for high power lasers, e.g., more than 10 meters for 1-peatawatt\n(PW) laser pulse and up to hundreds of meters for 10-100 PW lasers. The long\nfocal length originates from the low damage threshold of the optical off-axial\nparabolic (OAP) mirror and consequent large spot size. We propose implementing\nan OAP plasma mirror (PM) to form a telescope geometry, reducing the beam size\nand hence constraining the focal length to meter-range for LWFA driven by\nlasers beyond 1PW. Three-dimensional particle-in-cell simulations are performed\nto characterize the reflection of a 1-PW laser by the plasma OAP and find that\noptimal condition is achieved within only 1-m optical length. The new method\nsuccessfully generates 9GeV electron bunch in the subsequent LWFA stage with\nconsistent acceleration gradients to that of the 1-PW laser via ordinary\nfocusing. The proposed geometry provides a solution of compact LWFAs available\nfor even 100-PW laser systems.", "category": "physics_plasm-ph" }, { "text": "Wave Turbulence in Inertial Electron Magnetohydrodynamics: A wave turbulence theory is developed for inertial electron\nmagnetohydrodynamics (IEMHD) in the presence of a relatively strong and uniform\nexternal magnetic field $\\boldsymbol{B_0} = B_0 \\hat{\\boldsymbol{e}}_\\|$. This\nregime is relevant for scales smaller than the electron inertial length $d_e$.\nWe derive the kinetic equations that describe the three-wave interactions\nbetween inertial whistler or kinetic Alfv\\'en waves. We show that for both\ninvariants, energy and momentum, the transfer is anisotropic (axisymmetric)\nwith a direct cascade mainly in the direction perpendicular ($\\perp$) to\n$\\boldsymbol{B_0}$. The exact stationary solutions (Kolmogorov-Zakharov\nspectra) are obtained for which we prove the locality. We also found the\nKolmogorov constant $C_K \\simeq 8.474$. In the simplest case, the study reveals\nan energy spectrum in $k_\\perp^{-5/2} k_\\|^{-1/2}$ and a momentum spectrum\nenslaved to the energy dynamics in $k_\\perp^{-3/2} k_\\|^{-1/2}$. These\nsolutions correspond to a magnetic energy spectrum $\\sim k_\\perp^{-9/2}$, which\nis steeper than the EMHD prediction made for scales larger than $d_e$. We\nconclude with a discussion on the application of the theory to space plasmas.", "category": "physics_plasm-ph" }, { "text": "Coordinate-choice independent expression for drift orbit flux and\n flux-force relation in neoclassical toroidal viscosity theory: A coordinate-choice independent expression does not depend how the magnetic\nsurface is parametrized by (\\theta,\\zeta). Flux-force relation in neoclassical\ntoroidal viscosity(NTV) theory has been generalized in a coordinate-choice\nindependent way. The expression for the surface averaged drift orbit flux in\n1/\\nu regime is derived without the requirement of straight field line\ncoordinates. The resulted formula is insensitive to how the magnetic surface is\nparametrized and broadens the cases where flux-force relation can be applied.\nConstruction of straight field line coordinates is avoided when the formula is\nused for numerical computation.", "category": "physics_plasm-ph" }, { "text": "High frequency mode generation by toroidal Alfven eigenmodes: Nonlinear generation of high frequency mode (HFM) by toroidal Alfven\neigenmode (TAE) observed in HL-2A tokamak is analyzed using nonlinear\ngyrokinetic theory. It is found that, the HFM can be dominated by $|nq-m|=1$\nperturbations with predominantly ideal magnetohydrodynamic if the two primary\nTAEs are co-propagating; while the HFM can be characterized by $nq-m=0$\nelectrostatic perturbations if the two primary TAEs are counter-propagating.\nHere, $n$ and $m$ are respectively the toroidal and poloidal mode numbers, and\n$q$ is the safety factor. The nonlinear process is sensitive to the equilibrium\nmagnetic geometry of the device.", "category": "physics_plasm-ph" }, { "text": "Electron Acceleration and Radiation Generation from Relativistic\n Laser-Plasma Interactions at High Repetition-Rate: This dissertation explores the interaction between high-intensity lasers and\nplasmas to accelerate electrons and produce radiation via experimental and\ncomputational efforts. The laser pulses used in this dissertation have\nultrashort duration (< 100 fs), near-infrared to mid-infrared wavelength (0.8\n$\\mu$m, 2 $\\mu$m, or 3.9 $\\mu$m), millijoules of energy, and high repetition\nrates (480 Hz or 20 Hz). The plasma sources applied are from solid-density\ntargets (overdense) or gaseous targets (underdense). With the\nhigh-repetition-rate capability, statistical methods are employed to optimize\ncertain aspect of the experiments and to interpret the physics.", "category": "physics_plasm-ph" }, { "text": "Recent progress in astrophysical plasma turbulence from solar wind\n observations: This paper summarises some of the recent progress that has been made in\nunderstanding astrophysical plasma turbulence in the solar wind, from in situ\nspacecraft observations. At large scales, where the turbulence is predominantly\nAlfvenic, measurements of critical balance, residual energy, and 3D structure\nare discussed, along with comparison to recent models of strong Alfvenic\nturbulence. At these scales, a few percent of the energy is also in compressive\nfluctuations, and their nature, anisotropy, and relation to the Alfvenic\ncomponent is described. In the small scale kinetic range, below the ion\ngyroscale, the turbulence becomes predominantly kinetic Alfven in nature, and\nmeasurements of the spectra, anisotropy, and intermittency of this turbulence\nare discussed with respect to recent cascade models. One of the major remaining\nquestions is how the turbulent energy is dissipated, and some recent work on\nthis question, in addition to future space missions which will help to answer\nit, are briefly discussed.", "category": "physics_plasm-ph" }, { "text": "Investigation of positive and negative modes of nanosecond pulsed\n discharge in water and electrostriction model of initiation: This work investigates the development of nanosecond pulsed discharges in\nwater ignited with the application of both positive and negative polarity\npulses to submerged pin to plane electrodes. Optical diagnostics are used to\nstudy two main aspects of these discharges: the initiation phase, and the\ndevelopment phase. Nanosecond pulses up to 24 kV with 4 ns rise time, 10 ns\nduration and 5 ns fall time are used to ignite discharges in a 1.5 mm gap\nbetween a copper plate and a tungsten needle with radius of curvature of 25 um.\nFast ICCD imaging is used to trace the discharge development over varying\napplied pulse amplitudes for both positively and negatively applied pulses to\nthe pin electrode. The discharge is found to progress similar to that of\ndischarges in long gaps in gases, both in structure and development. The more\nimportant initiation phase is investigated via Schlieren transmission imaging.\nThe region near the tip of the electrode is investigated for slightly\nunder-breakdown conditions, and changes in the liquids refractive index and\ndensity are observed over the duration of the applied pulse. An attempt to\nexplain the results is made based on the electrostriction model of discharge\ninitiation.", "category": "physics_plasm-ph" }, { "text": "Longitudinal electric conductivity and dielectric permeability in\n quantum plasma with constant collision frequency in Mermin' approach: Detailed deducing of formulas for longitudinal electric conductivity and\ndielectric permeability in the quantum degenerate collisional plasma with\nconstant collision frequency in Mermin' approach is given. The kinetic\nSchr\\\"{o}dinger-Boltzmann equation in momentum space in relaxation\napproximation is used. It is shown that when collision frequency of plasma\nparticles tends to zero (plasma passes to collisionless one), the deduced\nformula for dielectric function passes to the known Lindhard' formula for\ncollisionless plasmas. It is shown that the deduced formula for dielectric\npermeability coincides with known Mermin's formula. Graphic research of the\nreal and imaginary parts of dielectric function is made. Graphic comparison of\nthe real and imaginary parts of dielectric function for quantum and classical\nplasma also is made. The module of derivative dielectric function also has been\ninvestigated graphically.", "category": "physics_plasm-ph" }, { "text": "Approximate symmetries of guiding-centre motion: Quasisymmetry builds a third invariant for charged-particle motion besides\nenergy and magnetic moment. We address quasisymmetry at the level of\napproximate symmetries of first-order guiding-centre motion. We find that the\nconditions to leading order are the same as for exact quasisymmetry if one\ninsists that the symmetry is purely spatial. We also generalise to allow for\napproximate phase-space symmetries, and derive weaker conditions. The latter\nrecover \"weak quasisymmetry\" as a subcase, thus we prove it is spatial only to\nleading order, but also that it implies the existence of a wider class of\nindependent approximate conserved quantities. Finally, we demonstrate that\nmagnetohydrostatics imposes quasisymmetry to leading order.", "category": "physics_plasm-ph" }, { "text": "Reply to Comment on `Formation of bound states of electrons in\n spherically symmetric oscillations of plasma': I reply here to the comment of Dr Shmatov on my recent work and demonstrate\nthe invalidity of his criticism of the classical physics description of the\nformation of bound states of electrons participating in spherically symmetric\noscillations of plasma.", "category": "physics_plasm-ph" }, { "text": "In situ Evidence of Breaking the Ion Frozen-in Condition via the\n Non-gyrotropic Pressure Effect in Magnetic Reconnection: For magnetic reconnection to proceed, the frozen-in condition for both ion\nfluid and electron fluid in a localized diffusion region must be violated by\ninertial effects, thermal pressure effects, or inter-species collisions. It has\nbeen unclear which underlying effects unfreeze ion fluid in the diffusion\nregion. By analyzing in-situ THEMIS spacecraft measurements at the dayside\nmagnetopause, we present clear evidence that the off-diagonal components of the\nion pressure tensor is mainly responsible for breaking the ion frozen-in\ncondition in reconnection. The off-diagonal pressure tensor, which corresponds\nto a nongyrotropic pressure effect, is a fluid manifestation of ion\ndemagnetization in the diffusion region. From the perspective of the ion\nmomentum equation, the reported non-gyrotropic ion pressure tensor is a\nfundamental aspect in specifying the reconnection electric field that controls\nhow quickly reconnection proceeds.", "category": "physics_plasm-ph" }, { "text": "Formation Mechanism of Atmospheric Pressure Plasma Jet: Atmospheric pressure plasma jet can protrude some 5.0 cm into air. It holds\npromise for multivarious innovative applications, but its formation mechanism\nremains unsettled. We show that the plasma jet is essentially a streamer corona\ntotally independent of, but obscured by, dielectric barrier discharge.\nConsequently, the jets can be equally successfully generated even with one\nsingle bare metal electrode attached to the tube orifice, both downstream and\nupstream simultaneously, and at a significantly reduced voltage. These results\nwill help understand the underlying physics and facilitate a safer and more\nflexible implementation of this marvelous plasma source.", "category": "physics_plasm-ph" }, { "text": "Spacecraft observations and analytic theory of crescent-shaped electron\n distributions in asymmetric magnetic reconnection: Supported by a kinetic simulation, we derive an exclusion energy parameter\n$\\cal{E}_X$ providing a lower kinetic energy bound for an electron to cross\nfrom one inflow region to the other during magnetic reconnection. As by a\nMaxwell Demon, only high energy electrons are permitted to cross the inner\nreconnection region, setting the electron distribution function observed along\nthe low density side separatrix during asymmetric reconnection. The analytic\nmodel accounts for the two distinct flavors of crescent-shaped electron\ndistributions observed by spacecraft in a thin boundary layer along the low\ndensity separatrix.", "category": "physics_plasm-ph" }, { "text": "Plasma-wall transition and sheath formation: A review of the theoretical and computational aspects of plasma-wall\ntransition is presented. The conditions for the existence of plasma sheaths in\nfront of a solid surface are established. Various regimes are analyzed\n-collisionless and collisional. fluid and kinetic, with and without ionization.\nThe partition of the plasma-wall transition into distinct regions (Debye\nsheath, quasineutral presheath. magnetic presheath, etc...) is also discussed.\nApplications to the theory of electrostatic probes are illustrated by means of\na recently developed kinetic model. The influence of the probe on ion\ntemperature measurements is studied in detail.", "category": "physics_plasm-ph" }, { "text": "Stable Langmuir solitons in plasma with diatomic ions: We study stable axially and spherically symmetric spatial solitons in plasma\nwith diatomic ions. The stability of a soliton against the collapse is provided\nby the interaction of induced electric dipole moments of ions with rapidly\noscillating electric field of a plasmoid. We derive the new cubic-quintic\nnonlinear Schrodinger equation which governs the soliton dynamics and\nnumerically solve it. Then we discuss the possibility of implementation of such\nplasmoids in realistic atmospheric plasma. In particular, we suggest that\nspherically symmetric Langmuir solitons, described in the present work, can be\nexcited at the formation stage of long-lived atmospheric plasma structures. The\nimplication of our model for the interpretation of the results of experiments\nfor the plasmoids generation is discussed.", "category": "physics_plasm-ph" }, { "text": "Particle flows in a dc discharge in laboratory and microgravity\n conditions: We describe a series of experiments on dust particles flows in a positive\ncolumn of a horizontal dc discharge operating in laboratory and microgravity\nconditions. The main observation is that the particle flow velocities in\nlaboratory experiments are systematically higher than in microgravity\nexperiments, for otherwise identical discharge conditions. The paper provides\nan explanation for this interesting and unexpected observation. The explanation\nis based on a physical model, which properly takes into account main\nplasma-particle interaction mechanisms relevant to the described experimental\nstudy. Comparison of experimentally measured particle velocities and those\ncalculated using the proposed model demonstrates reasonable agreement, both in\nlaboratory and microgravity conditions, in the entire range of discharge\nparameters investigated.", "category": "physics_plasm-ph" }, { "text": "Quantum Kinetic Theory of Plasmas: As is well-known, for plasmas of high density and modest temperature, the\nclassical kinetic theory needs to be extended. Such extensions can be based on\nthe Schr\\\"odinger Hamiltonian, applying a Wigner transform of the density\nmatrix, in which case the Vlasov equation is replaced by the celebrated\nWigner-Moyal equation. Extending the treatment to more complicated models, we\ninvestigate aspects such as spin dynamics (based on the Pauli Hamiltonian),\nexchange effects (using the Hartree-Fock approximation), Landau quantization,\nand quantum relativistic theory. In the relativistic theory we first study\ncases where the field strength is well beyond Schwingers critical field. Both\nweakly relativistic theory (gamma factors close to unity) and strongly\nrelativistic theory are investigated, using assumptions that allow for a\nseparation of electron and positron states. Finally, we study the\nDirac-Heisenberg-Wigner (DHW) formalism, which is a fully quantum relativistic\ntheory, allowing for field strengths of the order of the Schwinger critical\nfield or even larger. As a result, the quantum kinetic theory is extended to\ncover phenomena such as Zitterbewegung and electron-positron pair creation.\nWhile the focus of this review is on the quantum kinetic models, we illustrate\nthe theories with various applications throughout the manuscript.", "category": "physics_plasm-ph" }, { "text": "Verification of the standard theory of plasma emission with\n particle-in-cell simulations: The standard theory of plasma emission is based on kinetic couplings between\na single beam of energetic electrons and unmagnetized thermal plasmas,\ninvolving multi-step nonlinear wave-particle and wave-wave interactions. The\ntheory has not yet been completely verified with fully-kinetic electromagnetic\nparticle-in-cell (PIC) simulations. Earlier studies, greatly limited by\navailable computational resources, are controversial regarding whether the\nfundamental emission can be generated according to the standard theory. To\nresolve the controversy, we conducted PIC simulations with a large domain of\nsimulation and a large number of macroparticles, among the largest ones of\nsimilar studies. We found significant fundamental emission if the relative beam\ndensity is small enough (say, $\\le$ 0.01), in line with earlier study with a\nmuch-smaller domain; the relative intensity (normalized by the total initial\nbeam energy) of all modes, except the mode associated with the\nbeam-electromagnetic Weibel instability, decreases with increasing relative\ndensity of the beam. We also found significant transverse magnetic component\nassociated with the superluminal Langmuir turbulence, which has been mistakenly\nregarded as evidence of the F emission in earlier study. Further investigations\nare required to reveal their origin.", "category": "physics_plasm-ph" }, { "text": "Laser wakefield acceleration driven by few-cycle laser pulses in\n overdense plasmas: We measure the emission of energetic electrons from the interaction between\nultrashort laser pulses and a solid density plasma in the relativistic regime.\nWe detect an electron beam that only appears with few-cycle pulses (< 10 fs)\nand large plasma scale lengths ($L > \\lambda_0$). Numerical simulations, in\nagreement with the experiments, reveal that these electrons are accelerated by\na laser wakefield. Plasma waves are indeed resonantly excited by the few-cycle\nlaser pulses in the near-critical-density region of the plasma. Electrons are\nthen injected by ionization into the plasma waves and accelerated to\nrelativistic energies. This study provides an unprecedented insight into the\nphysics happening in the few-cycle regime.", "category": "physics_plasm-ph" }, { "text": "Dust-acoustic waves and stability in the permeating dusty plasma: II.\n Power-law distributions: The dust-acoustic waves and their stability driven by a flowing dusty plasma\nwhen it cross through a static (target) dusty plasma (the so-called permeating\ndusty plasma) are investigated when the components of the dusty plasma obey the\npower-law q-distributions in nonextensive statistics. The frequency, the growth\nrate and the stability condition of the dust-acoustic waves are derived under\nthis physical situation, which express the effects of the nonextensivity as\nwell as the flowing dusty plasma velocity on the dust-acoustic waves in this\ndusty plasma. The numerical results illustrate some new characteristics of the\ndust-acoustic waves, which are different from those in the permeating dusty\nplasma when the plasma components are the Maxwellian distribution. In addition,\nwe show that the flowing dusty plasma velocity has a significant effect on the\ndust-acoustic waves in the permeating dusty plasma with the power-law\nq-distribution.", "category": "physics_plasm-ph" }, { "text": "Electron-positron Pair Creation by Counterpropagating Laser Pulses: Role\n of Carrier Envelope phase: The effect of carrier envelope phase (CEP) on the spatio-temporal\ndistribution of the electron-positron pairs created by untraintense\ncounterpropagating femtosecond laser pulses is studied. When the laser pulses\nare linearly polarized, the temporal distribution of the pairs is found to be\nsensitive to CEP. On the hand, it is found to be largely insensitive to CEP for\ncircularly polarized pulses.", "category": "physics_plasm-ph" }, { "text": "Resistive MHD Modelling of Quasi-Single Helicity State in the KTX\n Regimes: The potential formation of the quasi-single-helicity (QSH) state in the Keda\nTorus eXperiment (KTX) is investigated in resistive MHD simulations using the\nNIMROD code. We focus on the effects of finite resistivity on the mode\nstructure and characteristics of the dominant linear and nonlinear resistive\ntearing-mode in a finite $\\beta$, cylindrical configuration of reversed field\npinch model for KTX. In the typical resistive regimes of KTX where Lundquist\nnumber $S=5 \\times 10^4$, the plasma transitions to a steady QSH state after\nevolving through an initial transient phase with multiple helicities. The\ndominant mode of the QSH state develops from the dominant linear tearing mode\ninstability. In lower $\\beta$ regime, the QSH state are intermittent and short\nin duration; in higher $\\beta$ regime, the QSH state persists for a longer time\nand should be more observable in experiment.", "category": "physics_plasm-ph" }, { "text": "Equation-of-state model for shock compression of hot dense matter: A quantum equation-of-state model is presented and applied to the calculation\nof high-pressure shock Hugoniot curves beyond the asymptotic fourfold density,\nclose to the maximum compression where quantum effects play a role. An\nanalytical estimate for the maximum attainable compression is proposed. It\ngives a good agreement with the equation-of-state model.", "category": "physics_plasm-ph" }, { "text": "Self Organization of Edge and Internal Pedestals in a Sandpile: The temperature profiles of magnetically confined plasmas can display\ndistinctive longlived pedestals at the edge and internally. Here we show that\nsuch structures can arise naturally through avalanching transport in a sandpile\nmodel. A single control parameter that is constant across the sandpile\ndetermines the occurrence and regularity of these effects, as well as the\nentrainment of global confinement to edge pedestal dynamics. The system\ndynamics indicate that the pedestals are a consequence of an inverse cascade in\nreal space, and that self organization is necessary for their occurrence.", "category": "physics_plasm-ph" }, { "text": "3D simulations of positive streamers in air in a strong external\n magnetic field: We study how external magnetic fields from 0 to 40 T influence positive\nstreamers in atmospheric pressure air, using 3D PIC-MCC (particle-in-cell,\nMonte Carlo collision) simulations. When a magnetic field $\\vec{B}$ is applied\nperpendicular to the background electric field $\\vec{E}$, the streamers deflect\ntowards the $+\\vec{B}$ and $-\\vec{B}$ directions which results in a branching\ninto two main channels. With a stronger magnetic field the angle between the\nbranches increases, and for the 40 T case the branches grow almost parallel to\nthe magnetic field. Due to the $\\vec{E}\\times\\vec{B}$ drift of electrons we\nalso observe a streamer deviation in the opposite $-\\vec{E}\\times\\vec{B}$\ndirection, where the minus sign appears because positive streamers propagate\nopposite to the electron drift velocity. The deviation due to this\n$\\vec{E}\\times\\vec{B}$ effect is smaller than the deviation parallel to\n$\\vec{B}$. In both cases of $\\vec{B}$ perpendicular and parallel to $\\vec{E}$,\nthe streamer radius decreases with the magnetic field strength. We relate our\nobservations to the effects of electric and magnetic fields on electron\ntransport and reaction coefficients.", "category": "physics_plasm-ph" }, { "text": "On the stability and emittance growth of different particle phase-space\n distributions in a long magnetic quadrupole channel: The behavior of K-V, waterbag, parabolic, conical and Gaussian distributions\nin periodic quadrupole channels is studied by particle simulations. It is found\nthat all these different distributions exhibit the known K-V instabilities. But\nthe action of the K-V type modes becomes more and more damped in the order of\nthe types of distributions quoted above. This damping is so strong for the\nGaussian distribution that the emittance growth factor after a large number of\nperiods is considerably lower than in the case of an equivalent K-V\ndistribution. In addition, the non K-V distributions experience in only one\nperiod of the channel a rapid initial emittance growth, which becomes very\nsignificant at high beam intensities. This growth is attributed to the\nhomogenization of the space-charge density, resulting in a conversion of\nelectric-field energy into transverse kinetic and potential energy. Two simple\nanalytical formulae are derived to estimate the upper and lower boundary values\nfor this effect and are compared with the results obtained from particle\nsimulations.", "category": "physics_plasm-ph" }, { "text": "The action principle for generalized fluid motion including\n gyroviscosity: A general set of fluid equations that allow for energy-conserving momentum\ntransport by gyroscopic motion of fluid elements is obtained. The equations are\nproduced by a class of action principles that yield a large subset of the known\nfluid and magnetofluid models, including gyroviscosity. Analysis of the action\nprinciple yields broad, model-independent results regarding the conservation\nlaws of energy and linear and angular momenta. The formalism is illustrated by\nstudying fluid models with intrinsic angular momentum that may appear in the\ncontexts of condensed matter, biological, and other areas of physics.", "category": "physics_plasm-ph" }, { "text": "Dispersion relation of an electromagnetic wave in unmagnetized cold\n plasma: The dispersion relation of an electromagnetic wave in an unmagnetized neutral\nplasma is well known to be w^2=wp^2+(ck)^2. A modified dispersion relation is\npresented taking into account the ion restoring force in the transverse\ndirection.", "category": "physics_plasm-ph" }, { "text": "A generalized Grad-Shafranov equation with plasma flow under a conformal\n coordinate transformation: We employ a conformal mapping transformation to solve a generalized\nGrad-Shafranov equation with incompressible plasma flow of arbitrary direction\nand construct particular up-down asymmetric D-shaped and diverted tokamak\nequilibria. The proposed method can also be employed as an alternative\nquasi-analytic method to solving two dimensional elliptic partial differential\nequations.", "category": "physics_plasm-ph" }, { "text": "Landau Damping in a Turbulent Setting: To address the problem of Landau damping in kinetic turbulence, the forcing\nof the linearized Vlasov equation by a stationary random source is considered.\nIt is found that the time-asymptotic density response is dominated by resonant\nparticle interactions that are synchronized with the source. The energy\nconsumption of this response is calculated, implying an effective damping rate,\nwhich is the main result of this paper. Evaluating several cases, it is found\nthat the effective damping rate can differ from the Landau damping rate in\nmagnitude and also, remarkably, in sign. A limit is demonstrated in which the\ndensity and current become phase-locked, which causes the effective damping to\nbe negligible; this potentially resolves an energy paradox that arises in the\napplication of critical balance to a kinetic turbulence cascade.", "category": "physics_plasm-ph" }, { "text": "Comments on the comments by Lackner et al. on the series of papers about\n \"A novel direct drive ultra-fast heating concept for ICF\": In this paper, we provide a response to the comments made by Lackner et al.\nregarding our series of recent papers on \"A novel direct drive ultra-fast\nheating concept for ICF\". Specifically, we comment on the necessity of fuel\npre-compression in the ICF context.", "category": "physics_plasm-ph" }, { "text": "Transformer ratio growth due to ion motion in plasma wakefield\n accelerators: We report a recently discovered counterintuitive effect where breaking of a\nLangmuir wave in a plasma wakefield accelerator leads to an increase in the\naccelerating field rather than wave dissipation. The effect relies on the\nability of transversely breaking waves to draw wave energy from nearby regions\ndue to the inflow of electrons oscillating collectively and the outflow of\nelectrons moving individually.", "category": "physics_plasm-ph" }, { "text": "Equation of state of a strongly magnetized hydrogen plasma: The influence of a constant uniform magnetic field on the thermodynamic\nproperties of a partially ionized hydrogen plasma is studied. Using the method\nof Green' s function various interaction contributions to the thermodynamic\nfunctions are calculated. The equation of state of a quantum magnetized plasma\nis presented within the framework of a low density expansion up to the order\ne^4 n^2 and, additionally, including ladder type contributions via the bound\nstates in the case of strong magnetic fields (2.35*10^{5} T << B << 2.35*10^{9}\nT). We show that for high densities (n=10^{27-30} m^{-3}) and temperatures\nT=10^5 - 10^6 K typical for the surface of neutron stars nonideality effects\nas, e.g., Debye screening must be taken into account.", "category": "physics_plasm-ph" }, { "text": "Collective excitations of a spherically confined Yukawa plasma: The results of a recent fluid theory for the multipole modes of a Yukawa\nplasma in a spherical confinement [H. K\\\"{a}hlert and M. Bonitz, Phys. Rev. E\n\\textbf{82}, 036407 (2010)] are compared with molecular dynamics simulations\nand the exact $N$-particle eigenmodes in the crystalline phase. Simulations\nconfirm the existence of high order modes found in cold fluid theory. We\ninvestigate the influence of screening, coupling and friction on the mode\nspectra in detail. Good agreement between theory and simulation is found for\nweak to moderate screening and low order modes. The relations between the\nbreathing mode in the fluid theory, simulation and the crystal eigenmode are\ninvestigated in further detail.", "category": "physics_plasm-ph" }, { "text": "Numerical simulations of a continuously injected relativistic electron\n beam relaxation into a plasma with large-scale density gradients: In this paper the influence of large-scale decreasing and increasing\ngradients of the density of magnetized plasma on the relaxation process of a\ncontinuously injected relativistic electron beam with an energy of 611 keV\n($v_b=0.9c$) and a pitch-angle distribution is studied using particle-in-cell\nnumerical simulations. It is found that for the selected parameters in the case\nof a smoothly decreasing gradient and in a homogeneous plasma the formation of\nspatially limited plasma oscillations of large amplitude occurs. In such cases,\nmodulation instability develops and a long-wave longitudinal modulation of the\nion density is formed. In addition, the large amplitude of plasma waves\naccelerates plasma electrons to energies on the order of the beam energy. In\nthe case of increasing and sharply decreasing gradients, a significant decrease\nin the amplitude of plasma oscillations and the formation of a turbulent ion\ndensity spectrum are observed. The possibility of acceleration of beam\nelectrons to energies more than 2 times higher than the initial energy of the\nbeam particles is also demonstrated. This process takes place not only during\nbeam propagation in growing plasma density, but also in homogeneous plasma due\nto interaction of beam particles with plasma oscillations of large amplitude.", "category": "physics_plasm-ph" }, { "text": "Fast-Ion Deuterium Alpha spectroscopic observations of the effects of\n fishbones in the Mega-Ampere Spherical Tokamak: Using the recently-installed Fast-Ion Deuterium Alpha (FIDA) spectrometer,\nthe effects of low-frequency (20-50 kHz) chirping energetic particle modes with\ntoroidal mode number n \\geq 1 on the fast-ion population in MAST plasmas are\nconsidered. Results from the FIDA diagnostic are presented and discussed in the\nlight of the present theoretical understanding of these modes, known as\nfishbones, in plasmas with reversed shear. Measurements of the fast-ion\npopulation reveal strong redistribution of fast ions in both real and velocity\nspace as a result of the fishbones. Time-resolved measurements throughout the\nevolution of a fishbone show radial redistribution of fast ions with energies\nup to 95% of the primary beam injection energy. Correlations between changes in\nthe FIDA signal and the peak time derivative of the magnetic field perturbation\nare observed in a limited range of operating scenarios. The transient reduction\nin signal caused by a fishbone may in some cases reach 50% of the signal\nintensity before mode onset.", "category": "physics_plasm-ph" }, { "text": "Combined effect of horizontal magnetic field and vorticity on Rayleigh\n Taylor instability: In this research, the height, curvature and velocity of the bubble tip in\nRayleigh-Taylor instability at arbitrary Atwood number with horizontal magnetic\nfield are investigated. To support the earlier simulation and experimental\nresults, the vorticity generation inside the bubble is introduced. It is found\nthat, in early nonlinear stage, the temporal evolution of the bubble tip\nparameters depend essentially on the strength and initial perturbation of the\nmagnetic field, although the asymptotic nature coincides with the non magnetic\ncase. The model proposed here agrees with the previous linear, nonlinear and\nsimulation observations.", "category": "physics_plasm-ph" }, { "text": "Passive and active electromagnetic stabilization of free-surface liquid\n metal flows: Free-surface liquid metal flows tend to be uneven due to instabilities and\nother effects. Some applications, however, require constant, uniform liquid\nmetal thickness. This is for example the case of liquid walls in nuclear fusion\nreactors. With this motivation, here we present experimental results on the\nstabilization of a free-surface flow of Galinstan. The flow was sustained by an\nelectromagnetic induction pump featuring rotating permanent magnets. Evidence\nis reported of the flowing Galinstan layer becoming flatter in the presence of\na sufficiently strong magnetic field, either alone (passive stabilization) or\nin combination with an electrical current passing through the liquid metal\n(active stabilization). The results are interpreted in terms of an effective\nviscosity and effective gravity, respectively.", "category": "physics_plasm-ph" }, { "text": "The impact of the Hall effect on high energy density plasma jets: Using a 1-MA, 100ns-rise-time pulsed power generator, radial foil\nconfigurations can produce strongly collimated plasma jets. The resulting jets\nhave electron densities on the order of 10^20 cm^-3, temperatures above 50 eV\nand plasma velocities on the order of 100 km/s, giving Reynolds numbers of the\norder of 10^3, magnetic Reynolds and P\\'eclet numbers on the order of 1. While\nHall physics does not dominate jet dynamics due to the large particle density\nand flow inside, it strongly impacts flows in the jet periphery where plasma\ndensity is low. As a result, Hall physics affects indirectly the geometrical\nshape of the jet and its density profile. The comparison between experiments\nand numerical simulations demonstrates that the Hall term enhances the jet\ndensity when the plasma current flows away from the jet compared to the case\nwhere the plasma current flows towards it.", "category": "physics_plasm-ph" }, { "text": "Analysis of the temperature influence on Langmuir probe measurements on\n the basis of gyrofluid simulations: The influence of the temperature and its fluctuations on the ion saturation\ncurrent and the floating potential, which are typical quantities measured by\nLangmuir probes in the turbulent edge region of fusion plasmas, is analysed by\nglobal nonlinear gyrofluid simulations for two exemplary parameter regimes. The\nnumerical simulation facilitates a direct access to densities, temperatures and\nthe plasma potential at different radial positions around the separatrix. This\nallows a comparison between raw data and the calculated ion saturation current\nand floating potential within the simulation. Calculations of the\nfluctuation-induced radial particle flux and its statistical properties reveal\nsignificant differences to the actual values at all radial positions of the\nsimulation domain, if the floating potential and the temperature averaged\ndensity inferred from the ion saturation current is used.", "category": "physics_plasm-ph" }, { "text": "Simulation study of a bright attosecond $\u03b3$-ray source generation\n by irradiating an intense laser on a cone target: The interaction between an ultrastrong laser and a cone-like target is an\nefficient approach to generate high power radiations like attosecond pulses and\nterahertz waves. The object is to study the $\\gamma$-ray generation under this\nconfiguration with the help of 2D particle-in-cell simulations. It is\ndeciphered that electrons experience three stages including injection,\nacceleration and scattering to emit high energy photons via nonlinear compton\nscattering (NCS). These spatial-separated attosecond $\\gamma$-ray pulses own\nhigh peak brilliance ($>10^{22}$ photons/($\\rm s\\cdot\\rm mm^2\\cdot\\rm\nmrad^2\\cdot0.1\\%BW$)) and high energy (6MeV) under the case of normalized laser\nintensity $a_0=30$ ($\\mathrm{I=2\\times10^{21}W/cm^2}$). Besides, the cone\ntarget turns out to be an order of magnitude more efficient in energy transfer\ncompared with a planar one.", "category": "physics_plasm-ph" }, { "text": "Dynamical Systems' approach to relativistic nonlinear wave-particle\n interaction in weakly collisional plasmas: In this report, we present a dynamical systems' approach to study the exact\nnonlinear wave-particle interaction in relativistic regime. We give a\nparticular attention to the effect of wave obliquity on the dynamics of the\norbits by studying the specific cases of parallel ($\\theta=0$) and\nperpendicular ($\\theta=-\\pi/2$) propagations in comparison to the general case\nof oblique propagation $\\theta=]-\\pi/2, 0[$. We found that the fixed points of\nthe system correspond to Landau resonance, and that the dynamics can evolve\nfrom trapping to surfatron acceleration for propagation angles obeying a Hopf\nbifurcations condition. Cyclotron-resonant particles are also studied by the\nconstruction of a pseudo-potential structure in the Lorentz factor $\\gamma$. We\nderived a condition for which Arnold diffusion results in relativistic\nstochastic acceleration. Hence, two general conclusions are drawn : 1) The\npropagation angle $\\theta$ can significantly alter the dynamics of the orbits\nat both Landau and cyclotron-resonances. 2) Considering the short-time scales\nupon which the particles are accelerated, these two mechanisms for Landau and\ncyclotron resonant orbits could become potential candidates for problems of\nparticle energization in weakly collisional space and cosmic plasmas.", "category": "physics_plasm-ph" }, { "text": "Seeding of the Self-Modulation in a Long Proton Bunch by Charge\n Cancellation with a Short Electron Bunch: In plasma wakefield accelerators (e.g. AWAKE) the proton bunch\nself-modulation is seeded by the ionization front of a high-power laser pulse\nionizing a vapour and by the resulting steep edge of the driving bunch profile\ninside the created plasma. In this paper, we present calculations in 2D linear\ntheory for a concept of a different self-modulation seeding mechanism based on\nelectron injection. The whole proton bunch propagates through a preformed\nplasma and the effective beam current is modulated by the external injection of\na short electron bunch at the centre of the proton beam. The resulting sharp\nedge in the effective beam current in the trailing part of the proton bunch is\ndriving large wakefields that can lead to a growth of the seeded\nself-modulation (SSM). Furthermore, we discuss the feasibility for applications\nin AWAKE Run 2.", "category": "physics_plasm-ph" }, { "text": "Non-linear modeling of the threshold between ELM mitigation and ELM\n suppression by Resonant Magnetic Perturbations in ASDEX Upgrade: The interaction between Edge Localized Modes (ELMs) and Resonant Magnetic\nPerturbations (RMPs) is modeled with the magnetohydrodynamic code JOREK using\nexperimental parameters from ASDEX Upgrade discharges. The ELM mitigation or\nsuppression is optimal when the amplification of both tearing and peeling-kink\nresponses result in a better RMP penetration. The ELM mitigation or suppression\nis not only due to the reduction of the pressure gradient, but predominantly\narises from the toroidal coupling between the ELMs and the RMP-induced mode at\nthe plasma edge, forcing the edge modes to saturate at a low level. The\nbifurcation from ELM mitigation to ELM suppression is observed when the RMP\namplitude is increased. ELM mitigation is characterized by rotating modes at\nthe edge, while the mode locking to RMPs is induced by the resonant braking of\nthe electron perpendicular flow in the ELM suppression regime.", "category": "physics_plasm-ph" }, { "text": "Highly charged ions in a weakly coupled plasma: an exact solution: The ion sphere model introduced long ago by Salpeter is placed in a rigorous\ntheoretical setting. The leading corrections to this model for very highly\ncharged but dilute ions in thermal equilibrium with a weakly coupled,\none-component background plasma are explicitly computed, and the subleading\ncorrections are shown to be negligibly small. Such analytic results for very\nstrong coupling are rarely available, and they can serve as benchmarks for\ntesting computer models in this limit.", "category": "physics_plasm-ph" }, { "text": "From linear to nonlinear Breit-Wheeler pair production in laser-solid\n interactions: During the ultraintense laser interaction with solids (overdense plasmas),\nthe competition between two possible quantum electrodynamics (QED) mechanisms\nresponsible for $e^\\pm$ pair production, i.e., linear and nonlinear\nBreit-Wheeler (BW) processes, remains to be studied. Here, we have implemented\nthe linear BW process via a Monte Carlo algorithm into the QED particle-in-cell\n(PIC) code YUNIC, enabling us to self-consistently investigate both pair\nproduction mechanisms in the plasma environment. By a series of 2D QED-PIC\nsimulations, the transition from the linear to the nonlinear BW process is\nobserved with the increase of laser intensities in the typical configuration of\na linearly polarized laser interaction with solid targets. A critical\nnormalized laser amplitude about $a_0\\sim$ 400-500 is found under a large range\nof preplasma scale lengths, below which the linear BW process dominates over\nthe nonlinear BW process. This work provides a practicable technique to model\nlinear QED processes via integrated QED-PIC simulations. Moreover, it calls for\nmore attention to be paid to linear BW pair production in near future\n10-PW-class laser-solid interactions.", "category": "physics_plasm-ph" }, { "text": "I-mode investigation on the Experimental Advanced Superconducting\n Tokamak: By analyzing large quantities of discharges in the unfavorable ion $ \\vec\nB\\times \\nabla B $ drift direction, the I-mode operation has been confirmed in\nEAST tokamak. During the L-mode to I-mode transition, the energy confinement\nhas a prominent improvement by the formation of a high-temperature edge\npedestal, while the particle confinement remains almost identical to that in\nthe L-mode. Similar with the I-mode observation on other devices, the $ E_r $\nprofiles obtained by the eight-channel Doppler backscattering system\n(DBS8)\\cite{J.Q.Hu} show a deeper edge $ E_r $ well in the I-mode than that in\nthe L-mode. And a weak coherent mode (WCM) with the frequency range of 40-150\nkHz is observed at the edge plasma with the radial extend of about 2-3 cm. WCM\ncould be observed in both density fluctuation and radial electric field\nfluctuation, and the bicoherence analyses showed significant couplings between\nWCM and high frequency turbulence, implying that the $ E_r $ fluctuation and\nthe caused flow shear from WCM should play an important role during I-mode. In\naddition, a low-frequency oscillation with a frequency range of 5-10 kHz is\nalways accompanied with WCM, where GAM intensity is decreased or disappeared.\nMany evidences show that the a low-frequency oscillation may be a novel kind of\nlimited cycle oscillation but further investigations are needed to explain the\nnew properties such as the harmonics and obvious magnetical perturbations.", "category": "physics_plasm-ph" }, { "text": "On Rayleigh-Taylor interfacial mixing: The Rayleigh-Taylor instability develops when fluids are accelerated counter\nto their density gradients; intense interfacial fluid mixing ensues with time.\nThe Rayleigh-Taylor mixing controls a broad range of processes in fluids,\nplasmas, materials, at astrophysical and at atomic scales. In this perspective\npaper we briefly review theoretical and experimental approaches, and apply\ntheory and experiment to investigate order and disorder in Rayleigh-Taylor\nflows. The theory finds that properties of heterogeneous anisotropic\naccelerated Rayleigh-Taylor mixing depart from those of homogeneous isotropic\ninertial turbulence, including strong correlations, weak fluctuations, and\nsensitivity to deterministic conditions. The experiment unambiguously observes\nthe heterogeneity and anisotropy of Rayleigh-Taylor mixing at very high\nReynolds numbers, and the stabilizing effects of acceleration and accelerated\nshear on the interfacial dynamics. The theory and the experiment agree with one\nanother and evince that Rayleigh- Taylor mixing may exhibit order and\nlaminarize, similarly to other accelerated flows, thus opening new perspectives\nfor research of complex processes in nature and technology.", "category": "physics_plasm-ph" }, { "text": "Enhancement of electron energy to multi-GeV regime by a dual-stage\n laser-wakefield accelerator pumped by petawatt laser pulses: Laser wakefield acceleration offers the promise of a compact electron\naccelerator for generating a multi-GeV electron beam using the huge field\ngradient induced by an intense laser pulse, compared to conventional rf\naccelerators. However, the energy and quality of the electron beam from the\nlaser wakefield accelerator have been limited by the power of the driving laser\npulses and interaction properties in the target medium. Recent progress in\nlaser technology has resulted in the realization of a petawatt (PW) femtosecond\nlaser, which offers new capabilities for research on laser wakefield\nacceleration. Here, we present a significant increase in laser-driven electron\nenergy to the multi-GeV level by utilizing a 30-fs, 1-PW laser system. In\nparticular, a dual-stage laser wakefield acceleration scheme (injector and\naccelerator scheme) was applied to boost electron energies to over 3 GeV with a\nsingle PW laser pulse. Three-dimensional particle-in-cell simulations\ncorroborate the multi-GeV electron generation from the dual-stage laser\nwakefield accelerator driven by PW laser pulses.", "category": "physics_plasm-ph" }, { "text": "New linear stability parameter to describe low-$\u03b2$ electromagnetic\n microinstabilities driven by passing electrons in axisymmetric toroidal\n geometry: In magnetic confinement fusion devices, the ratio of the plasma pressure to\nthe magnetic field energy, $\\beta$, can become sufficiently large that\nelectromagnetic microinstabilities become unstable, driving turbulence that\ndistorts or reconnects the equilibrium magnetic field. In this paper, a theory\nis proposed for electromagnetic, electron-driven linear instabilities that have\ncurrent layers localised to mode-rational surfaces and binormal wavelengths\ncomparable to the ion gyroradius. The model retains axisymmetric toroidal\ngeometry with arbitrary shaping, and consists of orbit-averaged equations for\nthe mode-rational surface layer, with a ballooning space kinetic matching\ncondition for passing electrons. The matching condition connects the current\nlayer to the large scale electromagnetic fluctuations, and is derived in the\nlimit that $\\beta$ is comparable to the square root of the electron-to-ion-mass\nratio. Electromagnetic fluctuations only enter through the matching condition,\nallowing for the identification of an effective $\\beta$ that includes the\neffects of equilibrium flux surface shaping. The scaling predictions made by\nthe asymptotic theory are tested with comparisons to results from linear\nsimulations of micro-tearing and electrostatic microinstabilities in MAST\ndischarge #6252, showing excellent agreement. In particular, it is demonstrated\nthat the effective $\\beta$ can explain the dependence of the local\nmicro-tearing mode (MTM) growth rate on the ballooning parameter $\\theta_0$ --\npossibly providing a route to optimise local flux surfaces for reduced\nMTM-driven transport.", "category": "physics_plasm-ph" }, { "text": "Shear Alfven wave continuous spectrum within magnetic islands: The radial structure of the continuous spectrum of shear Alfven waves is\ncalculated in this paper within the separatrix of a magnetic island.\nGeometrical effects due to the noncircularity of the flux surface's cross\nsection are retained to all orders. On the other hand, we keep only curvature\neffects responsible for the beta-induced gap in the low-frequency part of the\ncontinuous spectrum. Modes with different helicity from that of the magnetic\nisland are considered. The main result is that, inside a magnetic island, there\nis a continuous spectrum very similar to that of tokamak plasmas, where a\ngeneralized safety factor q can be defined and where a wide frequency gap is\nformed, analogous to the ellipticity induced Alfven eigenmode gap in tokamaks.\nThe presence of this gap is due to the strong eccentricity of the island cross\nsection. The importance of the existence of such a gap is recognized in\npotentially hosting magnetic-island induced Alfven eigenmodes (MiAE). Due to\nthe frequency dependence of the shear Alfven wave continuum on the\nmagnetic-island size, the possibility of utilizing MiAE frequency scalings as a\nnovel magnetic-island diagnostic is also discussed.", "category": "physics_plasm-ph" }, { "text": "Relativistic quantum plasma dispersion functions: Relativistic quantum plasma dispersion functions are defined and the\nlongitudinal and transverse response functions for an electron (plus positron)\ngas are written in terms of them. The dispersion is separated into\nLandau-damping, pair-creation and dissipationless regimes. Explicit forms are\ngiven for the RQPDFs in the cases of a completely degenerate distribution and a\nnondegenerate thermal (J\\\"uttner) distribution. Particular emphasis is placed\non the relation between dissipation and dispersion, with the dissipation\ntreated in terms of the imaginary parts of RQPDFs. Comparing the dissipation\ncalculated in this way with the existing treatments leads to the identification\nof errors in the literature, which we correct. We also comment on a controversy\nas to whether the dispersion curves in a superdense plasma pass through the\nregion where pair creation is allowed.", "category": "physics_plasm-ph" }, { "text": "Wakefield acceleration in atmospheric plasmas: a possible source of MeV\n electrons: Intense electromagnetic pulses interacting with a plasma can create a wake of\nplasma oscillations. Electrons trapped in such oscillations can be accelerated\nunder certain conditions to very high energies. We study the conditions for the\nwakefield acceleration to produce MeV electrons in atmospheric plasmas. This\nmechanism may explain the origin of MeV or runaway electrons needed in the\ncurrent theories for the production of Terrestrial Gamma ray Flashes.", "category": "physics_plasm-ph" }, { "text": "Realization of SOC behavior in a dc glow discharge plasma: Experimental observations consistent with Self Organized Criticality (SOC)\nhave been obtained in the electrostatic floating potential fluctuations of a dc\nglow discharge plasma. Power spectrum exhibits a power law which is compatible\nwith the requirement for SOC systems. Also the estimated value of the Hurst\nexponent (self similarity parameter), H being greater than 0.5, along with an\nalgebraic decay of the autocorrelation function, indicate the presence of\ntemporal long-range correlations, as may be expected from SOC dynamics. This\ntype of observations in our opinion has been reported for the first time in a\nglow discharge system.", "category": "physics_plasm-ph" }, { "text": "DEMO ion cyclotron heating: status of ITER-type antenna design: The ITER ICRF system will gain in complexity relative to the existing systems\non modern devices, and the same will hold true for DEMO. The accumulated\nexperience can help greatly in designing an ICRF system for DEMO. In this paper\nthe current status of the pre-conceptual design of the DEMO ICRF antenna and\nsome related components is presented. While many aspects strongly resemble the\nITER system, in some design solutions we had to take an alternative route to be\nable to adapt to DEMO specific. One of the key points is the toroidal antenna\nextent needed for the requested ICRF heating performance, achieved by splitting\nthe antenna in halves, with appropriate installation. Modelling of the so far\nlargest ICRF antenna in RAPLICASOL and associated challenges are presented.\nCalculation are benchmarked with TOPICA. Results of the analysis of the latest\nmodel and an outlook for future steps are given.", "category": "physics_plasm-ph" }, { "text": "Fast ion transport in quasisymmetric equilibria in the presence of a\n resonant Alfv\u00e9nic perturbation: Significant progress has been made in designing magnetic fields that provide\nexcellent confinement of the guiding enter trajectories of alpha particles\nusing quasisymmetry (QS). Given the reduction in this transport channel, we\nassess the impact of resonant Alfv\\'{e}n eigenmodes (AEs) on the guiding center\nmotion. The AE amplitudes are chosen to be consistent with experimental\nmeasurements and large-scale simulations. We evaluate the drift resonance\ncondition, phase-space island width, and island overlap criterion for\nquasisymmetric configurations. Kinetic Poincar\\'{e} plots elucidate features of\nthe transport, including stiff transport above a critical perturbation\namplitude. Our analysis highlights key departures from the AE-driven transport\nin tokamaks, such as the avoidance of phase-space island overlap in\nquasihelical configurations and the enhanced transport due to wide phase-space\nislands in low magnetic shear configurations. In configurations that are closer\nto QS, with QS deviations $\\delta B/B_0 \\lesssim 10^{-3}$, the transport is\nprimarily driven by the AE, while configurations that are further from QS,\n$\\delta B/B_0 \\sim 10^{-2}$, experience significant transport due to the\nQS-breaking fields in addition to the AE.", "category": "physics_plasm-ph" }, { "text": "Effects of the Second Harmonic on the GAM in Electron Scale Turbulence: The effects higher order harmonics have been self-consistently included in\nthe derivation of the electron branch of the electron Geodesic Acoustic Mode\n(el-GAM) in an Electron-Temperature-Gradient (ETG) turbulence background. The\nwork is based on a two-fluid model including finite $\\beta$-effects while\nretaining non-adiabatic ions. In solving the linear dispersion relation, it is\nfound that the due to the coupling to the $m=2$ mode the real frequency may be\nsignificantly altered and yield higher values.", "category": "physics_plasm-ph" }, { "text": "Ha spectroscopy for hot plasma parameters measurement: The new spectroscopic method for measurement of hot plasma parameters is\ndeveloped. The method based on Ha profile monitoring. The profile was\naccurately calculated for a wide range of plasma parameters (n$_e \\sim $\n10$^{14} \\div $ 10$^{17}$ cm$^{-3}$, T$_e \\sim 1\\div$500 eV) Use of the method\nin the experiment gives the electron density and ion temperature dependence\nfrom the time. Measurements was in a good agreement with diamagnetic loop\ndates.\n Key words: H$_\\alpha$ spectroscopy, line profile, density and high\ntemperature measurement.", "category": "physics_plasm-ph" }, { "text": "Impact Of A Uniform Plasma Resistivity In MHD Modelling Of Helical\n Solutions For The Reversed Field Pinch Dynamo: Till now the magnetohydrodynamic (MHD) simulation of the reversed field pinch\n(RFP) has been performed by assuming axis-symmetric radial time independent\ndissipation profiles. In helical states this assumption is not correct since\nthese dissipations should be flux functions, and should exhibit a helical\nsymmetry as well. Therefore more correct simulations should incorporate\nself-consistent dissipation profiles. As a first step in this direction, the\ncase of uniform dissipation profiles was considered by using the 3D nonlinear\nvisco-resistive MHD code SpeCyl. It is found that a flattening of the\nresistivity profile results in the reduction of the dynamo action, which brings\nto marginally-reversed or even non-reversed equilibrium solutions. The physical\norigin of this result is discussed in relation to the electrostatic drift\nexplanation of the RFP dynamo. This sets constraints on the functional choice\nof dissipations in future self-consistent simulations.", "category": "physics_plasm-ph" }, { "text": "Intrinsic nonlinearity of interaction of an electromagnetic field with\n quantum plasma and its research: The analysis of nonlinear interaction of transversal electromagnetic field\nwith quantum collisionless plasma is carried out. Formulas for calculation\nelectric current in quantum collisionless plasma at any temperature are\ndeduced. It has appeared, that the nonlinearity account leads to occurrence of\nthe longitudinal electric current directed along a wave vector. This second\ncurrent is orthogonal to the known transversal classical current, received at\nthe classical linear analysis. The case of degenerate electronic plasma is\nconsidered. The concept of longitudinal-transversal conductivity is entered.\nThe graphic analysis of the real and imaginary parts of dimensionless\ncoefficient of longitudinal-transversal conductivity is made. It is shown, that\nfor degenerate plasmas the electric current is calculated under the formula,\nnot containing quadratures. In this formula we have allocated known Kohn's\nsingularities (W. Kohn, 1959).", "category": "physics_plasm-ph" }, { "text": "Power-law scaling of plasma pressure on laser-ablated tin microdroplets: The measurement of the propulsion of metallic microdroplets exposed to\nnanosecond laser pulses provides an elegant method for probing the ablation\npressure in dense laser-produced plasma. We present the measurements of the\npropulsion velocity over three decades in the driving Nd:YAG laser pulse\nenergy, and observe a near-perfect power law dependence. Simulations performed\nwith the RALEF-2D radiation-hydrodynamic code are shown to be in good agreement\nwith the power law above a specific threshold energy. The simulations highlight\nthe importance of radiative losses which significantly modify the power of the\npressure scaling. Having found a good agreement between the experiment and the\nsimulations, we investigate the analytic origins of the obtained power law and\nconclude that none of the available analytic theories is directly applicable\nfor explaining our power exponent.", "category": "physics_plasm-ph" }, { "text": "Lagrangean description of nonlinear dust--ion acoustic waves in dusty\n plasmas: An analytical model is presented for the description of nonlinear\ndust-ion-acoustic waves propagating in an unmagnetized, collisionless, three\ncomponent plasma composed of electrons, ions and inertial dust grains. The\nformulation relies on a Lagrangean approach of the plasma fluid model. The\nmodulational stability of the wave amplitude is investigated. Different types\nof localized envelope electrostatic excitations are shown to exist.", "category": "physics_plasm-ph" }, { "text": "Magnetohydrodynamic \"cat eyes\" and stabilizing effects of plasma flow: The cat-eyes steady state solution in the framework of hydrodynamics\ndescribing an infinite row of identical vortices is extended to the\nmagnetohydrodynamic equilibrium equation with incompressible flow of arbitrary\ndirection. The extended solution covers a variety of equilibria including one-\nand two-dimensional generalized force-free and Harris-sheet configurations\nwhich are preferable from those usually employed as initial states in\nreconnection studies. Although the vortex shape is not affected by the magnetic\nfield, the flow in conjunction with the equilibrium nonlinearity has a strong\nimpact on isobaric surfaces by forming pressure islands located within the\ncat-eyes vortices. More importantly, a magnetic-field-aligned flow of\nexperimental fusion relevance and the flow shear have significant stabilizing\neffects in the region of pressure islands. The stable region is enhanced by an\nexternal axial (\"toroidal\") magnetic field.", "category": "physics_plasm-ph" }, { "text": "Global simulations of kinetic-magnetohydrodynamic processes with\n energetic electrons in tokamak plasmas: The energetic electrons (EEs) generated through auxiliary heating have been\nfound to destabilize various Alfven eigenmodes (AEs) in recent experiments,\nwhich in turn lead to the EE transport and degrade the plasma energy\nconfinement. In this work, we propose a global fluid-kinetic hybrid model for\nstudying corresponding kinetic-magnetohydrodynamic (MHD) processes by coupling\nthe drift-kinetic EEs to the Landau-fluid model of bulk plasmas in a\nnon-perturbative manner. The numerical capability of Landau-fluid bulk plasmas\nis obtained based on a well-benchmarked eigenvalue code MAS [Multiscale\nAnalysis of plasma Stabilities, J. Bao et al. Nucl. Fusion accepted 2023], and\nthe EE responses to the electromagnetic fluctuations are analytically derived,\nwhich not only contribute to the MHD interchange drive and parallel current but\nalso lead to the newly kinetic particle compression with the precessional drift\nresonance in the leading order. The hybrid model is casted into a nonlinear\neigenvalue matrix equation and solved iteratively using Newton's method. By\ncalibrating the EE precession frequency against the particle equation of motion\nin general geometry and applying more realistic trapped particle distribution\nin the poloidal plane, MAS simulations of EE-driven beta-induced Alfven\neigenmodes (e-BAE) show excellent agreements with gyrokinetic particle-in-cell\nsimulations, and the non-perturbative effects of EEs on e-BAE mode structure,\ngrowth rate and damping rate are demonstrated. With these efforts, the upgraded\nMAS greatly improves the computation efficiency for plasma problems related to\ndeeply-trapped EEs, which is superior than initial-value simulations restricted\nby the stringent electron Courant condition regarding to the practical\napplication of fast linear analysis.", "category": "physics_plasm-ph" }, { "text": "Linear pair creation damping of high frequency plasma oscillation: We have studied the linear dispersion relation for Langmuir waves in plasmas\nof very high density, based on the Dirac-Heisenberg-Wigner formalism. The\nvacuum contribution to the physical observables leads to ultra-violet\ndivergences, that are removed by a charge renormalization. The remaining vacuum\ncontribution is small, and is in agreement with previously derived expressions\nfor the time-dependent vacuum polarization. The main new feature of the theory\nis a damping mechanism similar to Landau damping, but where the plasmon energy\ngive rise to creation of electron-positron pairs. The dependence of the damping\nrate (pair-creation rate) on wave-number, temperature, and density is analyzed.\nFinally, the analytical results of linearized theory are compared.", "category": "physics_plasm-ph" }, { "text": "Wavelets, Curvelets and Multiresolution Analysis Techniques in Fast Z\n Pinch Research: Z pinches produce an X ray rich plasma environment where backlighting imaging\nof imploding targets can be quite challenging to analyze. What is required is a\ndetailed understanding of the implosion dynamics by studying snapshot images of\nits in flight deformations away from a spherical shell. We have used wavelets,\ncurvelets and multiresolution analysis techniques to address some of these\ndifficulties and to establish the Shell Thickness Averaged Radius (STAR) of\nmaximum density, r*(N, {\\theta}), where N is the percentage of the shell\nthickness over which we average. The non-uniformities of r*(N, {\\theta}) are\nquantified by a Legendre polynomial decomposition in angle, {\\theta}, and the\nidentification of its largest coefficients. Undecimated wavelet decompositions\noutperform decimated ones in denoising and both are surpassed by the curvelet\ntransform. In each case, hard thresholding based on noise modeling is used.", "category": "physics_plasm-ph" }, { "text": "Coupling parameter lower bound in Yukawa-Vlasov plasmas: We have analyzed the Vlasov dispersion relation for Yukawa plasmas in three\nand two dimensions primarily for the purpose of identifying coupling parameter\ndomains where the existence of well-developed collective excitations is\nforbidden or allowed. First, we have established a rigorous lower bound for the\ncoupling parameter, below which there can be no real solution to the dispersion\nrelation. In the coupling domain where weakly damped solutions do exist, we\nhave focused on the long-wavelength acoustic regime where we have established\nmore restrictive lower-bound estimates of the coupling parameter. We have also\nderived a general formula for the corresponding acoustic phase velocity, valid\nover a wide range of coupling parameter/screening parameter ratios above the\nmore restrictive lower bound.", "category": "physics_plasm-ph" }, { "text": "Characterization of the State of Hydrogen: Fermionic path integral Monte Carlo simulations have been applied to study\nthe equilibrium properties of the hydrogen and deuterium in the density and\ntemperature range of 1.6 < rs < 14.0 and 5000K < T < 167000K. We use this\ntechnique to determine the phase diagram by identifying the plasma, the\nmolecular, atomic and metallic regime. We explain how one can identify the\nphases in the path integral formalism and discuss the state of hydrogen for 5\npoints in the temperature-density plane. Further we will provide arguments for\nthe nature of the transitions between the regimes.", "category": "physics_plasm-ph" }, { "text": "High-brilliance synchrotron radiation induced by the plasma\n magnetostatic mode: Using multi-dimensional PIC simulations we show that the magnetic\nundulator-type field of the plasma magnetostatic mode is indeed produced by the\ninteraction of a laser pulse with a relativistic ionization front, as predicted\nby linear theory for a cold plasma. When the front with this magnetostatic mode\nis followed by a relativistic electron beam, the interaction of the beam with\nthis magnetic field, produces FEL-type synchrotron radiation, providing a\ndirect signature of the magnetostatic mode. The possibility of generating\nreadily detectable ultrashort wavelength radiation using this mode, by\nemploying state-of-the-art laser systems, is demonstrated, thus opening the way\ntowards experimental observation of the hitherto unseen magnetostatic mode and\nthe use of this plasma FEL mechanism to provide a source of high-brilliance\nultrashort wavelength radiation.", "category": "physics_plasm-ph" }, { "text": "Signature of inertia on light dragging in rotating plasmas: The signature of light dragging in a rotating unmagnetized plasma is studied\nanalytically. In contrast with previous work which focused exclusively on the\ndrag effects arising from rigid rotation, we examine here the supplemental\ncontribution of inertia to the rest-frame dielectric properties of a rotating\nmedium. We reveal, for the first time, that these so far neglected\ncontributions actually play a dominant role on light dragging in rotating\nunmagnetized plasmas. Besides birefringence and enhanced polarization drag,\ninertia is notably demonstrated to be the cause of a non-zero drag, pointing to\nfundamental differences between linear and angular momentum coupling. We\nfinally discuss how, thanks to the more favourable scaling elicited here, it\nmay be possible to observe these effects in recently proposed laser driven\nrotating plasmas, identifying new promising directions for experimental\ninvestigations.", "category": "physics_plasm-ph" }, { "text": "Dust ion acoustic solitary structures in presence of isothermal\n positrons: The Sagdeev potential technique has been employed to study the dust ion\nacoustic solitary waves and double layers in an unmagnetized collisionless\ndusty plasma consisting of negatively charged static dust grains, adiabatic\nwarm ions, and isothermally distributed electrons and positrons. A\ncomputational scheme has been developed to draw the qualitatively different\ncompositional parameter spaces or solution spaces showing the nature of\nexistence of different solitary structures with respect to any parameter of the\npresent plasma system. The qualitatively distinct solution spaces give the\noverall scenario regarding the existence of different solitary structures. The\npresent system supports both positive and negative potential double layers. The\nnegative potential double layer always restricts the occurrence of negative\npotential solitary waves, i.e., any sequence of negative potential solitary\nwaves having monotonically increasing amplitude converges to a negative\npotential double layer. However, there exists a parameter regime for which the\npositive potential double layer is unable to restrict the occurrence of\npositive potential solitary waves. As a result, in this region of the parameter\nspace, there exist solitary waves after the formation of positive potential\ndouble layer, i.e., positive potential supersolitons have been observed. But\nthe amplitudes of these supersolitons are bounded. A general theory for the\nexistence of bounded supersolitons has been discussed analytically by imposing\nthe restrictions on the Mach number. For any small value of positron\nconcentration, there is no effect of very hot positrons on the dust ion\nacoustic solitary structures. The qualitatively different solution spaces are\ncapable of producing new results for the formation of solitary structures.", "category": "physics_plasm-ph" }, { "text": "Numerical calculation of the transport coefficients in thermal plasmas: -We have performed a new efficient method to calculate numerically the\ntransport coefficients at high temperature. The collision theory was treated to\nstudy singularities that occur when evaluating the collision cross section. The\ntransport coefficients (viscosity, diffusion coefficient, thermal and\nelectrical conductivity) depend strongly on nature of the interaction between\nthe particles that form the plasma and that is why it is necessary to determine\nthe interaction potential accurately.", "category": "physics_plasm-ph" }, { "text": "A flowing plasma model to describe drift waves in a cylindrical helicon\n discharge: A two-fluid model developed originally to describe wave oscillations in the\nvacuum arc centrifuge, a cylindrical, rapidly rotating, low temperature and\nconfined plasma column, is applied to interpret plasma oscillations in a RF\ngenerated linear magnetised plasma (WOMBAT), with similar density and field\nstrength. Compared to typical centrifuge plasmas, WOMBAT plasmas have slower\nnormalised rotation frequency, lower temperature and lower axial velocity.\nDespite these differences, the two-fluid model provides a consistent\ndescription of the WOMBAT plasma configuration and yields qualitative agreement\nbetween measured and predicted wave oscillation frequencies with axial field\nstrength. In addition, the radial profile of the density perturbation predicted\nby this model is consistent with the data. Parameter scans show that the\ndispersion curve is sensitive to the axial field strength and the electron\ntemperature, and the dependence of oscillation frequency with electron\ntemperature matches the experiment. These results consolidate earlier claims\nthat the density and floating potential oscillations are a resistive drift\nmode, driven by the density gradient. To our knowledge, this is the first\ndetailed physics model of flowing plasmas in the diffusion region away from the\nRF source. Possible extensions to the model, including temperature\nnon-uniformity and magnetic field oscillations, are also discussed.", "category": "physics_plasm-ph" }, { "text": "Electrostatic potential variation on the flux surface and its impact on\n impurity transport: The particle transport of impurities in magnetically confined plasmas under\nsome conditions does not find, neither quantitatively nor qualitatively, a\nsatisfactory theory-based explanation. This compromise the successful\nrealization of thermo-nuclear fusion for energy production since its\naccumulation is known to be one of the causes that leads to the plasma\nbreakdown. In standard reactor-relevant conditions this accumulation is in most\nstellarators intrinsic to the lack of toroidal symmetry, that leads to the\nneoclassical electric field to point radially inwards. This statement, that the\nstandard theory allows to formulate, has been contradicted by some experiments\nthat showed weaker or no accumulation under such conditions\n\\cite{Ida_pop_16_056111_2009, Yoshinuma_nf_49_062002_2009}. The charge state of\nthe impurities makes its transport more sensitive to the electric fields. Thus,\nthe short length scale turbulent electrostatic potential or its long\nwave-length variation on the flux surface $\\Phi_{1}$ -- that the standard\nneoclassical approach usually neglects -- might possibly shed some light on the\nexperimental findings. In the present work the focus is put on the second of\nthe two, and investigate its influence of the radial transport of C$^{6+}$. We\nshow that in LHD it is strongly modified by $\\Phi_{1}$, both resulting in\nmitigated/enhanced accumulation at internal/external radial positions; for\nWendelstein 7-X, on the contrary, $\\Phi_{1}$ is expected to be considerably\nsmaller and the transport of C$^{6+}$ not affected up to an appreciable extent;\nand in TJ-II the potential shows a moderate impact despite of the large\namplitude of $\\Phi_1$ for the parameters considered.", "category": "physics_plasm-ph" }, { "text": "Effects of the neutral point of dust charge in plasma sheath: We numerically investigate the dust charging in the sheath, by using the\nusual fluid approximation, it is extended to include self consistently the dust\ncharge variation. The grain charge becomes a new self consistent dynamic\nvariable, it is found that dust grains are first charged negatively at the\nsheath edge and then begin to be charged positively in the sheath. The\nnumerical results show that the initial velocity of dust charged grains, sized\ngrains and the density ratio of ion to electron at the sheath edge have\naffected the neutral point of dust charge. Moreover, the neutral point of dust\ncharge affects the spatial distribution of dust density.", "category": "physics_plasm-ph" }, { "text": "Observation of neutron bursts produced by laboratory high-voltage\n atmospheric discharge: Data on the observation of neutron bursts in the process of high-voltage\ndischarge in the air at an average electric field strength ~ 1 MV/m and\ndischarge current ~ 10 kA are presented. Two independent methods (CR-39 track\ndetectors and plastic scintillation detectors) registered neutrons within the\nrange from thermal energies up to the energies above 10 MeV with the flux of >=\n1E6 neutrons per shot into 4{\\pi} solid angle.", "category": "physics_plasm-ph" }, { "text": "A review on the Vortex and Coherent Structures in Dusty Plasma Medium: Dusty plasma which is nothing but an admixture of electrons, ions and massive\ncharged solid particles of sub-micron to micron sized in the background of\nneutrals. The dust grain medium exhibits fluid as well as solid-like\ncharacteristics depending on the background medium conditions. It supports\nvarious self-sustained non-linear dynamical structures as a result of the\nsaturation of instabilities. The vortical or vortex structure in the dusty\nplasma medium is one of self-sustained dynamical structures that are formed\neither by internal instabilities or external perturbation. In this review\nreport, the author discusses the theoretical, experimental, and computational\nresearch works on vortical and coherent structures in unmagnetized as well as\nin magnetized dusty plasma. The sources of vortex formation such as obstacle,\nion drag shear, dust charge gradient, RT and K-H instabilities are pointed out\nin detail. The studies on the evolution of vortices by researchers are also\ndiscussed.", "category": "physics_plasm-ph" }, { "text": "Collective plasma effects of electron-positron pairs in beam-driven QED\n cascades: Understanding the interplay of strong-field QED and collective plasma effects\nis important for explaining extreme astrophysical environments like magnetars.\nIt has been shown that QED pair plasmas is possible to be produced and observed\nby passing a relativistic electron beam through an intense laser field. This\npaper presents in detail multiple sets of 3D QED-PIC simulations to show the\ncreation of pair plasmas in the QED cascade. The beam driven method enables a\nhigh pair particle density and also a low particle gamma factor, which both\nplay equal rolls on exhibiting large collective plasma effects. Finite laser\nfrequency upshift is observed with both ideal parameters (24 PW laser laser\ncolliding with 300 GeV electron beam) and with existing technologies (3 PW\nlaser laser colliding with 30 GeV electron beam).", "category": "physics_plasm-ph" }, { "text": "Hot ion generation from nanostructured surfaces under intense,\n femtosecond irradiation: We present the effect of a nanostructured surface on the emission of ions and\nelectrons from intense (5-36 Petwatt per sq.cm) femtosecond laser produced\nplasmas. Electrons from optically polished copper targets coated with copper\nnanoparticles (CuNP) are observed to be hotter than those from uncoated\npolished targets. A nearly two-fold enhancement is observed for ions in the\nrange 14-74 keV, while ion yield decreases by a factor of 2 in the 74-2000 keV\nrange. The total ion yields measured using a large area Faraday cup are more\nfrom CuNP targets than those from polished Cu targets, indicating increased ion\nbeam divergence due to surface modulations.", "category": "physics_plasm-ph" }, { "text": "The sinusoid and the phasor: Mathieu equation is widely used to study several natural phenomenon. In this\npaper, we show that replacing the sinusoid in the Mathieu equation with a\nphasor can lead to solutions that behave in a totally different way. Solutions\nof Mathieu equation are either bounded or grow unboundedly at an exponential\nrate. Solutions of this new equation are always unbounded and grow linearly\nwith time.", "category": "physics_plasm-ph" }, { "text": "Synchronous post-acceleration of laser-driven protons in helical coil\n targets by controlling the current dispersion: Post-acceleration of protons in helical coil targets driven by intense,\nultrashort laser pulses can enhance the ion energy by utilizing the transient\ncurrent originating from the self-discharging of the targets. The acceleration\nlength of the protons can exceed a few millimeters, and the accelerating\ngradient is in the order of GeV/m. How to ensure the synchronization of the\naccelerating electric field with the protons is a crucial problem for an\nefficient post-acceleration. In this paper, we study how the electric field\nmismatch induced by the current dispersion affects the synchronous acceleration\nof the protons. We propose a scheme using a two-stage helical coil to control\nthe current dispersion. With optimized parameters, the energy gain of protons\nis enhanced by 4 times. And it is expected that the proton energy would reach\n45 MeV using a hundreds-terawatt laser, or over 100 MeV using a petawatt laser,\nby controlling the current dispersion.", "category": "physics_plasm-ph" }, { "text": "Fully kinetic versus reduced-kinetic modelling of collisionless plasma\n turbulence: We report the results of a direct comparison between different kinetic models\nof collisionless plasma turbulence in two spatial dimensions. The models\nconsidered include a first principles fully-kinetic (FK) description, two\nwidely used reduced models [gyrokinetic (GK) and hybrid-kinetic (HK) with fluid\nelectrons], and a novel reduced gyrokinetic approach (KREHM). Two different ion\nbeta ($\\beta_i$) regimes are considered: 0.1 and 0.5. For $\\beta_i=0.5$, good\nagreement between the GK and FK models is found at scales ranging from the ion\nto the electron gyroradius, thus providing firm evidence for a kinetic Alfv\\'en\ncascade scenario. In the same range, the HK model produces shallower spectral\nslopes, presumably due to the lack of electron Landau damping. For\n$\\beta_i=0.1$, a detailed analysis of spectral ratios reveals a slight\ndisagreement between the GK and FK descriptions at kinetic scales, even though\nkinetic Alfv\\'en fluctuations likely still play a significant role. The\ndiscrepancy can be traced back to scales above the ion gyroradius, where the FK\nand HK results seem to suggest the presence of fast magnetosonic and ion\nBernstein modes in both plasma beta regimes, but with a more notable deviation\nfrom GK in the low-beta case. The identified practical limits and strengths of\nreduced-kinetic approximations, compared here against the fully-kinetic model\non a case-by-case basis, may provide valuable insight into the main kinetic\neffects at play in turbulent collisionless plasmas, such as the solar wind.", "category": "physics_plasm-ph" }, { "text": "Constraining Alfv\u00e9nic Turbulence with Helicity Invariants: In this paper, we study the constraints imposed by the invariants\n(generalized helicities and energy) of extended magnetohydrodynamics on some\nglobal characteristics of turbulence. We show that the global turbulent kinetic\nand magnetic energies will approach equipartition only under certain\ncircumstances that depend on the ratio of the generalized helicities. In\nsystems with minimal thermal energy, we demonstrate that the three invariants\ncollectively determine the characteristic length scale associated with\nAlfv\\'enic turbulence.", "category": "physics_plasm-ph" }, { "text": "Flux and energy asymmetry in a low pressure capacitively coupled plasma\n discharge excited by sawtooth-like waveform -- a harmonic study: Control over plasma asymmetry in a low-pressure capacitively coupled plasma\n(CCP) discharges is vital for many plasma processing applications. In this\narticle, using the particle-in-cell simulation technique, we investigated the\nasymmetry generation by a temporally asymmetric waveform (sawtooth-like) in\ncollisionless CCP discharge. A study by varying the number of harmonics (N)\ncontained in the sawtooth waveform is performed. The simulation resultspredict\na non-linear increase in the plasma density and ion flux with N i.e., it first\ndecreases, reaching a minimum value for a critical value of N, and then\nincreases almost linearly with afurther rise in N. The ionization asymmetry\nincreases with N, and higher harmonics on the instantaneous sheath position are\nobserved for higher values of N. These higher harmonics generate multiple\nionization beams that are generated near the expanding sheath edge and are\nresponsible for an enhanced plasma density for higher values of N. The ion\nenergy distribution function (IEDF) depicts a bi-modal shape for different\nvalues of N. A strong DC self-bias is observed on the powered electrode, and\nits value with respect to the plasma potential decreases with an increase in N\ndue to which corresponding ion energy on the powered electrode decreases. The\nsimulation results conclude that by changing the number of harmonics of a\nsawtooth-like in collisionless CCP discharges, the ion flux asymmetry is not\ngenerated, whereas sheath symmetry could be significantly affected and\ntherefore a systematic variation in the ion energy asymmetry is observed. Due\nto an increase in the higher harmonic contents in the sawtooth waveform with N,\na transition from broad bi-modal to narrow-shaped IEDFs is found.", "category": "physics_plasm-ph" }, { "text": "Performance assessment of a tightly baffled, long-legged divertor\n configuration in TCV with SOLPS-ITER: Numerical simulations explore the possibility to test the tightly baffled,\nlong-legged divertor (TBLLD) concept in a future upgrade of the Tokamak \\`a\nconfiguration variable (TCV). The SOLPS-ITER code package is used to compare\nthe exhaust performance of several TBLLD configurations with existing unbaffled\nand baffled TCV configurations. The TBLLDs feature a range of radial gaps\nbetween the separatrix and the outer leg side walls. All considered TBLLDs are\npredicted to lead to a denser and colder plasma in front of the targets and\nimprove the power handling by factors of 2-3 compared to the present, baffled\ndivertor and by up to a factor of 12 compared to the original, unbaffled\nconfiguration. The improved TBLLD performance is mainly due to a better neutral\nconfinement with improved plasma-neutral interactions in the divertor region.\nBoth power handling capability and neutral confinement increases when reducing\nthe radial gap. The core compatibility of TBLLDs with nitrogen seeding is also\nevaluated and the detachment window with acceptable core pollution for the\nproposed TBLLDs is explored, showing a reduction of required upstream impurity\nconcentration up to 18% to achieve the detachment with thinner radial gap.", "category": "physics_plasm-ph" }, { "text": "A non-linear theory for the bubble regime of plasma wake fields in\n tailored plasma channels: We introduce a first full analytical bubble and blow-out model for a radially\ninhomogeneous plasma in a quasi-static approximation. For both cases we\ncalculate the accelerating and the focusing fields. In our model we also assume\na thin electron layer that surrounds the wake field and calculate the field\nconfiguration within. Our theory holds for arbitrary radial density profiles\nand reduces to known models in the limit of a homogeneous plasma. From a\nprevious study of hollow plasma channels with smooth boundaries for\nlaser-driven electron acceleration in the bubble regime we know that\npancake-like laser pulses lead to highest electron energies [Pukhov et al, PRL\n113, 245003 (2014)]. As it was shown, the bubble fields can be adjusted to\nbalance the laser depletion and dephasing lengths by varying the plasma density\nprofile inside a deep channel. Now we show why the radial fields in the vacuum\npart of a channel become defocussing.", "category": "physics_plasm-ph" }, { "text": "Observation of Three-dimensional Long-range Order in Smaller Ion Coulomb\n Crystals in an rf Trap: Three-dimensional long-range ordered structures in smaller and\nnear-spherically symmetric Coulomb crystals of ^{40}Ca^+ ions confined in a\nlinear rf Paul trap have been observed when the number of ions exceeds ~1000\nions. This result is unexpected from ground state molecular dynamics (MD)\nsimulations, but found to be in agreement with MD simulations of metastable ion\nconfigurations. Previously, three-dimensional long-range ordered structures\nhave only been reported in Penning traps in systems of ~50,000 ions or more.", "category": "physics_plasm-ph" }, { "text": "Avalanche boron fusion by laser picosecond block ignition with magnetic\n trapping for clean and economic reactor: After the very long consideration of the ideal energy source by fusion of the\nprotons of light hydrogen with the boron isotope 11 (boron fusion HB11) the\nvery first two independent measurements of very high reaction gains by lasers\nbasically opens a fundamental breakthrough. The non-thermal plasma block\nignition with extremely high power laser pulses above petawatt of picosecond\nduration in combination with up to ten kilotesla magnetic fields for trapping\nhas to be combined to use the measured high gains as proof of an avalanche\nreaction for an environmentally clean, low cost and lasting energy source as\npotential option against global warming. The unique HB11 avalanche reaction is\nare now based on elastic collisions of helium nuclei (alpha particles) limited\nonly to a reactor for controlled fusion energy during a very short time within\na very small volume.", "category": "physics_plasm-ph" }, { "text": "Experimental studies of plasma-antenna coupling with the JET Alfven\n Eigenmode Active Diagnostic: This paper presents a dedicated study of plasma-antenna (PA) coupling with\nthe Alfven Eigenmode Active Diagnostic (AEAD) in JET. Stable AEs and their\nresonant frequencies f, damping rates $\\gamma$ < 0, and toroidal mode numbers n\nare measured for various PA separations and limiter versus X-point magnetic\nconfigurations. Two stable AEs are observed to be resonantly excited at\ndistinct low and high frequencies in limiter plasmas. The values of f and n do\nnot vary with PA separation. However, $\\vert\\gamma\\vert$ increases with PA\nseparation for the low-f, but not high-f, mode, yet this may be due to slightly\ndifferent edge conditions. The high-f AE is detected throughout the transition\nfrom limiter to X-point configuration, though its damping rate increases; the\nlow-f mode, on the other hand, becomes unidentifiable. The linear resistive MHD\ncode CASTOR is used to simulate the frequency scan of an AEAD-like external\nantenna. For the limiter pulses, the high-f mode is determined to be an n = 0\nGAE, while the low-f mode is likely an n = 2 TAE. During the transition from\nlimiter to X-point configuration, CASTOR indicates that n = 1 and 2 EAEs are\nexcited in the edge gap. These results extend previous experimental studies in\nJET and Alcator C-Mod; validate the computational work performed by Dvornova et\nal 2020 Phys. Plasmas 27 012507; and provide guidance for the optimization of\nPA coupling in upcoming JET energetic particle experiments, for which the AEAD\nwill aim to identify the contribution of alpha particles to AE drive during the\nDT campaign.", "category": "physics_plasm-ph" }, { "text": "Direct observation of mode-coupling instability in two-dimensional\n plasma crystals: Dedicated experiments on melting of 2D plasma crystals were carried out. The\nmelting was always accompanied by spontaneous growth of the particle kinetic\nenergy, suggesting a universal plasma-driven mechanism underlying the process.\nBy measuring three principal dust-lattice (DL) wave modes simultaneously, it is\nunambiguously demonstrated that the melting occurs due to the resonance\ncoupling between two of the DL modes. The variation of the wave modes with the\nexperimental conditions, including the emergence of the resonant (hybrid)\nbranch, reveals exceptionally good agreement with the theory of mode-coupling\ninstability.", "category": "physics_plasm-ph" }, { "text": "Kinetic Model for Stochastic Heating in the INCA Discharge: A novel electron heating mechanism based on periodically structured vortex\nfields induced in a plane was first proposed in 2014 [U. Czarnetzki and Kh.\nTarnev, Physics of Plasmas 21, 123508 (2014)]. This theoretical concept has now\nbeen realized in an experiment which confirms efficient collisionless heating\nin such array structures [Ph. Ahr, T.V. Tsankov, J. Kuhfeld, U. Czarnetzki,\nsubmitted to Plasma Sources Science and Technology, arXiv:1806.02043v1 (2018)].\nThe new concept is called \"Inductively Coupled Array\": INCA. Here, the physical\nmechanism behind the collisionless (stochastic) heating is investigated by two\nanalytical models. Firstly, the electron heating rate in an array field\nstructure with an exponential spatial decay of the field in the direction\nperpendicular to the plane is investigated by stochastically averaging single\nelectron trajectories. The approach is similar to the Lieberman model for the\nclassical stochastic heating in standard inductively coupled plasmas. This\nanalysis shows that classical stochastic heating by thermal motion along the\nvertical direction makes a negligible contribution. However, there is a strong\ncollisonless non-local heating effect in the plane. In conclusion, heating is\nnon-local in the plane but local in the vertical direction. This insight allows\na straightforward solution of the collisionless Boltzmann equation which not\nonly confirms the results of the Lieberman model but provides also explicit\nexpressions for the complex conductivity. Based on the conductivity an\neffective stochastic collision frequency, the complex damping coefficient and\nthe related field penetration of the field into the plasma is calculated.\nFinally, elastic collisions with neutral background atoms are included in the\nmodel and a condition for dominance of stochastic heating over Ohmic heating is\nderived.", "category": "physics_plasm-ph" }, { "text": "Longitudinal and transversal current in collisional plasma, generated by\n two transversal electromagnetic waves: From kinetic Vlasov equation for collisional plasmas distribution function in\nsquare-law approximation on sizes of intensivities of electric fields is\nreceived. The known integral of collisions of relaxation type, so-called BGK\n(Bhatnagar, Gross, Krook) integral of collisions is considered. The formula for\ncalculation electric current at any temperature (any degree of degeneration of\nelectronic gas) is deduced. This formula contains an one-dimensional\nquadrature. It is shown, that the nonlinearity account leads to occurrence the\nlongitudinal electric current directed along a wave vector. This longitudinal\ncurrent is orthogonal to a known transversal classical current, received at the\nlinear analysis. When frequency of collisions tends to the zero, all received\nresults for collisional plasmas pass in corresponding formulas for\ncollisionless plasmas. The case of small values of wave number is considered.\nIt is shown, that the received quantity of longitudinal current at aspiration\nof frequency of collisions to zero also passes in corresponding expression of\ncurrent for collisionless plasmas. Graphic comparison of dimensionless quantity\nof current depending on wave number, frequency of oscillations of\nelectromagnetic field and frequencies of electron collisions with plasma\nparticles is carry out.", "category": "physics_plasm-ph" }, { "text": "Developments in laser-driven plasma accelerators: Laser-driven plasma accelerators provide acceleration gradients three orders\nof magnitude greater than conventional machines, offering the potential to\nshrink the length of accelerators by the same factor. To date,\nlaser-acceleration of electron beams to particle energies comparable to those\noffered by synchrotron light sources has been demonstrated with plasma\nacceleration stages only a few centimetres long. This article describes the\nprinciples of operation of laser-driven plasma accelerators, and reviews their\ndevelopment from their proposal in 1979 to recent demonstrations. The potential\napplications of plasma accelerators are described and the challenges which must\nbe overcome before they can become a practical tool are discussed.", "category": "physics_plasm-ph" }, { "text": "Non-linear extended MHD simulations of type-I edge localised mode cycles\n in ASDEX Upgrade and their underlying triggering mechanism: A triggering mechanism responsible for the explosive onset of edge localised\nmodes (ELMs) in fusion plasmas is identified by performing, for the first time,\nnon-linear magnetohydrodynamic simulations of repetitive type-I ELMs. Briefly\nprior to the ELM crash, destabilising and stabilising terms are affected at\ndifferent timescales by an increasingly ergodic magnetic field caused by\nnon-linear interactions between the axisymmetric background plasma and growing\nnon-axisymmetric perturbations. The separation of timescales prompts the\nexplosive, i.e. faster than exponential, growth of an ELM crash which lasts\n${\\sim}$ 500 ${\\mu}$s. The duration and size of the simulated ELM crashes\ncompare qualitatively well with type-I ELMs in ASDEX Upgrade. As expected for\ntype-I ELMs, a direct proportionality between the heating power in the\nsimulations and the ELM repetition frequency is obtained. The simulations\npresented here are a major step forward towards predictive modelling of ELMs\nand of the assessment of mitigation techniques in ITER and other future\ntokamaks.", "category": "physics_plasm-ph" }, { "text": "RHDLPP: A multigroup radiation hydrodynamics code for laser-produced\n plasmas: We introduce the RHDLPP, a flux-limited multigroup radiation hydrodynamics\nnumerical code designed for simulating laser-produced plasmas in diverse\nenvironments. The code bifurcates into two packages: RHDLPP-LTP for\nlow-temperature plasmas generated by moderate-intensity nanosecond lasers, and\nRHDLPP-HTP for high-temperature, high-density plasmas formed by high-intensity\nlaser pulses. The core radiation hydrodynamic equations are resolved in the\nEulerian frame, employing an operator-split method. This method decomposes the\nsolution into two substeps: first, the explicit resolution of the hyperbolic\nsubsystems integrating radiation and fluid dynamics, and second, the implicit\ntreatment of the parabolic part comprising stiff radiation diffusion, heat\nconduction, and energy exchange. Laser propagation and energy deposition are\nmodeled through a hybrid approach, combining geometrical optics ray-tracing in\nsub-critical plasma regions with a one-dimensional solution of the Helmholtz\nwave equation in super-critical areas. The thermodynamic states are ascertained\nusing an equation of state, based on either the real gas approximation or the\nquotidian equation of state (QEOS). Additionally, RHDLPP includes\nRHDLPP-SpeIma3D, a three-dimensional spectral simulation post-processing\nmodule, for generating both temporally-spatially resolved and time-integrated\nspectra and imaging, facilitating direct comparisons with experimental data.\nThe paper showcases a series of verification tests to establish the code's\naccuracy and efficiency, followed by application cases, including simulations\nof laser-produced aluminum (Al) plasmas, pre-pulse-induced target deformation\nof tin (Sn) microdroplets relevant to extreme ultraviolet lithography light\nsources, and varied imaging and spectroscopic simulations.", "category": "physics_plasm-ph" }, { "text": "Conditions for up-down asymmetry in the core of tokamak equilibria: A local magnetic equilibrium solution is sought around the magnetic axis in\norder to identify the key parameters defining the magnetic-surface's up-down\nasymmetry in the core of tokamak plasmas. The asymmetry is found to be\ndetermined essentially by the ratio of the toroidal current density flowing on\naxis to the fraction of the external field's odd perturbation that manages to\npropagate from the plasma boundary into the core. The predictions are tested\nand illustrated first with an analytical Solovev equilibrium and then using\nexperimentally relevant numerical equilibria. Hollow current-density\ndistributions, and hence reverse magnetic shear, are seen to be crucial to\nbring into the core asymmetry values that are usually found only near the\nplasma edge.", "category": "physics_plasm-ph" }, { "text": "Charge state distribution analysis of Al and Pb ions from the laser ion\n source at IMP: A prototype laser ion source that could demonstrate the possibility of\nproducing intense pulsed high charge state ion beams has been established with\na commercial Nd:YAG laser (E max = 3 J, 1064 nm, 8-10 ns) to produce laser\nplasma for the research of Laser Ion Source (LIS). At the laser ion source test\nbench, high purity (99.998 %) aluminum and lead targets have been tested for\nlaser plasma experiment. An Electrostatic Ion Analyzer (EIA) and Electron\nMultiply Tube (EMT) detector were used to analyze the charge state and energy\ndistribution of the ions produced by the laser ion source. The maximum charge\nstates of Al12+ and Pb7+ were achieved. The results will be presented and\ndiscussed in this paper.", "category": "physics_plasm-ph" }, { "text": "ASCOT: solving the kinetic equation of minority particle species in\n tokamak plasmas: A comprehensive description of methods, suitable for solving the kinetic\nequation for fast ions and impurity species in tokamak plasmas using Monte\nCarlo approach, is presented. The described methods include Hamiltonian\norbit-following in particle and guiding center phase space, test particle or\nguiding center solution of the kinetic equation applying stochastic\ndifferential equations in the presence of Coulomb collisions, neoclassical\ntearing modes and Alfv\\'en eigenmodes as electromagnetic perturbations relevant\nto fast ions, together with plasma flow and atomic reactions relevant to\nimpurity studies. Applying the methods, a complete reimplementation of the\nwell-established minority species code ASCOT is carried out as a response both\nto the increase in computing power during the last twenty years and to the\nweakly structured growth of the code, which has made implementation of\nadditional models impractical. Also, a benchmark between the previous code and\nthe reimplementation is accomplished, showing good agreement between the codes.", "category": "physics_plasm-ph" }, { "text": "Proton energy behavior by variation of the target density in laser\n acceleration: Ion acceleration using a laser pulse irradiating a thin disk target is\nexamined using three-dimensional and two-dimensional particle-in-cell\nsimulations. A laser pulse of $620$ TW, with an intensity of $5\\times 10^{21}$\nW/cm$^{2}$ and a duration time of $27$ fs is irradiated on a double-layer\ntarget. Simulations are performed by varying the ion density, i.e., electron\ndensity, of the first layer. It is shown that the obtained proton energy jumps\nat a certain density of the first layer, which is made of a \"light\" material\nsuch as carbon; that is, Coulomb explosion of the target and radiation pressure\nacceleration act effectively above a certain density. Moreover, even at the\nsame electron density, the reflection of the laser pulse from the first layer\nis small for a \"light\" material.", "category": "physics_plasm-ph" }, { "text": "Three-dimensional simulations of laser-plasma interactions at ultrahigh\n intensities: Three-dimensional (3D) particle-in-cell (PIC) simulations are used to\ninvestigate the interaction of ultrahigh intensity lasers ($> 10^{20}$\nW/cm$^{-2}$) with matter at overcritical densities. Intense laser pulses are\nshown to penetrate up to relativistic critical density levels and to be\nstrongly self-focused during this process. The heat flux of the accelerated\nelectrons is observed to have an annular structure when the laser is tightly\nfocused, showing that a large fraction of fast electrons is accelerated at an\nangle. These results shed light into the multi-dimensional effects present in\nlaser-plasma interactions of relevance to fast ignition of fusion targets and\nlaser-driven ion acceleration in plasmas.", "category": "physics_plasm-ph" }, { "text": "Hard bremsstrahlung from a high-voltage atmospheric discharge and its\n anisotropy: The results of the experiments on recording hard gamma radiation and\nmeasurements of its angular distribution at the initial stage of a laboratory\nhigh-voltage atmospheric discharge are presented. The experiments were\nperformed on an ERG installation at a voltage of $\\sim 1$ MV, an atmospheric\ndischarge current of up to 12 kA, and a gap of 0.55 m. The duration of the\nvoltage pulse was about 1~$\\mu$s with a pulse rise time of 150-200 ns. The\nradiation was recorded by an assembly of 10 identical scintillation detectors\ninstalled each 10$^\\circ$ around the circumference of a quarter of a circle\nwith a curvature of 1 m. In order to separate the radiation with energies from\n20 keV to 1.5 MeV, Al and Pb filters of different thicknesses were used. The\nobtained results show that, as a rule, a multi-beam radiation pattern and\nseveral bursts of radiation (each with a directional pattern) are recorded in\neach shot. In a considerable number of \"shots\", hard radiation with photon\nenergies comparable to or exceeding the maximum electron energy corresponding\nto the applied voltage is recorded. In these cases, a needle-like radiation\npattern is observed, including at large angles to the axis of the discharge.\nThis may indicate the acceleration of electrons in different plasma channels.", "category": "physics_plasm-ph" }, { "text": "Ionization equilibrium and equation of state of hydrogen plasmas in\n strong magnetic fields: We study hydrogen plasmas at magnetic fields B ~ 10^{12}-10^{13} Gauss,\ndensities ~ 10^{-3}-10^3 g/cc and temperatures T ~ 10^{5.5}-10^{6.5} K, typical\nof photospheres of middle-aged cooling neutron stars. We construct an\nanalytical free energy model of the partially ionized plasma, including into\nconsideration the decentred atomic states, which arise due to the thermal\nmotion across the strong field. We show that these states, neglected in\nprevious studies, may contribute appreciably into thermodynamics of the outer\natmospheric layers at density below 1 g/cc and typical B and T. We take into\naccount Coulomb non-ideality of the ionized component of the plasma affected by\nintense magnetic field. Ionization degree, occupancies and equation of state\nare calculated, and their dependences on the temperature, density and magnetic\nfield are studied.", "category": "physics_plasm-ph" }, { "text": "Differential-geometrical approach to the dynamics of dissipationless\n incompressible Hall magnetohydrodynamics I: Lagrangian mechanics on\n semidirect product of two volume preserving diffeomorphisms and conservation\n laws: The dynamics of a dissipationless incompressible Hall magnetohydrodynamic\n(HMHD) medium is formulated using Lagrangian mechanics on a semidirect product\nof two volume preserving diffeomorphism groups. In the case of $\\mathbb{T}^3$\nor $E^3$, the generalized Elsasser variables introduced by Galtier (S. Galtier\n2006 J. Plasma Phys. 72 721-769) yield remarkably simple expressions of basic\nformulas and equations such as the structure constants of Lie algebra, the\nequation of motion, and the conservation laws. Four constants of motion, where\nthree of the four are independent, are naturally derived from the generalized\nElsasser variables representation of the equation of motion for the HMHD\nsystem: total plasma energy, magnetic helicity, hybrid helicity, and the\nmodified cross helicity.", "category": "physics_plasm-ph" }, { "text": "A new ICRF scenario for bulk ion heating in D-T plasmas: How to utilize\n intrinsic impurities in fusion devices in our favour: A fusion reactor requires plasma pre-heating before the rate of\ndeuterium-tritium fusion reactions becomes significant. In ITER, radiofrequency\n(RF) heating of 3He ions, additionally puffed into the plasma, is one of the\nmain options considered for increasing bulk ion temperature during the ramp-up\nphase of the pulse. In this paper, we propose an alternative scenario for bulk\nion heating with RF waves, which requires no extra 3He puff and profits from\nthe presence of intrinsic Beryllium impurities in the plasma. The discussed\nmethod to heat Be impurities in D-T plasmas is shown to provide an even larger\nfraction of fuel ion heating.", "category": "physics_plasm-ph" }, { "text": "Wavelet Bicoherence Analysis as a Method for Investigating Coherent\n Structures in an Electron Beam with an Overcritical Current: Results are presented from numerical modeling of the effect of the\ninhomogeneity of the ion background on the complicated spatiotemporal dynamics\nof an electron beam with a virtual cathode in plane geometry. The possibility\nis demonstrated of increasing the generation frequency without changing the\nbeam current. The spatiotemporal structures that form in the beam and govern\nthe complicated stochastic dynamics of the nonuniform electron-plasma system\nunder consideration are investigated by the methods of wavelet bicoherence and\nby analyzing the calculated electron trajectories on the space-time diagrams", "category": "physics_plasm-ph" }, { "text": "Spin magnetohydrodynamics: Starting from the non-relativistic Pauli description of spin-1/2 particles, a\nset of fluid equations, governing the dynamics of such particles interacting\nwith external fields and other particles, is derived. The equations describe\nelectrons, positrons, holes, and similar conglomerates. In the case of\nelectrons, the magnetohydrodynamic limit of an electron-ion plasma is\ninvestigated. The results should be of interest and relevance both to\nlaboratory and astrophysical plasmas.", "category": "physics_plasm-ph" }, { "text": "Exchange effects in plasmas: the case of low-frequency dynamics: Recently, there has been a surge in the interest of non-equilibrium\ncollective quantum models, where particle dispersion and spin are examples of\neffects taken into account. Here, we derive a kinetic plasma model con- taining\nfermion exchange effects. Exchange interactions are of great importance in many\nsystems, and have no classical analogy. Our model therefore constitute a\npossible probe of collective quantum phenomena in new regimes. As an example,\nwe consider the influence of exchange effect on low frequency dynamics, in\npartic- ular ion acoustic waves. Comparisons to related computational\ntechniques are given and the differences are highlighted. Furthermore, we\ndiscuss the applicability of our model, its limitations and possible\nextensions.", "category": "physics_plasm-ph" }, { "text": "Ellipticity conditions for the extended MHD Grad-Shafranov-Bernoulli\n equilibrium equations: In this study, we find the points of transition between elliptic and\nhyperbolic regimes for the axisymmetric extended magnetohydrodynamic (MHD)\nequilibrium equations. The ellipticity condition is expressed via a single\ninequality but is more involved than the corresponding two-fluid ones due to\nthe imposition of the quasineutrality condition and is also more complicated\nthan the Hall MHD ellipticity condition, due to electron inertia. In fact, the\ninclusion of electron inertia is responsible for peculiar results; namely, even\nthe static equilibrium equations can become hyperbolic.", "category": "physics_plasm-ph" }, { "text": "Decomposition of Plasma Kinetic Entropy into Position and Velocity Space\n and the Use of Kinetic Entropy in Particle-in-Cell Simulations: We describe a systematic development of kinetic entropy as a diagnostic in\nfully kinetic particle-in-cell (PIC) simulations and use it to interpret plasma\nphysics processes in heliospheric, planetary, and astrophysical systems. First,\nwe calculate kinetic entropy in two forms -- the ``combinatorial'' form related\nto the logarithm of the number of microstates per macrostate and the\n``continuous'' form related to $f \\ln f$, where $f$ is the particle\ndistribution function. We discuss the advantages and disadvantages of each and\ndiscuss subtleties about implementing them in PIC codes. Using collisionless\nPIC simulations that are two-dimensional in position space and\nthree-dimensional in velocity space, we verify the implementation of the\nkinetic entropy diagnostics and discuss how to optimize numerical parameters to\nensure accurate results. We show the total kinetic entropy is conserved to\nthree percent in an optimized simulation of anti-parallel magnetic\nreconnection. Kinetic entropy can be decomposed into a sum of a position space\nentropy and a velocity space entropy, and we use this to investigate the nature\nof kinetic entropy transport during collisionless reconnection. We find the\nvelocity space entropy of both electrons and ions increases in time due to\nplasma heating during magnetic reconnection, while the position space entropy\ndecreases due to plasma compression. This project uses collisionless\nsimulations, so it cannot address physical dissipation mechanisms; nonetheless,\nthe infrastructure developed here should be useful for studies of collisional\nor weakly collisional heliospheric, planetary, and astrophysical systems.\nBeyond reconnection, the diagnostic is expected to be applicable to plasma\nturbulence and collisionless shocks.", "category": "physics_plasm-ph" }, { "text": "Non-invasive characterization of transverse beam emittance of electrons\n from a laser-plasma wakefield accelerator in the bubble regime using betatron\n x-ray radiation: We propose and use a technique to measure the transverse emittance of a\nlaser-wakefield accelerated beam of relativistic electrons. The technique is\nbased on the simultaneous measurements of the electron beam divergence given by\n$v_{\\perp}/v_{\\parallel}$, the measured longitudinal spectrum\n$\\gamma_\\parallel$ and the transverse electron bunch size in the bubble\n$r_{\\perp}$. The latter is obtained via the measurement of the source size of\nthe x-rays emitted by the accelerating electron bunch in the bubble. We measure\na \\textit{normalised} RMS beam transverse emittance $<0.5$ $\\pi$ mm$\\:$mrad as\nan upper limit for a spatially gaussian, spectrally quasi-monoenergetic\nelectron beam with 230 MeV energy in agreement with numerical modeling and\nanalytic theory in the bubble regime.", "category": "physics_plasm-ph" }, { "text": "Dependence on laser intensity of the number-weighted angular\n distribution of Compton-scattered photon beams: Inverse Compton scattering of an ultra-relativistic electron in the field of\na high-intensity laser produces photon beams with angular and spectral\ndistributions that are strongly dependent on the laser intensity. Here, we show\nthat the laser intensity at the interaction point can be accurately inferred\nfrom the measurement of the angular number-density distribution of\nCompton-scattered photon beams. The theory, corroborated by numerical\nsimulations, is accurate to within 10\\% in a wide range of laser intensities\n(dimensionless intensity $5 \\leq a_0 \\leq 50$) and electron energies (250 MeV\n$\\leq E \\leq $ 15 GeV), and accounts for experimental features such as the\nfinite transverse size of the electron beam, low-energy cut-offs in the photon\ndetector, and the possibility of a transverse misalignment between the electron\nbeam and the laser focus.", "category": "physics_plasm-ph" }, { "text": "LCODE: a parallel quasistatic code for computationally heavy problems of\n plasma wakefield acceleration: LCODE is a freely-distributed quasistatic 2D3V code for simulating plasma\nwakefield acceleration, mainly specialized at resource-efficient studies of\nlong-term propagation of ultrarelativistic particle beams in plasmas. The beam\nis modeled with fully relativistic macro-particles in a simulation window\ncopropagating with the light velocity; the plasma can be simulated with either\nkinetic or fluid model. Several techniques are used to obtain exceptional\nnumerical stability and precision while maintaining high resource efficiency,\nenabling LCODE to simulate the evolution of long particle beams over long\npropagation distances even on a laptop. A recent upgrade enabled LCODE to\nperform the calculations in parallel. A pipeline of several LCODE processes\ncommunicating via MPI (Message-Passing Interface) is capable of executing\nmultiple consecutive time steps of the simulation in a single pass. This\napproach can speed up the calculations by hundreds of times.", "category": "physics_plasm-ph" }, { "text": "Photon orbital angular momentum in a plasma vortex: We study theoretically the exchange of angular momentum between a photon beam\nand a plasma vortex, and demonstrate the possible excitation of photon angular\nmomentum states in a plasma. This can be relevant to laboratory and space\nplasma diagnostics; radio astronomy self-calibration; and generating photon\nangular momentum beams. A static plasma perturbation with helical structure,\nand a rotating plasma vortex are studied in detail and a comparison between\nthese two cases, and their relevance to the physical nature of photon OAM, is\nestablished.", "category": "physics_plasm-ph" }, { "text": "The Effect of Correlations on the Heat Transport in a Magnetized Plasma: In a classical ideal plasma, a magnetic field is known to reduce the heat\nconductivity perpendicular to the field whereas it does not alter the one along\nthe field. Here we show that, in strongly correlated plasmas that are observed\nat high pressure or/and low temperature, a magnetic field reduces the\nperpendicular heat transport much less and even {\\it enhances} the parallel\ntransport. These surprising observations are explained by the competition of\nkinetic, potential and collisional contributions to the heat conductivity. Our\nresults are based on first principle molecular dynamics simulations of a\none-component plasma.", "category": "physics_plasm-ph" }, { "text": "Extended propagation of powerful laser pulses in focusing Kerr media: Powerful incoherent laser pulses can propagate in focusing Kerr media much\nlonger distances than can coherent pulses, due to the fast phase mixing that\nprevents transverse filamentation. This distance is limited by 4-wave\nscattering, which accumulates waves at small transverse wavenumbers, where\nphase mixing is too slow to retain the incoherence and thus prevent the\nfilamentation. However, we identify how this theoretical limit can be overcome\nby countering this accumulation through transverse heating of the pulse by\nrandom fluctuations of the refractive index. In these new regimes, the laser\npulse propagation distances are significantly extended, making feasible a new\nclass of random lasers, in particular, ultra-powerful random lasers in plasmas.", "category": "physics_plasm-ph" }, { "text": "A walk in the parameter space of L-H transitions without stepping on or\n through the cracks: A mathematically and physically sound three-degree-of-freedom dynamical model\nthat emulates low- to high-confinement mode (L--H) transitions is elicited from\na singularity theory critique of earlier fragile models. We construct a smooth\nmap of the parameter space that is consistent both with the requirements of\nsingularity theory and with the physics of the process. The model is found to\ncontain two codimension 2 organizing centers and two Hopf bifurcations, which\nunderlie dynamical behavior that has been observed around L-H transitions but\nnot mirrored in previous models. The smooth traversal of parameter space\nprovided by this analysis gives qualitative guidelines for controlling access\nto H-mode and oscillatory regimes.", "category": "physics_plasm-ph" }, { "text": "Turbulence and particle acceleration in a relativistic plasma: In a collisionless plasma, the energy distribution function of plasma\nparticles can be strongly affected by turbulence. In particular, it can develop\na non-thermal power-law tail at high energies. We argue that turbulence with\ninitially relativistically strong magnetic perturbations (magnetization\nparameter $\\sigma \\gg 1$) quickly evolves into a state with ultra-relativistic\nplasma temperature but mildly relativistic turbulent fluctuations. We present a\nphenomenological and numerical study suggesting that in this case, the exponent\n$\\alpha$ in the power-law particle energy distribution function,\n$f(\\gamma)d\\gamma\\propto \\gamma^{-\\alpha}d\\gamma$, depends on magnetic\ncompressibility of turbulence. Our analytic prediction for the scaling exponent\n$\\alpha$ is in good agreement with the numerical results.", "category": "physics_plasm-ph" }, { "text": "On electrostatic modes in multi-ion and pair-ion collisional plasmas: The physics of plasmas containing positive and negative ions is discussed\nwith special attention to the recently produced pair-ion plasma containing ions\nof equal mass and opposite charge. The effects of the density gradient in the\ndirection perpendicular to the ambient magnetic field vector, observed in the\nexperiment, are discussed. In addition, the possible presence of electrons is\ndiscussed in the context of plasma modes propagating at an angle with respect\nto the magnetic field vector. The electrons may either be added to the plasma\nor enter the plasma attached to negative ions and then become released from the\nions as a result of collisions. It is shown that the electron plasma mode in a\ncold plasma may become a backward mode in the presence of a density gradient,\nand this behavior may be controlled either by the electron number density or\nthe mode number in the perpendicular direction. In plasmas with hot electrons\nan instability may develop, driven by the combination of electron collisions\nand the density gradient, and in the regime when the ions' response is similar\nto a sound mode, i.e., for un-magnetized ions, implying mode frequencies much\nabove the ion gyro-frequency or mode wave-lengths shorter than the ion\ngyro-radius. In the case of a pure pair-ion plasma, for lower frequencies and\nfor parameters close to those used in the recent experiments, the perturbed\nions may feel the effects of the magnetic field. In this case the plasma mode\nalso becomes backward, resembling features of an experimentally observed but\nyet unexplained backward mode.", "category": "physics_plasm-ph" }, { "text": "Two-stream instability with a growth rate insensitive to collisions in a\n dissipative plasma jet: The two-stream instability (Buneman instability) is traditionally derived as\na collisionless instability with the presumption that collisions inhibit this\ninstability. We show here via a combination of a collisional two-fluid model\nand associated experimental observations made in the Caltech plasma jet\nexperiment, that in fact, a low frequency mode of the two-stream instability is\nindifferent to collisions. Despite the collision frequency greatly exceeding\nthe growth rate of the instability, the instability can still cause an\nexponential growth of electron velocity and a rapid depletion of particle\ndensity. High collisionality nevertheless has an important effect as it enables\nthe development of a double layer when the cross-section of the plasma jet is\nconstricted by a kink-instigated Rayleigh-Taylor instability.", "category": "physics_plasm-ph" }, { "text": "Two-dimensional modeling of a free expansion Plasma Focus, applied to\n the Sumaj Lauray 720-J device with and without external electrodes: The dynamics of the current sheet of a Plasma Focus device is simulated with\na two-dimensional model, in the radial expansion and the axial acceleration\nphase. The simulation considers the free expansion of the current sheet in\nhydrogen gas, without cathodes, and the comparison of experimental data with\ncathodes and without cathodes, using the parameters of Sumaj Lauray Plasma\nFocus 720 J.", "category": "physics_plasm-ph" }, { "text": "Differential nature of inelastic collisions facilitating runaway\n electron generation in weakly-ionized plasmas: We report extention of the Dreicer generation theory to situation where the\nsmall energy exchange no more predominates. In weakly-ionized plamsas, the\nDreicer mechanism can be severely underestimated due to the broken assumption\nof dominant small energy exchange. This Letter numerically demonstrates that\nthe differential nature of inelastic collisions facilitates the Dreicer\ngeneration by developing the new Fokker-Planck-Boltzmann operator of\nelectron-hydrogen atom collisions based on experimental data. This work is\nenvisaged to predict runaway electron generations in future fusion reactors.", "category": "physics_plasm-ph" }, { "text": "Cross-Filament Stochastic Acceleration of Electrons in Kilojoule\n Picosecond Laser Interactions with Near Critical Density Plasmas: Understanding the interaction of kilojoule, picosecond laser pulse with\nlong-scale length preplasma or homogeneous near critical density (NCD) plasma\nis crucial for guiding experiments at national short-pulse laser facilities.\nUsing full three-dimensional particle-in-cell simulations, we demonstrate that\nin this regime, cross-filament stochastic acceleration is an important\nmechanism that contributes to the production of superponderomotive, high-flux\nelectron beams. Since the laser power significantly exceeds the threshold of\nthe relativistic self-focusing, multiple filaments are generated and can\npropagate independently over a long distance. Electrons jump across the\nfilaments during the acceleration, and their motion becomes stochastic. We find\nthat the effective temperature of electrons increases with the total\ninteraction time following a scaling like $T_{\\rm eff}\\propto\\tau_{i}^{0.65}$.\nBy irradiating a submillimeter thick NCD target, the space charge of electrons\nwith energy above 2.5 MeV reaches tens of $\\mu$C. Such high-flux electrons with\nsuperponderomotive energies significantly facilitate applications in\nhigh-energy-density science, nuclear science, secondary sources and diagnostic\ntechniques.", "category": "physics_plasm-ph" }, { "text": "Characterization of 3D filament dynamics in a MAST SOL fluxtube geometry: Non-linear simulations of filament propagation in a realistic MAST SOL flux\ntube geometry using the BOUT++ fluid modelling framework show an isolation of\nthe dynamics of the filament in the divertor region from the midplane region\ndue to three features of the magnetic geometry; the variation of magnetic\ncurvature along the field line, the expansion of the flux tube and strong\nmagnetic shear. Of the three effects, the latter two lead to a midplane\nballooning feature of the filament, whilst the former leads to a ballooning\naround the X-points. In simulations containing all three effects the filament\nis observed to balloon at the midplane, suggesting that the role of curvature\nvariation is sub-dominant to the flux expansion and magnetic shear. The\nmagnitudes of these effects are all strongest near the X-point which leads to\nthe formation of parallel density gradients. The filaments simulated, which\nrepresent filaments in MAST, are identified as resistive ballooning, meaning\nthat their motion is inertially limited, not sheath limited. Parallel density\ngradients can drive the filament towards a Boltzmann response when the\ncollisionalityof the plasma is low. The results here show that the formation of\nparallel density gradients is a natural and inevitable consequence of a\nrealistic magnetic geometry and therefore the transition to the Boltzmann\nresponse is a consequence of the use of realistic magnetic geometry. The\nBoltzmann response causes the filament to self-organise and spin on an axis.\nThe transition from interchange motion to the Boltzmann response occurs with\nincreasing temperature through a decrease in collisionality. This is confirmed\nby measuring the correlation between density and potential perturbations within\nthe filament, which is low in the anti-symmetric state associated with the\ninterchange mechanism, but high in the Boltzmann regime.", "category": "physics_plasm-ph" }, { "text": "Effective field theory of the classical two-dimensional plasma: Using techniques of effective field theory, we consider the thermodynamical\nproperties of a dilute two-dimensional plasma interacting via a $1/r$\npotential. The first one-loop correction to the partition function is already\nlogarithmically divergent in the effective theory. The finite part of the\none-loop induced couplings is then explicitly evaluated by matching the\ndensity-density correlator in the effective theory and in the full quantum\ntheory. This task is performed from the formulation of the Coulomb problem in\nmomentum space by projecting the two-dimensional momentum space onto the\nsurface of a three-dimensional sphere. We also report some higher order results\nthat, in the case of the one-component plasma immersed in a uniform\nneutralizing background, are resummed to obtain the complete leading-log\npartition function.", "category": "physics_plasm-ph" }, { "text": "Correlations Between Conduction Electrons in Dense Plasmas: Most treatments of electron-electron correlations in dense plasmas either\nignore them entirely (random phase approximation) or neglect the role of ions\n(jellium approximation). In this work, we go beyond both these approximations\nto derive a new formula for the electron-electron static structure factor which\nproperly accounts for the contributions of both ionic structure and\nquantum-mechanical dynamic response in the electrons. The result can be viewed\nas a natural extension of the quantum Ornstein-Zernike theory of ionic and\nelectronic correlations, and it is suitable for dense plasmas in which the ions\nare classical and the conduction electrons are quantum-mechanical. The\ncorresponding electron-electron pair distribution functions are compared with\nthe results of path integral Monte Carlo simulations, showing good agreement\nwhenever no strong electron resonance states are present. We construct\napproximate potentials of mean force which describe the effective screened\ninteraction between electrons. Significant deviations from Debye-H\\\"uckel\nscreening are present at temperatures and densities relevant to high energy\ndensity experiments involving warm and hot dense plasmas. The presence of\ncorrelations between conduction electrons is likely to influence the\nelectron-electron contribution to the electron and thermal conductivity. It is\nexpected that excitation processes involving the conduction electrons (e.g.,\nfree-free absorption) will also be affected.", "category": "physics_plasm-ph" }, { "text": "Impact of positive ion energy on carbon-surface production of negative\n ions in deuterium plasmas: This work focuses on the production of negative-ions on graphite and diamond\nsurfaces bombarded by positive ions in a low pressure (2 Pa) low power (20 W)\ncapacitively coupled deuterium plasma. A sample is placed opposite a mass\nspectrometer and negatively biased so that surface produced negative ions can\nbe self-extracted from the plasma and measured by the mass spectrometer. The\nratio between negative-ion counts at mass spectrometer and positive ion current\nat sample surface defines a relative negative-ion yield. Changes in\nnegative-ion production yields versus positive ion energy in the range 10-60 eV\nare analysed. While the negative-ion production yield is decreasing for diamond\nsurfaces when increasing the positive ion impact energy, it is strongly\nincreasing for graphite. This increase is attributed to the onset of the\nsputtering mechanisms between 20 and 40 eV which creates negative ions at\nrather low energy that are efficiently collected by the mass spectrometer. The\nsame mechanism occurs for diamond but is mitigated by a strong decrease of the\nionization probability due to defect creation and loss of diamond electronic\nproperties.", "category": "physics_plasm-ph" }, { "text": "Sequential terahertz pulse generation by photoionization and coherent\n transition radiation in underdense relativistic plasmas: Terahertz (THz) emission by two-color, ultrashort optical pulses interacting\nwith underdense helium gases at ultrahigh intensities ($>\n10^{19}\\,\\mathrm{W/cm}^2$) is investigated by means of 3D particle-in-cell\nsimulations. The THz field is shown to be produced by two mechanisms occurring\nsequentially, namely, photoionization-induced radiation (PIR) by the two-color\npulse and coherent transition radiation (CTR) by the wakefield-accelerated\nelectrons escaping the plasma. For plasmas of atomic densities $>\n10^{17}\\,\\mathrm{cm}^{-3}$, CTR proves to be the dominant process, providing\nTHz bursts with field strength as high as $100\\,\\mathrm{GV/m}$ and energy in\nexcess of $1\\,\\mathrm{mJ}$. Analytical models are developed for both the PIR\nand CTR processes, which correctly reproduce the simulation data.", "category": "physics_plasm-ph" }, { "text": "3D cylindrical BGK model of electron phase-space holes with finite\n velocity and polarization drift: Nonlinear electron kinetic structures are regularly observed in space and\nexperimental magnetized plasmas, called electron phase-space holes (EHs). The\nexistence of EHs is conditioned and varies according to the ambient magnetic\nfield and the parameters of the electron beam(s) that may generate them. The\nobjective of this paper is to extend the 3D Bernstein-Greene-Kruskal (BGK)\nmodel with cylindrical geometry developed by Chen et al. (2004,2004) to include\nsimultaneously finite effects due to (i) the strength of the ambient magnetic\nfield $\\vec{B}_0$, by modifying the Poisson equation with a term derived from\nthe electron polarization current, and (ii) the drift velocity $\\vec{u}_e$ of\nthe background plasma electrons with respect to the EH, by considering\nvelocity-shifted Maxwellian distributions for the boundary conditions. This\nallows us to more realistically determine the distributions of trapped and\npassing particles forming the EHs, as well as the width-amplitude relationships\nfor their existence.", "category": "physics_plasm-ph" }, { "text": "A Novel Method for Solving the Linearized 1D Vlasov--Poisson Equation: We present a novel method for solving the linearized Vlasov--Poisson\nequation, based on analyticity properties of the equilibrium and initial\ncondition through Cauchy-type integrals, that produces algebraic expressions\nfor the distribution and field, i.e., the solution is expressed without\nintegrals. Standard extant approaches involve deformations of the Bromwich\ncontour that give erroneous results for certain physically reasonable\nconfigurations or eigenfunction expansions that are misleading as to the\ntemporal structure of the solution. Our method is more transparent, lacks these\ndefects, and predicts previously unrecognized behavior.", "category": "physics_plasm-ph" }, { "text": "Automated control and optimisation of laser driven ion acceleration: The interaction of relativistically intense lasers with opaque targets\nrepresents a highly non-linear, multi-dimensional parameter space. This limits\nthe utility of sequential 1D scanning of experimental parameters for the\noptimisation of secondary radiation, although to-date this has been the\naccepted methodology due to low data acquisition rates. High repetition-rate\n(HRR) lasers augmented by machine learning present a valuable opportunity for\nefficient source optimisation. Here, an automated, HRR-compatible system\nproduced high fidelity parameter scans, revealing the influence of laser\nintensity on target pre-heating and proton generation. A closed-loop Bayesian\noptimisation of maximum proton energy, through control of the laser wavefront\nand target position, produced proton beams with equivalent maximum energy to\nmanually-optimized laser pulses but using only 60% of the laser energy. This\ndemonstration of automated optimisation of laser-driven proton beams is a\ncrucial step towards deeper physical insight and the construction of future\nradiation sources.", "category": "physics_plasm-ph" }, { "text": "Electron parallel closures for arbitrary collisionality: Electron parallel closures for heat flow, viscosity, and friction force are\nexpressed as kernel-weighted integrals of thermodynamic drives, the temperature\ngradient, relative electron-ion flow velocity, and flow-velocity gradient.\nSimple, fitted kernel functions are obtained for arbitrary collisionality from\nthe 6400 moment solution and the asymptotic behavior in the collisionless\nlimit. The fitted kernels circumvent having to solve higher order moment\nequations in order to close the electron fluid equations. For this reason, the\nelectron parallel closures provide a useful and general tool for theoretical\nand computational models of astrophysical and laboratory plasmas.", "category": "physics_plasm-ph" }, { "text": "Addendum to `Mapping electron dynamics in highly transient EUV\n photon-induced plasmas: a novel diagnostic approach using multi-mode\n microwave cavity resonance spectroscopy': A new approach for an in-line beam monitor for ionizing radiation was\nintroduced in a recent publication (Beckers, J., et al. \"Mapping electron\ndynamics in highly transient EUV photon-induced plasmas: a novel diagnostic\napproach using multi-mode microwave cavity resonance spectroscopy.\" Journal of\nPhysics D: Applied Physics 52.3 (2018): 034004.). Due to the recent detection\nand investigation of an additional third decay regime of the afterglow of an\nextreme ultraviolet photon-induced plasma described in a later article\n(Platier, B., et al. \"Transition from ambipolar to free diffusion in an\nEUV-induced argon plasma.\" Applied Physics Letters 116.10 (2020).) there is an\nadditional reason for a minimum number of photons for this approach to work.\nNear or below this threshold, we explain that the response time of the\ndiagnostic method is a limiting factor. Further, a second limit for the number\nof photons within a pulse is formalized related to the trapping of highly\nenergetic free electrons.", "category": "physics_plasm-ph" }, { "text": "Particle acceleration and radiation friction effects in the\n filamentation instability of pair plasmas: The evolution of the filamentation instability produced by two\ncounter-streaming pair plasmas is studied with particle-in-cell (PIC)\nsimulations in both one (1D) and two (2D) spatial dimensions. Radiation\nfriction effects on particles are taken into account. After an exponential\ngrowth of both the magnetic field and the current density, a nonlinear\nquasi-stationary phase sets up characterized by filaments of opposite currents.\nDuring the nonlinear stage, a strong broadening of the particle energy spectrum\noccurs accompanied by the formation of a peak at twice their initial energy. A\nsimple theory of the peak formation is presented. The presence of radiative\nlosses does not change the dynamics of the instability but affects the\nstructure of the particle spectra.", "category": "physics_plasm-ph" }, { "text": "Low-frequency shear Alfv\u00e9n waves at DIII-D: theoretical interpretation\n of experimental observations: The linear properties of the low-frequency shear Alfv\\'en waves such as those\nassociated with the beta-induced Alfv\\'en eigenmodes (BAEs) and the\nlow-frequency modes observed in reversed-magnetic-shear DIII-D discharges (W.\nHeidbrink, et al 2021 Nucl. Fusion 61 066031) are theoretically investigated\nand delineated based on the theoretical framework of the general fishbone-like\ndispersion relation (GFLDR). By adopting representative experimental\nequilibrium profiles, it is found that the low-frequency modes and BAEs are,\nrespectively, the reactive-type and dissipative-type unstable modes with\ndominant Alfv\\'enic polarization, thus the former being more precisely called\nlow-frequency Alfv\\'en modes (LFAMs). More specifically, due to different\ninstability mechanisms, the maximal drive of BAEs occurs, in comparison to\nLFAMs, when the minimum of the safety factor ($q_{min}$) deviates from a\nrational number. Meanwhile, the BAE eigenfunction peaks at the radial position\nof the maximum energetic particle pressure gradient, resulting in a large\ndeviation from the $q_{min}$ surface. Moreover, the ascending frequency\nspectrum patterns of the experimentally observed BAEs and LFAMs can be\ntheoretically reproduced by varying $q_{min}$ and also be well interpreted\nbased on the GFLDR. The present analysis illustrates the solid predictive\ncapability of the GFLDR and its practical usefulness in enhancing the\ninterpretative capability of both experimental and numerical simulation\nresults.", "category": "physics_plasm-ph" }, { "text": "A 3D kinetic Monte Carlo study of streamer discharges in CO$_2$: We theoretically study the inception and propagation of positive and negative\nstreamers in CO$_2$. Our study is done in 3D, using a newly formulated kinetic\nMonte Carlo discharge model where the electrons are described as drifting and\ndiffusing particles that adhere to the local field approximation. Our emphasis\nlies on electron attachment and photoionization. For negative streamers we find\nthat dissociative attachment in the streamer channels leads to appearance of\nlocalized segments of increased electric fields, while an analogous feature is\nnot observed for positive-polarity discharges. Positive streamers, unlike\nnegative streamers, require free electrons ahead of them in order to propagate.\nIn CO$_2$, just as in air, these electrons are supplied through\nphotoionization. However, ionizing radiation in CO$_2$ is absorbed quite\nrapidly and is also weaker than in air, which has important ramifications for\nthe emerging positive streamer morphology (radius, velocity, and fields). We\nperform a computational analysis which shows that positive streamers can\npropagate due to photoionization in CO$_2$. Conversely, photoionization has no\naffect on negative streamer fronts, but plays a major role in the coupling\nbetween negative streamers and the cathode. Photoionization in CO$_2$ is\ntherefore important for the propagation of both positive and negative\nstreamers. Our results are relevant in several applications, e.g., CO$_2$\nconversion and high-voltage technology (where CO$_2$ is used in pure form or\nadmixed with other gases).", "category": "physics_plasm-ph" }, { "text": "Ultrafast opacity in borosilicate glass induced by picosecond bursts of\n laser-driven ions: Direct investigation of ion-induced dynamics in matter on picosecond (ps,\n10-12 s) timescales has been precluded to date by the relatively long\nnanosecond (ns, 10-9 s) scale ion pulses typically provided by radiofrequency\naccelerators1. By contrast, laser-driven ion accelerators provide bursts of ps\nduration2, but have yet to be applied to the study of ultrafast ion-induced\ntransients in matter. We report on the evolution of an electron-hole plasma\nexcited in borosilicate glass by such bursts. This is observed as an onset of\nopacity to synchronised optical probe radiation and is characterised by the 3.0\n+/- 0.8 ps ion pump rise-time . The observed decay-time of 35 +/- 3 ps i.e. is\nin excellent agreement with modelling and reveals the rapidly evolving electron\ntemperature (>10 3 K) and carrier number density (>10 17cm-3). This result\ndemonstrates that ps laser accelerated ion bursts are directly applicable to\ninvestigating the ultrafast response of matter to ion interactions and, in\nparticular, to ultrafast pulsed ion radiolysis of water3-5, the radiolytic\ndecompositions of which underpin biological cell damage and hadrontherapy for\ncancer treatment6.", "category": "physics_plasm-ph" }, { "text": "Polarized positron beams via intense two-color laser pulses: Generation of ultrarelativistic polarized positrons during interaction of an\nultrarelativistic electron beam with a counterpropagating two-color petawatt\nlaser pulse is investigated theoretically. Our Monte Carlo simulation based on\na semi-classical model, incorporates photon emissions and pair productions,\nusing spin-resolved quantum probabilities in the local constant field\napproximation, and describes the polarization of electrons and positrons for\nthe pair production and photon emission processes, as well as the classical\nspin precession in-between. The main reason of the polarization is shown to be\nthe spin-asymmetry of the pair production process in strong external fields,\ncombined with the asymmetry of the two-color laser field. Employing a feasible\nscenario, we show that highly polarized positron beams, with a polarization\ndegree of $\\zeta\\approx 60\\%$, can be produced in a femtosecond time scale,\nwith a small angular divergence, $\\sim 74$ mrad, and high density $\\sim\n10^{14}$ cm$^{-3}$. The laser-driven positron source, along with laser\nwakefield acceleration, may pave the way to small scale facilities for high\nenergy physics studies.", "category": "physics_plasm-ph" }, { "text": "Kinetic Simulation of Ion Thruster Plume Neutralization in a Vacuum\n Chamber: The electrical environment of a ground vacuum testing chamber creates\nfacility effects for gridded ion thrusters. For example, it is well known that\nthe plume from the thruster generates current paths that are very different\nfrom what occurs in space, and the neutralization of this plume is also\ndifferent. For reasons such as this, it is important to clarify how the\nexperimental testing environment affects plasma flows, but understanding this\neffect solely through ground experiments is difficult. To that end, this study\nutilizes particle-in-cell and direct simulation Monte Carlo methods to simulate\nxenon beam ions and electrons emitted from a neutralizer. First, we compare\nsimulations conducted within the chamber to those conducted in space,\ndemonstrating that grounded chamber walls increase the electric potential and\nelectron temperature. Next, we investigate the impact of the neutralizer's\nposition and the background pressure on the plume in the vacuum chamber. We\nfind that as the neutralizer position moves closer to the location of maximum\npotential, more electrons are extracted, resulting in increased neutralization\nof the plume. We also observe that high background pressure generates slow\ncharge-exchange ions, creating ion sheaths on the side walls that alter ion\ncurrent paths. Finally, we discuss how the potential at the thruster and\nneutralizer exits affects the plume. The relative potential of the neutralizer\nto the vacuum chamber wall is observed to significantly influence the behavior\nof the electrons, thereby altering the degree of plume neutralization. These\nfindings are shown to be consistent with experimental results in the literature\nand demonstrate the promise of high-performance simulation.", "category": "physics_plasm-ph" }, { "text": "A model for the fast evaluation of prompt losses of energetic ions in\n stellarators: A good understanding of the confinement of energetic ions in non-axisymmetric\nmagnetic fields is key for the design of reactors based on the stellarator\nconcept. In this work, we develop a model that, based on the radially-local\nbounce-averaged drift-kinetic equation, classifies orbits and succeeds in\npredicting configuration-dependent aspects of the prompt losses of energetic\nions in stellarators. Such a model could in turn be employed in the\noptimization stage of the design of new devices.", "category": "physics_plasm-ph" }, { "text": "Non-Maxwellian electron distributions resulting from direct laser\n acceleration in near-critical plasmas: The irradiation of few nm thick targets by a finite-contrast high-intensity\nshort-pulse laser results in a strong pre-expansion of these targets at the\narrival time of the main pulse. The targets decompress to near and lower than\ncritical densities plasmas extending over few micrometers, i.e. multiple\nwavelengths. The interaction of the main pulse with such a highly localized but\ninhomogeneous target leads to the generation of a short channel and further\nself-focusing of the laser beam. Experiments at the GHOST laser system at UT\nAustin using such targets measured non-Maxwellian, peaked electron distribution\nwith large bunch charge and high electron density in the laser propagation\ndirection. These results are reproduced in 2D PIC simulations using the EPOCH\ncode, identifying Direct Laser Acceleration (DLA) as the responsible mechanism.\nThis is the first time that DLA has been observed to produce peaked spectra as\nopposed to broad, maxwellian spectra observed in earlier experiments. This\nhigh-density electrons have potential applications as injector beams for a\nfurther wakefield acceleration stage as well as for pump-probe applications.", "category": "physics_plasm-ph" }, { "text": "Characterization of the equilibrium configuration for modulated beams in\n a plasma wakefield accelerator: We analyze the equilibrium configuration for a modulated beam with sharp\nboundaries exposed to the fields self-generated by the interaction with a\nplasma. Through a semi-analytical approach we show the presence of multiple\nequilibrium configurations and we determine the one more suitable for wakefield\nexcitation. Once pointed out the absence of confinement for the front of the\nbeam and the consequently divergence driven by the emittance, we study the\nevolution of the equilibrium configuration while propagating in the plasma,\ndiscarding all the others time-dependencies. We show the onset of a rigid\nbackward drift of the equilibrium configuration and we provide an explanation\nin the increasing length of the first bunch.", "category": "physics_plasm-ph" }, { "text": "The shear Alfv\u00e9n continuum with a magnetic island chain in tokamak\n plasmas: The shear Alfv\\'en continuum spectrum is studied for a tokamak with a single\nisland chain using the ideal Magnetohydrodynamics (MHD) theory. We have taken\ninto account the toroidal geometry and toroidal mode coupling with the island\nconsidered as a highly-shaped stellarator. Various new frequency gaps open up\ninside the island due to its asymmetry both poloidally and toroidally, such as\nthe Mirror-induced Alfv\\'en Eigenmode (MAE) gap and the Helicity-induced\nAlfv\\'en Eigenmode (HAE) gap. We have shown that the MAE gap acts as the\ncontinuation of the outside Toroidal Alfv\\'en Eigenmode (TAE) gap into the\nisland. However, the combined TAE/MAE gap is getting narrower as the island\ngrows, leaving only half of its original width with a moderate island size as\nmuch as 3.2% of the minor radius. In addition, the two-dimensional\neigenfunction of the continuum mode on the lower tip of the MAE gap now has\nhighly localised structures around the island's long axis, contrary to the\nusual oscillatory global solutions found with no or a low level of toroidal\nasymmetry - an indication of the continuous spectrum becoming discrete and\ndense. These results have implications for the frequency, mode structure and\ncontinuum damping of global TAEs residing in the gap.", "category": "physics_plasm-ph" }, { "text": "Characterization of admissible quasisymmetries: We solve \"half\" the problem of finding three-dimensional quasisymmetric\nmagnetic fields that do not necessarily satisfy force balance. This involves\ndetermining which hidden symmetries are admissible as quasisymmetries, and then\nshowing explicitly how to construct quasisymmetric magnetic fields given an\nadmissible symmetry. The admissibility conditions take the form of a system of\noverdetermined nonlinear partial differential equations involving second\nderivatives of the symmetry's infinitesimal generator.", "category": "physics_plasm-ph" }, { "text": "Rate equations model for multiple magnetic mirrors in various\n thermodynamic scenarios: Axial particle loss is one of the main challenges for fusion aimed, linear\nmagnetic mirror plasma configurations. One way to mitigate this disadvantage\nand increase the confinement time is to use a multiple mirrors setup. The idea\nis to reduce the outgoing flux by collisions in the outer magnetic cells. Here,\nwe develop a semi-kinetic rate equation model for the ions' density dynamics,\nincluding scattering within the magnetic cell and the transmission between\nneighboring cells. The dominant parameter is the ions' mean free path, which\ndepends on the temperature and density in each cell. The steady-state flow is\nstudied analytically and numerically for three thermodynamic scenarios:\nisothermal plasma, adiabatic expansion, and constant diffusion. It is found\nthat the confinement time varies about five-fold over the different scenarios,\nwhere the adiabatic cooling is the best confining scenario.", "category": "physics_plasm-ph" }, { "text": "Charged Particle Motion in a Plasma: Electron-Ion Energy Partition: A charged particle traversing a plasma loses its energy to both plasma\nelectrons and ions. We compute the energy partition, the fractions $E_e/E_0$\nand $E_\\smI/E_0$ of the initial energy $E_0$ of this `impurity particle' that\nare deposited into the electrons and ions when it has slowed down into an\nequilibrium distribution that we shall determine. We use a well-defined\nFokker-Planck equation for the phase space distribution of the charged impurity\nparticles in a weakly to moderately coupled plasma. The Fokker-Planck equation\nholds to first sub-leading order in the dimensionless plasma coupling constant,\nwhich means we compute to order $n\\ln n$ (leading) and $n$ (sub-leading) in the\nplasma density $n$. Previously, the order $n$ terms had been estimated, not\ncalculated. Since the charged particle does not come to rest, the energy loss\nobtained by an integration of a $dE/dx$ has an ambiguity of order of the plasma\ntemperature. Our Fokker-Planck formulation provides an unambiguous, precise\ndefinition of the energy fractions. For equal electron and ion temperatures, we\nfind that our precise results agree well with a fit obtained by Fraley,\nLinnebur, Mason, and Morse. The case with differing electron and ion\ntemperatures, a case of great importance for nuclear fusion, will be\ninvestigated in detail in the present paper. The energy partitions for this\ngeneral case, partitions that have not been obtained before, will be presented.\nWe find that now the proper solution of the Fokker-Planck equation yields a\nquasi-static equilibrium distribution to which fast particles relax that has\nneither the electron nor the ion temperature. This \"schizophrenic\" final\nensemble of slowed particles gives a new mechanism to bring the electron and\nion temperatures together. The rate at which this new mechanism brings the\nelectrons and ions in the plasma into thermal equilibrium will be computed.", "category": "physics_plasm-ph" }, { "text": "Field emission mechanism from nanotubes through gas ionization induced\n nanoscale surface charging: Experimental evidences reported in this letter show that the gas ionization\ninduced positive charge accumulation can lead to the electron field emission\nfrom carbon nanotubes (CNTs) in an electrode system with proper range of gap\nspacing, where the CNT film with ethocel was covered with ZnO nanorods. The\nhypothesis for illustration is suggested that: 1) the cosmic ray ionization\nfrequency increases 108~1010 times due to the metastable population resulted\nfrom the interaction between the gases and the CNTs; 2) the flux of positive\ncharges is enhanced in the converged field due to the ZnO nanostructures. The\nresulted positive charge local density is high enough to trigger the field\nemission of the CNTs. The methodology may be useful in particle detectors and\nionization gas sensors.", "category": "physics_plasm-ph" }, { "text": "Multidimensional effects on proton acceleration using high-power intense\n laser pulses: Dimensional effects in particle-in-cell (PIC) simulation of target normal\nsheath acceleration (TNSA) of protons are considered. As the spatial divergence\nof the laser-accelerated hot sheath electrons and the resulting space-charge\nelectric field on the target backside depend on the spatial dimension, the\nmaximum energy of the accelerated protons obtained from three-dimensional (3D)\nsimulations is usually much less that from two-dimensional (2D) simulations. By\nclosely examining the TNSA of protons in 2D and 3D PIC simulations, we deduce\nan empirical ratio between the maximum proton energies obtained from the 2D and\n3D simulations. This ratio may be useful for estimating the maximum proton\nenergy in realistic (3D) TNSA from the results of the corresponding 2D\nsimulation. It is also shown that the scaling law also applies to TNSA from\nstructured targets.", "category": "physics_plasm-ph" }, { "text": "Numerical study of the effect of mass of the background gas on the\n lateral interactions of two plasma plumes at high pressure: The characteristic of the lateral interaction of two plasma plumes in $Ar$\nbackground gas at high pressures was reported in recent publication [Yadav {\\it\net. al.}, J. Phys. D: Appl. Phys. {\\bf 50}, 053421 (2017)]. Further we have\ninvestigated the interaction characteristics of plumes in $He$, $Ne$, $Ar$ and\n$Xe$ background gases to see the effect of mass on the interaction. The present\nwork illustrate the applicability of the present model for theoretical\nunderstanding of dynamics, structure, density variation, shock wave formations\nand their interactions of two propagating plasma plumes in a wide range of\nambient conditions. The formation of interaction region, geometrical shape and\nstrength of the shock fronts and subsequent regular and Mach reflections in\naccordance with the nature and pressure of ambient gas are successfully\ncaptured in the simulations. The observed results are supported by the reported\nexperimental observations under identical conditions.", "category": "physics_plasm-ph" }, { "text": "Propagation of Relativistic Shock Wave Induced by Laser Spark in\n Quiescent Air: Laser-induced breakdown has shown many potential applications in the various\nfield of science and engineering. As the breakdown occurs in gas or aerosol, a\nrapid hydrodynamic expansion as shock (blast) wave initiated from the\ndeposition location. The nature of the shock wave is one of the controlling\nfactors in many physical processes; unfortunately, its nature has still not\nbeen clearly understood. In this study, an error was found during the\ncalculation of shock wave properties with the classical non-relativistic\napproach. The error in calculation was due to the initial relativistic\npropagation of high-temperature plasma. Initially, the plasma and shock wave\ntravel together with higher acceleration up to the point of inflation, and in\nlater time shockwave dissociate itself from the plasma. However, this\naccelerating effect is neglected in the earlier studies. To address the\nspontaneous accelerating and deaccelerating nature of the shock wave, the\ntheoretical details of the relativistic approach of shock wave propagation is\npresented.", "category": "physics_plasm-ph" }, { "text": "Theoretical Studies of Long Lived Plasma Structures: We construct the model of a long lived plasma structure based on spherically\nsymmetric oscillations of electrons in plasma. Oscillations of electrons are\nstudied in frames of both classical and quantum approaches. We obtain the\ndensity profile of electrons and the dispersion relations for these\noscillations. The differences between classical and quantum approaches are\ndiscussed. Then we study the interaction between electrons participating in\nspherically symmetric oscillations. We find that this interaction can be\nattractive and electrons can form bound states. The applications of the\nobtained results to the theory of natural plasmoids are considered.", "category": "physics_plasm-ph" }, { "text": "On fast radial propagation of parametrically excited geodesic acoustic\n mode: The spatial and temporal evolution of parametrically excited geodesic\nacoustic mode (GAM) initial pulse is investigated both analytically and\nnumerically. Our results show that the nonlinearly excited GAM propagates at a\ngroup velocity which is, typically, much larger than that due to finite ion\nLarmor radius as predicted by the linear theory. The nonlinear dispersion\nrelation of GAM driven by a finite amplitude drift wave pump is also derived,\nshowing a nonlinear frequency increment of GAM. Further implications of these\nfindings for interpreting experimental observations are also discussed.", "category": "physics_plasm-ph" }, { "text": "Direct Implicit and Explicit Energy-Conserving Particle-in-Cell Methods\n for Modeling of Capacitively-Coupled Plasma Devices: Achieving large-scale kinetic modelling is a crucial task for the development\nand optimization of modern plasma devices. With the trend of decreasing\npressure in applications such as plasma etching, kinetic simulations are\nnecessary to self-consistently capture the particle dynamics. The standard,\nexplicit, electrostatic, momentum-conserving Particle-In-Cell method suffers\nfrom restrictive stability constraints on spatial cell size and temporal time\nstep, requiring resolution of the electron Debye length and electron plasma\nperiod respectively. This results in a very high computational cost, making the\ntechnique prohibitive for large volume device modeling. We investigate the\nDirect Implicit algorithm and the explicit Energy Conserving algorithm as\nalternatives to the standard approach, both of which can reduce computational\ncost with a minimal (or controllable) impact on results. These algorithms are\nimplemented into the well-tested EDIPIC-2D and LTP-PIC codes, and their\nperformance is evaluated via 2D capacitively coupled plasma discharge\nsimulations. The investigation revels that both approaches enable the\nutilization of cell sizes larger than the Debye length, resulting in reduced\nruntime, while incurring only minor inaccuracies in plasma parameters. The\nDirect Implicit method also allows for time steps larger than the electron\nplasma period, however care must be taken to avoid numerical heating or\ncooling. It is demonstrated that by appropriately adjusting the ratio of cell\nsize to time step, it is possible to mitigate this effect to an acceptable\nlevel.", "category": "physics_plasm-ph" }, { "text": "Benchmarking and validation of global model code for negative hydrogen\n ion sources: Benchmarking and validation are prerequisite for using simulation codes as\npredictive tools. In this work, we have developed a Global Model for Negative\nHydrogen Ion Source (GMNHIS) and performed benchmarking of GMNHIS against\nanother independently developed code, Global Enhanced Vibrational Kinetic Model\n(GEVKM). This is the first study to present quite comprehensive benchmarking\ntest of this kind for models of negative hydrogen ion sources (NHIS), and very\ngood agreement has been achieved for electron temperature, vibrational\ndistribution function (VDF) of hydrogen molecules, and n_H^-/n_e ratio. The\nsmall discrepancies in number densities of negative hydrogen ions, positive\nions, as well as hydrogen atoms can be attributed to the differences in the\npredicted electron number density for given discharge power. Higher electron\nnumber density obtained with GMNHIS is possibly due to fewer dissociation\nchannels accounted for in GMNHIS, leading to smaller energy loss. In addition,\nwe validated GMNHIS against experimental data obtained in an electron cyclotron\nresonance (ECR) discharge used for H^- production. The model qualitatively (and\neven quantitatively for certain conditions) reproduces the experimental H^-\nnumber density. The H^- number density as a function of pressure first\nincreases at pressures below 12 mTorr, and then saturates for higher pressures.\nThis dependence was analyzed by evaluating contributions from different\nreaction pathways to the creation and loss of the H^- ions. The developed codes\ncan be used for predicting the H^- production, improving the performance of\nNHIS, and ultimately optimizing the parameters of negative ion beams for ITER.", "category": "physics_plasm-ph" }, { "text": "High-Power Tunable Laser Driven THz Generation in Corrugated Plasma\n Waveguides: The excitation of THz radiation by the interaction of an ultra short laser\npulse with the modes of a miniature corrugated plasma waveguide is considered.\nThe axially corrugated waveguide supports the electromagnetic (EM) modes with\nappropriate polarization and subluminal phase velocities that can be phase\nmatched to the ponderomotive potential associated with laser pulse, making\nsignificant THz generation possible. This process is studied via full format\nParticle-in-Cell (PIC) simulations that, for the first time, model the\nnonlinear dynamics of the plasma and the self-consistent evolution of the laser\npulse in the case where the laser pulse energy is entirely depleted. It is\nfound that the generated THz is characterized by lateral emission from the\nchannel, with a spectrum that may be narrow or broad depending on the laser\nintensity. A range of realistic laser pulse and plasma parameters is considered\nwith the goal of maximizing the conversion efficiency of optical energy to THz\nradiation. As an example, a fixed drive pulse (0.55 J) with a spot size of 15\n$\\mu m$ and duration of 15 $fs$ produces 37.8 mJ of THz radiation in a 1.5 cm\ncorrugated plasma waveguide with an on axis average density of\n$1.4\\times10^{18} cm^{-3}$.", "category": "physics_plasm-ph" }, { "text": "Plasma physics in strong-field regimes: theories and simulations: In strong electromagnetic fields, unique plasma phenomena and applications\nemerge, whose description requires recently developed theories and simulations\n[Y. Shi, Ph.D. thesis, Princeton University (2018)]. In the classical regime,\nto quantify effects of strong magnetic fields on three-wave interactions, a\nconvenient formula is derived by solving the fluid model to the second order in\ngeneral geometry. As an application, magnetic resonances are exploited to\nmediate laser pulse compression, using which higher intensity pulses can be\nproduced in wider frequency ranges, as confirmed by particle-in-cell\nsimulations. In even stronger fields, relativistic-quantum effects become\nimportant, and a plasma model based on scalar quantum electrodynamics (QED) is\ndeveloped, which unveils observable corrections to Faraday rotation and\ncyclotron absorption in strongly magnetized plasmas. Beyond the perturbative\nregime, lattice QED is extended as a numerical tool for plasma physics, using\nwhich the transition from wakefield acceleration to electron-positron pair\nproduction is captured when laser intensity exceeds the Schwinger threshold.", "category": "physics_plasm-ph" }, { "text": "The impact of rational surfaces on radial heat transport in TJ-II: In this work, we study the outward propagation of temperature perturbations.\nFor this purpose, we apply an advanced analysis technique, the Transfer\nEntropy, to ECE measurements performed in ECR heated discharges at the\nlow-shear stellarator TJ-II. We observe that the propagation of these\nperturbations is not smooth, but is slowed down at specific radial positions,\nnear 'trapping zones' characterized by long time lags with respect to the\nperturbation origin. We also detect instances of rapid or instantaneous\n(non-local) propagation, in which perturbations appear to 'jump over' specific\nradial regions.\n The analysis of perturbations introduced in a resistive Magneto-Hydrodynamic\nmodel of the plasma leads to similar results. The radial regions corresponding\nto slow radial transport are identified with maxima of the flow shear\nassociated with rational surfaces (mini-transport barriers). The non-local\ninteractions are ascribed to MHD mode coupling effects.", "category": "physics_plasm-ph" }, { "text": "Ultra-short pulse generation from mid-IR to THz range using plasma wakes\n and relativistic ionization fronts: This paper discusses numerical and experimental results on frequency\ndownshifting and upshifting of a 10 $\\mu$m infrared laser to cover the entire\nwavelength (frequency) range from $\\lambda$=1-150 $\\mu$m ($\\nu$=300-2 THz)\nusing two different plasma techniques. The first plasma technique utilizes\nfrequency downshifting of the drive laser pulse in a nonlinear plasma wake.\nBased on this technique, we have proposed and demonstrated that in a tailored\nplasma structure multi-millijoule energy, single-cycle, long-wavelength IR\n(3-20 $\\mu$m) pulses can be generated by using an 810 nm Ti:sapphire drive\nlaser. Here we extend this idea to the THz frequency regime. We show that\nsub-joule, terawatts, single-cycle terahertz (2-12 THz, or 150-25 $\\mu$m)\npulses can be generated by replacing the drive laser with a picosecond 10\n$\\mu$m CO$_2$ laser and a different shaped plasma structure. The second plasma\ntechnique employs frequency upshifting by colliding a CO$_2$ laser with a\nrather sharp relativistic ionization front created by ionization of a gas in\nless than half cycle (17 fs) of the CO$_2$ laser. Even though the electrons in\nthe ionization front carry no energy, the frequency of the CO$_2$ laser can be\nupshifted due to the relativistic Doppler effect as the CO$_2$ laser pulse\nenters the front. The wavelength can be tuned from 1-10 $\\mu$m by simply\nchanging the electron density of the front. While the upshifted light with $5\n<\\lambda(\\mu$m$)< 10$ propagates in the forward direction, that with $1\n<\\lambda(\\mu$m$)< 5$ is back-reflected. These two plasma techniques seem\nextremely promising for covering the entire molecular fingerprint region.", "category": "physics_plasm-ph" }, { "text": "Semi-analytical derivation of the 2D all-FLR ICRH wave equation as a\n high-order partial differential equation: For 1-dimensional applications, Bude's method [Bude et al, Plasma Phys.\nControl. Fusion, 63 (2021) 035014] has been shown to be capable of accurately\nsolving the all-FLR (Finite Larmor Radius) integro-differential wave equation\nas a high-order differential equation allowing to represent all physically\nrelevant (fast, slow and Bernstein) modes upon making a polynomial fit that is\naccurate in the relevant part of k-space. The adopted fit is superior to the\nTaylor series expansion traditionally adopted to truncate the series of finite\nLarmor radius corrections, while the differential rather than\nintegro-differential approach allows for significant gain in required\ncomputational time when solving the wave equation. The method was originally\nproposed and successfully tested in 1D for radio frequency (RF) waves and in\nabsence of the poloidal field [D. Van Eester & E. Lerche, Nucl. Fusion, 61\n(2021) 016024]. In the present paper, the derivation of the extension of that\nprocedure to 2D and for finite poloidal field - semi-analytically yielding the\ncoefficients of the relevant high-order partial differential equation - is\ndiscussed in preparation of future numerical application.", "category": "physics_plasm-ph" }, { "text": "Onset of negative dispersion in one-component-plasma revisited: A simple approach to describe the long-wavelength dispersion of the\nlongitudinal (plasmon) mode of the classical one-component-plasma (OCP), with\nthe main objective to correctly capture the onset of negative dispersion, is\nproposed. The approach is applicable to both three-dimensional and\ntwo-dimensional OCP. The predicted onset of negative dispersion compares well\nwith the available results from numerical simulations and more sophisticated\ntheoretical models.", "category": "physics_plasm-ph" }, { "text": "Deposition rate controls nucleation and growth during\n amorphous/nanocrystalline competition in sputtered Zr-Cr thin films: Dual-phase Zr-based thin films synthesized by magnetron co-sputtering and\nshowing competitive growth between amorphous and crystalline phases have been\nreported recently. In such films, the amorphous phase grows as columns, while\nthe crystalline phase grows as separated cone-shaped crystalline regions made\nof smaller crystallites. In this paper, we investigate this phenomenon and\npropose a model for the development of the crystalline regions during thin film\ngrowth. We evidence using X-ray diffraction (XRD), scanning electron microscopy\n(SEM) and transmission electron microscopy (TEM), that this competitive\nselfseparation also exists in co-sputtered Zr-Cr thin films with Cr contents of\n~84-86 at.%, corresponding to the transition between the amorphous and\ncrystalline compositions, and in the Zr-V system. Then, to assess the\nsturdiness of this phenomenon, its existence and geometrical characteristics\nare evaluated when varying the film composition and the deposition rate. The\nvariation of geometrical features, such as the crystalline cone angle, the size\nand density of crystallites, is discussed. Is it shown that a variation in the\ndeposition rate changes the nucleation and growth kinetics of the crystallites.\nThe surface coverage by the crystalline phase at a given thickness is also\ncalculated for each deposition rate. Moreover, comparison is made between\nZr-Cr, Zr-V, Zr-Mo and Zr-W dual-phase thin films to compare their nucleation\nand growth kinetics.", "category": "physics_plasm-ph" }, { "text": "Mechanism for Sequestering Magnetic Energy at Large Scales in Shear-Flow\n Turbulence: Straining of magnetic fields by large-scale shear flow, generally assumed to\nlead to intensification and generation of small scales, is re-examined in light\nof the persistent observation of large-scale magnetic fields in astrophysics.\nIt is shown that, in magnetohydrodynamic turbulence, unstable shear flows have\nthe unexpected effect of sequestering magnetic energy at large scales, due to\ncounteracting straining motion of nonlinearly excited large-scale stable\neigenmodes. This effect is quantified via dissipation rates, energy transfer\nrates, and visualizations of magnetic field evolution by artificially removing\nthe stable modes. These analyses show that predictions based upon physics of\nthe linear instability alone miss substantial dynamics, including those of\nmagnetic fluctuations.", "category": "physics_plasm-ph" }, { "text": "Influence of 3D plasmoid dynamics on the transition from collisional to\n kinetic reconnection: Within the resistive magnetohydrodynamic model, high-Lundquist number\nreconnection layers are unstable to the plasmoid instability, leading to a\nturbulent evolution where the reconnection rate can be independent of the\nunderlying resistivity. However, the physical relevance of these results\nremains questionable for many applications. First, the reconnection electric\nfield is often well above the runaway limit, implying that collisional\nresistivity is invalid. Furthermore, both theory and simulations suggest that\nplasmoid formation may rapidly induce a transition to kinetic scales, due to\nthe formation of thin current sheets. Here, this problem is studied for the\nfirst time using a first-principles kinetic simulation with a Fokker-Planck\ncollision operator in 3D. The low-$\\beta$ reconnecting current layer thins\nrapidly due to Joule heating before onset of the oblique plasmoid instability.\nLinear growth rates for standard ($k_y = 0$) tearing modes agree with\nsemi-collisional boundary layer theory, but the angular spectrum of oblique\n($|k_y|>0$) modes is significantly narrower than predicted. In the non-linear\nregime, flux-ropes formed by the instability undergo complex interactions as\nthey are advected and rotated by the reconnection outflow jets, leading to a\nturbulent state with stochastic magnetic field. In a manner similar to previous\n2D results, super-Dreicer fields induce a transition to kinetic reconnection in\nthin current layers that form between flux-ropes. These results may be testable\nwithin new laboratory experiments.", "category": "physics_plasm-ph" }, { "text": "Low-temperature plasma simulation based on physics-informed neural\n networks: frameworks and preliminary applications: Plasma simulation is an important and sometimes only approach to\ninvestigating plasma behavior. In this work, we propose two general AI-driven\nframeworks for low-temperature plasma simulation: Coefficient-Subnet\nPhysics-Informed Neural Network (CS-PINN) and Runge-Kutta Physics-Informed\nNeural Network (RK-PINN). The CS-PINN uses either a neural network or an\ninterpolation function (e.g. spline function) as the subnet to approximate\nsolution-dependent coefficients (e.g. electron-impact cross sections,\nthermodynamic properties, transport coefficients, et al.) in plasma equations.\nOn the basis of this, the RK-PINN incorporates the implicit Runge-Kutta\nformalism in neural networks to achieve a large-time-step prediction of\ntransient plasmas. Both CS-PINN and RK-PINN learn the complex non-linear\nrelationship mapping from spatio-temporal space to equation's solution. Based\non these two frameworks, we demonstrate preliminary applications by four cases\ncovering plasma kinetic and fluid modeling. The results verify that both\nCS-PINN and RK-PINN have good performance in solving plasma equations.\nMoreover, the RK-PINN has ability of yielding a good solution for transient\nplasma simulation with not only large time step but also limited noisy sensing\ndata.", "category": "physics_plasm-ph" }, { "text": "The possibility of hypersonic electrostatic solitons in a plasma with\n turbulence heating: Here, we show that electrostatic solitons in a plasma with turbulent heating\nof the electrons through an accelerating electric field, can form with very\nhigh velocities, reaching up to several order of magnitudes larger than the\nion-sound speed. We call these solitons hypersonic solitons. The possible\nparameter regime, where this work may be relevant, can be found the so-called\n``dead zones'' of a protoplanetary disk. These zones are stable to\nmagnetorotational instability but the resultant turbulence can in effect heat\nthe electrons make them follow a highly non-Maxwellian velocity distribution.\nWe show that these hypersonic solitons can also reach very high velocities.\nWith electron velocity distribution described by Davydov distribution function,\nwe argue that these solitons can be an effective mechanism for energy\nequilibration in such a situation through soliton decay and radiation.", "category": "physics_plasm-ph" }, { "text": "A nonlinear plasma retroreflector for single pulse Compton\n backscattering: Compton scattered x-rays can be generated using a configuration consisting of\na single, ultra-intense laser pulse, and a shaped gas target. The gas target\nincorporates a hydrodynamically formed density spike, which nonlinearly\nscatters the incident pump radiation, to produce a counter-propagating\nelectromagnetic wiggler. This self-generated wiggler field Compton scatters\nfrom electrons accelerated in the laser wakefield of the pump radiation. The\nnonlinear scattering mechanism in the density spike is examined theoretically\nand numerically in order to optimize the Compton scattered radiation. It is\nfound that narrow-band x-rays are produced by moderate intensity pump radiation\nincident on the quarter-critical surface of the density spike, while high\nfluence, broadband x-rays are produced by high intensity pump radiation\nreflected near the critical surface.", "category": "physics_plasm-ph" }, { "text": "HV discharges triggered by dual- and triple-frequency laser filaments: We study the use of frequency upconversion schemes of near-IR picosecond\nlaser pulses and compare their ability to guide and trigger electric discharges\nthrough filamentation in air. Upconversion, such as Second Harmonic Generation,\nis favorable for triggering electric discharges for given amount of available\nlaser energy, even taking into account the losses inherent to frequency\nconversion. We focus on the practical question of optimizing the use of energy\nfrom a given available laser system and the potential advantage to use\nfrequency conversion schemes.", "category": "physics_plasm-ph" }, { "text": "Investigation of a short argon arc with hot anode. Part I: numerical\n simulations of non-equilibrium effects in the near-electrode regions: Atmospheric pressure arcs have recently found application in the production\nof nanoparticles. Distinguishing features of such arcs are small length and hot\nablating anode characterized by intensive electron emission and radiation from\nits surface. We performed one-dimensional modeling of argon arc, which shows\nthat near-electrode effects of thermal and ionization non-equilibrium play\nimportant role in operation of a short arc, because the non-equilibrium regions\nare up to several millimeters long and are comparable with the arc length. The\nnear-anode region is typically longer than the near-cathode region and its\nlength depends more strongly on the current density. The model was extensively\nverified and validated against previous simulation results and experimental\ndata. Volt-Ampere characteristic (VAC) of the near-anode region depends on the\nanode cooling mechanism. The anode voltage is negative. In case of strong anode\ncooling (water-cooled anode) when anode is cold, temperature and plasma density\ngradients increase with current density resulting in decrease of the anode\nvoltage (absolute value increases). Falling VAC of the near-anode region\nsuggests the arc constriction near the anode. Without anode cooling, the anode\ntemperature increases significantly with current density, leading to drastic\nincrease in the thermionic emission current from the anode. Correspondingly,\nthe anode voltage increases to suppress the emission - and the opposite trend\nin the VAC is observed. The results of simulations were found to be independent\nof sheath model used: collisional (fluid) or collisionless model gave the same\nplasma profiles for both near-anode and near-cathode regions.", "category": "physics_plasm-ph" }, { "text": "Evidence of oblique electron acoustic solitary waves triggered by\n magnetic reconnection in Earth's magnetosphere: Motivated by the recent Magnetospheric Multiscale (MMS) observations of\noblique electron acoustic waves, we addressed the generation mechanism of the\nobserved waves by utilizing the reductive perturbation technique. The nonlinear\nZakharov-Kuznetsov (ZK) equation is derived for collisionless, magnetised\nplasma composed of cool inertial background electrons, the cool inertial\nelectron beam, hot inertialess suprathermal electrons represented by a\n$\\kappa$-distribution, and stationary ions. Moreover, the instability growth\nrate is derived by using the small-k perturbation expansion method. Our\nfindings reveal that the structure of the electrostatic wave profile is\nsignificantly influenced by the external magnetic field, the unperturbed hot,\ncool, and electron beam densities, the obliquity angle, and the rate of\nsuperthermality. Such parameters also affect the instability growth rate. This\nstudy clarifies the characteristics of the oblique electron solitary waves that\nmay be responsible for changing the electron and ion distribution functions,\nwhich alter the magnetic reconnection process. Moreover, increasing the growth\nrate with the plasma parameters could be a source of anomalous resistivity that\nenhances the rate of magnetic reconnection.", "category": "physics_plasm-ph" }, { "text": "An advection-diffusion model for cross-field runaway electron transport\n in perturbed magnetic fields: Disruption-generated runaway electrons (RE) present an outstanding issue for\nITER. The predictive computational studies of RE generation rely on\norbit-averaged computations and, as such, they lack the effects from the\nmagnetic field stochasticity. Since stochasiticity is naturally present in\npost-disruption plasma, and externally induced stochastization offers a\nprominent mechanism to mitigate RE avalanche, we present an advection-diffusion\nmodel that can be used to couple an orbit-following code to an orbit-averaged\ntool in order to capture the cross-field transport and to overcome the latter's\nlimitation. The transport coefficients are evaluated via a Monte Carlo method.\nWe show that the diffusion coefficient differs significantly from the\nwell-known Rechester-Rosenbluth result. We also demonstrate the importance of\nincluding the advection: it has a two-fold role both in modelling transport\nbarriers created by magnetic islands and in amplifying losses in regions where\nthe islands are not present.", "category": "physics_plasm-ph" }, { "text": "Demonstration of an optical mixing technique to drive Kinetic\n Electrostatic Electron Nonlinear waves in laser produced plasmas: A nitrogen gas Raman cell system has been constructed to shift a 70 J 527 nm\nlaser beam to 600 nm with 20 J of energy. The 600 nm probe and a 200J, 527 nm\npump beam were optically mixed in a laser produced (gas jet) plasma. The\nbeating of the two laser beams formed a ponderomotive force that can drive\nKinetic Electrostatic Electron Nonlinear (KEEN) waves discovered in\nVlasov-Poisson simulations by Afeyan et al [1,2]. KEEN waves were detected in\nthese experiments where traditional plasma theory would declare there to be a\nspectral gap (ie no linear waves possible). The detection was done using\nThomson scattering with probe wavelengths of both 351 nm and 263.5 nm.", "category": "physics_plasm-ph" }, { "text": "Self-waveguiding of relativistic laser pulses in neutral gas channel: We demonstrate that an ultrashort high intensity laser pulse can propagate\nfor hundreds of Rayleigh ranges in a prepared neutral hydrogen channel by\ngenerating its own plasma waveguide as it propagates; the front of the pulse\ngenerates a waveguide that confines the rest of the pulse. A wide range of\nsuitable initial index structures will support this \"self-waveguiding\" process;\nthe necessary feature is that the gas density on axis is a minimum. Here, we\ndemonstrate self-waveguiding of pulses of at least $1.5\\times10^{17} W/cm^2$\n(normalized vector potential $a_0\\sim0.3)$ over 10 cm, or $\\sim100$ Rayleigh\nranges, limited only by our laser energy and length of our gas jet. We predict\nand observe characteristic oscillations corresponding to mode-beating during\nself-waveguiding. The self-waveguiding pulse leaves in its wake a fully ionized\nlow density plasma waveguide which can guide another pulse injected immediately\nfollowing; we demonstrate optical guiding of such a follow-on probe pulse", "category": "physics_plasm-ph" }, { "text": "Magnetohydrodynamic instabilities in a self-gravitating rotating cosmic\n plasma: The generation of magnetohydrodynamic (MHD) waves and their instabilities are\nstudied in galactic gaseous rotating plasmas with the effects of the magnetic\nfield, the self gravity, the diffusion-convection of cosmic rays as well as the\ngas and cosmic-ray pressures. The coupling of the Jeans, Alfv{\\'e}n and\nmagnetosonic waves, and the conditions of damping or instability are studied in\nthree different cases, namely when the propagation direction is perpendicular,\nparallel and oblique to the static magnetic field, and are shown to be\nsignificantly modified by the effects of the Coriolis force due to the rotation\nof cosmic fluids and the cosmic-ray diffusion. The coupled modes can be damped\nor anti-damped depending on the wave number is above or below the Jeans\ncritical wave number that is reduced by the effects of the Coriolis force and\nthe cosmic-ray pressure. It is found that the deviation of the axis of rotation\nfrom the direction of the static magnetic field gives rise to the coupling\nbetween the Alfv{\\'e}n wave and the classical Jeans mode which otherwise\nresults into the modified slow and fast Alfv{\\'e}n waves as well as the\nmodified classical Jeans modes. Furthermore, due to the effects of the cosmic\nrays diffusion, there appears a new wave mode (may be called the fast Jeans\nmode) in the intermediate frequency regimes of the slow and fast Alfv{\\'e}n\nwaves, which seems to be dispersionless in the long-wavelength propagation and\nhas a lower growth rate of instability in the high density regimes of galaxies.\nThe dispersion properties and the instabilities of different kinds of MHD waves\nreported here can play pivotal roles in the formation of various galactic\nstructures at different length scales.", "category": "physics_plasm-ph" }, { "text": "A scaling law for the dust cloud in radio frequency discharge under\n microgravity conditions: We employ the approximation of overlapped scattering potentials of charged\ndust particles exposed to streaming ions to deduce the \"equation of state\" for\na stationary dust cloud in the radio frequency discharge apart from the void\ndust boundary. The obtained equation defines the potential of a dust particle\nas a function of the ion number density, the mass of a carrier gas atom, and\nthe electron temperature. A scaling law that relates the particle number\ndensity to the particle radius and electron temperature in different systems is\nformulated. Based on the proposed approach the radius of a cavity around a\nlarge particle in the bulk of a cloud is estimated. The results of calculation\nare in a reasonable agreement with the experimental data available in\nliterature.", "category": "physics_plasm-ph" }, { "text": "Selective Atomic Heating in Plasmas: Implications for Quantum Theory: A new model of quantum mechanics, Classical Quantum Mechanics, is based on\nthe (nearly heretical) postulate that electrons are physical objects that obey\nclassical physical laws. Indeed, ionization energies, excitation energies etc.\nare computed based on picturing electrons as bubbles of charge that\nsymmetrically surround a nucleus. Hence, for example, simple algebraic\nexpressions based on Newtonian force balances are used to predict ionization\nenergies and stable excitation states with remarkable precision. One of the\nmost startling predictions of the model is that there are stable sizes of the\nhydrogen atom electron (bubble diameter) that are smaller (called hydrinos)\nthan that calculated for the standard ground state. Experimental evidence in\nsupport of this novel physical/classical version of quantum is alleged to be\nfound in the existence of super heated hydrogen atoms reported by many teams in\na variety of plasmas. It is postulated that the energy required for creating\nsuper heated H aoms comes from the shrinkage of ground state H atoms to form\nhydrinos. This claim is discussed with reference to a brief review of the\npublished studies of line broadening.", "category": "physics_plasm-ph" }, { "text": "Understanding the surface wave characteristics using 2D particle-in-cell\n simulation and deep neural network: The characteristics of the surface waves along the interface between a plasma\nand a dielectric material have been investigated using kinetic Particle-In-Cell\n(PIC) simulations. A microwave source of GHz frequency has been used to trigger\nthe surface wave in the system. The outcome indicates that the surface wave\ngets excited along the interface of plasma and the dielectric tube and appears\nas light and dark patterns in the electric field profiles. The dependency of\nradiation pressure on the dielectric permittivity and supplied input frequency\nhas been investigated. Further, we assessed the capabilities of neural networks\nto predict the radiation pressure for a given system. The proposed Deep Neural\nNetwork model is aimed at developing accurate and efficient data-driven plasma\nsurface wave devices.", "category": "physics_plasm-ph" }, { "text": "Direct laser acceleration: a model for the electron injection from the\n walls of a cylindrical guiding structure: We use analytical methods and particle-in-cell simulation to investigate the\norigin of electrons accelerated by the process of direct laser acceleration\ndriven by high-power laser pulses in preformed narrow cylindrical plasma\nchannels. The simulation shows that the majority of accelerated electrons are\noriginally located along the interface between the channel wall and the channel\ninterior. The analytical model based on the electron hydrodynamics illustrates\nthe underlying physical mechanism of the release of electrons from the channel\nwall when irradiated by an intense laser, the subsequent electron dynamics, and\nthe corresponding evolution of the channel density profile. The quantitative\npredictions of the total charge of released electrons and the average electron\ndensity inside the channel are validated by comparison with the simulation\nresults.", "category": "physics_plasm-ph" }, { "text": "Behaviour of Ion Acoustic Soliton in a two-electron temperature plasmas\n of Multi-pole line cusp Plasma Device (MPD): This article presents the experimental observations and characterization of\nIon Acoustic Soliton (IAS) in a unique Multi-pole line cusp Plasma Device (MPD)\ndevice in which the magnitude of the pole-cusp magnetic field can be varied.\nAnd by varying the magnitude of the pole-cusp magnetic field, the proportions\nof two-electron-temperature components in the filament-produced plasmas of MPD\ncan be varied. The solitons are experimentally characterized by measuring their\namplitude-width relation and Mach numbers. The nature of the solitons is\nfurther established by making two counter-propagating solitons interact with\neach other. Later, the effect of the two-temperature electron population on\nsoliton amplitude and width is studied by varying the magnitude of the pole\ncusp-magnetic field. It has been observed that different proportions of\ntwo-electron-temperature significantly influence the propagation of IAS. The\namplitude of the soliton has been found to be following inversely with the\neffective electron temperature (Teff)", "category": "physics_plasm-ph" }, { "text": "Dust-acoustic rogue waves in non-thermal plasmas: The nonlinear propagation of dust-acoustic (DA) waves (DAWs) and associated\nDA rogue waves (DARWs), which are governed by the nonlinear Schr\\\"{o}dinger\nequation, is theoretically investigated in a four component plasma medium\ncontaining inertial warm negatively charged dust grains and inertialess\nnon-thermal distributed electrons as well as iso-thermal positrons and ions.\nThe modulationally stable and unstable parametric regimes of DAWs are\nnumerically studied for the plasma parameters. Furthermore, the effects of\ntemperature ratios of ion-to-electron and ion-to-positron, and the number\ndensity of ion and dust grains on the DARWs are investigated. It is observed\nthat the physical parameters play a very crucial role in the formation of\nDARWs. These results may be useful in understanding the electrostatic\nexcitations in dusty plasmas in space and laboratory situations.", "category": "physics_plasm-ph" }, { "text": "Effect of detachment on Magnum-PSI ELM-like pulses: II. Spectroscopic\n analysis and role of molecular assisted reactions: The linear plasma machine Magnum-PSI can replicate similar conditions to\nthose found in a tokamak at the end of the divertor leg. A dedicated capacitor\nbank, in parallel to the plasma source, can release a sudden burst of energy,\nleading to a rapid increase in plasma temperature and density, resulting in a\ntransient heat flux increase of half of an order of magnitude, a so called\nELM-like pulse. Throughout both the steady state and the pulse, the neutral\npressure in the target chamber is then increased, causing the target to\ntransition from an attached to a detached state. In the first paper related to\nthis study\\cite{Federici} direct measurements of the plasma properties are used\nto qualitatively determine the effect of detachment on the ELM-like pulse. This\nis used to show the importance of molecular assisted reactions. Molecular\nprocesses, and especially molecular activated dissociation, are found to be\nimportant in the exchange of potential energy with the plasma, while less so in\nradiating the energy from the ELM-like pulse. At low target chamber pressure,\nthe plasma generated via ionisation during the part of the ELM-like pulse with\nthe higher temperature is more than that produced by the plasma source, a\nunique case in linear machines. At high target chamber pressure molecular\nactivated recombination contributes up to a third of the total recombination\nrate, contributing to the reduction of the target particle flux. Some metrics\nthat estimate the energy lost by the plasma per interactions with neutrals,\npotentially relevant for the portion of the tokamak divertor leg below\n$\\sim10eV$, are then tentatively obtained.", "category": "physics_plasm-ph" }, { "text": "Gyrokinetic simulations of electric current generation in ion\n temperature gradient driven turbulence: Gyrokinetic simulations in the collisionless limit demonstrate the physical\nmechanisms and the amplitude of the current driven by turbulence. Simulation\nresults show the spatio-temporal variation of the turbulence driven current and\nits connection to the divergence of the Reynolds stress and the turbulence\nacceleration. Fine structures (a few ion Larmor radii) of the turbulence\ninduced current are observed near the rational surfaces with the arbitrary\nwavelength solver of the quasi-neutrality equation. The divergence of the\nReynolds stress plays a major role in the generation of these fine structures.\nThe so-called \"spontaneous\" current is featured with large local magnitude near\nthe rational surfaces.", "category": "physics_plasm-ph" }, { "text": "Critical Need for a National Initiative in Low Temperature Plasma\n Research: In the white paper we describe a national program in Low Temperature Plasma\n(LTP). The program should take advantage of the research opportunities of 3\nrapidly growing areas (nanomaterial plasma synthesis, plasma medicine,\nmicroelectronics). The main theme is to achieve a fundamental understanding of\nLow Temperature Plasmas as they are applied to these different applications.\nThis understanding will allow U.S. industry to meet the challenges of\ninternational competition.", "category": "physics_plasm-ph" }, { "text": "The Suppression of Radiation Reaction and Laser Field Depletion in\n Laser-Electron beam interaction: The effects of radiation reaction (RR) have been studied extensively by using\nthe ultraintense laser interacts with the counter-propagating relativistic\nelectron. At the laser intensity at the order of $10^{23}$ W/cm$^2$, the\neffects of RR are significant in a few laser period for a relativistic\nelectron. However, the laser at such intensity is tightly focused and the laser\nenergy is usually assumed to be fixed. Then, the signal of RR and energy\nconservation cannot be guaranteed. To assess the effects of RR in a tightly\nfocused laser pulse and the evolution of the laser energy, we simulate this\ninteraction with a beam of $10^9$ electrons by means of Particle-in-Cell (PIC)\nmethod. We observed that the effects of RR are suppressed due to the\nponderomotive force and accompanied by a non-negligible amount of laser field\nenergy reduction. This is due to the ponderomotive force that prevents the\nelectrons from approaching the center of the laser pulse and leads to the\ninteraction at weaker field region. At the same time, the laser energy is\nabsorbed through ponderomotive acceleration. Thus, the kinetic energy of the\nelectron beam has to be carefully selected such that the effects of RR become\nobvious.", "category": "physics_plasm-ph" }, { "text": "Electric field in spatially inhomogeneous non-neutral plasma: We present a general expression for the probability distribution function of\nelectric field in a plasma cloud formed by the impact of a laser pulse on a gas\nor a solid body. We also present the results of numerical calculation of this\nfunction for the case of non-interacting particles depending on the plasma\ncloud size. It takes into account the ionic microfield and the macrofield\narising from the charge imbalance. As the charge imbalance increases, the\neffect of a sharp increase in the distribution function for large field values\nis observed. Good agreement between the calculation of the shift of the\nspectral line and the experiment is obtained. The results obtained are of\ncrucial importance for diagnosing plasma in various applications.", "category": "physics_plasm-ph" }, { "text": "Gravity compensation in complex plasmas by application of a temperature\n gradient: Micron sized particles are suspended or even lifted up in a gas by\nthermophoresis. This allows the study of many processes occurring in strongly\ncoupled complex plasmas at the kinetic level in a relatively stress-free\nenvironment. First results are presented. The technique is also of interest for\ntechnological applications.", "category": "physics_plasm-ph" }, { "text": "On a hydrodynamic description of waves propagating perpendicular to the\n magnetic field in relativistically hot plasmas: The novel hydrodynamic model of plasmas with the relativistic temperatures\nconsisted of four equations for the material fields: the concentration and the\nvelocity field \\emph{and} the average reverse relativistic $\\gamma$ functor and\nthe flux of the reverse relativistic $\\gamma$ functor is applied to study\nhigh-frequency part of spectrum of electromagnetic waves propagating\nperpendicular to the external magnetic field. The thermal effects considered\nfor the temperatures close to the rest energy of electrons considerably change\nthe dispersion equation in compare with the nonrelativistic temperatures.\nAnalytical analysis of the changes is presented.", "category": "physics_plasm-ph" }, { "text": "Kinetic Solvers with Adaptive Mesh in Phase Space for Low-Temperature\n Plasmas: We describe the implementation of 1d1v and 1d2v Vlasov and Fokker-Planck\nkinetic solvers with adaptive mesh refinement in phase space (AMPS) and\ncoupling these kinetic solvers to Poisson equation solver for electric field.\nWe demonstrate that coupling AMPS kinetic and electrostatic solvers can be done\nefficiently without splitting phase-space transport. We show that Eulerian\nfluid and kinetic solvers with dynamically adaptive Cartesian mesh can be used\nfor simulations of collisionless plasma expansion into vacuum. The\nVlasov-Fokker-Planck solver is demonstrated for the analysis of electron\nacceleration and scattering as well as the generation of runaway electrons in\nspatially inhomogeneous electric fields.", "category": "physics_plasm-ph" }, { "text": "Excitation of Flow Instabilities due to Nonlinear Scale Invariance: A novel route to instabilities and turbulence in fluid and plasma flows is\npresented in kinetic Vlasov-Maxwell model. New kind of flow instabilities is\nshown to arise due to the availability of new kinetic energy sources which are\nabsent in conventional treatments. The present approach is based on a scale\ninvariant nonlinear analytic formalism developed to address irregular motions\non a chaotic attractor or in turbulence in a more coherent manner. We have\nstudied two specific applications of this turbulence generating mechanism. The\nwarm plasma Langmuir wave dispersion relation is shown to become unstable in\nthe presence of these multifractal measures. In the second application, these\nmultifractal measures are shown to induce naturally non-Gaussian i.e. a\nstretched -Gaussian distribution and anomalous transport for tracer particles\nfrom the turbulent advection-diffusion transport equation in a Vlasov plasma\nflow.", "category": "physics_plasm-ph" }, { "text": "Gyrokinetic understanding of the edge pedestal transport driven by\n resonant magnetic perturbations in a realistic divertor geometry: Self-consistent simulations of neoclassical and electrostatic turbulent\ntransport in a DIII-D H-mode edge plasma under resonant magnetic perturbations\n(RMPs) have been performed using the global total-f gyrokinetic\nparticle-in-cell code XGC, in order to study density-pump out and electron heat\nconfinement. The RMP field is imported from the extended magneto-hydrodynamics\n(MHD) code M3D-C1, taking into account the linear two-fluid plasma response.\nWith both neoclassical and turbulence physics considered together, the XGC\nsimulation reproduces two key features of experimentally observed edge\ntransport under RMPs: increased radial particle transport in the pedestal\nregion that is sufficient to account for the experimental pump-out rate, and\nsuppression of the electron heat flux in the steepest part of the edge\npedestal. In the simulation, the density fluctuation amplitude of modes moving\nin the electron diamagnetic direction increases due to interaction with RMPs in\nthe pedestal shoulder and outward, while the electron temperature fluctuation\namplitude decreases.", "category": "physics_plasm-ph" }, { "text": "Nondiffusive suprathermal ion transport in simple magnetized toroidal\n plasmas: We investigate suprathermal ion dynamics in simple magnetized toroidal\nplasmas in the pres- ence of electrostatic turbulence driven by the ideal\ninterchange instability. Turbulent fields from fluid simulations are used in\nthe non-relativistic equation of ion motion to compute suprathermal tracer ion\ntrajectories. Suprathermal ion dispersion starts with a brief ballistic phase,\nduring which particles do not interact with the plasma, followed by a\nturbulence interaction phase. In this one simple system, we observe the entire\nspectrum of suprathermal ion dynamics, from subdiffusion to superdiffusion,\ndepending on beam energy and turbulence amplitude. We estimate the duration of\nthe ballistic phase and identify basic mechanisms during the interaction phase\nthat determine the character of suprathermal ion dispersion upon the beam\nenergy and turbulence fluctuation amplitude.", "category": "physics_plasm-ph" }, { "text": "Numerical investigation of isolated filament motion in a realistic\n tokamak geometry: This paper presents a numerical investigation of isolated filament dynamics\nin a simulation geometry representative of the scrape-off layer (SOL) of the\nMega Amp Spherical Tokamak (MAST) previously studied in [N.R.Walkden et.al,\nPlasma Phys. Control. Fusion, 55 (2013) 105005]. This paper focuses on the\nevolution of filament cross-sections at the outboard midplane and investigates\nthe scaling of the centre of mass velocity of the filament cross-section with\nfilament width and electron temperature. By decoupling the vorticity equation\ninto even and odd parity components about the centre of the filament in the\nbi-normal direction parallel density gradients are shown to drive large\nvelocities in the bi-normal (approximately poloidal) direction which scale\nlinearly with electron temperature. In this respect increasing the electron\ntemperature causes a departure of the filament dynamics from 2D behaviours.\nDespite the strong impact of 3D effects the radial motion of the filament is\nshown to be relatively well predicted by 2D scalings. The radial velocity is\nfound to scale positively with both electron temperature and cross-sectional\nwidth, suggesting an inertially limited nature. Comparison with the two-region\nmodel [J. R. Myra et.al, Phys.Plasmas, 13 (2006) 112502] achieves reasonable\nagreement when using a corrected parallel connection length due to the neglect\nof diamagnetic currents driven in the divertor region of the filament. Analysis\nof the transport of particles due to the motion of the filament shows that the\nbackground temperature has a weak overall effect on the radial particle flux\nwhilst the filament width has a strong effect.", "category": "physics_plasm-ph" }, { "text": "Relativistic Thermodynamics of Magnetized Fermi Electron Gas: To study the relativistic thermodynamic properties of a Fermi gas in a strong\nmagnetic field, we construct the relativistic thermodynamic potential by the\nrelativistic Fermi distribution function taking into account that the motion of\nparticles in a plane perpendicular to the magnetic field is quantized. With\nthis general potential at hand, we investigate all the thermodynamic quantities\nas a function of densities, temperatures and the magnetic field. We obtain a\nnovel set of adiabatic equations. Having the expression of the pressure and\nadiabatic state equations, we determine the sound velocity for several cases\nrevealing a new type of sound velocity. Finally, we disclose the magnetic\ncooling in the quantized electron Fermi gas, which is based on an adiabatic\nmagnetization in contrast to the known adiabatic demagnetization.", "category": "physics_plasm-ph" }, { "text": "Shielding of a small charged particle in weakly ionized plasmas: In this paper we present a concise overview of our recent results concerning\nthe electric potential distribution around a small charged particle in weakly\nionized plasmas. A number of different effects which influence plasma screening\nproperties are considered. Some consequences of the results are discussed,\nmostly in the context of complex (dusty) plasmas.", "category": "physics_plasm-ph" }, { "text": "On the relationship between the multi-region relaxed variational\n principle and resistive inner layer theory: We show that the variational energy principle of multi-region relaxed\nmagnetohydrodynamic (MRxMHD) model can be used to predict finite-pressure\nlinear tearing instabilities. In this model, the plasma volume is sliced into\nsub-volumes separated by \"ideal interfaces\", and in each volume the magnetic\nfield relaxes to a Taylor state where the pressure gradient $\\nabla p = 0$. The\nMRxMHD model is implemented in the SPEC code so that the equilibrium solution\nin each region is computed while the preserving force balance across the\ninterfaces. As SPEC computes the Hessian matrix (a discretized stability\nmatrix), the stability of an MRxMHD equilibrium can also be computed with SPEC.\nIn this article, using SPEC, we investigate the effect of local pressure\ngradients and the $\\nabla p = 0$ in the vicinity of the resonant surface of a\ntearing mode. For low beta plasma, we have been able to illustrate a\nrelationship between the resistive singular layer theory [Coppi et al. (1966)\nNucl. Fusion 6 101, Glasser et al. The Physics of Fluids 18, 875-888 (1975)],\nand the MRxMHD model. Within the singular layer, the volume-averaged magnetic\nhelicity and the flux-averaged toroidal flux are shown to be the invariants for\nthe linear tearing modes in SPEC simulations. Our technique to compute MRxMHD\nstability is first tested numerically in cylindrical tokamak and its\napplication in toroidal geometry is demonstrated. We demonstrate an agreement\nbetween the stability boundary obtained with SPEC simulation and the resistive\ninner layer theories.", "category": "physics_plasm-ph" }, { "text": "Generation of polarized electron beams through self-injection in the\n interaction of a laser with a pre-polarized plasma: Polarized electron beam production via laser wakefield acceleration in\npre-polarized plasma is investigated by particle-in-cell simulations. The\nevolution of the electron beam polarization is studied based on the\nThomas-Bargmann-Michel-Telegdi equation for the transverse and longitudinal\nself-injection, and the depolarization process is found to be influenced by the\ninjection schemes. In the case of transverse self-injection as found typically\nin the bubble regime, the spin precession of the accelerated electrons is\nmainly influenced by the wakefield. However, in the case of longitudinal\ninjection in the quasi-one-dimensional regime (for example, F. Y. Li \\emph{et\nal}., Phys. Rev. Lett. 110, 135002 (2013)), the direction of electron spin\noscillates in the laser filed. Since the electrons move around the laser axis,\nthe net influence of the laser field is nearly zero and the contribution of the\nwakefield can be ignored. Finally, an ultra-short electron beam with\npolarization of $99\\%$ can be obtained using longitudinal self-injection.", "category": "physics_plasm-ph" }, { "text": "An efficient method for tracing high-resolution invariant manifolds of\n three-dimensional flows: In Hamiltonian systems subjected to periodic perturbations the stable and\nunstable manifolds of the unstable periodic orbits provide the dynamical\n\"skeleton\" that drives the mixing process and bounds the chaotic regions of the\nphase space. Determining the behavior of these objects is valuable in physical\napplications involving asymmetric solenoidal fields or time-dependent\nHamiltonian systems. Here we introduce a simple method to calculate an unstable\nperiodic orbit given an initial guess on its position. Then we present an\nefficient adaptive method to build its high-resolution invariant manifolds to\narbitrary length and compare it to a random sampling method with the same\ncomputational cost. The adaptive method gives a high-quality representation of\nthe manifolds and reveals fine details that become lost in the random sampling\nmethod. Finally, we introduce an approximation to the adaptive method to build\nthe manifolds avoiding redundant calculations and reducing logarithmically the\nnumber of computations needed to represent these surfaces.", "category": "physics_plasm-ph" }, { "text": "Stability and evolution of electromagnetic solitons in relativistic\n degenerate laser plasmas: The dynamical behaviors of electromagnetic (EM) solitons formed due to\nnonlinear interaction of linearly polarized intense laser light and\nrelativistic degenerate plasmas are studied. In the slow motion approximation\nof relativistic dynamics, the evolution of weakly nonlinear EM envelope is\ndescribed by the generalized nonlinear Schr{\\\"o}dinger (GNLS) equation with\nlocal and nonlocal nonlinearities. Using the Vakhitov-Kolokolov criteria, the\nstability of an EM soliton solution of the GNLS equation is studied. Different\nstable and unstable regions are demonstrated with the effects of soliton\nvelocity, soliton eigenfrequency, as well as the degeneracy parameter\n$R=p_{Fe}/m_ec$, where $p_{Fe}$ is the Fermi momentum and $m_e$ the electron\nmass, and $c$ is the speed of light in vacuum. It is found that the stability\nregion shifts to an unstable one and is significantly reduced as one enters\nfrom the regimes of weakly relativistic $(R\\ll1)$ to ultrarelativistic\n$(R\\gg1)$ degeneracy of electrons. The analytically predicted results are in\ngood agreement with the simulation results of the GNLS equation. It is shown\nthat the standing EM soliton solutions are stable. However, the moving solitons\ncan be stable or unstable depending on the values of soliton velocity, the\neigenfrequency or the degeneracy parameter. The latter with strong degeneracy\n$(R>1)$ can eventually lead to soliton collapse.", "category": "physics_plasm-ph" }, { "text": "Magnetized Gas Collimation of Interstellar Outflow Scaled by\n Laser-produced Plasma: Young stellar objects/planetary nebula outflow anisotropies usually involve\nwind-wind interactions and magnetic collimation, but detailed structures of\nwind and magnetic fields inside collimation region remain undetermined. We\nnumerically investigated its laboratory counterpart, based on poloidal field\ncollimation in magnetocentrifugal launching model. Our analog consist of fast\nwind: Aluminum plasma generated by laser and magnetized ambient: molecular\nHelium and B=5-60 Tesla embedded field. Elevating magnetic field strength or\ndecreasing gas density can alter expansion morphology, from sphere to prolonged\ncavity and ultimately to collimated jet. Outflow patterns can be quantitatively\npredicted based on the knowledge of its surroundings through a set of external\nMach numbers. We conclude that such mixed gas and magnetic field dynamics are\nconsistent with astronomical observations of protostars and planetary nebulae\nin certain evolution stages, and here provide a scalable framework allowing\nfitting of flow-field structures in astronomical unresolved regions by assuming\ntheir possible geometries on a repeatable laboratory platform.", "category": "physics_plasm-ph" }, { "text": "Observations of Pressure Anisotropy Effects within Semi-Collisional\n Magnetized-Plasma Bubbles: Magnetized plasma interactions are ubiquitous in astrophysical and laboratory\nplasmas. Various physical effects have been shown to be important within\ncolliding plasma flows influenced by opposing magnetic fields, however,\nexperimental verification of the mechanisms within the interaction region has\nremained elusive. Here we discuss a laser-plasma experiment whereby\nexperimental results verify that Biermann battery generated magnetic fields are\nadvected by Nernst flows and anisotropic pressure effects dominate these flows\nin a reconnection region. These fields are mapped using time-resolved proton\nprobing in multiple directions. Various experimental, modelling and analytical\ntechniques demonstrate the importance of anisotropic pressure in\nsemi-collisional, high-$\\beta$ plasmas, causing a reduction in the magnitude of\nthe reconnecting fields when compared to resistive processes. Anisotropic\npressure dynamics are crucial in collisionless plasmas, but are often neglected\nin collisional plasmas. We show pressure anisotropy to be essential in\nmaintaining the interaction layer, redistributing magnetic fields even for\nsemi-collisional, high energy density physics (HEDP) regimes", "category": "physics_plasm-ph" }, { "text": "X-ray imaging and radiation transport effects on cylindrical implosions: Magnetization of inertial confinement implosions is a promising means of\nimproving their performance, owing to the potential reduction of energy losses\nwithin the target and mitigation of hydrodynamic instabilities. In particular,\ncylindrical implosions are useful for studying the influence of a magnetic\nfield thanks to their axial symmetry. Here we present experimental results from\ncylindrical implosions on the OMEGA-60 laser using a 40-beam, 14.5 kJ, 1.5 ns\ndrive and an initial seed magnetic field of B0=24 T along the axis of the\ntargets, compared with reference results without an imposed B-field. Implosions\nwere characterized using time-resolved X-ray imaging from two orthogonal lines\nof sight. We found that the data agree well with magnetohydrodynamic\nsimulations once radiation transport within the imploding plasma is considered.\nWe show that for a correct interpretation of the data in this type of\nexperiments, explicit radiation transport must be taken into account.", "category": "physics_plasm-ph" }, { "text": "Comparison of linear modes in kinetic plasma models: We compare, in an extensive and systematic way, linear theory results\nobtained with the hybrid (ion-kinetic and electron-fluid), the gyrokinetic and\nthe fully-kinetic plasma models. We present a test case with parameters that\nare relevant for solar wind turbulence at small scales, which is a topic now\nrecognized to need a kinetic treatment, to a certain extent. We comment on the\ncomparison of low-frequency single modes (Alfv\\'{e}n/ion-cyclotron,\nion-acoustic, and fast modes) for a wide range of propagation angles, and on\nthe overall spectral properties of the linear operators, for\nquasi-perpendicular propagation. The methodology and the results presented in\nthis paper will be valuable when choosing which model should be used in regimes\nwhere the assumptions of each model are not trivially satisfied.", "category": "physics_plasm-ph" }, { "text": "Spin-orbit interaction in a high-power laser irradiated micro-scale\n plasma waveguide: Light carries angular momentum as spin and orbital components. The spin-orbit\ninteraction (SOI) of light refers to phenomena in which the spin (left or right\ncircular polarisation) affects the spatial degrees of freedom. Recently,\ninterest in SOI has surged, as it provides physical insight into the behaviour\nof polarised light at subwavelength scales, and allows for spin-controlled\nmanipulation of light. Most studies are performed with low intensity, leaving\nthe role SOI plays in the relativistic laser-plasma interaction, characterised\nby nonlinearity, less understood. Here, using 3D particle-in-cell simulations,\nwe report SOI effects in this unprecedented regime. Specifically, a circularly\npolarised Gaussian laser irradiating a micro-scale plasma waveguide drives a\nspin-controlled chiral surface wave, allowing the reflected harmonic photons to\ngain orbital angular momentum (OAM). These effects produce intense optical\nvortices in the extreme ultraviolet regime - an area of significant fundamental\nand applied physics potential.", "category": "physics_plasm-ph" }, { "text": "Employing machine learning for theory validation and identification of\n experimental conditions in laser-plasma physics: The validation of a theory is commonly based on appealing to clearly\ndistinguishable and describable features in properly reduced experimental data,\nwhile the use of ab-initio simulation for interpreting experimental data\ntypically requires complete knowledge about initial conditions and parameters.\nWe here apply the methodology of using machine learning for overcoming these\nnatural limitations. We outline some basic universal ideas and show how we can\nuse them to resolve long-standing theoretical and experimental difficulties in\nthe problem of high-intensity laser-plasma interactions. In particular we show\nhow an artificial neural network can \"read\" features imprinted in laser-plasma\nharmonic spectra that are currently analysed with spectral interferometry.", "category": "physics_plasm-ph" }, { "text": "Nonlinear structures: explosive, soliton and shock in a quantum\n electron-positron-ion magnetoplasma: Theoretical and numerical studies are performed for the nonlinear structures\n(explosive, solitons and shock) in quantum electron-positron-ion\nmagnetoplasmas. For this purpose, the reductive perturbation method is employed\nto the quantum hydrodynamical equations and the Poisson equation, obtaining\nextended quantum Zakharov-Kuznetsov equation. The latter has been solved using\nthe generalized expansion method to obtain a set of analytical solutions, which\nreflect the possibility of the propagation of various nonlinear structures. The\nrelevance of the present investigation to the white dwarfs is highlighted.", "category": "physics_plasm-ph" }, { "text": "On the synergy between easier plasma operation and affordable coil-set\n requirements enabled by Negative Triangularity in the prospective ARC fusion\n reactor: A numerical workflow is developed to explore the viability of running\nmultiple plasma configurations in the ARC fusion pilot plant. Suitable cost\nfunctions for various poloidal field coil sets are evaluated based on currents\nrequired in the coils, induced stresses, and flexibility in plasma\nconfigurations. It is shown, for the first time, that a given set of poloidal\nfield coils can sustain equilibria with both signs of triangularity and that\nequilibria at Negative Triangularity have comparable coil requirements to those\nat Positive Triangularity, despite ARC's Positive Triangularity design\nequilibrium. These results contribute to demonstrate that the Negative\nTriangularity Tokamak is a promising candidate for the commercialization of\nfusion power.", "category": "physics_plasm-ph" }, { "text": "Hybrid codes (massless electron fluid): Hybrid codes are widely used to model ion-scale phenomena in space plasmas.\nHybrid codes differ from full particle (PIC) codes in that the electrons are\nmodeled as a fluid that is usually assumed to be massless, while the electric\nfield is not advanced in time, but instead calculated at the new time level\nfrom the advanced ion quantities and the magnetic field. In this chapter we\nconcentrate on such hybrid models with massless electrons, beginning with a\ndiscussion of the basics of a simple hybrid code algorithm. We then show\nexamples of recent use of hybrid codes for large-scale space plasma simulations\nof structures formed at planetary bow shock--foreshock systems, magnetic\nreconnection at the magnetopause, and complex phenomena in the magnetosheath\ndue to the interaction of kinetic processes associated with the bow shock,\nmagnetic reconnection, and turbulence. A discussion then follows of a number of\nother hybrid codes based on different algorithms that are presently in active\nuse to investigate a variety of plasma processes in space as well as some\nrecent work on the development of new models. We conclude with a few brief\ncomments concerning the future development and use of hybrid codes.", "category": "physics_plasm-ph" }, { "text": "Scaling the Yield of Laser-Driven Electron-Positron Jets to Laboratory\n Astrophysical Applications: We report new experimental results obtained on three different laser\nfacilities that show directed laser-driven relativistic electron-positron jets\nwith up to 30 times larger yields than previously obtained and a quadratic (~\nE^2) dependence of the positron yield on the laser energy. This favorable\nscaling stems from a combination of higher energy electrons due to increased\nlaser intensity and the recirculation of MeV electrons in the mm-thick target.\nBased on this scaling, first principles simulations predict the possibility of\nusing such electron-positron jets, produced at upcoming high-energy laser\nfacilities, to probe the physics of relativistic collisionless shocks in the\nlaboratory.", "category": "physics_plasm-ph" }, { "text": "Radial equilibrium of relativistic particle bunches in plasma wakefield\n accelerators: Drive particle beams in linear or weakly nonlinear regimes of the plasma\nwakefield accelerator quickly reach a radial equilibrium with the wakefield,\nwhich is described in detail for the first time. The equilibrium beam state and\nself-consistent wakefields are obtained by combining analytical relationships,\nnumerical integration, and first-principle simulations. In the equilibrium\nstate, the beam density is strongly peaked near the axis, the beam radius is\nconstant along the beam, and longitudinal variation of the focusing strength is\nbalanced by varying beam emittance. The transverse momentum distribution of\nbeam particles depends on the observation radius and is neither separable, nor\nGaussian.", "category": "physics_plasm-ph" }, { "text": "Simulation of Pair Production in Extreme Strong EM Fields: In this article we review a theoretical framework for pair production from\nstrong external electromagnetic fields. We propose a numerical method to solve\nthe resulting equations of motion and present results for both cases of\nspatially homogeneous and inhomogeneous electric fields.", "category": "physics_plasm-ph" }, { "text": "Dissipation in PIC simulations of moderate to low \\b{eta} plasma\n turbulence: We simulate decaying turbulence in electron-positron pair plasmas using a\nfully- kinetic particle-in-cell (PIC) code. We run two simulations with\nmoderate-to-low plasma beta. The energy decay rate is found to be similar in\nboth the cases. The perpendicular wave-number spectrum of magnetic energy shows\na slope of k^-1.3 in both the cases. The particle energy distribution function\nshows the formation of a non-thermal feature in the case of lower plasma beta,\nwith a slope close to E^-1. The role of thin turbulent current sheets in this\nprocess is investigated. The heating by E_{\\parallel}.J_{\\parallel} term\ndominates the E_{\\perp}.J_{\\perp} term. Regions of strong\nE_{\\parallel}.J_{\\parallel} are spatially well-correlated with regions of\nintense current sheets, which also appear correlated with regions of strong\nE_{\\parallel} in the low beta simulation, suggesting an important role of\nmagnetic reconnection in the dissipation of low beta plasma turbulence.", "category": "physics_plasm-ph" }, { "text": "Perpendicular Diffusion of Energetic Particles: A Complete Analytical\n Theory: Over the past two decades scientists have achieved a significant improvement\nof our understanding of the transport of energetic particles across a mean\nmagnetic field. Due to test-particle simulations as well as powerful non-linear\nanalytical tools our understanding of this type of transport is almost\ncomplete. However, previously developed non-linear analytical theories do not\nalways agree perfectly with simulations. Therefore, a correction factor $a^2$\nwas incorporated into such theories with the aim to balance out inaccuracies.\nIn this paper a new analytical theory for perpendicular transport is presented.\nThis theory contains the previously developed unified non-linear transport\ntheory, the most advanced theory to date, in the limit of small Kubo number\nturbulence. For two-dimensional turbulence new results are obtained. In this\ncase the new theory describes perpendicular diffusion as a process which is\nsub-diffusive while particles follow magnetic field lines. Diffusion is\nrestored as soon as the turbulence transverse complexity becomes important. For\nlong parallel mean free paths one finds that the perpendicular diffusion\ncoefficient is a reduced field line random walk limit. For short parallel mean\nfree paths, on the other hand, one gets a hybrid diffusion coefficient which is\na mixture of collisionless Rechester & Rosenbluth and fluid limits. Overall the\nnew analytical theory developed in the current paper is in agreement with\nheuristic arguments. Furthermore, the new theory agrees almost perfectly with\npreviously performed test-particle simulations without the need of the\naforementioned correction factor $a^2$ or any other free parameter.", "category": "physics_plasm-ph" }, { "text": "First results from beam emission spectroscopy in SPIDER negative ion\n source: The SPIDER experiment, part of the Neutral Beam Test Facility (NBTF) at\nConsorzio RFX (Padua, Italy), is the prototype of the negative ion source for\nthe ITER neutral beam injectors; the source is coupled to a 100 kV three-grid\nacceleration system. A Beam Emission Spectroscopy (BES) diagnostic was\ninstalled in SPIDER to study and optimize energy distribution, aiming,\nuniformity and divergence of the H-/D- beam extracted from the source. The\ndiagnostic is based on the analysis of the Doppler shifted H{\\alpha}/D{\\alpha}\nlight emitted in the interaction between the beam particles and the H2/D2\nmolecules of the background. In 2019 the BES diagnostic in SPIDER was installed\nand calibrated, and it allowed to characterize the first hydrogen beams\nextracted from the SPIDER source, in cesium free conditions. The number of\nactive beamlets composing the beam was reduced from 1280 to 80, affecting the\nBES diagnostic capabilities. This paper presents the BES diagnostic setup and\ndiscusses the first collected results. Under limited extracted current density\n($\\sim$ 10 A/m^2) and ion energy ($\\leq$35 keV), no significant vertical beam\ndeflection caused by the magnetic filter field in the source was detected. In\nsome cases the beamlets were observed to be elongated in horizontal direction;\nbeamlet divergence values down to 20 mrad and 30 mrad e-folding were measured\nin vertical and horizontal direction, respectively; the intensity of the\nDoppler shifted radiation was found to be strongly correlated to the beam\ncurrent and to the beam divergence. The progressive compensation of beamlet\ndeflections (caused by electron suppression filter fields) with increasing\nvoltage in the extraction gap was studied.", "category": "physics_plasm-ph" }, { "text": "Improvement of Converter Surface Plasma Sources: A large volume surface plasma source (SPS) with a biased converter was\ndeveloped for the Los Alamos linear accelerator. A large gas-discharge chamber\nwith a multipole magnetic wall and 2 heated cathodes can support a discharge\ngenerating plasma. A cooled converter with a diameter of 5 cm and a potential\nof up to -300 V bombarded by positive ions and emits secondary negative ions,\naccelerates them and focuses in an emission aperture with a diameter of 6.4 mm.\nFrom this SPS, up to 18 mA of H- ions are extracted at a duty cycle of up to\n10%. The emitted H- beam current is attenuated through H- destruction in thick\nlayer of gas and discharge plasma. The H- beam intensity and H- generation\nefficiency can be increased by decreasing the gas and plasma layer thickness\nbetween the converter surface and the emission aperture. It is possible to\nimprove beam characteristics by making a small modification to the converted\nSPS. We propose to use a thin Penning discharge in front of the converter. The\nmagnetic field for the Penning discharge is created by permanent magnets.\nDecreasing the plasma end gas layer between the converter and the emission\naperture can decrease the H- beam loss and thus increase the extracted beam\nintensity up to 2 times", "category": "physics_plasm-ph" }, { "text": "An integrable localized approximation for interaction of two nearly\n anti-parallel sheets of the generalized vorticity in 2D ideal\n electron-magnetohydrodynamic flows: The formalism of frozen-in vortex lines for two-dimensional (2D) flows in\nideal incompressible electron magnetohydrodynamics (EMHD) is formulated. A\nlocalized approximation for nonlinear dynamics of two close sheets of the\ngeneralized vorticity is suggested and its integrability by the hodograph\nmethod is demonstrated.", "category": "physics_plasm-ph" }, { "text": "Controlled Betatron X-Ray Radiation from Tunable Optically Injected\n Electrons: The features of Betatron x-ray emission produced in a laser-plasma\naccelerator are closely linked to the properties of the relativistic electrons\nwhich are at the origin of the radiation. While in interaction regimes explored\npreviously the source was by nature unstable, following the fluctuations of the\nelectron beam, we demonstrate in this Letter the possibility to generate x-ray\nBetatron radiation with controlled and reproducible features, allowing fine\nstudies of its properties. To do so, Betatron radiation is produced using\nmonoenergetic electrons with tunable energies from a laser-plasma accelerator\nwith colliding pulse injection [J. Faure et al., Nature (London) 444, 737\n(2006)]. The presented study provides evidence of the correlations between\nelectrons and x-rays, and the obtained results open significant perspectives\ntoward the production of a stable and controlled femtosecond Betatron x-ray\nsource in the keV range.", "category": "physics_plasm-ph" }, { "text": "Dust-acoustic rogue waves in an electron depleted plasma: A rigorous theoretical investigation is made to study the characteristics of\ndust-acoustic (DA) waves (DAWs) in an electron depleted unmagnetized opposite\npolarity dusty plasma system that contains super-thermal ($\\kappa$-distributed)\nions, mobile positively and negatively charged dust grains for the first time.\nThe reductive perturbation method is employed to obtain the NLSE to explore the\nmodulational instability (MI) conditions for DAWs as well as the formation and\ncharacteristics of gigantic rogue waves. The nonlinear and dispersion\nproperties of the dusty plasma medium are the prime reasons behind the\nformation of rogue waves. The height and thickness of the DARWs associated with\nDAWs as well as the MI conditions of DAWs are numerically analyzed by changing\ndifferent dusty plasma parameters, such as dust charges, dust and ion number\ndensities, and ion-temperature, etc. The implications of the results for\nvarious space dusty plasma systems (viz., mesosphere, F-rings of Saturn, and\ncometary atmosphere, etc.) as well as laboratory dusty plasma produced by\nlaser-matter interaction are briefly mentioned.", "category": "physics_plasm-ph" }, { "text": "Tracing dynamics of laser-induced fields on ultra-thin foils using\n complementary imaging with streak deflectometry: We present a detailed study of the electric and magnetic fields, which are\ncreated on plasma vacuum interfaces as a result of highly intense\nlaser-matter-interactions. For the field generation ultra-thin polymer foils\nwere irradiated with high intensity femtosecond and picosecond laser pulses\nwith ultra-high contrast. To determine the temporal evolution and the spatial\ndistribution of these fields the proton streak deflectometry method has been\ndeveloped further and applied in two different imaging configurations. It\nenabled us to gather complementary information about the investigated field\nstructure, in particular about the influence of different field components\n(parallel and normal to the target surface) and the impact of a moving ion\nfront. The applied ultra-high laser contrast significantly increased the\nreproducibility of the experiment and improved the accuracy of the imaging\nmethod. In order to explain the experimental observations, which were obtained\nby applying ultra-short laser pulses, two different analytical models have been\nstudied in detail. Their ability to reproduce the streak deflectometry\nmeasurements was tested on the basis of three-dimensional particle simulations.\nA modification and combination of the two models allowed for an extensive and\naccurate reproduction of the experimental results in both imaging\nconfigurations. The controlled change of the laser pulse duration from 50\nfemtoseconds to 2.7 picoseconds led to a transition of the dominating force\nacting on the probing proton beam at the rear side of the polymer foil. The\napplied proton deflectometry method allowed for an unambiguous determination of\nthe magnetic field polarity at the rear side of the ultra-thin foil.", "category": "physics_plasm-ph" }, { "text": "Theoretical Resolution of Magnetic Reconnection in High Energy Plasmas: The formation of macroscopic reconnected magnetic structures (islands) have\nbeen observed in advanced experiments on weakly collisional, well confined\nplasmas while established theories of the drift-tearing modes, which depend\nstrongly on the electron temperature gradient and can describe the formation of\nthese structures, had predicted practically inaccessible excitation thresholds\nfor them in these regimes. The relevant theoretical dilemma is resolved as\nmesoscopic modes that depend critically on the ratio of the transverse (to the\nmagnetic field) to the longitudinal thermal\nconductivity${D^e_{\\perp}/D^e_{\\|}$, can produce large scale magnetic\nreconnection. These modes are envisioned to emerge from a background, which can\nbe coherent, of collisionless microscopic reconnecting modes driven by the\nelectron temperature gradient, that create a sequence of adjacent strings of\nmagnetic islands and increase considerably the ratio ${D^e_{\\perp}/D^e_{\\|}$\nover its classical value. The mesoscopic reconnecting mode is treated by a\nsingular perturbation analysis involving three asymptotic regions and the small\nparameters ${(D^e_{\\perp}/D^e_{\\|})}^{1/4}$ and ${\\epsilon}^{1/4}_{*}$, where\n${\\epsilon}_{*} {\\equiv}D_m/D_A$, $D_m$ is the magnetic diffusion coefficient,\n$D_A\\sim\\texttt{v}^{2}_{A}r_{Te}/(D_Bk_{\\perp})$,\n$r_{Te}\\equiv(-d\\texttt{ln}T_e/dr)^{-1}$, $k_{\\perp}$ is the transverse mode\nnumber, $\\texttt{v}^{2}_{A}=B^{2}/(4\\pi{nm}_{i})}$ and $D_B=cT_e/(eB)$.", "category": "physics_plasm-ph" }, { "text": "Currents from relativistic laser-plasma interaction as novel metrology\n for system stability of high-repetition-rate laser secondary sources: This work shows for the first time experimentally the close relation between\nreturn currents from relativistic laser-driven target polarization and the\nquality of the relativistic laser plasma interaction for laser driven secondary\nsources. Such currents rise in all interaction schemes where targets of any\nkind are charged by escaping laser-accelerated relativistic electrons.\nTherefore, return currents can be used as a metrological online tool in the\noptimization of many laser-driven secondary sources and for diagnosing their\nstability. We demonstrate the destruction free measurement of return currents\nat the example of a tape target system irradiated by the 1 PW VEGA3 laser at\nCLPU at its maximum capabilities for laser-driven ion acceleration. Such\nendeavour paves the ground for feedback systems that operate at the\nhigh-repetition-rate of PW-class laser systems.", "category": "physics_plasm-ph" }, { "text": "Ionization enhanced ion collection by a small floating grain in plasmas: It is demonstrated that the ionization events in the vicinity of a small\nfloating grain can increase the ion flux to its surface. In this respect the\neffect of electron impact ionization is fully analogous to that of the\nion-neutral resonant charge exchange collisions. Both processes create slow ion\nwhich cannot overcome grain' electrical attraction and eventually fall onto its\nsurface. The relative importance of ionization and ion-neutral collisions is\nroughly given by the ratio of the corresponding frequencies. We have evaluated\nthis ratio for neon and argon plasmas to demonstrate that ionization enhanced\nion collection can indeed be an important factor affecting grain charging in\nrealistic experimental conditions.", "category": "physics_plasm-ph" }, { "text": "From Liouville's theorem to quasilinear, nonlinear stochastic, and\n fractional transport, a multi-scale kinetics of plasma turbulence: From Liouville's equation, a phase-space multi-scale transport equation is\nsystematically derived. The proposed phase-space multi-scale transport equation\nbased on the first principle indicates that the nonlinear stochastic transport\nis due to the micro-turbulence scattering while the familiar quasilinear\ntransport is due to the long-range correlation of the meso-scale coherent\nmodes; and more interestingly, it demonstrates a systematic derivation of the\nfractional transport equation that may provide a new view-angle to understand\nthe anomalous transport observed in plasma turbulence. The multi-scale kinetics\nsuggests a new approach to confinement improvement through the formation of\nnonlinearly self-organized stable large-scale structures.", "category": "physics_plasm-ph" }, { "text": "Stellarator equilibrium axis-expansion to all orders in distance from\n the axis for arbitrary plasma beta: A systematic theory of the asymptotic expansion of the magnetohydrodynamic\n(MHD) equilibrium in the distance from the magnetic axis is developed to\ninclude arbitrary smooth currents near the magnetic axis. Compared to the\nvacuum and the force-free system, an additional magnetic differential equation\nmust be solved to obtain the pressure-driven currents. It is shown that there\nexist variables in which the rest of the MHD system closely mimics the vacuum\nsystem. Thus, a unified treatment of MHD fields is possible. The mathematical\nstructure of the near-axis expansions to arbitrary order is examined carefully\nto show that the double-periodicity of physical quantities in a toroidal domain\ncan be satisfied order by order. The essential role played by the normal form\nin solving the magnetic differential equations is highlighted. Several explicit\nexamples of vacuum, force-free, and MHD equilibrium in different geometries are\npresented.", "category": "physics_plasm-ph" }, { "text": "Parallel electric fields are inefficient drivers of energetic electrons\n in magnetic reconnection: We present two-dimensional kinetic simulations, with a broad range of initial\nguide fields, that isolate the role of parallel electric fields ($E_\\parallel$)\nin energetic electron production during collisionless magnetic reconnection. In\nthe strong guide field regime, $E_\\parallel$ drives essentially all of the\nelectron energy gain, yet fails to generate an energetic component. We suggest\nthat this is due to the weak energy scaling of particle acceleration from\n$E_\\parallel$ compared to that of a Fermi-type mechanism responsible for\nenergetic electron production in the weak guide-field regime. This result has\nimportant implications for energetic electron production in astrophysical\nsystems and reconnection-driven dissipation in turbulence.", "category": "physics_plasm-ph" }, { "text": "Magnetoplasmons in rotating dusty plasmas: A rotating dusty plasma apparatus was constructed to provide the possibility\nof experimental emulation of extremely high magnetic fields by means of the\nCoriolis force, observable in a co-rotating measurement frame. We present\ncollective excitation spectra for different rotation rates with a magnetic\ninduction equivalent of up to 3200 Tesla. We identify the onset of\nmagnetoplasmon-equivalent mode dispersion in the rotating macroscopic\ntwo-dimensional single-layer dusty plasma. The experimental results are\nsupported by molecular dynamics simulations of 2D magnetized Yukawa systems.", "category": "physics_plasm-ph" }, { "text": "Topological defect motifs in two-dimensional Coulomb clusters: The most energetically favourable arrangement of low-density electrons in an\ninfinite two-dimensional plane is the ordered triangular Wigner lattice.\nHowever, in most instances of contemporary interest one deals instead with\nfinite clusters of strongly interacting particles localized in potential traps,\nfor example, in complex plasmas. In the current contribution we study\ndistribution of topological defects in two-dimensional Coulomb clusters with\nparabolic lateral confinement. The minima hopping algorithm based on molecular\ndynamics is used to efficiently locate the ground- and low-energy metastable\nstates, and their structure is analyzed by means of the Delaunay triangulation.\nThe size, structure and distribution of geometry-induced lattice imperfections\nstrongly depends on the system size and the energetic state. Besides isolated\ndisclinations and dislocations, classification of defect motifs includes defect\ncompounds --- grain boundaries, rosette defects, vacancies and interstitial\nparticles. Proliferation of defects in metastable configurations destroys the\norientational order of the Wigner lattice.", "category": "physics_plasm-ph" }, { "text": "Acceleration of Particles in Imbalanced Magnetohydrodynamic Turbulence: The present work investigates the acceleration of test particles in balanced\nand imbalanced Alfv\\'{e}nic turbulence, relevant to the solar-wind problem.\nThese turbulent states, obtained numerically by prescribing the injection rates\nfor the ideal invariants, are evolved dynamically with the particles. While the\nenergy spectrum for balanced and imbalanced states is known, the impact made on\nparticle heating is a matter of debate, with different considerations giving\ndifferent results. By performing direct numerical simulations, resonant and\nnon-resonant particle accelerations are automatically considered and the\ncorrect turbulent phases are taken into account. For imbalanced turbulence, it\nis found that the acceleration rate of charged particles is reduced and the\nheating rate diminished. This behaviour is independent of the particle\ngyroradius, although particles that have a stronger adiabatic motion (smaller\ngyroradius) tend to experience a larger heating.", "category": "physics_plasm-ph" }, { "text": "Towards an integrated modeling of the plasma-solid interface: Solids facing a plasma are a common situation in many astrophysical systems\nand laboratory setups. Moreover, many plasma technology applications rely on\nthe control of the plasma-surface interaction. However, presently often a\nfundamental understanding of them is missing, so most technological\napplications are being developed via trial and error. In the majority of plasma\nsimulations surface processes are either neglected or treated via\nphenomenological parameters such as sticking coefficients, sputter rates or\nsecondary electron emission coefficients. However, those parameters are known\nonly in some cases and with very limited accuracy. Similarly, while surface\nphysics simulations have often studied the impact of single ions or neutrals,\nso far, the influence of a plasma medium and correlations between successive\nimpacts have not been taken into account. Such an approach cannot have\npredictive power. In this paper we discuss in some detail the physical\nprocesses a the plasma-solid interface which brings us to the necessity of\ncoupled plasma-solid simulations. We briefly summarize relevant theoretical\nmethods from solid state and surface physics that are suitable to contribute to\nsuch an approach and identify four methods. The first are mesoscopic\nsimulations such as kinetic Monte Carlo (KMC) and molecular dynamics (MD) that\nare able to treat complex processes on large scales but neglect electronic\neffects. The second are quantum kinetic methods based on the quantum Boltzmann\nequation that give access to a more accurate treatment of surface processes\nusing simplifying models for the solid. The third approach are ab initio\nsimulations of surface process that are based on density functional theory\n(DFT) and time-dependent DFT. The fourths are nonequilibrium Green functions\nthat able to treat correlation effects in the material and at the interface.", "category": "physics_plasm-ph" }, { "text": "Laser Acceleration toward PeV Feeling the Texture of Vacuum: Identified is a set of ballpark parameters for laser, plasma, and accelerator\ntechnologies that are defined for accelerated electron energies reaching as\nhigh as PeV. These parameters are carved out from the scaling laws that govern\nthe physics of laser acceleration, theoretically suggested and experimentally\nexplored over a wide range in the recent years. We extrapolate this knowledge\ntoward PeV energies. In the density regime on the order of 10^16 cm^-3, it is\npossible to consider the application of the existing NIF (or LMJ) or its\nextended lasers to their appropriate retrofitting for this purpose. Although\nthe ambition of luminosity is not pursued, such energies by themselves may\nallow us to begin to feel and study the physics of the 'texture of vacuum'.\nThis is an example of fundamental physics exploration without the need of\nluminosity paradigm. By converting accelerated electrons with extreme energies\nto like energy gamma photons, and let them propagate through vacuum over a\nsufficient distance, these extremely high energy (and therefore short\nwavelength) photons experience smallest vacuum structures and fluctuations. If\nwe can measure the arrival time differential and thus the gamma photon speed as\na function of different energies such as 0.1 PeV vs 1 PeV, say within\nattoseconds accuracy, we can collect valuable data if and how gamma photons\nstill obeys the premise of relativity or the vacuum texture begins to alter\nsuch fundamentals. The only method currently available to look at this problem\nmay be to study astrophysical data of the primordial gamma ray bursts (GRBs),\nwhich are compared with the presently suggested approach.", "category": "physics_plasm-ph" }, { "text": "Transverse instability magnetic field thresholds of electron phase-space\n holes: A detailed comparison is presented of analytical and particle-in-cell (PIC)\nsimulation investigation of the transverse instability, in two dimensions, of\ninitially one-dimensional electron phase-space hole equilibria. Good\nquantitative agreement is found between the shift-mode analysis and the\nsimulations for the magnetic field ($B$) threshold at which the instability\nbecomes overstable (time-oscillatory) and for the real and imaginary parts of\nthe frequency. The simulation $B$-threshold for full stabilization exceeds the\npredictions of shift-mode analysis by 20 to 30\\%, because the mode becomes\nsubstantially narrower in spatial extent than a pure shift. This threshold\nshift is qualitatively explained by the kinematic mechanism of instability.", "category": "physics_plasm-ph" }, { "text": "Nonlinear evolution of the ablative Rayleigh-Taylor instability with\n preheating: Evolution of the single-mode ablative Rayleigh-Taylor instability in the\npresence of preheating is investigated numerically. It is found that the\ninstability evolution depends strongly on the preheating length $L$ in front of\nthe ablation-front interface. For weak preheating ($L/L_{\\rm SH}\\leq f_0$,\nwhere $L_{\\rm SH}$ is the Spitzer-Harm length and $f_0\\sim 20$ is a critical\nvalue), the linear growth rate has a sharp maximum and a secondary spike-bubble\nstructure is generated. Interaction of this growing structure with the master\nspike-bubble leads to rupture of the latter. However, for strong preheating\n($L/L_{\\rm SH}>20$), the linear growth rate has no sharp maximum and no\nsecondary spike-bubble is generated. Instead, the master spike-bubble evolves\ninto an elongated jet. These results are interpreted in terms of the evolution\nand interaction of the instability-generated harmonic modes.", "category": "physics_plasm-ph" }, { "text": "The contribution of magnetic monopoles to ponderomotive forces in\n plasmas: It is well--known that when magnetic monopoles are introduced in plasma\nequations the propagation of electromagnetic waves is modified. This occurs\nbecause of Maxwell equations acquire a symmetrical structure due to the\nexistence of electric and magnetic charge current densities. In this work we\nstudy the nonlinear phenomena of ponderomotive forces associated to the\npresence of magnetic monopoles in a plasma. We generalize the Washimi-Karpman\nresult for the ponderomotive force on electric charges to take into account the\nsymmetrical form of Maxwell equations in the presence of magnetic charges. It\nis shown that the general ponderomotive force on this plasma depends\nnon--trivially on the magnetic monopoles, through the slowly temporal and\nspatial variations of the electromagnetic field amplitudes. The magnetic\ncharges introduce corrections even if the plasma is unmagnetized. This last\nforce induces a magnetization of the plasma, and therefore a cyclotron\nfrequency (proportional to the square of the magnetic charge) is induced for\nelectrons. Finally, the magnetic monopoles also experience a ponderomotive\nforce due to the electrons. This force accelerates the magnetic charges along\nthe direction of propagation of the electromagnetic waves. The possible\nconsequences for experimental detection are discussed.", "category": "physics_plasm-ph" }, { "text": "Electrostatic wave interaction via asymmetric vector solitons as\n precursor to rogue wave formation in non-Maxwellian plasmas: An asymmetric pair of coupled nonlinear Schr{\\\"o}dinger (CNLS) equations has\nbeen derived through a multiscale perturbation method applied to a plasma fluid\nmodel, in which two wavepackets of distinct carrier wavenumbers and amplitudes\nare allowed to co-propagate and interact. The original fluid model was set up\nfor a non-magnetized plasma consisting of cold inertial ions evolving against a\n$\\kappa-$distributed electron background in 1D. The reduction procedure\nresulting in the CNLS equations has provided analytical expressions for the\ndispersion, self-modulation and cross-coupling coefficients in terms of the\ncarrier wavenumbers.\n The system admits various types of vector solitons (VSs), physically\nrepresenting nonlinear localized electrostatic plasma modes. The possibility\nfor either bright (B) or dark (D) type excitations for either of the two waves\nprovides four combinations for the envelope pair (BB, BD, DB, DD). Moreover,\nthe soliton parameters are also calculated for each type of VS in its\nrespective area of existence. The dependence of the VS characteristics on the\ncarrier wavenumbers and the spectral index $\\kappa$ has been explored. In\ncertain cases, the amplitude of one component may exceed its counterpart\n(second amplitude) by a factor 2.5 or higher, indicating that extremely\nasymmetric waves may be formed due to modulational interactions among the\nwavepackets.\n As $\\kappa$ decreases from large values, modulational instability (MI) occurs\nin larger areas of the parameter plane(s) and with higher growth rates. The\ndistribution of different types of VSs on the parameter plane(s) also varies\nsignificantly with decreasing $\\kappa$, and in fact dramatically for $\\kappa$\nbetween $3$ and $2$. Deviation from the Maxwell-Boltzmann picture therefore\nseems to favor MI as a precursor to the formation of bright (predominantly)\ntype envelope excitations and freak waves.", "category": "physics_plasm-ph" }, { "text": "Simulation of plasma water interaction with discharge in the existing\n bubble in water: The plasma-liquid interaction is an important issue in plasma technology. The\nsimulation of discharge in spherical bubbles in the water that produced\nplasma-activated water (PAW) is investigated using finite element methods (FEM)\nfor a simulated 2D dielectric barrier discharge in three different geometries.\nThe electron density changes with voltage, frequency, dielectric thickness, and\nbubble radius are investigated in different time duration. The results show\nthat electron density increases linearly by increasing voltage, frequency and\nbubble radius, while it is vice versa for dielectric thickness. High plasma\ndensity indicates sufficient plasma-water interaction.", "category": "physics_plasm-ph" }, { "text": "Effects of high intensity Lasers on the Entanglement fidelity of quantum\n plasmas: The dynamics of entanglement during the low energy scattering processes in\nbipartite systems at the presence of a laser field is studied, using the\nKramers-Henneberger unitary transformation as the semi classical counterpart of\nthe Block-Nordsieck transformation, in the quantizied field formalism. The\nStationary-state Schrodinger equation for quantum scattering process is\nobtained for such systems. Then, by using partial wave analysis, we introduce\nnew form of entanglement fidelity containing high intense laser field.\nTherefore, the effective potential of hot quantum plasmas including plasmon and\nquantum screening effect is used to show entanglement fidelity ratio as a\nfunction of the laser amplitude, plasmon parameter and the Debye length\nparameter for elastic electron-ion collisions. It is shown that the amplitude\nof laser beam or free electron oscillation play important roles in the\nevolution of entanglement of the system.", "category": "physics_plasm-ph" }, { "text": "Effect of electron-beam energy chirp on signatures of radiation reaction\n in laser-based experiments: Current experiments investigating radiation reaction employ high energy\nelectron beams together with tightly focused laser pulses in order to reach the\nquantum regime, as expressed through the quantum nonlinearity parameter $\\chi$.\nSuch experiments are often complicated by the large number of latent variables,\nincluding the precise structure of the electron bunch. Here we examine a\ncorrelation between the electron spatial and energy distributions, called an\nenergy chirp, investigate its significance to the laser-electron beam\ninteraction and show that the resulting effect cannot be trivially ignored when\nanalysing current experiments. In particular, we show that the energy chirp has\na large effect on the second moment of the electron energy, but a lesser impact\non the first electron energy moment or the photon critical energy. These\nresults show the importance of improved characterisation and control over\nelectron bunch parameters on a shot-to-shot basis in such experiments.", "category": "physics_plasm-ph" }, { "text": "First-Principles Calculation of Principal Hugoniot and K-Shell X-ray\n Absorption Spectra for Warm Dense KCl: Principal Hugoniot and K-shell X-ray absorption spectra of warm dense KCl are\ncalculated using the first-principles molecular dynamics method. Evolution of\nelectronic structures as well as the influence of the approximate description\nof ionization on pressure (caused by the underestimation of the energy gap\nbetween conduction bands and valence bands) in the first-principles method are\nillustrated by the calculation. Pressure ionization and thermal smearing are\nshown as the major factors to prevent the deviation of pressure from global\naccumulation along the Hugoniot. In addition, cancellation between electronic\nkinetic pressure and virial pressure further reduces the deviation. The\ncalculation of X-ray absorption spectra shows that the band gap of KCl persists\nafter the pressure ionization of the $3p$ electrons of Cl and K taking place at\nlower energy, which provides a detailed understanding to the evolution of\nelectronic structures of warm dense matter.", "category": "physics_plasm-ph" }, { "text": "Volume Ignition via Time-like Detonation in Pellet Fusion: Relativistic fluid dynamics and the theory of relativistic detonation fronts\nare used to estimate the space-time dynamics of the burning of the D-T fuel in\nLaser driven pellet fusion experiments. The initial \"High foot\" heating of the\nfuel makes the compressed target transparent to radiation, and then a rapid\nignition pulse can penetrate and heat up the whole target to supercritical\ntemperatures in a short time, so that most of the interior of the target\nignites almost simultaneously and instabilities will have no time to develop.\nIn these relativistic, radiation dominated processes both the interior,\ntime-like burning front and the surrounding space-like part of the front will\nbe stable against Rayleigh-Taylor instabilities. To achieve this rapid, volume\nignition the pulse heating up the target to supercritical temperature should\nprovide the required energy in less than ~ 10 ps.", "category": "physics_plasm-ph" }, { "text": "En-route to the fission-fusion reaction mechanism: a status update on\n laser-driven heavy ion acceleration: The fission-fusion reaction mechanism was proposed in order to generate\nextremely neutron-rich nuclei close to the waiting point N = 126 of the rapid\nneutron capture nucleosynthesis process (r-process). The production of such\nisotopes and the measurement of their nuclear properties would fundamentally\nhelp to increase the understanding of the nucleosynthesis of the heaviest\nelements in the universe. Major prerequisite for the realization of this new\nreaction scheme is the development of laser-based acceleration of ultra-dense\nheavy ion bunches in the mass range of A = 200 and above. In this paper, we\nreview the status of laser-driven heavy ion acceleration in the light of the\nfission-fusion reaction mechanism. We present results from our latest\nexperiment on heavy ion acceleration, including a new milestone with\nlaser-accelerated heavy ion energies exceeding 5 MeV/u.", "category": "physics_plasm-ph" }, { "text": "eduPIC: an introductory particle based code for radio-frequency plasma\n simulation: For the self-consistent description of various plasma sources operated in the\nlow-pressure (nonlocal, kinetic) regime, the Particle-In-Cell simulation\napproach, combined with the Monte Carlo treatment of collision processes\n(PIC/MCC), has become an important tool during the past decades. PIC/MCC\nsimulation codes have been developed and maintained by many research groups,\nsome of these codes are available to the community as freeware resources. While\nthis computational approach has already been present for a number of decades,\nthe rapid evolution of the computing infrastructure makes it increasingly more\npopular and accessible, as simulations of simple systems can be executed now on\npersonal computers or laptops. During the past few years we have experienced an\nincreasing interest in lectures and courses dealing with the basics of particle\nsimulations, including the PIC/MCC technique. In a response to this, this paper\n(i) provides a tutorial on the physical basis and the algorithms of the PIC/MCC\ntechnique and (ii) presents a basic (spatially one-dimensional) electrostatic\nPIC/MCC simulation code for Capacitively Coupled Plasmas, whose source is made\nfreely available in various programming languages. We share the code in C/C++\nversions, as well as in a version written in Rust, which is a rapidly emerging\ncomputational language. Our code intends to be a \"starting tool\" for those who\nare interested in learning the details of the PIC/MCC technique and would like\nto develop the \"skeleton\" code further, for their research purposes.", "category": "physics_plasm-ph" }, { "text": "Linearized model Fokker-Planck collision operators for gyrokinetic\n simulations. I. Theory: A new analytically and numerically manageable model collision operator is\ndeveloped specifically for turbulence simulations. The like-particle collision\noperator includes both pitch-angle scattering and energy diffusion and\nsatisfies the physical constraints required for collision operators: it\nconserves particles, momentum and energy, obeys Boltzmann's H-theorem\n(collisions cannot decrease entropy), vanishes on a Maxwellian, and efficiently\ndissipates small-scale structure in the velocity space. The process of\ntransforming this collision operator into the gyroaveraged form for use in\ngyrokinetic simulations is detailed. The gyroaveraged model operator is shown\nto have more suitable behavior at small scales in phase space than previously\nsuggested models. A model operator for electron-ion collisions is also\npresented.", "category": "physics_plasm-ph" }, { "text": "Structure of pressure-gradient-driven current singularity in ideal\n magnetohydrodynamic equilibrium: Singular currents typically appear on rational surfaces in non-axisymmetric\nideal magnetohydrodynamic equilibria with a continuum of nested flux surfaces\nand a continuous rotational transform. These currents have two components: a\nsurface current (Dirac $\\delta$-function in flux surface labeling) that\nprevents the formation of magnetic islands and an algebraically divergent\nPfirsch--Schl\\\"uter current density when a pressure gradient is present across\nthe rational surface. At flux surfaces adjacent to the rational surface, the\ntraditional treatment gives the Pfirsch--Schl\\\"uter current density scaling as\n$J\\sim1/\\Delta\\iota$, where $\\Delta\\iota$ is the difference of the rotational\ntransform relative to the rational surface. If the distance $s$ between flux\nsurfaces is proportional to $\\Delta\\iota$, the scaling relation\n$J\\sim1/\\Delta\\iota\\sim1/s$ will lead to a paradox that the Pfirsch--Schl\\\"uter\ncurrent is not integrable. In this work, we investigate this issue by\nconsidering the pressure-gradient-driven singular current in the\nHahm\\textendash Kulsrud\\textendash Taylor problem, which is a prototype for\nsingular currents arising from resonant magnetic perturbations. We show that\nnot only the Pfirsch--Schl\\\"uter current density but also the diamagnetic\ncurrent density are divergent as $\\sim1/\\Delta\\iota$. However, due to the\nformation of a Dirac $\\delta$-function current sheet at the rational surface,\nthe neighboring flux surfaces are strongly packed with\n$s\\sim(\\Delta\\iota)^{2}$. Consequently, the singular current density\n$J\\sim1/\\sqrt{s}$, making the total current finite, thus resolving the paradox.", "category": "physics_plasm-ph" }, { "text": "Temporal evolution of drift-type modes in plasma flows with strong\n time-dependent velocity shear: The linear and renormalized nonlinear analysis of the temporal evolution of\ndrift-type modes in plasma flows with strong time-varying velocity shear is\ndeveloped. Analysis is performed in the time domain without spectral\ndecomposition in time and admits time variation of the flow velocity with time\nscales which may be comparable with turbulent time scales. 52.35.Ra", "category": "physics_plasm-ph" }, { "text": "Particle and guiding-center orbits in crossed electric and magnetic\n fields: The problem of the charged-particle motion in crossed electric and magnetic\nfields is investigated, and the validity of the guiding-center representation\nis assessed in comparison with the exact particle dynamics. While the magnetic\nfield is considered to be straight and uniform, the (perpendicular) radial\nelectric field is nonuniform. The Hamiltonian guiding-center theory of\ncharged-particle motion is presented for arbitrary radial electric fields, and\nexplicit examples are provided for the case of a linear radial electric field.", "category": "physics_plasm-ph" }, { "text": "Effect of Scrape-Off-Layer Current on Reconstructed Tokamak Equilibrium: Methods are described that extend fields from reconstructed equilibria to\ninclude scrape-off-layer current through extrapolated parametrized and\nexperimental fits. The extrapolation includes both the effects of the\ntoroidal-field and pressure gradients which produce scrape-off-layer current\nafter recomputation of the Grad-Shafranov solution. To quantify the degree that\ninclusion of scrape-off-layer current modifies the equilibrium, the\n$\\chi$-squared goodness-of-fit parameter is calculated for cases with and\nwithout scrape-off-layer current. The change in $\\chi$-squared is found to be\nminor when scrape-off-layer current is included however flux surfaces are\nshifted by up to 3 cm. The impact on edge modes of these scrape-off-layer\nmodifications is also found to be small and the importance of these methods to\nnonlinear computation is discussed.", "category": "physics_plasm-ph" }, { "text": "An adjoint method for neoclassical stellarator optimization: Stellarators are a promising route to steady-state fusion power. However, to\nachieve the required confinement, the magnetic geometry must be highly\noptimized. This optimization requires navigating high-dimensional spaces, often\nnecessitating the use of gradient-based methods. The gradient of the\nneoclassical fluxes is expensive to compute with classical methods, requiring\n$O(N)$ flux computations, where $N$ is the number of parameters. To reduce the\ncost of the gradient computation, we present an adjoint method for computing\nthe derivatives of moments of the neoclassical distribution function for\nstellarator optimization. The linear adjoint method allows derivatives of\nquantities which depend on solutions of a linear system, such as moments of the\ndistribution function, to be computed with respect to many parameters from the\nsolution of only two linear systems. This reduces the cost of computing the\ngradient to the point that the finite-collisionality neoclassical fluxes can be\nused within an optimization loop.\n With the neoclassical adjoint method, we compute solutions of the drift\nkinetic equation and an adjoint drift kinetic equation to obtain derivatives of\nneoclassical quantities with respect to geometric parameters. When the number\nof parameters in the derivative is large ($\\mathcal{O}(10^2)$), this adjoint\nmethod provides up to a factor of 200 reduction in cost. We demonstrate\nadjoint-based optimization of the field strength to obtain minimal bootstrap\ncurrent on a surface. With adjoint-based derivatives, we also compute the local\nsensitivity to magnetic perturbations on a flux surface and identify regions\nwhere tight tolerances on error fields are required for control of the\nbootstrap current or radial transport. Furthermore, the solve for the ambipolar\nelectric field is accelerated using a Newton method with derivatives obtained\nfrom the adjoint method.", "category": "physics_plasm-ph" }, { "text": "Spatial distribution of Dust Density Wave Properties in Fluid Complex\n Plasmas: Complex plasmas consist of microparticles embedded in a low-temperature\nplasma and allow investigating various effects by tracing the motion of these\nmicroparticles. Dust density waves appear in complex plasmas as self-excited\nacoustic waves in the microparticle fluid at low neutral gas pressures. Here we\nshow that various properties of these waves depend on the position of the\nmicroparticle cloud with respect to the plasma sheath and explain this finding\nin terms of the underlying ion-drift instability. These results may be helpful\nin better understanding the propagation of dust density waves in complex\nplasmas and beyond, for instance, in astrophysical dusty plasmas.", "category": "physics_plasm-ph" }, { "text": "Interplay between the Weibel instability and the Biermann battery in\n realistic laser-solid interactions: A novel setup allows the Weibel instability and its interplay with the\nBiermann battery to be probed in laser-driven collisionless plasmas. Ab initio\nparticle-in-cell (PIC) simulations of the interaction of short ($ \\le ps$)\nintense $(a_0 \\ge 1)$ laser-pulses with overdense plasma targets show\nobservable Weibel generated magnetic fields. This field strength surpasses that\nof the Biermann battery, usually dominant in experiments, as long as the\ngradient scale length is much larger than the local electron inertial length;\nthis is achievable by carefully setting the appropriate gradients in the front\nof the target e.g. by tuning the delay between the main laser pulse and the\npre-pulse.", "category": "physics_plasm-ph" }, { "text": "Topological Implications of the Total Generalized Electron-Flow Magnetic\n Helicity Invariant in Electron Magnetohydrodynamics: Topological implications of the total generalized electron-flow magnetic\nhelicity He in electron magnetohydrodynamics(EMHD) are explored. The invariance\nof He is shown to imply the invariance of the sum of the linkage of the\nmagnetic field lines, the linkage of electron-flow vorticity field lines and\nthe mutual linkage among these two sets of field lines. This result appears to\nsupport a change in the magnetic field topology and hence pave the way for\nmagnetic reconnection in EMHD via a change in the concomitant electron-flow\nvorticity topology.", "category": "physics_plasm-ph" }, { "text": "A study of magnetically-supported dc discharge in cylindrical and\n inverted cyclindrical configuration: We have investigated apparently stochastic fluctuations of\nmagnetically-supported dc discharge in cylindrical coaxial configuration. In\nthe system the electric field had radial direction while the magnetic field was\napplied axially. The discharge vessel length was 12 centimetres. Working gas\nwas typically argon at pressure of several Pa, magnetic field 10-50 mT. The\ncontribution describes experimental results - frequency and phase analysis of\nthe instabilities, which we detected in our experimental system in both the\nconventional and inverted magnetron configurations. We bring also 2-D PIC model\nof the dc discharge under conditions, which can be achieved in the experimental\napparatus.", "category": "physics_plasm-ph" }, { "text": "Activation of MHD reconnection on ideal timescales: Magnetic reconnection in laboratory, space and astrophysical plasmas is often\ninvoked to explain explosive energy release and particle acceleration. However,\nthe timescales involved in classical models within the macroscopic MHD regime\nare far too slow to match the observations. Here we revisit the tearing\ninstability by performing visco-resistive two-dimensional numerical simulations\nof the evolution of thin current sheets, for a variety of initial\nconfigurations and of values of the Lunquist number $S$, up to $10^7$. Results\nconfirm that when the critical aspect ratio of $S^{1/3}$ is reached in the\nreconnecting current sheets, the instability proceeds on ideal (Alfv\\'enic)\nmacroscopic timescales, as required to explain observations. Moreover, the same\nscaling is seen to apply also to the local, secondary reconnection events\ntriggered during the nonlinear phase of the tearing instability, thus\naccelerating the cascading process to increasingly smaller spatial and temporal\nscales. The process appears to be robust, as the predicted scaling is measured\nboth in inviscid simulations and when using a Prandtl number $P=1$ in the\nviscous regime.", "category": "physics_plasm-ph" }, { "text": "Fast magneto-acoustic wave turbulence and the Iroshnikov-Kraichnan\n spectrum: An analytical theory of wave turbulence is developed for pure compressible\nmagnetohydrodynamics in the small $\\beta$ limit. In contrast to previous works\nwhere the multiple scale method was not mentioned and slow magneto-acoustic\nwaves were included, I present here a theory for fast magneto-acoustic waves\nonly for which an asymptotic closure is possible in three dimensions. I\nintroduce the compressible Elsasser fields (canonical variables) and show their\nlinear relationship with the mass density and the compressible velocity. The\nkinetic equations of wave turbulence for three-wave interactions are obtained\nand the detailed conservation is shown for the two invariants, energy and\nmomentum (cross-helicity). An exact stationary solution (Kolmogorov-Zakharov\nspectrum) exists only for the energy. I find a $k^{-3/2}$ energy spectrum\ncompatible with the Iroshnikov-Kraichnan (IK) phenomenological prediction; this\nleads to a mass density spectrum with the same scaling. Despite the presence of\na relatively strong uniform magnetic field, this turbulence is characterized by\nan energy spectrum with a power index that is independent of the angular\ndirection; its amplitude, however, shows an angular dependence. I prove the\nexistence of the IK solution using the locality condition, show that the energy\nflux is positive and hence the cascade direct, and find the Kolmogorov\nconstant. This theory offers a plausible explanation for recent observations in\nthe solar wind at small $\\beta$ where isotropic spectra with a $-3/2$ power law\nindex are found and associated with fast magneto-acoustic waves. This theory\nmay also be used to explain the IK spectrum often observed near the Sun.\nBesides, it provides a rigorous theoretical basis for the well-known\nphenomenological IK spectrum, which coincides with the Zakharov-Sagdeev\nspectrum for acoustic wave turbulence.", "category": "physics_plasm-ph" }, { "text": "Amplification of coupled nonlinear oscillations of charged particle beam\n in crossed magnetic fields: A non-relativistic, charged-particle beam is placed into a crossed magnetic\nfield. For such a system, the nonlinear electrostatic oscillations generation\nin the different degrees of the beam freedom may be triggered by the\nenergy/momentum exchange between the beam's particles and these external\nfields. The influence of oscillation dynamics of these fields and beam have\nbeen studied based on the cold-fluid hydrodynamic description. As a result, the\nnecessary conditions under resonant amplification of the beam's natural\noscillations are identified. Present results demonstrate that the beam density\nincreases when the amplitude of radial and axial velocities increase. This\nprocess decreases the radius of the beam over the course of time. The technical\napplication of the process applies in real accelerators such as a gyrotrons,\nFELs, and cyclotrons, where transverse size is limited by the size of the\nvacuum chamber. Thus redistribution of energy between the external field and\nthe kinetic energy of the beam can effectively accelerate the beam by using an\nexternal magnetic field. These fields with both axial and radial directions use\nfurther this beam as an effective light source by identifying the resonance\nfrequency to improve stability, focus particles, and wave propagation.", "category": "physics_plasm-ph" }, { "text": "Multiphysics simulations of collisionless plasmas: Collisionless plasmas, mostly present in astrophysical and space\nenvironments, often require a kinetic treatment as given by the Vlasov\nequation. Unfortunately, the six-dimensional Vlasov equation can only be solved\non very small parts of the considered spatial domain. However, in some cases,\ne.g. magnetic reconnection, it is sufficient to solve the Vlasov equation in a\nlocalized domain and solve the remaining domain by appropriate fluid models. In\nthis paper, we describe a hierarchical treatment of collisionless plasmas in\nthe following way. On the finest level of description, the Vlasov equation is\nsolved both for ions and electrons. The next courser description treats\nelectrons with a 10-moment fluid model incorporating a simplified treatment of\nLandau damping. At the boundary between the electron kinetic and fluid region,\nthe central question is how the fluid moments influence the electron\ndistribution function. On the next coarser level of description the ions are\ntreated by an 10-moment fluid model as well. It may turn out that in some\nspatial regions far away from the reconnection zone the temperature tensor in\nthe 10-moment description is nearly isotopic. In this case it is even possible\nto switch to a 5-moment description. This change can be done separately for\nions and electrons. To test this multiphysics approach, we apply this full\nphysics-adaptive simulations to the Geospace Environmental Modeling (GEM)\nchallenge of magnetic reconnection.", "category": "physics_plasm-ph" }, { "text": "Research on pinches driven by SPPED 2 generator : hard X-ray and neutron\n emission in plasma focus configuration: SPEED2 is a generator based on Marx technology and was designed in the\nUniversity of Dusseldorf. SPEED2 consists on 40 +/- Marx modules connected in\nparallel (4.1 mF equivalent Marx generator capacity, 300 kV, 4 MA in short\ncircuit, 187 kJ, 400 ns rise time, dI/dt~1013 A/s). Currently the SPEED2 is\noperating at the Comision Chilena de Energia Nuclear, CCHEN, Chile, being the\nmost powerful and energetic device for dense transient plasma in the Southern\nHemisphere. Most of the previous works developed in SPEED2 at Dusseldorf were\ndone in a plasma focus configuration for soft X-ray emission and the neutron\nemission from SPEED2 was not completely studied. The research program at CCHEN\nconsiders experiments in different pinch configurations (plasma focus, gas\npuffed plasma focus, gas embedded Z-pinch, wire arrays) at current of hundred\nof kiloamperes to mega-amperes, using the SPEED2 generator. The Chilean\noperation has begun implementing and developing diagnostics in a conventional\nplasma focus configuration operating in deuterium in order to characterize the\nneutron emission and the hard X-ray production. Silver activation counters,\nplastics CR39 and scintillator-photomultiplier detectors are used to\ncharacterize the neutron emission. Images of metallic plates with different\nthickness are obtained on commercial radiographic film, Agfa Curix ST-G2, in\norder to characterize an effective energy of the hard X-ray outside of the\ndischarge .", "category": "physics_plasm-ph" }, { "text": "Hybrid simulation of energetic particles interacting with\n magnetohydrodynamics using a slow manifold algorithm and GPU acceleration: The hybrid method combining particle-in-cell and magnetohydrodynamics can be\nused to study the interaction between energetic particles and global plasma\nmodes. In this paper we introduce the M3D-C1-K code, which is developed based\non the M3D-C1 finite element code solving the magnetohydrodynamics equations,\nwith a newly developed kinetic module simulating energetic particles. The\nparticle pushing is done using a new algorithm by applying the Boris pusher to\nthe classical Pauli particles to simulate the slow-manifold of particle orbits,\nwith long-term accuracy and fidelity. The particle pushing can be accelerated\nusing GPUs with a significant speedup. The moments of the particles are\ncalculated using the $\\delta f$ method, and are coupled into the\nmagnetohydrodynamics simulation through pressure or current coupling schemes.\nSeveral linear simulations of magnetohydrodynamics modes driven by energetic\nparticles have been conducted using M3D-C1-K, including fishbone, toroidal\nAlfv\\'en eigenmodes and reversed shear Alfv\\'en eigenmodes. Good agreement with\nprevious results from other eigenvalue, kinetic and hybrid codes has been\nachieved.", "category": "physics_plasm-ph" }, { "text": "Practical dispersion relations for strongly coupled plasma fluids: Very simple explicit analytical expressions are discussed, which are able to\ndescribe the dispersion relations of longitudinal waves in strongly coupled\nplasma systems such as one-component plasma and weakly screened Yukawa fluids\nwith a very good accuracy. Applications to other systems with soft pairwise\ninteractions are briefly discussed.", "category": "physics_plasm-ph" }, { "text": "Quasi-classical Molecular Dynamics Simulations of the Electron Gas:\n Dynamic properties: Results of quasi-classical molecular dynamics simulations of the quantum\nelectron gas are reported. Quantum effects corresponding to the Pauli and the\nHeisenberg principle are modeled by an effective momentum-dependent\nHamiltonian. The velocity autocorrelation functions and the dynamic structure\nfactors have been computed. A comparison with theoretical predictions was\nperformed.", "category": "physics_plasm-ph" }, { "text": "Interparticle-Fields Amplified Radiation Reaction: In classical electrodynamics, energy losses due to the emission of\nelectromagnetic radiation can be accounted for by solving the Landau-Lifshitz\nequation of motion. Analytically, this equation is typically solved while\ntreating each particle independently in an external field; numerically, one\noften includes a self-consistent mean field, as seen with particle-in-cell\n(PIC) codes. In both cases, interparticle fields from point-like particles are\nneglected. By considering the collision of a neutral relativistic\nelectron-positron bunch with an intense laser pulse, we demonstrate that the\ninclusion of interparticle fields can coherently amplify a broad range of\nradiated frequencies by orders of magnitude. This corresponds to an amplified\nenergy loss by particles within the bunch, with interparticle fields that feed\ninto the radiation reaction force.", "category": "physics_plasm-ph" }, { "text": "Cold plasma waves in the chiral Maxwell-Carroll-Field-Jackiw\n electrodynamics: In this work, we study the propagation and absorption of plasma waves in the\nchiral Maxwell-Carroll-Field-Jackiw (MCJF) electrodynamics. The Maxwell\nequations are rewritten for a cold, uniform, and collisionless fluid plasma\nmodel, allowing us to determine the new refractive indices and propagating\nmodes. The cases of propagation parallel and orthogonal to the magnetic field\nare examined considering a purely timelike CFJ background that plays the role\nof the magnetic conductivity chiral parameter. The collective electromagnetic\nmodes are associated with four distinct refractive indices associated with\nright-circularly polarized and left-circularly polarized waves. For each index,\nthe propagation and absorption zones are illustrated for some specific\nparameter values. In low-frequency regime, we have obtained modified helicons\nwith right- and left-circularly polarizations. The optical behavior is\ninvestigated by means of the rotatory power (RP) and dichroism coefficient. The\nexistence of a negative refraction zone enhances the rotatory power. It is also\nobserved RP sign reversal, a feature of rotating plasmas.", "category": "physics_plasm-ph" }, { "text": "Particle dynamics governed by radiation losses in extreme-field current\n sheets: Particles moving in current sheets under extreme conditions, such as those in\nthe vicinity of pulsars or those predicted on upcoming multipetawatt laser\nfacilities, may be subject to significant radiation losses. We present an\nanalysis of particle motion in fields of a relativistic neutral\nelectron-positron current sheet in the case when radiative effects must be\naccounted for. In the Landau-Lifshitz radiation reaction force model, when\nquantum effects are negligible, an analytical solution for particle\ntrajectories is derived. Based on this solution, for the case when quantum\neffects are significant an averaged quantum solution in the semiclassical\napproach is obtained. The applicability region of the solutions is determined\nand analytical trajectories are found to be in good agreement with those of\nnumerical simulations with account for radiative effects. Based on these\nresults we gain new insights into current sheet phenomena expected on upcoming\nlaser facilities.", "category": "physics_plasm-ph" }, { "text": "Local Gyrokinetic Collisional Theory of the Ion-Temperature Gradient\n Mode: We present a study of the linear properties of ion temperature gradient (ITG)\nmodes with collisions modelled by the linearized gyrokinetic (GK) Coulomb\ncollision operator (Frei et al. 2021) in the local limit. The study is based on\na Hermite-Laguerre polynomial expansion of the perturbed ion distribution\nfunction applied to the linearized GK Boltzmann equation, yielding a hierarchy\nof coupled equations for the expansion coefficients, referred to as\ngyro-moments. We explore analytically the collisionless and high-collisional\nlimits of the gyro-moment hierarchy. Parameter scans revealing the dependence\nof the ITG growth rate on the collisionality are reported, showing strong\ndamping at small scales as the collisionality increases. These properties are\ncompared with the predictions based on the Sugama, the momentum-conserving\npitch-angle scattering, the Hirshman- Sigmar-Clarke, and the Daugherty\ncollision operators. The importance of finite Larmor radius (FLR) terms in the\ncollision operators is pointed out by the appearance of a short wavelength (SW)\nITG branch when collisional FLR terms are neglected, this branch being\ncompletely suppressed by collisional FLR effects. We demonstrate that energy\ndiffusion is important at high collisionality and small scale lengths and that,\namong the collision operators considered in this work, the GK Sugama collision\noperator yields, in general, the smallest deviation on the ITG growth rate\ncompared to the GK Coulomb collision operator. Convergence studies of the\ngyro-moment method are reported.", "category": "physics_plasm-ph" }, { "text": "Creation of a homogeneous plasma column by means of hohlraum radiation\n for ion-stopping measurements: In this work, we present the results of two-dimensional\nradiation-hydrodynamics simulations of a hohlraum target whose outgoing\nradiation is used to produce a homogeneously ionized carbon plasma for ion-beam\nstopping measurements. The cylindrical hohlraum with gold walls is heated by a\nfrequency-doubled ($\\lambda_l = 526.5$ $\\mu m$) $1.4$ $ns$ long laser pulse\nwith the total energy of $E_l = 180$ $J$. At the laser spot, the peak matter\nand radiation temperatures of, respectively, $T \\approx 380$ $eV$ and $T_r\n\\approx 120$ $eV$ are observed. X-rays from the hohlraum heat the attached\ncarbon foam with a mean density of $\\rho_C = 2$ $mg/cm^3$ to a temperature of\n$T \\approx 25$ $eV$. The simulation shows that the carbon ionization degree ($Z\n\\approx 3.75$) and its column density stay relatively stable (within variations\nof about $\\pm7\\%$) long enough to conduct the ion-stopping measurements. Also,\nit is found that a special attention should be paid to the shock wave, emerging\nfrom the X-ray heated copper support plate, which at later times may\nsignificantly distort the carbon column density traversed by the fast ions.", "category": "physics_plasm-ph" }, { "text": "Lagrangian and Hamiltonian constraints for guiding-center Hamiltonian\n theories: A consistent guiding-center Hamiltonian theory is derived by Lie-transform\nperturbation method, with terms up to second order in magnetic-field\nnonuniformity. Consistency is demonstrated by showing that the guiding-center\ntransformation presented here satisfies separate Jacobian and Lagrangian\nconstraints that have not been explored before. A new first-order term\nappearing in the guiding-center phase-space Lagrangian is identified through a\ncalculation of the guiding-center polarization. It is shown that this new\npolarization term also yields a simpler expression of the guiding-center\ntoroidal canonical momentum, which satisfies an exact conservation law in\naxisymmetric magnetic geometries. Lastly, an application of the guiding-center\nLagrangian constraint on the guiding-center Hamiltonian yields a natural\ninterpretation for its higher-order corrections.", "category": "physics_plasm-ph" }, { "text": "Ultrafast Electron Cooling in an Expanding Ultracold Plasma: Plasma dynamics critically depends on density and temperature, thus\nwell-controlled experimental realizations are essential benchmarks for\ntheoretical models. The formation of an ultracold plasma can be triggered by\nionizing a tunable number of atoms in a micrometer-sized volume of a\nBose-Einstein condensate (BEC) by a single femtosecond laser pulse. The large\ndensity combined with the extremely low temperature of the BEC give rise to an\ninitially strongly coupled plasma in a so far unexplored regime bridging\nultracold neutral plasma and ionized nanoclusters. Here, we report on ultrafast\ncooling of electrons, trapped on orbital trajectories in the long-range Coulomb\npotential of the dense ionic core, with a cooling rate of 400 K/ps.\nFurthermore, our experimental setup grants direct access to the electron\ntemperature that relaxes from 5250 K to below 10 K in less than 500 ns.", "category": "physics_plasm-ph" }, { "text": "Remarks on the derivation and evaluation of the Stacey-Sigmar model for\n tokamak equilibrium: The Stacey-Sigmar model for tokamak equilibrium as presented in the\nliterature relies heavily on the neoclassical theory of the electrostatic\nfield. Its neglect of Gauss's law is inconsistent with the potential\nformulation of electrodynamics. Its treatment of the dynamic electric field\ngenerated by the central heating and poloidal field coils also is suspect. Its\nderivation of the viscosity term remains incomplete and does not account for\nthe varying pitch angle of the magnetic field. A derivation of the viscosity\nterm which respects the pitch angle can account for the radial force commonly\nascribed to the radial electrostatic field, thereby obviating the desire to\nneglect Gauss's law.", "category": "physics_plasm-ph" }, { "text": "Influence of secondary gas on control characteristics of ICP-RF plasma\n torches: Thermal plasma characteristics inside an atmospheric pressure Radio-Frequency\nArgon plasma torch have been studied by numerically solving axis-symmetric 2-D\nMagnetohydrodynamics equations, energy transport equations and species\nconservation equation coupled with 2D Maxwell equations in the vector potential\nform. A renormalization group k - epsilon model was employed to study the\nturbulence within the torch. Helium is mixed with Argon for injection as the\nsheath gas, and its effect on the control characteristics of the torch has been\nstudied. A control volume approach and semi-implicit pressure linked equations\nrevised (SIMPLER) algorithm was used to solve the above set of equations to\nobtain the flow, temperature, turbulence and EM source fields within the torch.\nFinally, a comparison of the results obtained in our present calculation has\nbeen made with other works of similar nature. The torch geometry, flow rates\nand power dissipation values used in the present calculation are similar to the\nstandard ones available in literature.", "category": "physics_plasm-ph" }, { "text": "Quasi-monoenergetic electron beams production in a sharp density\n transition: Using a laser plasma accelerator, experiments with a 80 TW and 30 fs laser\npulse demonstrated quasi-monoenergetic electron spectra with maximum energy\nover 0.4 GeV. This is achieved using a supersonic He gas jet and a sharp\ndensity ramp generated by a high intensity laser crossing pre-pulse focused 3\nns before the main laser pulse. By adjusting this crossing pre-pulse position\ninside the gas jet, among the laser shots with electron injection more than 40%\ncan produce quasi-monoenergetic spectra. This could become a relatively\nstraight forward technique to control laser wakefield electron beams\nparameters.", "category": "physics_plasm-ph" }, { "text": "Field-line localized destabilization of ballooning modes in 3D tokamaks: Field-line localized ballooning modes have been observed at the edge of high\nconfinement mode plasmas in ASDEX Upgrade with rotating 3D perturbations\ninduced by an externally applied n = 2 error field and during a moderate level\nof edge localized mode-mitigation. The observed ballooning modes are localized\nto the field-lines which experience one of the two zero-crossings of the radial\nflux surface displacement during one rotation period. The localization of the\nballooning modes agrees very well with the localization of the largest growth\nrates from infinite-n ideal ballooning stability calculations using a realistic\n3D ideal magnetohydrodynamic equilibrium. This analysis predicts a lower\nstability with respect to the axisymmetric case. The primary mechanism for the\nlocal lower stability is the 3D distortion of the local magnetic shear.", "category": "physics_plasm-ph" }, { "text": "Avoiding Tokamak disruptions by applying static magnetic fields that\n align locked modes with stabilizing wave-driven currents: Non-rotating (`locked') magnetic islands often lead to complete losses of\nconfinement in tokamak plasmas, called major disruptions. Here locked islands\nwere suppressed for the first time, by a combination of applied\nthree-dimensional magnetic fields and injected millimetre waves. The applied\nfields were used to control the phase of locking and so align the island\nO-point with the region where the injected waves generated non-inductive\ncurrents. This resulted in stabilization of the locked island, disruption\navoidance, recovery of high confinement and high pressure, in accordance with\nthe expected dependencies upon wave power and relative phase between O-point\nand driven current.", "category": "physics_plasm-ph" }, { "text": "Dynamics of Nanometer-Scale Foil Targets Irradiated with\n Relativistically Intense Laser Pulses: In this letter we report on an experimental study of high harmonic radiation\ngenerated in nanometer-scale foil targets irradiated under normal incidence.\nThe experiments constitute the first unambiguous observation of odd-numbered\nrelativistic harmonics generated by the $\\vec{v}\\times\\vec{B}$ component of the\nLorentz force verifying a long predicted property of solid target harmonics.\nSimultaneously the observed harmonic spectra allow in-situ extraction of the\ntarget density in an experimental scenario which is of utmost interest for\napplications such as ion acceleration by the radiation pressure of an\nultraintense laser.", "category": "physics_plasm-ph" }, { "text": "First principle simulation of ultra-cold ion crystals in a Penning trap\n with Doppler cooling and a rotating wall potential: A direct numerical simulation of many interacting ions in a Penning trap with\na rotating wall is presented. The ion dynamics is modelled classically. Both\naxial and planar Doppler laser cooling are modeled using stochastic momentum\nimpulses based on two-level atomic scattering rates. The plasmas being modeled\nare ultra-cold two-dimensional crystals made up of 100's of ions. We compare\nDoppler cooled results directly to a previous linear eigenmodes analysis.\nAgreement in both frequency and mode structure are obtained. Additionally, when\nDoppler laser cooling is applied, the laser cooled steady state plasma axial\ntemperature agrees with the Doppler cooling limit. Numerical simulations using\nthe approach described and benchmarked here will provide insights into the\ndynamics of large trapped-ion crystals, improving their performance as a\nplatform for quantum simulation and sensing.", "category": "physics_plasm-ph" }, { "text": "Anticorrelated emission of high-harmonics and fast electron beams from\n plasma mirrors: We report for the first time on the anticorrelated emission of high-order\nharmonics and energetic electron beams from a solid-density plasma with a sharp\nvacuum interface$-$plasma mirror$-$driven by an intense ultrashort laser pulse.\nWe highlight the key role played by the nanoscale structure of the plasma\nsurface during the interaction by measuring the spatial and spectral properties\nof harmonics and electron beams emitted by a plasma mirror. We show that the\nnanoscale behavior of the plasma mirror can be controlled by tuning the scale\nlength of the electronic density gradient, which is measured in-situ using\nspatial-domain interferometry.", "category": "physics_plasm-ph" }, { "text": "Bohm-like Neoclassical Transport in Highly Collisional Toroidal Plasmas\n with High Density Gradients: Conventional neoclassical theory in the Pfirsch-Schl\\\"{u}ter regime fails to\naccurately model collision-induced transport in toroidal plasmas with high\ndensity gradients. In this scenario, we find that collision suppresses the\nreturn flow, leading to the dominance of the transport flux by the vacuum\ntoroidal field drift with a reduced Bohm-like scaling. The new regime is also\nconfirmed by full-orbit particle simulations, and can be employed to improve\nthe accurate modeling of impurity transport in toroidal magnetized plasmas.", "category": "physics_plasm-ph" }, { "text": "Ponderomotive laser channelling and multi-channelling in homogeneous\n underdense plasma: We have studied laser pulse channeling in underdense plasma by means of\nanalytical theory and 3D PIC simulations. The most significant result of this\npaper is the demonstration of the single fully evacuated stationary channel\nsolution can be reached an asymptotic state in PIC simulations. In the lower\ndensity and for a laser power above channeling power, we were able to reproduce\nin PIC simulations the analytical curve. We showed that single channels are\nstable structures against symmetric perturbations. We also studied the\nformation of the ring structure in theory and simulations. An evacuated ring\nenclosed by an electron filament was observed in our 3D simulations. However\nthey always coexist with the main laser mode. The threshold power for ring\nstructure formation is found. Our studies on stability of the rings against\nasymmetric perturbations show that ring structure is not stable against\nazimuthal perturbations. The growth rate of the instability is shorter for\nhigher densities and it is therefore more effective at higher densities.", "category": "physics_plasm-ph" }, { "text": "Dust vortex flow analysis in weakly magnetized plasma: Analysis of driven dust vortex flow is presented in a weakly magnetized\nplasma. The 2D hydrodynamic model is applied to the confined dust cloud in a\nnon-uniform magnetic field in order to recover the dust vortex flow driven in a\nconservative force field setup, in absence of any non-conservative fields or\ndust charge variation. Although the time independent electric and magnetic\nfields included in the analysis provide conservative forcing mechanisms, when\nthe a drift based mechanism, recently observed in a dusty plasma experiment by\n[M. Puttscher and A. Melzer, Physics of Plasmas, 21,123704(2014)] is\nconsidered, the dust vortex flow solutions are shown to be recovered. We have\nexamined the case where purely ambipolar electric field, generated by\npolarization produced by electron E*B drift, drives the dust flow. A sheared\nE*B drift flow is facilitated by the magnetic field gradient, driving the\nvortex flow in the absence of ion drag. The analytical stream-function\nsolutions have been analyzed with varying magnetic field strength, its gradient\nand kinematic viscosity of the dust fluid. The effect of B field gradient is\nanalyzed which contrasts that of E field gradient present in the plasma sheath.", "category": "physics_plasm-ph" }, { "text": "Low-frequency one-electrode discharge in long tubes at low gas pressure: The specific form of one electrode capacitive discharge was studied in long\ntubes filled with high purity neon or argon at pressure 1-4 Torr. The main\nfeature of the discharge is the low rate (less than 10 kHz) of the voltage\npulses of given polarity which are applied to only one electrode of the tube,\nwhile another one remains free or missing. This type of the discharge was named\none-electrode discharge (OED), and seems not to be described in previous\nstudies. The discharge is observed as a glowing plasma column which occupies\neither entire tube or its part dependent on actual voltage amplitude and on its\nrate. Current-volt characteristics, ignition thresholds and the OED length\nchanging patterns demonstrate features unknown for RF discharges. It was found\nthat the plasma generation mechanism in OED consists in formation and traveling\nof the set of ionization waves (IW). It was shown that the wave motion is\ncharacterized by a pronounced attenuation the patterns of which were\ninvestigated by the time-position diagrams method. The attenuation specifies\nthe length of the occupied plasma area as well as other OED parameters.\nKinematic model of the wave propagation is proposed. This model allows to\nestimate the electric field in different OED points as well as average electron\nconcentration via the current measurements; the typical values of the above\nparameters are $5 V/cm$ and $10^9-10^{10}$ $cm^{-3}$. It was shown that\nnonhomogeneous electric field behind the IW front creates the conditions for\nappearance of plasma channel striations observed in the experiment. The\nstationary striations found in neon are more distinctive for negative polarity\nOED. It was shown that ionization-drift mechanism with account of metastable\nstates existence in plasma seems to be much adequate for the OED striation\ndescription.", "category": "physics_plasm-ph" }, { "text": "Gravitational Instability Analysis in Multi-Ion Dense Quantum\n Magnetoplasma: Electrostatic Gravitational or Rayleigh-Taylor (RT) instability in an\ninhomogeneous magnetized multi-ions plasma with some fraction of quantum\nmechanical electrons. The effect of Bohm potential, temperature degeneracy and\nmagnetic field are carried out. A generalized dispersion relation is deduced\nunder the drift approximation. The presence of negative ions with their\ndifferent streaming velocities make the dispersion relation a cubic equation.\nDifferent roots of both real and imaginary parts of the RT mode are studied by\nusing the Cardano method of solving the cubic equation. The growth rates of RT\ninstability are examined analytically and numerically. It is shown that the\nbasic features of these waves are significantly modified by the positive and\nnegative ions drift speed as well as by the magnetic field and density.\nRelevance of the work regarding to dense astrophysical plasmas is pointed out.", "category": "physics_plasm-ph" }, { "text": "Terahertz pulsed photogenerated current in microdiodes at room\n temperature: Space-charge modulation of the current in a vacuum diode under photoemission\nleads to the formation of beamlets with time periodicity corresponding to THz\nfrequencies. We investigate the effect of the emitter temperature and internal\nspace-charge forces on the formation and persistence of the beamlets. We find\nthat temperature effects are most important for beam degradation at low values\nof the applied electric field, whereas at higher fields intra-beamlet\nspace-charge forces are dominant. The current modulation is most robust when\nthere is only one beamlet present in the diode gap at a time, corresponding to\na macroscopic version of the Coulomb blockade. It is shown that a vacuum\nmicrodiode can operate quite well as a tunable THz oscillator at room\ntemperature with an applied electric field above 10 MV/m and a diode gap of the\norder of 100 nanometers.", "category": "physics_plasm-ph" }, { "text": "Spiky development at the interface in Rayleigh-Taylor instability:\n Layzer approximation with second harmonic: Layzer's approximation method for investigation of two fluid interface\nstructures associated with Rayleigh Taylor instability for arbitrary Atwood\nnumber is extended with the inclusion of second harmonic mode leaving out the\nzeroth harmonic one. The modification makes the fluid velocities vanish at\ninfinity and leads to avoidance of the need to make the unphysical assumption\nof the existence of a time dependent source at infinity.\n The present analysis shows that for an initial interface perturbation with\ncurvature exceeding 1/(2sqrt{A}), where $A$ is the Atwood number there occurs\nan almost free fall of the spike with continuously increasing sharpening as it\nfalls. The curvature at the tip of the spike also increases with Atwood number.\nCertain initial condition may also result in occurrence of finite time\nsingularity as found in case of conformal mapping technique used earlier.\nHowever bubble growth rate is not appreciably affected.", "category": "physics_plasm-ph" }, { "text": "Kinetic characteristics of ions in an inertial electrostatic confinement\n device: The kinetic analyses are quite important when it comes to understand the\nparticle behavior in any device as they start to deviate from continuum nature.\nIn the present study, kinetic simulations are performed using Particle-in-Cell\n(PIC) method to analyze the behavior of ions inside a cylindrical Inertial\nElectrostatic Confinement Fusion (IECF) device which is being developed as a\ntabletop neutron source. Here, the lighter ions, like deuterium are accelerated\nby applying an electrostatic field between the chamber wall (anode) and the\ncathode (cylindrical gridded wire), placed at the center of the device. The\nplasma potential profiles obtained from the simulated results indicate the\nformation of multiple potential well structures inside the cathode grid\ndepending upon the applied cathode potential (from $-1$ to $-5~kV$). The ion\ndensity at the core region of the device is found to be of the order of\n$10^{16}~m^{-3}$, which closely resembles the experimental observations.\nSpatial variation of Ion Energy Distribution Function (IEDF) has been measured\nin order to observe the characteristics of ions at different cathode voltages.\nFinally, the simulated results are compared and found to be in good agreement\nwith the experimental profiles. The present analysis can serve as a reference\nguide to optimize the technological parameters of the discharge process in IECF\ndevices.", "category": "physics_plasm-ph" }, { "text": "Pulsed RF Schemes for Tearing Mode Stabilization: The RF stabilization of tearing modes with current condensation has the\npotential to increase stabilization efficiency and loosen power localization\nrequirements. Such benefits stem from the cooperative feedback between the RF\ndeposition and resulting island temperature perturbation governed by diffusion.\nA self consistent treatment of the damping of an rf ray as it traverses the\nisland shows that low damping scenarios can require unfavorably high powers to\novercome initial power leakage and effectively capitalize on the nonlinear\neffect. In this work it is demonstrated that for such regimes,modulated\nstabilization schemes can achieve significant improvements in heating and\ncurrent drive contributions to stabilization for the same average power as a\ncontinuous wave scheme. The impact of modulation frequency and duty cycle on\nthe performance is explored, the results of which suggest modulation strategies\nin which the pulsing periods are kept on the order of a diffusive time.", "category": "physics_plasm-ph" }, { "text": "Effect of temperature anisotropy on the dynamics of geodesic acoustic\n modes: In this work, we revisit the linear gyro-kinetic theory of geodesic acoustic\nmodes (GAMs) and derive a general dispersion relation for an arbitrary\nequilibrium distribution function of ions. A bi-Maxwellian distribution of ions\nis then used to study the effects of ion temperature anisotropy on GAM\nfrequency and growth rate. We find that ion temperature anisotropy yields\nsensible modifications to both the GAM frequency and growth rate as both tend\nto increase with anisotropy and these results are strongly affected by the\nelectron to ion temperature ratio.", "category": "physics_plasm-ph" }, { "text": "Field induced phase transition in the few photon regime: Some features of the field induced phase transition accompanied by the vacuum\ncreation of an electron-positron plasma (EPP) in strong time-dependent electric\nfields have been discussed in the work [1] in the domain of the tunneling\nmechanism ($\\omega \\ll m$, where $\\omega$ is the characteristic frequency of\nthe external field and $m$ is the electron mass). In the present contribution\nthe features of the this process will be considered in the few photon domain\nwhere $\\omega \\sim m$. We observe a narrowing of the transient domain of the\nfast oscillations and, mainly, a considerable growth of the effectiveness of\nthe EPP production. Under these circumstances, we see an increase of the\neffectiveness of the EPP creation in the particular case of a bifrequent\nexcitation, where both mechanisms (tunneling and few photon) act simultaneously\n[2,3].", "category": "physics_plasm-ph" }, { "text": "On the correspondence between classical geometric phase of gyro-motion\n and quantum Berry phase: We show that the geometric phase of the gyro-motion of a classical charged\nparticle in a uniform time-dependent magnetic field described by Newton's\nequation can be derived from a coherent Berry phase for the coherent states of\nthe Schroedinger equation or the Dirac equation. This correspondence is\nestablished by constructing coherent states for a particle using the energy\neigenstates on the Landau levels and proving that the coherent states can\nmaintain their status of coherent states during the slow varying of the\nmagnetic field. It is discovered that orbital Berry phases of the eigenstates\ninterfere coherently to produce an observable effect (which we termed \"coherent\nBerry phase\"), which is exactly the geometric phase of the classical\ngyro-motion. This technique works for particles with and without spin. For\nparticles with spin, on each of the eigenstates that makes up the coherent\nstates, the Berry phase consists of two parts that can be identified as those\ndue to the orbital and the spin motion. It is the orbital Berry phases that\ninterfere coherently to produce a coherent Berry phase corresponding to the\nclassical geometric phase of the gyro-motion. The spin Berry phases of the\neigenstates, on the other hand, remain to be quantum phase factors for the\ncoherent states and have no classical counterpart.", "category": "physics_plasm-ph" }, { "text": "Non-stationary Rayleigh-Taylor instability in supernovae ejecta: The Rayleigh-Taylor instability plays an important role in the dynamics of\nseveral astronomical objects, in particular, in supernovae (SN) evolution. In\nthis paper we develop an analytical approach to study the stability analysis of\nspherical expansion of the SN ejecta by using a special transformation in the\nco-moving coordinate frame. We first study a non-stationary spherical expansion\nof a gas shell under the pressure of a central source. Then we analyze its\nstability with respect to a no radial, non spherically symmetric perturbation\nof the of the shell. We consider the case where the polytropic constant of the\nSN shell is $\\gamma=5/3$ and we examine the evolution of a arbitrary shell\nperturbation. The dispersion relation is derived. The growth rate of the\nperturbation is found and its temporal and spatial evolution is discussed. The\nstability domain depends on the ejecta shell thickness, its acceleration, and\nthe perturbation wavelength.", "category": "physics_plasm-ph" }, { "text": "A Systematic Approach to Numerical Dispersion in Maxwell Solvers: The finite-difference time-domain (FDTD) method is a well established method\nfor solving the time evolution of Maxwell's equations. Unfortunately the scheme\nintroduces numerical dispersion and therefore phase and group velocities which\ndeviate from the correct values. The solution to Maxwell's equations in more\nthan one dimension results in non-physical predictions such as numerical\ndispersion or numerical Cherenkov radiation emitted by a relativistic electron\nbeam propagating in vacuum.\n Improved solvers, which keep the staggered Yee-type grid for electric and\nmagnetic fields, generally modify the spatial derivative operator in the\nMaxwell-Faraday equation by increasing the computational stencil. These\nmodified solvers can be characterized by different sets of coefficients,\nleading to different dispersion properties. In this work we introduce a norm\nfunction to rewrite the choice of coefficients into a minimization problem. We\nsolve this problem numerically and show that the minimization procedure leads\nto phase and group velocities that are considerably closer to $c$ as compared\nto schemes with manually set coefficients available in the literature.\nDepending on a specific problem at hand (e.g. electron beam propagation in\nplasma, high-order harmonic generation from plasma surfaces, etc), the norm\nfunction can be chosen accordingly, for example, to minimize the numerical\ndispersion in a certain given propagation direction. Particle-in-cell\nsimulations of an electron beam propagating in vacuum using our solver are\nprovided.", "category": "physics_plasm-ph" }, { "text": "On the Raman Instability in Degenerate Relativistic Plasmas: The stimulated Raman scattering instability in a fully degenerate electron\nplasma is studied applying relativistic hydrodynamic and Maxwell equations. We\ndemonstrated that the instability develops for weakly as well as strongly\nrelativistic degenerate plasma. It is shown that in the field of strong\nradiation relativistically degenerate plasma effectively responses as in the\ncase of weak degeneracy.", "category": "physics_plasm-ph" }, { "text": "An \\mathcal{O}(N) Maxwell solver with improved numerical dispersion\n properties: A Maxwell solver derived from finite element method with \\mathcal{O}(N)\ncomputing cost is developed to improve the numerical dispersion properties in\nrelativistic particle-in-cell (PIC) simulations. The correction of the\ndispersion relation of the electromagnetic wave is achieved using the\nneighboring cells via an iteration scheme without decomposing into Fourier\nmodes. The local nature of the communication is ideally suited to massively\nparallel computer architectures. This Maxwell solver constrains the Numerical\nCherenkov instability (NCI) for the ultra-relativistic drifting pair plasma in\nx direction to large wave vectors for two dimensional grid. The growth rate of\nNCI is suppressed by using the low pass filtering.", "category": "physics_plasm-ph" }, { "text": "A Short Introduction to Plasma Physics: This chapter contains a short discussion of some fundamental plasma\nphenomena. In section 2 we introduce key plasma properties like\nquasi-neutrality, shielding, particle transport processes and sheath formation.\nIn section 3 we describe the simplest plasma models: collective phenomena\n(drifts) deduced from single-particle trajectories and fundamentals of plasma\nfluid dynamics. The last section discusses wave phenomena in homogeneous,\nunbounded, cold plasma.", "category": "physics_plasm-ph" }, { "text": "Raman amplification in the coherent wavebreaking regime: In regimes far beyond the wavebreaking threshold of Raman amplification, we\nshow that significant amplifcation can occur after the onset of wavebreaking,\nbefore phase mixing destroys the coherent coupling between pump, probe and\nplasma wave. Amplification in this regime is therefore a transient effect, with\nthe higher-efficiency \"coherent wavebreaking\" (CWB) regime accessed by using a\nshort, intense probe. Parameter scans illustrate the marked difference in\nbehaviour between below wavebreaking, in which the energy-transfer efficiency\nis high but total energy transfer is low, wavebreaking, in which efficiency is\nlow, and CWB, in which moderate efficiencies allow the highest total energy\ntransfer.", "category": "physics_plasm-ph" }, { "text": "Vibrational model of heat transfer in strongly coupled Yukawa fluids\n (dusty plasma liquids): A concise overview of the vibrational model of heat transfer in simple fluids\nwith soft pairwise interactions is presented. The model is applied to evaluate\nthe thermal conductivity coefficient of the strongly coupled Yukawa fluid,\nwhich often serves as a simplest model of a real liquid-like dusty (complex)\nplasma. A reasonable agreement with the available data from molecular dynamics\nnumerical simulations is observed. Universality of the properly reduced thermal\nconductivity coefficient with respect to the effective coupling parameter is\nexamined. Relations between the vibrational model and the excess entropy\nscaling of the thermal conductivity coefficient are discussed.", "category": "physics_plasm-ph" }, { "text": "Classification of Orbits in Poincar\u00e9 Maps using Machine Learning: Poincar\\'e plots, also called Poincar\\'e maps, are used by plasma physicists\nto understand the behavior of magnetically confined plasma in numerical\nsimulations of a tokamak. These plots are created by the intersection of field\nlines with a two-dimensional poloidal plane that is perpendicular to the axis\nof the torus representing the tokamak. A plot is composed of multiple orbits,\neach created by a different field line as it goes around the torus. Each orbit\ncan have one of four distinct shapes, or classes, that indicate changes in the\ntopology of the magnetic fields confining the plasma. Given the (x,y)\ncoordinates of the points that form an orbit, the analysis task is to assign a\nclass to the orbit, a task that appears ideally suited for a machine learning\napproach. In this paper, we describe how we overcame two major challenges in\nsolving this problem - creating a high-quality training set, with few\nmislabeled orbits, and converting the coordinates of the points into features\nthat are discriminating, despite the variation within the orbits of a class and\nthe apparent similarities between orbits of different classes. Our automated\napproach is not only more objective and accurate than visual classification,\nbut is also less tedious, making it easier for plasma physicists to analyze the\ntopology of magnetic fields from numerical simulations of the tokamak.", "category": "physics_plasm-ph" }, { "text": "Influence of toroidal flow on stationary density of collisionless\n plasmas: Starting from the given passive particle equilibrium particle cylindrical\nprofiles, we built self-consistent stationary conditions of the Maxwell-Vlasov\nequation at thermodynamic equilibrium with non-flat density profiles. The\nsolutions to the obtained equations are then discussed. It appears that the\npresence of an azimuthal (poloidal) flow in the plasma can insure radial\nconfinement, while the presence of a longitudinal (toroidal) flow can enhance\ngreatly the confinement. Moreover in the global physically reasonable\nsituation, we find that no unstable point can emerge in the effective\nintegrable Hamiltonian of the individual particles, hinting at some stability\nof the confinement when considering a toroidal geometry in the large aspect\nratio limit.", "category": "physics_plasm-ph" }, { "text": "Nonlinear cyclotron-resonance accelerations by a generalized EM wave: Particle accelerations by a 1D, EM, dispersive pulse in an external magnetic\nfield are investigated. It is found that the well-known cyclotron resonance may\nbe classified into three regimes as the length and/or the amplitude of the\npulse are varied. Namely, as the pulse amplitude increases, the transit-time\ncyclotron-resonance acceleration [CRA] evolves to phase trapping, and reflect\nparticles. The amplitude and wave dispersion as well as the pulse length\nstrongly affect those accelerations. The interesting phenomena of quantization\nof resonance velocities in between the two regimes are also investigated. This\nnew mechanism may lead to wave amplification at some discrete frequencies other\nthan the cyclotron frequency.", "category": "physics_plasm-ph" }, { "text": "Ponderomotive force of quasi-particles in a plasma: We derive the force exerted in the background plasma by an arbitrary\ndistribution of non interacting quasi-particles, corresponding to either\ncollective excitations of the plasma (plasmons, phonons) or em dressed\nparticles (photons, neutrinos). Our approach is based on the effective\nHamiltonian describing the quasi-classical dynamics of the individual particles\nin the presence of a background medium. We recover the usual results for the\nrelativistic ponderomotive force of a photon gas, and we derive the force, due\nto weak interactions, exerted by the electron-neutrinos in a background medium\ncontaining electrons, positrons and neutrons with arbitrary distribution\nfunctions. Generalization to other background species and other neutrino\nflavors is also discussed.", "category": "physics_plasm-ph" }, { "text": "Relationship between Horizontal Flow Velocity and Cell Lifetime for\n Supergranulation: A study of 50 supergranular cells obtained from SOHO Dopplergrams was\nundertaken in order to investigate the relationship between the lifetime ($T$)\nand the horizontal flow velocity ($v_h$) of the cells. For this sample we find\nthat the two parameters are correlated with a relation $v_h\\propto T^{0.5}$ and\n$T$ is identified with the eddy turn-over time. This is in agreement with the\nturbulent convective model of the solar atmosphere where the velocity spectrum\nof supergranular field given by '$v_h \\propto L^{1/3}$' can be identified with\nthe Kolmogorov spectrum for the eddy size $L$.", "category": "physics_plasm-ph" }, { "text": "Poloidal inhomogeneity of the particle fluctuation induced fluxes near\n of the LCFS at lower hybrid heating and improved confinement transition at\n the FT-2 tokamak: This paper deals with the new spectral and microturbulence experimental data\nand their analysis, which show, that the radial electric field Er generated at\nthe LH heating (LHH) in the FT-2 is high enough to form the transport barriers.\nThe ETB is formed when LHH is switched off. The radial fluctuation-induced\nEB drift flux densities near LCFS in SOL are measured at two different\npoloidal angles. For this purpose two Langmuir probes located at low and high\nfield sides of the torus are used. Registration of the poloidal and radial\ncomponents of the electric field and density fluctuations at the same time\nduring one discharge permits to measure the poloidal asymmetry of the transport\nreduction mechanism of the radial and poloidal particle fluxes in the SOL. The\nabsolute E(~) fluctuation levels show dependence on the sign of Er\nshear. The modification of the microscale turbulence by the poloidal Er x B\nrotation shear EB at the L - H transition near LCFS is also\nstudied by X-mode fluctuation Reflectometry. The new data were obtained by\nspatial spectroscopic technique.", "category": "physics_plasm-ph" }, { "text": "Cosmic-ray pitch-angle scattering in imbalanced MHD turbulence\n simulations: Pitch-angle scattering rates for cosmic-ray particles in magnetohydrodynamic\n(MHD) simulations with imbalanced turbulence are calculated for fully evolving\nelectromagnetic turbulence. We compare with theoretical predictions derived\nfrom the quasilinear theory of cosmic-ray diffusion for an idealized slab\nspectrum and demonstrate how cross helicity affects the shape of the\npitch-angle diffusion coefficient. Additional simulations in evolving magnetic\nfields or static field configurations provide evidence that the scattering\nanisotropy in imbalanced turbulence is not primarily due to coherence with\npropagating Alfven waves, but an effect of the spatial structure of electric\nfields in cross-helical MHD turbulence.", "category": "physics_plasm-ph" }, { "text": "How do the barrier thickness and dielectric material influence the\n filamentary mode and CO2 conversion in a flowing DBD?: Dielectric barrier discharges (DBDs) are commonly used to generate cold\nplasmas at atmospheric pressure. Whatever their configuration (tubular or\nplanar), the presence of a dielectric barrier is mandatory to prevent too much\ncharge build up in the plasma and the formation of a thermal arc. In this\narticle, the role of the barrier thickness (2.0, 2.4 and 2.8 mm) and of the\nkind of dielectric material (alumina, mullite, pyrex, quartz) is investigated\non the filamentary behavior in the plasma and on the CO2 conversion in a\ntubular flowing DBD, by means of mass spectrometry measurements correlated with\nelectrical characterization and IR imaging. Increasing the barrier thickness\ndecreases the capacitance, while preserving the electrical charge. As a result,\nthe voltage over the dielectric increases and a larger number of\nmicrodischarges is generated, which enhances the CO2 conversion. Furthermore,\nchanging the dielectric material of the barrier, while keeping the same\ngeometry and dimensions, also affects the CO2 conversion. The highest CO2\nconversion and energy efficiency are obtained for quartz and alumina, thus not\nfollowing the trend of the relative permittivity. From the electrical\ncharacterization, we clearly demonstrate that the most important parameters are\nthe somewhat higher effective plasma voltage (yielding a somewhat higher\nelectric field and electron energy in the plasma) for quartz, as well as the\nhigher plasma current (and thus larger electron density) and the larger number\nof microdischarge filaments (mainly for alumina, but also for quartz). The\nlatter could be correlated to the higher surface roughness for alumina and to\nthe higher voltage over the dielectric for quartz.", "category": "physics_plasm-ph" }, { "text": "Experimental observation of a first order phase transition in a complex\n plasma mono-layer crystal: The formation and melting of a mono-layered charged dust particle crystal in\na DC glow discharge Argon plasma is studied. The nature of the\nmelting/formation process is established as a first order phase transition from\nthe nature of the variations in the Coulomb coupling parameter, the dust\ntemperature, the structural order parameter and from the existence of a\nhysteresis behavior. Our experimental results are distinctly different from\nexisting theoretical predictions for 2D crystals based on the KTHNY mechanism\nor the Grain boundary induced melting and indicate a novel mechanism that is\nakin to a fluctuation induced first order phase transition that has not been\nobserved before in complex plasmas.", "category": "physics_plasm-ph" }, { "text": "Memory effects in the velocity relaxation process of the dust particle\n in dusty plasma: In this paper, by comparing the time scales associated with the velocity\nrelaxation and correlation time of the random force due to dust charge\nfluctuations, memory effects in the velocity relaxation of an isolated dust\nparticle exposed to the random force due to dust charge fluctuations are\nconsidered, and the velocity relaxation process of the dust particle is\nconsidered as a non-Markovian stochastic process. Considering memory effects in\nthe velocity relaxation process of the dust particle yields a retarded friction\nforce, which is introduced by a memory kernel in the fractional Langevin\nequation. The fluctuation-dissipation theorem for the dust grain is derived\nfrom this equation. The mean-square displacement and the velocity\nautocorrelation function of the dust particle are obtained, and their\nasymptotic behavior, the dust particle temperature due to charge fluctuations,\nand the diffusion coefficient are studied in the long-time limit. As an\ninteresting feature, it is found that by considering memory effects in the\nvelocity relaxation process of the dust particle, fluctuating force on the dust\nparticle can cause an anomalous diffusion in a dusty plasma. In this case, the\nmean-square displacement of the dust grain increases slower than linearly with\ntime, and the velocity autocorrelation function decays as a power-law instead\nof the exponential decay. Finally, in the Markov limit, these results are in\ngood agreement with those obtained from previous works for Markov (memoryless)\nprocess of the velocity relaxation.", "category": "physics_plasm-ph" }, { "text": "Ion-kinetic D'Angelo mode: An extension of hydrodynamic D'Angelo mode of inhomogeneous sheared plasma\nflow along the magnetic field into the short-wavelength limit, where the\nhydrodynamic treatment is not valid, has been considered. We find that D'Angelo\nmode in this wavelength range is excited by inverse ion Landau damping and\nbecomes the shear flow driven ion-kinetic mode.", "category": "physics_plasm-ph" }, { "text": "On the viability of the single wave model for the beam plasma\n instability: We analyze the interaction of a cold fast electron beam with a thermalized\nplasma, in the presence of many Langmuir modes. The work aims at characterizing\nthe deviation of the system behavior from the single mode approximation, both\nwith respect to a consistent spectral analysis of the most unstable mode\nharmonics and with respect to the presence of a dense spectrum, containing\nlinearly unstable and stable modes. We demonstrate how, on the one hand, the\ntotal energy fraction absorbed by the harmonics is negligible at all (by\nevaluating its total amount) and, on the other hand, the additional Langmuir\nmodes can be excited via an avalanche mechanism, responsible for a transport in\nthe particle velocity space. In particular, we show that the spectral\nbroadening outlines a universal shape and the distribution function, associated\nto the avalanche mechanism, has an asymptotic plateau, differently from the\ncoherent structures characterizing the single wave model.", "category": "physics_plasm-ph" }, { "text": "Plasma opacity calculations using the Starrett and Saumon average-atom\n model with ion correlations: We present the opacities of iron, aluminum, and bromine plasmas calculated\nusing the Starrett and Saumon average-atom model allowing for ion correlations.\nWe show that the use of earlier average-atom ion-correlation model of Rozsnyai,\nas has recently been done in the solar opacity calculations, overestimates the\neffect of ion correlations on plasma opacities. The reason for this\noverestimation is discussed.", "category": "physics_plasm-ph" }, { "text": "Steady Hall Magnetohydrodynamics Near a X-type Magnetic Neutral Line: Hall magnetohydrodynamics (MHD) properties near a two-dimensional (2D) X-type\nmagnetic neutral line in the steady state are considered via heuristic and\nrigorous developments. Upon considering the steady-state as the asymptotic\nlimit of the corresponding \\textit{time-dependent} problem and using a rigorous\ndevelopment, Hall effects are shown to be able to sustain the hyperbolicity of\nthe magnetic field (and hence a more open X-point configuration) near the\nneutral line in the steady state. The heuristic development misses this subtle\nconnection of the steady state with the corresponding \\textit{time-dependent}\nproblem and predicts only an elongated current-sheet configuration (as in\nresistive MHD). However, the heuristic development turns out to be useful in\nproviding insight into the lack of dependence of the reconnection rate on the\nmechanism breaking the frozen-in condition of the magnetic field lines. The\nlatter result can be understood in terms of the ability of the ions and\nelectrons to transport equal amounts of magnetic flux per unit time out of the\nreconnection region.", "category": "physics_plasm-ph" }, { "text": "Experiment to Form and Characterize a Section of a Spherically Imploding\n Plasma Liner: We describe an experiment to form and characterize a section of a spherically\nimploding plasma liner by merging six supersonic plasma jets that are launched\nby newly designed contoured-gap coaxial plasma guns. This experiment is a\nprelude to forming a fully spherical imploding plasma liner using many dozens\nof plasma guns, as a standoff driver for plasma-jet-driven magneto-inertial\nfusion. The objectives of the six-jet experiments are to assess the evolution\nand scalings of liner Mach number and uniformity, which are important metrics\nfor spherically imploding plasma liners to compress magnetized target plasmas\nto fusion conditions. This paper describes the design of the coaxial plasma\nguns, experimental characterization of the plasma jets, six-jet experimental\nsetup and diagnostics, initial diagnostic data from three- and six-jet\nexperiments, and the high-level objectives of associated numerical modeling.", "category": "physics_plasm-ph" }, { "text": "Ab initio path integral Monte Carlo simulations of hydrogen snapshots at\n warm dense matter conditions: We combine ab initio path integral Monte Carlo (PIMC) simulations with fixed\nion configurations from density functional theory molecular dynamics (DFT-MD)\nsimulations to solve the electronic problem for hydrogen under warm dense\nmatter conditions [M.B\\\"ohme et. al. Phys.Rev.Lett.(in print)]. The problem of\npath collapse due to the Coulomb attraction is avoided by utilizing the pair\napproximation, which is compared against the simpler Kelbg pair-potential. We\nfind very favourable convergence behaviour towards the former. Since we do not\nimpose any nodal restrictions, our PIMC simulations are afflicted with the\nnotorious fermion sign problem, which we analyse in detail. While\ncomputationally demanding, our results constitute an exact benchmark for other\nmethods and approximations such as DFT. Our set-up gives us the unique\ncapability to study important properties of warm dense hydrogen such as the\nelectronic static density response and exchange--correlation (XC) kernel\nwithout any model assumptions, which will be very valuable for a variety of\napplications such as the interpretation of experiments and the development of\nnew XC functionals.", "category": "physics_plasm-ph" }, { "text": "Acceleration of protons to high energies by an ultra-intense femtosecond\n laser pulse: The paper reports the results of two-dimensional particle-in-cell simulations\nof proton beam acceleration at the interactions of a 130 fs laser pulse of\nintensity from the range of 10^21-10^23 W/cm^2, predicted for the Extreme Light\nInfrastructure (ELI) lasers currently built in Europe, with a thin hydrocarbon\n(CH) target. A special attention is paid to the effect of the laser pulse\nintensity and polarization (linear-LP, circular-CP) as well as the target\nthickness on the proton energy spectrum, the proton beam spatial distribution\nand the proton pulse shape and intensity. It is shown that for the highest,\nultra-relativistic intensities (10^23 W/cm^2) the effect of laser polarization\non the proton beam parameters is relatively weak and for both polarizations\nquasi-monoenergetic proton beams of the mean proton energy about 2 GeV and\ndE/E=0.3 for LP and dE/E=0.2 for CP are generated from the 0.1 micrometer CH\ntarget. At short distances from the irradiated target (below 50 micrometers),\nthe proton pulse is very short (below 20 fs), and the proton beam intensities\nreach extremely high values above 10^21 W/cm^2, which are much higher than\nthose attainable in conventional accelerators. Such proton beams can open the\ndoor for new areas of research in high energy-density physics and nuclear\nphysics as well as can also prove useful for applications in materials research\ne.g. as a tool for high-resolution proton radiography.", "category": "physics_plasm-ph" }, { "text": "Self-consistent kinetic simulations of lower hybrid drift instability\n resulting in electron current driven by fusion products in tokamak plasmas: We present particle-in-cell (PIC) simulations of minority energetic protons\nin deuterium plasmas, which demonstrate a collective instability responsible\nfor emission near the lower hybrid frequency and its harmonics. The simulations\ncapture the lower hybrid drift instability in a regime relevant to tokamak\nfusion plasmas, and show further that the excited electromagnetic fields\ncollectively and collisionlessly couple free energy from the protons to\ndirected electron motion. This results in an asymmetric tail antiparallel to\nthe magnetic field. We focus on obliquely propagating modes under conditions\napproximating the outer mid-plane edge in a large tokamak, through which there\npass confined centrally born fusion products on banana orbits that have large\nradial excursions. A fully self-consistent electromagnetic relativistic PIC\ncode representing all vector field quantities and particle velocities in three\ndimensions as functions of a single spatial dimension is used to model this\nsituation, by evolving the initial antiparallel travelling ring-beam\ndistribution of 3MeV protons in a background 10keV Maxwellian deuterium plasma\nwith realistic ion-electron mass ratio. The simulations thus demonstrate a key\nbuilding block of alpha channelling scenarios for burning fusion plasmas in\ntokamaks.", "category": "physics_plasm-ph" }, { "text": "Nonlinear regime of the mode-coupling instability in 2D plasma crystals: The transition between linear and nonlinear regimes of the mode-coupling\ninstability (MCI) operating in a monolayer plasma crystal is studied. The mode\ncoupling is triggered at the centre of the crystal and a melting front is\nformed, which travels through the crystal. At the nonlinear stage, the mode\ncoupling results in synchronisation of the particle motion and the kinetic\ntemperature of the particles grows exponentially. After melting of the\ncrystalline structure, the mean kinetic energy of the particles continued to\ngrow further, preventing recrystallisation of the melted phase. The effect\ncould not be reproduced in simulations employing a simple point-like wake\nmodel. This shows that at the nonlinear stage of the MCI a heating mechanism is\nworking which was not considered so far.", "category": "physics_plasm-ph" }, { "text": "Complex structure of the carbon arc discharge for synthesis of nanotubes: Comprehensive non-invasive spectroscopic techniques and electrical\nmeasurements of the carbon arc revealed two distinguishable plasma synthesis\nregions in the radial direction normal to the arc axis. These regions, which\nare defined as the arc core and the arc periphery, are shown to have very\ndifferent compositions of carbon species with different densities and\ntemperatures. The colder arc periphery is dominated by carbon diatomic\nmolecules (C2), which are in the minority in the composition of the hot arc\ncore. These differences are due to a highly non-uniform distribution of the arc\ncurrent, which is mainly conducted through the arc core populated with carbon\natoms and ions. Therefore, the ablation of the graphite anode is governed by\nthe arc core, while the formation of carbon molecules occurs in the colder arc\nperiphery. This result is consistent with previous predictions that the plasma\nenvironment in the arc periphery is suitable for synthesis of carbon nanotubes.", "category": "physics_plasm-ph" }, { "text": "Microparticle cloud imaging and tracking for data-driven plasma science: Large data sets give rise to the `fourth paradigm' of scientific discovery\nand technology development, extending other approaches based on human\nintuition, fundamental laws of physics, statistics and intense computation.\nBoth experimental and simulation data are growing explosively in plasma science\nand technology, motivating data-driven discoveries and inventions, which are\ncurrently in infancy. Here we describe recent progress in microparticle cloud\nimaging and tracking (mCIT, $\\mu$CIT) for laboratory plasma experiments. Three\ntypes of microparticle clouds are described: from exploding wires, in dusty\nplasmas and in atmospheric plasmas. The experimental data sets are obtained\nwith one or more imaging cameras at a rate up to 100k frames per second (fps).\nA physics-constrained motion tracker, a Kohonen neural network (KNN) or\nself-organizing map (SOM), the feature tracking kit (FTK), and U-Net are\ndescribed and compared with each other for particle tracking using the\ndatasets. Particle density and signal-to-noise ratio have been identified as\ntwo important factors that affect the tracking accuracy. Fast Fourier transform\n(FFT) is used to reveal how U-Net, a deep convolutional neural network (CNN)\ndeveloped for non-plasma applications, achieves the improvements for noisy\nscenes. The fitting parameters for a simple polynomial track model are found to\ngroup into clusters that reveal the geometry information about the camera\nsetup. The mCIT or $\\mu$CIT techniques, when enhanced with data models, can be\nused to study the microparticle- or Debye-length scale plasma physics. The\ndatasets are also available for ML code development and comparisons of\nalgorithms.", "category": "physics_plasm-ph" }, { "text": "Plasma probe characteristics in low density hydrogen pulsed plasmas: Probe theories are only applicable in the regime where the probe's\nperturbation of the plasma can be neglected. However, it is not always possible\nto know, a priori, that a particular probe theory can be successfully applied,\nespecially in low density plasmas. This is especially difficult in the case of\ntransient, low density plasmas. Here, we applied probe diagnostics in\ncombination with a 2D particle-in-cell model, to an experiment with a pulsed\nlow density hydrogen plasma. The calculations took into account the full\nchamber geometry, including the plasma probe as an electrode in the chamber. It\nwas found that the simulations reproduce the time evolution of the probe IV\ncharacteristics with good accuracy. The disagreement between the simulated and\nprobe measured plasma density is attributed to the limited applicability of\nprobe theory to measurements of low density pulsed plasmas. Indeed, in the case\nstudied here, probe measurements would lead to a large overestimate of the\nplasma density. In contrast, the simulations of the plasma evolution and the\nprobe characteristics do not suffer from such strict applicability limits.\nThese studies show that probe theory cannot be justified through probe\nmeasurements.", "category": "physics_plasm-ph" }, { "text": "Self-guided wakefield experiments driven by petawatt class ultra-short\n laser pulses: We investigate the extension of self-injecting laser wakefield experiments to\nthe regime that will be accessible with the next generation of petawatt class\nultra-short pulse laser systems. Using linear scalings, current experimental\ntrends and numerical simulations we determine the optimal laser and target\nparameters, i.e. focusing geometry, plasma density and target length, that are\nrequired to increase the electron beam energy (to > 1 GeV) without the use of\nexternal guiding structures.", "category": "physics_plasm-ph" }, { "text": "Generation of ultrabrilliant polarized attosecond electron bunch via\n dual-wake injection: Laser wakefield acceleration is paving the way for the next generation of\nelectron accelerators, for their own sake and as radiation sources. A\ncontrollable dual-wake injection scheme is put forward here to generate an\nultrashort triplet electron bunch with high brightness and high polarization,\nemploying a radially polarized laser as a driver. We find that the dual wakes\ncan be driven by both transverse and longitudinal components of the laser field\nin the quasi-blowout regime, sustaining the laser-modulated wakefield which\nfacilitates the sub-cycle and transversely-split injection of the triplet\nbunch. {Polarization of the triplet bunch can be highly preserved due to the\nlaser-assisted collective spin precession and the non-canceled transverse\nspins. In our three-dimensional particle-in-cell simulations, the triplet\nelectron bunch, with duration about $500$ as, six-dimensional brightness\nexceeding $10^{14}$ A/m$^2$/0.1$\\%$ and polarization over $80\\%$, can be\ngenerated using a few-terawatt laser}. Such an electron bunch could play an\nessential role in many applications, such as ultrafast imaging, nuclear\nstructure and high-energy physics studies, and the operation of coherent\nradiation sources.", "category": "physics_plasm-ph" }, { "text": "Kinetics of a Model Weakly Ionized Plasma in the Presence of Multiple\n Equilibria: We study, globaly in time, the velocity distribution $f(v,t)$ of a spatially\nhomogeneous system that models a system of electrons in a weakly ionized\nplasma, subjected to a constant external electric field $E$. The density $f$\nsatisfies a Boltzmann type kinetic equation containing a full nonlinear\nelectron-electron collision term as well as linear terms representing\ncollisions with reservoir particles having a specified Maxwellian distribution.\nWe show that when the constant in front of the nonlinear collision kernel,\nthought of as a scaling parameter, is sufficiently strong, then the $L^1$\ndistance between $f$ and a certain time dependent Maxwellian stays small\nuniformly in $t$. Moreover, the mean and variance of this time dependent\nMaxwellian satisfy a coupled set of nonlinear ODE's that constitute the\n``hydrodynamical'' equations for this kinetic system. This remain true even\nwhen these ODE's have non-unique equilibria, thus proving the existence of\nmultiple stabe stationary solutions for the full kinetic model. Our approach\nrelies on scale independent estimates for the kinetic equation, and entropy\nproduction estimates. The novel aspects of this approach may be useful in other\nproblems concerning the relation between the kinetic and hydrodynamic scales\nglobably in time.", "category": "physics_plasm-ph" }, { "text": "Nonlinear magnetoplasmons in strongly coupled Yukawa plasmas: The existence of plasma oscillations at multiples of the magnetoplasmon\nfrequency in a strongly coupled two-dimensional magnetized Yukawa plasma is\nreported, based on extensive molecular dynamics simulations. These modes are\nthe analogues of Bernstein modes which are renormalized by strong interparticle\ncorrelations. Their properties are theoretically explained by a dielectric\nfunction incorporating the combined effect of a magnetic field, strong\ncorrelations and finite temperature.", "category": "physics_plasm-ph" }, { "text": "Reduced Physics Model of the Tokamak Scrape-off-Layer for Pulse Design: The dynamic interplay between the core and the edge plasma has important\nconsequences in the confinement and heating of fusion plasma. The transport of\nthe Scrape-Off-Layer (SOL) plasma imposes boundary conditions on the core\nplasma, and neutral transport through the SOL influences the core plasma\nsourcing. In order to better study these effects in a self-consistent,\ntime-dependent fashion with reasonable turn-around time, a reduced model is\nneeded. In this paper we introduce the SOL Box Model, a reduced SOL model that\ncalculates the plasma temperature and density in the SOL given the core-to-edge\nparticle and power fluxes and recycling coefficients. The analytic nature of\nthe Box Model allows one to readily incorporate SOL physics in time-dependent\ntransport solvers for pulse design applications in the control room. Here we\ndemonstrate such a coupling with the core transport solver TRANSP and compare\nthe results with density and temperature measurements, obtained through Thomson\nscattering and Langmuir probes, of an NSTX discharge. Implications for future\ninterpretive and predictive simulations are discussed.", "category": "physics_plasm-ph" }, { "text": "Required toroidal confinement for fusion and omnigeneity: Deuterium-tritium (DT) burning requires a long energy confinement times\ncompared to collision times, so the particle distribution functions must\napproximate local-Maxwellians. Non-equilibrium thermodynamics is applicable,\nwhich gives relations among transport, entropy production, the collision\nfrequency, and the deviation from a Maxwellian. The distribution functions are\ngiven by the Fokker-Planck equation, which is an advection-diffusion equation.\nA large hyperbolic operator, the Vlasov operator with the particle trajectories\nas its characteristics, equals a small diffusive operator, the collision\noperator. The collisionless particle trajectories would be chaotic in\nstellarators without careful optimization. This would lead to rapid entropy\nproduction and transport -- far beyond what is consistent with a\nself-sustaining DT burn. Omnigeneity is the weakest general condition that is\nconsistent with a sufficiently small entropy production associated with the\nthermal particle trajectories. Omnigeneity requires that the contours of\nconstant magnetic field strength be unbounded in at least one of the two\nangular coordinates in magnetic surfaces and that there be a symmetry in the\nfield-strength wells along the field lines. Even in omnigenous plasmas,\nfluctuations due to microturbulence can produce chaotic particle trajectories\nand the gyro-Bohm transport seen in many stellarator and tokamak experiments.\nThe higher the plasma temperature above 10~keV, the smaller the transport must\nbe compared to gyro-Bohm for a self-sustaining DT burn. The hot alphas of DT\nfusion heat the electrons. When the ion-electron equilibration time is long\ncompared to the ion energy confinement time, a self-sustaining DT burn is not\npossible, which sets a limit on the electron temperature.", "category": "physics_plasm-ph" }, { "text": "Analytic stability boundaries for compressional and global Alfv\u00e9n\n eigenmodes driven by fast ions. I. Interaction via ordinary and anomalous\n cyclotron resonances: Conditions for net fast ion drive are derived for beam-driven, sub-cyclotron\ncompressional (CAE) and global (GAE) Alfv\\'en eigenmodes, such as those\nroutinely observed in spherical tokamaks such as NSTX(-U) and MAST. Both co-\nand counter-propagating CAEs and GAEs are investigated, driven by the ordinary\nand anomalous Doppler-shifted cyclotron resonance with fast ions. Whereas prior\nresults were restricted to vanishingly narrow distributions in velocity space,\nbroad parameter regimes are identified in this work which enable an analytic\ntreatment for realistic fast ion distributions generated by neutral beam\ninjection. The simple, approximate conditions derived in these regimes for beam\ndistributions of realistic width compare well to the numerical evaluation of\nthe full analytic expressions for fast ion drive. Moreover, previous results in\nthe very narrow beam case are corrected and generalized to retain all terms in\n$\\omega/\\omega_{ci}$ and $k_\\parallel/k_\\perp$, which are often assumed to be\nsmall parameters but can significantly modify the conditions of drive and\ndamping when they are non-negligible. Favorable agreement is demonstrated\nbetween the approximate stability criterion, simulation results, and a large\ndatabase of NSTX observations of cntr-GAEs.", "category": "physics_plasm-ph" }, { "text": "Chaotic dynamics of small sized charged Yukawa Dust Clusters: In a recent work, [1] the equilibrium of a cluster of charged dust particles\nmutually interacting with screened Coulomb force and radially confined by an\nexternally applied electric field in a 2-D configuration was studied. It was\nshown that the particles arranged themselves on discrete radial rings forming a\nlattice structure. In some cases with the specific number of particles, no\nstatic equilibrium was observed; instead, angular rotation of particles\npositioned at various rings was observed. In a two-ringed structure, it was\nshown that the direction of rotation was opposite. The direction of rotation\nwas also observed to change apparently at random time intervals. A detailed\ncharacterization of the dynamics of small-sized Yukawa clusters has been\ncarried out in the present work. In particular, it has been shown that the\ndynamical time reversal of angular rotation exhibits chaotic behavior.", "category": "physics_plasm-ph" }, { "text": "Elevating zero dimensional global scaling predictions to self-consistent\n theory-based simulations: We have developed an innovative workflow, STEP-0D, within the OMFIT\nintegrated modelling framework. Through systematic validation against the\nInternational Tokamak Physics Activity (ITPA) global H-mode confinement\ndatabase, we demonstrated that STEP-0D, on average, predicts the energy\nconfinement time with a mean relative error (MRE) of less than 19%. Moreover,\nthis workflow showed promising potential in predicting plasmas for proposed\nfusion reactors such as ARC, EU-DEMO, and CFETR, indicating moderate H-factors\nbetween 0.9 and 1.2. STEP-0D allows theory-based prediction of tokamak\nscenarios, beginning with zero-dimensional (0D) quantities. The workflow\ninitiates with the PRO-create module, generating physically consistent plasma\nprofiles and equilibrium using the same 0D quantities as the IPB98(y,2)\nconfinement scaling. This sets the starting point for the STEP (Stability,\nTransport, Equilibrium, and Pedestal) module, which further iterates between\ntheory-based physics models of equilibrium, core transport, and pedestal to\nyield a self-consistent solution. Given these attributes, STEP-0D not only\nimproves the accuracy of predicting plasma performance but also provides a path\ntowards a novel fusion power plant (FPP) design workflow. When integrated with\nengineering and costing models within an optimization, this new approach could\neliminate the iterative reconciliation between plasma models of varying\nfidelity. This potential for a more efficient design process underpins\nSTEP-0D's significant contribution to future fusion power plant development.", "category": "physics_plasm-ph" }, { "text": "Wave modelling in a cylindrical non-uniform helicon discharge: A radio frequency (RF) field solver based on Maxwell's equations and a cold\nplasma dielectric tensor is em- ployed to describe wave phenomena observed in a\ncylindrical non-uniform helicon discharge. The experiment is carried out on a\nrecently built linear plasma-material interaction machine: the MAGnetized\nPlasma In- teraction Experiment (MAGPIE) [B. D. Blackwell, J. F. Caneses, C.\nSamuell, J. Wach, J. Howard, and C. S. Corr, submitted on 25 March 2012 to\nPlasma Sources Science and Technology], in which both plasma density and static\nmagnetic field are functions of axial position. The field strength increases by\na factor of 15 from source to target plate, and plasma density and electron\ntemperature are radially non-uniform. With an enhancement factor of 9.5 to the\nelectron-ion Coulomb collision frequency, 12% reduction in the antenna radius,\nand the same other conditions as employed in the experiment, the solver\nproduces axial and radial profiles of wave amplitude and phase that are\nconsistent with measurements. Ion-acoustic turbulence, which can happen if\nelectron drift velocity exceeds the speed of sound in magnetized plasmas, may\naccount for the factor of 9.5 used to match simulated results with experimental\ndata. To overcome the single m vacuum solu- tion limitations of the RF solver,\nwhich can only compute the glass response to the same mode number of the\nantenna, we have adjusted the antenna radius to match the wave field strength\nin the plasma.(not finished because of the limited number of characters, please\nsee the full paper)", "category": "physics_plasm-ph" }, { "text": "Magneto-HydroDynamic activity and Energetic Particles - Application to\n Beta Alfven Eigenmodes: The goal of magnetic fusion research is to extract the power released by\nfusion reactions and carried by the product of these reactions, liberated at\nenergies of the order of a few MeV. The feasibility of fusion energy production\nrelies on our ability to confine these energetic particles, while keeping the\nthermonuclear plasma in safe operating conditions. For that purpose, it is\nnecessary to understand and find ways to control the interaction between\nenergetic particles and the thermonuclear plasma. Reaching these two goals is\nthe general motivation for the work conducted during the PhD. More\nspecifically, our focus is on one type of instability, the Beta Alfven\nEigenmode (BAE), which can be driven by energetic particles and impact on the\nconfinement of both energetic and thermal particles. In this work, we study the\ncharacteristics of BAEs analytically and derive its dispersion relation and\nstructure. Next, we analyze the linear stability of the mode in the presence of\nenergetic particles. First, a purely linear description is used, which makes\npossible to get an analytical linear criterion for BAE destabilization in the\npresence of energetic particles. This criterion is compared with experiments\nconducted in the Tore-Supra tokamak during the PhD. Secondly, because the\nlinear analysis reveals some features of the BAE stability which are subject to\na strong nonlinear modification, the question is raised of the possibility of a\nsub-critical activity of the mode. We propose a simple scenario which makes\npossible the existence of meta-stable modes, verified analytically and\nnumerically. Such a scenario is found to be relevant to the physics and scales\ncharacterizing BAEs.", "category": "physics_plasm-ph" }, { "text": "Explosion of relativistic electron vortices in laser plasmas: The interaction of high intensity laser radiation with underdense plasma may\nlead to the formation of electron vortices. Though being quasistationary on an\nelectron timescales, these structures tend to expand on a proton timescale due\nto Coloumb repulsion of ions. Using a simple analytical model of a stationary\nvortex as initial condition, 2D PIC simulations are performed. A number of\neffects are observed such as vortex boundary field intensification, multistream\ninstabilities at the vortex boundary, and bending of the vortex boundary with\nthe subsequent transformation into smaller electron vortices.", "category": "physics_plasm-ph" }, { "text": "\"Magneto-elastic\" waves in an anisotropic magnetised plasma: The linear waves that propagate in a two fluid magnetised plasma allowing for\na non-gyrotropic perturbed ion pressure tensor are investigated. For\nperpendicular propagation and perturbed fluid velocity a low frequency\n(magnetosonic) and a high frequency (ion Bernstein) branch are identified and\ndiscussed. For both branches a comparison is made with the results of a\ntruncated Vlasov treatment. For the low frequency branch we show that a\nconsistent expansion procedure allows us to recover the correct expression of\nthe Finite Larmor Radius corrections to the magnetosonic dispersion relation.", "category": "physics_plasm-ph" }, { "text": "Photo-transmutation of long-lived radionuclide Cs-135 by laser-plasma\n driven electron source: Relativistic electrons, accelerated by the laser ponderomotive force, can be\nfocused onto a high-Z convertor to generate high-brightness beams of\ngamma-rays, which in turn can be used to induce photonuclear reactions. In this\nwork, the possibility of photo-transmutation of long-lived radionuclide Cs-135\nby laser-plasma driven electron source has been demonstrated through Geant4\nsimulations. High energy electron generation, bremsstrahlung and photonuclear\nreaction have been observed at four different laser intensities of 10^{20}\nW/cm^2, 5 times 10^{20} W/cm^2, 10^{21} W/cm^2 and 5 times 10^{21} W/cm^2,\nrespectively. It was shown that the laser intensity and the target geometry\nhave strong effect on the transmutation reaction yield. At different laser\nintensities the recommended target sizes were found to obtain the maximum\nreaction yield. The remarkable feature of this work is to evaluate the optimal\nlaser intensity to produce maximum reaction yield of 10^8 per Joule in laser\npulse energy, which is 10^{21} W/cm^2. Our study suggests photo-transmutation\ndriven by laser-based electron source as a promising approach for experimental\nresearch into transmutation reactions, with potential applications to nuclear\nwaste management.", "category": "physics_plasm-ph" }, { "text": "Kinetic modelling of runaway electrons in dynamic scenarios: Improved understanding of runaway-electron formation and decay processes are\nof prime interest for the safe operation of large tokamaks, and the dynamics of\nthe runaway electrons during dynamical scenarios such as disruptions are of\nparticular concern. In this paper, we present kinetic modelling of scenarios\nwith time-dependent plasma parameters; in particular, we investigate hot-tail\nrunaway generation during a rapid drop in plasma temperature. With the goal of\nstudying runaway-electron generation with a self-consistent electric-field\nevolution, we also discuss the implementation of a collision operator that\nconserves momentum and energy and demonstrate its properties. An operator for\navalanche runaway-electron generation, which takes the energy dependence of the\nscattering cross section and the runaway distribution into account, is\ninvestigated. We show that the simplified avalanche model of Rosenbluth &\nPutvinskii [Nucl. Fusion 1997 37 1355] can give inaccurate results for the\navalanche growth rate (either lower or higher) for many parameters, especially\nwhen the average runaway energy is modest, such as during the initial phase of\nthe avalanche multiplication. The developments presented pave the way for\nimproved modelling of runaway-electron dynamics during disruptions or other\ndynamic events.", "category": "physics_plasm-ph" }, { "text": "A Second-Order Symplectic Integrator for Guiding-Center Equations: This paper had no abstract originally. A second-order symplectic integration\nalgorithm for guiding center motion is presented. The algorithm is based on the\nPoincar\\'e (mid-point) generating function.", "category": "physics_plasm-ph" }, { "text": "Dust acoustic solitary structures in presence of nonthermal ions,\n isothermally distributed electrons and positrons: Arbitrary amplitude dust acoustic solitary structures have been investigated\nin a four component multi-species plasma consisting of negatively charged dust\ngrains, nonthermal ions, isothermally distributed electrons and positrons\nincluding the effect of dust temperature. We have used the Sagdeev\npseudo-potential method to discuss the arbitrary amplitude steady state dust\nacoustic solitary structures in the present plasma system. We have designed a\ncomputational scheme to draw the existence domains of different dust acoustic\nsolitary structures. We have observed only negative potential solitary waves\nfor isothermal ions. But for strong nonthermality of ions the system supports\npositive potential solitary waves, positive potential double layers and\ncoexistence of solitary waves of both polarities. The positive potential\nsolitary waves are restricted by the positive potential double layers but\nnegative potential double layer has not been found for any parameter regime.\nThe system does not support dust acoustic supersoliton of any polarity. The\nconcentration of positrons plays an important role in the formation of positive\npotential double layers. Finally, the phase portraits of the dynamical system\nhave been presented to confirm the existence of different dust acoustic\nsolitary structures.", "category": "physics_plasm-ph" }, { "text": "Alternating-Order Interpolation in a Charge-Conserving Scheme for\n Particle-In-Cell Simulations: We discuss the interpolation of the electric and magnetic fields within a\ncharge-conserving Particle-In-Cell scheme. The choice of the interpolation\nprocedure for the fields acting on a particle can be constrained by analyzing\nconservation of the energy and the particle generalized momentum. The better\nconservative properties are achieved, if the alternating-order form-factor is\nused for interpolation, which combines the lower-order and higher-order\ninterpolation from integer and semi-integer points of a staggered grid. This\napproach allows us to significantly reduce noise in the charge conserving\nscheme and improves both the results quality and the computational efficiency.", "category": "physics_plasm-ph" }, { "text": "Quantum dot photoluminescence as a versatile probe to visualize the\n interaction between plasma and nanoparticles on a surface: We experimentally demonstrate that the interaction between plasma and\nnanometer-sized semiconductor quantum dots (QDs) is directly connected to a\nchange in their photoluminescence (PL) spectrum. This is done by taking\nin-situ, high resolution, and temporally-resolved spectra of the light emitted\nby laser-excited QDs on an electrically floating sample exposed to a low\npressure argon plasma. Our results show a fast redshift of the PL emission peak\nindicating the quantum-confined Stark effect due to direct plasma-charging of\nthese nanostructures and the substrate surface, while other plasma-induced\n(thermal and ion) effects on longer time scales could clearly be distinguished\nfrom these charging effects. The presented results and method open up novel\npathways to direct visualization and understanding of fundamental\nplasma-particle interactions on nanometer length scales.", "category": "physics_plasm-ph" }, { "text": "Three-wave interactions and strange attractor: It is shown that the incorporation of linear sink/source terms in the\nthree-wave resonance interaction model results in the time dependence of the\nwave amplitudes, which could exhibit the properties of a strange attractor.\nThis finding demonstrates that the transition to turbulent dynamics of the\nwaves could be related not only to the coupling of wave triads but also to the\nestablishing of the strange attractor-like dynamics within individual wave\ntriads.", "category": "physics_plasm-ph" }, { "text": "Effect of mismatch on Doppler backscattering in MAST and MAST-U plasmas: The Doppler backscattering (DBS) diagnostic, also referred to as Doppler\nreflectometry, measures turbulent density fluctuations of intermediate length\nscales. However, when the beam's wavevector is not properly aligned\nperpendicular to the magnetic field, the backscattered power is attenuated. In\nprevious work, we used beam tracing and reciprocity to derive this mismatch\nattenuation quantitatively. In this paper, we applied our model, in the small\nbut finite mismatch limit, to a several new cases. We compared our predictions\nwith multiple O-mode channels for the first time. We then identified a $\\sim\n3^{\\circ}$ error in the MAST Q-band's quasioptics, showing that our model is\nuseful for commissioning DBS diagnostics. For both O- and X-mode, we compared\nexperimental data with our model's predictions at multiple times during the\nshots, unlike our previous work, where only a single time was analysed.\nFinally, we analysed other contributions to the backscattered signal,\nevaluating how much they affect our measurements of mismatch attenuation,\ngiving comparisons with data from both MAST and MAST-U. This paper's detailed\nstudy systematically validates and demonstrates the usefulness of our model for\nquantitatively interpreting DBS data from spherical tokamaks.", "category": "physics_plasm-ph" }, { "text": "Physics of plasma burn-through and DYON simulations for the JET\n ITER-like wall: This paper presents the DYON simulations of the plasma burn-through phase at\nJoint European Torus (JET) with the ITER-like wall. The main purpose of the\nstudy is to validate the simulations with the ITER-like wall, made of\nberyllium. Without impurities, the burn-through process of a pure deuterium\nplasma is described using DYON simulations, and the criterion for deuterium\nburn-through is derived analytically. The plasma burn-through with impurities\nare simulated using wall-sputtering models in the DYON code, which are modified\nfor the ITER-like wall. The wall-sputtering models and the validation against\nJET data are presented. The impact of the assumed plasma parameters in DYON\nsimulations are discussed by means of parameter scans. As a result, the\noperation space of prefill gas pressure and toroidal electric field for plasma\nburn-through in JET is compared to the Townsend avalanche criterion.", "category": "physics_plasm-ph" }, { "text": "Effect of the electronic pressure on the energy and magnetic moment of\n charged test particles in turbulent electromagnetic fields: In this work we perform direct numerical simulations of three-dimensional\nmagnetohydrodynamics with a background magnetic field, representing solar wind\nplasma, and introduce test particles to explore how a turbulent electromagnetic\nenvironment affects them. Our focus is on the terms of the electric field\npresent in the generalized Omh's Law that are usually dismissed as unimportant.\nThese are the Hall and the electronic pressure (EP) terms, but we concentrate\nprimarily on the latter. We discover that the EP term generates an acceleration\nof the particles, which represent protons, in the direction parallel to the\nbackground magnetic field, in contrast to the known preferential perpendicular\nenergization. By studying the electric field itself, we are able to detect the\ntype of structures of the EP field that produce such parallel acceleration.\nThese are thin and elongated structures placed on top of a monotonic and\nnear-zero background. A statistical study to understand the real significance\nof the electronic pressure term is also performed.", "category": "physics_plasm-ph" }, { "text": "Axial magnetic field and toroidally streaming fast ions in the dense\n plasma focus are natural consequences of conservation laws in the curved\n axisymmetric geometry of the current sheath: Direct measurement of axial magnetic field in the PF-1000 dense plasma focus\n(DPF), and its reported correlation with neutron emission, call for a fresh\nlook at previous reports of existence of axial magnetic field component in the\nDPF from other laboratories, and associated data suggesting toroidal\ndirectionality of fast ions participating in fusion reactions, with a view to\nunderstand the underlying physics. In this context, recent work dealing with\napplication of the hyperbolic conservation law formalism to the DPF is extended\nin this paper to a curvilinear coordinate system, which reflects the shape of\nthe DPF current sheath. Locally-unidirectional shock propagation in this\ncoordinate system enables construction of a system of 7 one-dimensional\nhyperbolic conservation law equations with geometric source terms, taking into\naccount all the components of magnetic field and flow velocity.\nRankine-Hugoniot jump conditions for this system lead to expressions for the\naxial magnetic field and three components of fluid velocity having high ion\nkinetic energy.", "category": "physics_plasm-ph" }, { "text": "Scaling of Laser Produced Pair Plasmas And Comparison With Experimental\n Data: We report simulation results of pair production by ultra-intense lasers\nirradiating a gold target using the GEANT4 Monte-Carlo code. Certain\nexperimental features of the positron and electron energy spectra are\nreproduced, as well as trends with regard to target thickness and hot electron\ntemperature Te. For Te in the range 5-10 MeV, the optimal target thickness for\npair production is found to be about 3 mm. Further Monte-Carlo simulations may\naid in the optimization of laser-driven positron sources.", "category": "physics_plasm-ph" }, { "text": "Vlasov simulations of Kinetic Alfv\u00e9n Waves at proton kinetic scales: Kinetic Alfv\\'en waves represent an important subject in space plasma\nphysics, since they are thought to play a crucial role in the development of\nthe turbulent energy cascade in the solar wind plasma at short wavelengths (of\nthe order of the proton inertial length $d_p$ and beyond). A full understanding\nof the physical mechanisms which govern the kinetic plasma dynamics at these\nscales can provide important clues on the problem of the turbulent dissipation\nand heating in collisionless systems. In this paper, hybrid Vlasov-Maxwell\nsimulations are employed to analyze in detail the features of the kinetic\nAlfv\\'en waves at proton kinetic scales, in typical conditions of the solar\nwind environment. In particular, linear and nonlinear regimes of propagation of\nthese fluctuations have been investigated in a single-wave situation, focusing\non the physical processes of collisionless Landau damping and wave-particle\nresonant interaction. Interestingly, since for wavelengths close to $d_p$ and\nproton plasma beta $\\beta$ of order unity the kinetic Alfv\\'en waves have small\nphase speed compared to the proton thermal velocity, wave-particle interaction\nprocesses produce significant deformations in the core of the particle velocity\ndistribution, appearing as phase space vortices and resulting in flat-top\nvelocity profiles. Moreover, as the Eulerian hybrid Vlasov-Maxwell algorithm\nallows for a clean almost noise-free description of the velocity space,\nthree-dimensional plots of the proton velocity distribution help to emphasize\nhow the plasma departs from the Maxwellian configuration of thermodynamic\nequilibrium due to nonlinear kinetic effects.", "category": "physics_plasm-ph" }, { "text": "Langmuir wave filamentation in the kinetic regime. II. Weak and Strong\n Pumping of Nonlinear Electron Plasma Waves as the Route to Filamentation: We consider two kinds of pumped Langmuir waves (LWs) in the kinetic regime,\n$k\\lambda_D\\gtrsim0.2,$ where $k$ is the LW wavenumber and $\\lambda_D$ is the\nDebye length. They are driven to finite amplitude by a coherent external\npotential whose amplitude is either weak or strong. These dynamically prepared\nnonlinear LWs develop a transverse (filamentation) instability whose nonlinear\nevolution destroys the LW's transverse coherence. Instability growth rates in\nthe weakly pumped regime are the same as those of BGK modes considered in Part\nI, while strongly pumped LWs have higher filamentation grow rates.", "category": "physics_plasm-ph" }, { "text": "Stochastic and Discrete Time Models of Long-Range Turbulent Transport in\n the Scrape-Off Layer: Two dimensional stochastic time model of scrape-off layer (SOL) turbulent\ntransport is studied. Instability arisen in the system with respect to the\nstochastic perturbations of both either density or vorticity reveals itself in\nthe strong outward bursts of particle density propagating ballistically across\nthe SOL. The stability and possible stabilization of the cross- field turbulent\nsystem depend very much upon the reciprocal correlation time between density\nand vorticity fluctuations. Pdf of the particle flux for the large magnitudes\nof flux events can be modelled with a simple discrete time toy model of random\nwalks concluding at a boundary. The spectra of wandering times feature the pdf\nof particle flux in the model and qualitatively reproduce the experimental\nstatistics of transport events.", "category": "physics_plasm-ph" }, { "text": "Kinetic analysis of spin current contribution to spectrum of\n electromagnetic waves in spin-1/2 plasma, Part II: Dispersion dependencies: The dielectric permeability tensor for spin polarized plasmas derived in\nterms of the spin-1/2 quantum kinetic model in six-dimensional phase space in\nPart I of this work is applied for study of spectra of high-frequency\ntransverse and transverse-longitudinal waves propagating perpendicular to the\nexternal magnetic field. Cyclotron waves are studied at consideration of waves\nwith electric field directed parallel to the external magnetic field. It is\nfound that the separate spin evolution modifies the spectrum of cyclotron\nwaves. These modifications increase with the increase of the spin polarization\nand the number of the cyclotron resonance. Spin dynamics with no account of the\nanomalous magnetic moment gives a considerable modification of spectra either.\nThe account of anomalous magnetic moment leads to a fine structure of each\ncyclotron resonance. So, each cyclotron resonance splits on three waves.\nDetails of this spectrum and its changes with the change of spin polarization\nare studied for the first and second cyclotron waves. A cyclotron resonance\nexisting at $\\omega\\approx0.001\\mid\\Omega_{e}\\mid$ due to the anomalous\nmagnetic moment is also described, where $\\mid\\Omega_{e}\\mid$ is the cyclotron\nfrequency. The ordinary waves does not have any considerable modification. The\nelectrostatic and electromagnetic Berstein modes are studied during the\nanalysis of waves propagating perpendicular to the external magnetic field with\nthe electric field perturbation directed perpendicular to the external field. A\nmodification of the oscillatory structure caused by the equilibrium spin\npolarization is found in both regimes. Similar modification is found for the\nextraordinary wave spectrum.", "category": "physics_plasm-ph" }, { "text": "The causal impact of magnetic fluctuations in slow and fast L-H\n transitions at TJ-II: This work focuses on the relationship between L-H (or L-I) transitions and\nMHD activity in the low magnetic shear TJ-II stellarator. It is shown that the\npresence of a low order rational surface in the plasma edge (gradient) region\nlowers the threshold density for H-mode access. MHD activity is systematically\nsuppressed near the confinement transition.\n We apply a causality detection technique (based on the Transfer Entropy) to\nstudy the relation between magnetic oscillations and locally measured plasma\nrotation velocity (related to Zonal Flows). For this purpose, we study a large\nnumber of discharges in two magnetic configurations, corresponding to 'fast'\nand 'slow' transitions. With the 'slow' transitions, the developing Zonal Flow\nprior to the transition is associated with the gradual reduction of magnetic\noscillations. The transition itself is marked by a strong spike of 'information\ntransfer' from magnetic to velocity oscillations, suggesting that the magnetic\ndrive may play a role in setting up the final sheared flow responsible for the\nH-mode transport barrier. Similar observations were made for the 'fast'\ntransitions. Thus, it is shown that magnetic oscillations associated with\nrational surfaces play an important and active role in confinement transitions,\nso that electromagnetic effects should be included in any complete transition\nmodel.", "category": "physics_plasm-ph" }, { "text": "Towards a microscopic theory of particle charging: We recently questioned the treatment of a dust particle as a perfect absorber\nfor electrons and ions and proposed a surface model for the charge of a dust\nparticle in a quiescent plasma which combines the microscopic physics at the\ngrain boundary (sticking into and desorption from external surface states) with\nthe macrophysics of the discharge (plasma collection fluxes). Within this model\nthe charge and partial screening of the particle can be calculated without\nrelying on the condition that the total electron collection flux balances on\nthe grain surface the total ion collection flux. Grain charges obtained from\nour approach compared favorably with experimental data. The purpose of this\npaper is to describe our model in more detail, in particular, the hypotheses on\nwhich it is built, contrast it with the standard charging models based on flux\nbalancing on the grain surface, and to analyze additional experimental data.", "category": "physics_plasm-ph" }, { "text": "Differential forms for plasma physics: Differential forms provide a coordinate-free way to express many quantities\nand relations in mathematical physics. In particular, they are useful in plasma\nphysics. This tutorial gives a guide so that you can read the plasma physics\nliterature that uses them and apply them yourself.", "category": "physics_plasm-ph" }, { "text": "Effect of the bubble deformation in the3D nonlinear laser wake-field\n acceleration: A new analytical approach for bubble deformation was used for optimization of\nthe electron acceleration in the 3D highly nonlinear laser wake-field regime.\nInjection of the electron bunch with initial velocity in the bubble was\nconsidered in the inhomogeneous plasma with linearly density ramp. The\nresearchers show that deformation of the bubble shape has an efficient role on\nthe trapping of the electrons in the acceleration region. The influence of the\nlinearly density ramp on the electron bunch trapping ratio and its mean energy\nwas considered by the numerical method.", "category": "physics_plasm-ph" }, { "text": "The role of three-dimensional transport in driving enhanced electron\n acceleration during magnetic reconnection: Magnetic reconnection is an important driver of energetic particles in many\nastrophysical phenomena. Using kinetic particle-in-cell (PIC) simulations, we\nexplore the impact of three-dimensional reconnection dynamics on the efficiency\nof particle acceleration. In two-dimensional systems, Alfv\\'enic outflows expel\nenergetic electrons into flux ropes where they become trapped and disconnected\nfrom acceleration regions. However, in three-dimensional systems these flux\nropes develop axial structure that enables particles to leak out and return to\nacceleration regions. This requires a finite guide field so that particles may\nmove quickly along the flux rope axis. We show that greatest energetic electron\nproduction occurs when the guide field is of the same order as the reconnecting\ncomponent: large enough to facilitate strong transport, but not so large as to\nthrottle the dominant Fermi mechanism responsible for efficient electron\nacceleration. This suggests a natural explanation for the envelope of electron\nacceleration during the impulsive phase of eruptive flares.", "category": "physics_plasm-ph" }, { "text": "Electron and ion thermal forces in complex (dusty) plasmas: Expressions for the ion and electron thermal forces acting on a charged\ngrain, suspended in a weakly ionized plasma subject to temperature gradients,\nare derived. The main emphasize is on the conditions pertinent to the\ninvestigations of complex (dusty) plasmas in gas discharges. Estimates show\nthat for the electron temperature gradients $\\sim {\\mathcal O}$(eV/cm)\ntypically encountered in laboratory gas discharges, the electron thermal force\ncan become an important player among other forces acting on micron-size grains.", "category": "physics_plasm-ph" }, { "text": "A two-fluid analysis of waves in a warm ion-electron plasma: Following recent work, we discuss waves in a warm ideal two-fluid plasma\nconsisting of electrons and ions starting from a completely general, ideal\ntwo-fluid dispersion relation. The plasma is characterised by five variables:\nthe electron and ion magnetisations, the squared electron and ion sound speeds,\nand a parameter describing the angle between the propagation vector and the\nmagnetic field. The dispersion relation describes 6 pairs of waves which we\nlabel S, A, F, M, O, and X. Varying the angle, it is argued that parallel and\nperpendicular propagation (with respect to the magnetic field) exhibit unique\nbehaviour. This behaviour is characterised by the crossing of wave modes which\nis prohibited at oblique angles. We identify up to 6 different parameter\nregimes where a varying number of exact mode crossings in the special parallel\nor perpendicular orientations can occur. We point out how any ion-electron\nplasma has a critical magnetisation (or electron cyclotron frequency) at which\nthe cutoff ordering changes, leading to different crossing behaviour. These are\nrelevant for exotic plasma conditions found in pulsar and magnetar\nenvironments. Our discussion is fully consistent with ideal relativistic MHD\nand contains light waves. Additionally, exploiting the general nature of the\ndispersion relation, phase and group speed diagrams can be computed at\narbitrary wavelengths for any parameter regime. Finally, we recover earlier\napproximate dispersion relations that focus on low-frequency limits and make\ndirect correspondences with some selected kinetic theory results.", "category": "physics_plasm-ph" }, { "text": "Suppression of core turbulence by profile shaping in Wendelstein 7-X: In the Wendelstein 7-X magnetic confinement experiment, a reduction of\nturbulent density fluctuations as well as anomalous impurity diffusion is\nassociated with a peaking of the plasma density profile. These effects\ncorrelate with improved confinement and appear largely due to a reduction of\nanomalous transport as the change in neoclassical transport is small. The\nobserved decrease of turbulent heat flux with increased density gradients is in\nagreement with nonlinear gyrokinetic simulations, and has been attributed to\nthe unique geometry of W7-X that limits the severity of trapped electron modes.", "category": "physics_plasm-ph" }, { "text": "Implementation of energy transfer technique in ORB5 to study\n collisionless wave-particle interactions in phase-space: A new diagnostic has been developed to investigate the wave-particle\ninteraction in the phase-space in gyrokinetic particle-in-cell codes. Based on\nthe projection of energy transfer terms onto the velocity space, the technique\nhas been implemented and tested in the global code ORB5 and it gives an\nopportunity to localise velocity domains of maximum wave-plasma energy exchange\nfor separate species. Moreover, contribution of different species and\nresonances can be estimated as well, by integrating the energy transfer terms\nin corresponding velocity domains. This Mode-Plasma-Resonance (MPR) diagnostic\nhas been applied to study the dynamics of the Energetic-particle-induced\nGeodesic Acoustic Modes (EGAMs) in an ASDEX Upgrade shot, by analysing the\ninfluence of different species on the mode time evolution. Since the equations\non which the diagnostic is based, are valid in both linear and nonlinear cases,\nthis approach can be applied to study nonlinear plasma effects. As a possible\nfuture application, the technique can be used, for instance, to investigate the\nnonlinear EGAM frequency chirping, or the plasma heating due to the damping of\nthe EGAMs.", "category": "physics_plasm-ph" }, { "text": "Modelling arbitrarily shaped and tightly focused laser pulses in\n electromagnetic codes: Investigation of laser matter interaction with electromagnetic codes requires\nto implement sources for the electromagnetic fields. A way to do so is to\nprescribe the fields at the numerical box boundaries in order to achieve the\ndesired fields inside the numerical box. Here we show that the often used\nparaxial approximation can lead to unexpected field profiles with strong impact\non the laser matter interaction results. We propose an efficient numerical\nalgorithm to compute the required laser boundary conditions consistent with the\nMaxwell's equations for arbitrarily shaped, tightly focused laser pulses.", "category": "physics_plasm-ph" }, { "text": "Hydrogen and deuterium in shock wave experiments, ab initio simulations\n and chemical picture modeling: We present equation of state data of shock compressed hydrogen and deuterium.\nThese have been calculated in the physical picture by using {\\it ab initio}\nmolecular dynamics simulations based on finite temperature density functional\ntheory as well as in the chemical picture via the Saha-D model. The results are\ncompared in detail with data of shock wave experiments obtained for condensed\nand gaseous precompressed hydrogen and deuterium targets in a wide range of\nshock compressions from low pressures up to megabars.", "category": "physics_plasm-ph" }, { "text": "Pressure-Strain Interaction: II. Decomposition in Magnetic Field-Aligned\n Coordinates: In weakly collisional and collisionless magnetized plasmas, the\npressure-strain interaction describes the rate of conversion between bulk flow\nand thermal energy density. In this study, we derive an analytical expression\nfor the pressure-strain interaction in a coordinate system with an axis aligned\nwith the local magnetic field. The result is eight groups of terms\ncorresponding to different physical mechanisms that can contribute to the\npressure-strain interaction. We provide a physical description of each term.\nThe results are immediately of interest to weakly collisional and collisionless\nmagnetized plasmas and the fundamental processes that happen therein, including\nmagnetic reconnection, magnetized plasma turbulence, and collisionless shocks.\nThe terms in the field-aligned coordinate decomposition are likely accessible\nto measurement with satellite observations.", "category": "physics_plasm-ph" }, { "text": "Helium-3 and Helium-4 acceleration by high power laser pulses for hadron\n therapy: The laser driven acceleration of ions is considered a promising candidate for\nan ion source for hadron therapy of oncological diseases. Though proton and\ncarbon ion sources are conventionally used for therapy, other light ions can\nalso be utilized. Whereas carbon ions require 400 MeV per nucleon to reach the\nsame penetration depth as 250 MeV protons, helium ions require only 250 MeV per\nnucleon, which is the lowest energy per nucleon among the light ions. This fact\nalong with the larger biological damage to cancer cells achieved by helium\nions, than that by protons, makes this species an interesting candidate for the\nlaser driven ion source. Two mechanisms (Magnetic Vortex Acceleration and\nhole-boring Radiation Pressure Acceleration) of PW-class laser driven ion\nacceleration from liquid and gaseous helium targets are studied with the goal\nof producing 250 MeV per nucleon helium ion beams that meet the hadron therapy\nrequirements. We show that He3 ions, having almost the same penetration depth\nas He4 with the same energy per nucleon, require less laser power to be\naccelerated to the required energy for the hadron therapy.", "category": "physics_plasm-ph" }, { "text": "Gyrokinetic simulations in stellarators using different computational\n domains: In this work, we compare gyrokinetic simulations in stellarators using\ndifferent computational domains, namely, flux tube, full-flux-surface, and\nradially global domains. Two problems are studied: the linear relaxation of\nzonal flows and the linear stability of ion temperature gradient (ITG) modes.\nSimulations are carried out with the codes EUTERPE, GENE, GENE-3D, and stella\nin magnetic configurations of LHD and W7-X using adiabatic electrons. The zonal\nflow relaxation properties obtained in different flux tubes are found to differ\nwith each other and with the radially global result, except for sufficiently\nlong flux tubes, in general. The flux tube length required for convergence is\nconfiguration-dependent. Similarly, for ITG instabilities, different flux tubes\nprovide different results, but the discrepancy between them diminishes with\nincreasing flux tube length. Full-flux-surface and flux tube simulations show\ngood agreement in the calculation of the growth rate and frequency of the most\nunstable modes in LHD, while for W7-X differences in the growth rates are found\nbetween the flux tube and the full-flux-surface domains. Radially global\nsimulations provide results close to the full-flux-surface ones. The radial\nscale of unstable ITG modes is studied in global and flux tube simulations\nfinding that in W7-X, the radial scale of the most unstable modes depends on\nthe binormal wavenumber, while in LHD no clear dependency is found.", "category": "physics_plasm-ph" }, { "text": "Magnetohydrodynamic Stability at a Separatrix: Part II: In the first part to this paper\\cite{part1} it was shown how a simple\nMagnetohydrodynamic model could be used to determine the stability of a Tokamak\nplasma's edge to a Peeling (External Kink) mode. Stability was found to be\ndetermined by the value of $\\Delta'$, a normalised measure of the discontinuity\nin the radial derivative of the radial perturbation to the magnetic field at\nthe plasma-vacuum interface. Here we calculate $\\Delta'$, but in a way that\navoids the numerical divergences that can arise near a separatrice's X-point.\nThis is accomplished by showing how the method of conformal transformations may\nbe generalised to allow their application to systems with a non-zero boundary\ncondition, and using the technique to obtain analytic expressions for both the\nvacuum energy and $\\Delta'$. A conformal transformation is used again to obtain\nan equilibrium vacuum field surrounding a plasma with a separatrix. This allows\nthe subsequent evaluation of the vacuum energy and $\\Delta'$. For a\nplasma-vacuum boundary that approximates a separatrix, the growth rate $\\gamma$\nnormalised by the Aflven frequency $\\gamma_A$ is then found to have\n$\\ln(\\gamma/\\gamma_A)=-{1/2} \\ln (q'/q)$. Consequences for Peeling mode\nstability are discussed.", "category": "physics_plasm-ph" }, { "text": "Experimental and synthetic measurements of polarized synchrotron\n emission from runaway electrons in Alcator C-Mod: This paper presents the first experimental analysis of polarized synchrotron\nemission from relativistic runaway electrons (REs) in a tokamak plasma.\nImportantly, we show that the polarization information of synchrotron radiation\ncan be used to diagnose spatially-localized RE pitch angle distributions.\nSynchrotron-producing REs were generated during low density, Ohmic, diverted\nplasma discharges in the Alcator C-Mod tokamak. The ten-channel Motional Stark\nEffect diagnostic was used to measure spatial profiles of the polarization\nangle $\\theta_{\\mathrm{pol}}$ and the fraction $\\mathrm{f}_{\\mathrm{pol}}$ of\ndetected light that was linearly-polarized. Spatial transitions in\n$\\theta_{\\mathrm{pol}}$ of 90$\\deg$---from horizontal to vertical polarization\nand vice versa---are observed in experimental data and are well-explained by\nthe gyro-motion of REs and high directionality of synchrotron radiation.\nPolarized synchrotron emission is modeled with the synthetic diagnostic SOFT;\nits output Green's (or detector response) functions reveal a critical RE pitch\nangle at which $\\theta_{\\mathrm{pol}}$ flips by 90$\\deg$ and\n$\\mathrm{f}_{\\mathrm{pol}}$ is minimal. Using SOFT, we determine the dominant\nRE pitch angle which reproduces measured $\\theta_{\\mathrm{pol}}$ and\n$\\mathrm{f}_{\\mathrm{pol}}$ values. The spatiotemporal evolutions of\n$\\theta_{\\mathrm{pol}}$ and $\\mathrm{f}_{\\mathrm{pol}}$ are explored in detail\nfor one C-Mod discharge. For channels viewing REs near the magnetic axis and\nflux surfaces $q$ = 1 and 4/3, disagreements between synthetic and experimental\nsignals suggest that the sawtooth instability may be influencing RE dynamics.\nFurthermore, other sources of pitch angle scattering, not considered in this\nanalysis, could help explain discrepancies between simulation and experiment.", "category": "physics_plasm-ph" }, { "text": "Comment on \"Deuterium--tritium fusion reactors without external fusion\n breeding\" by Eliezer et al: Inclusion of inverse Compton effects in the calculation of\ndeuterium-deuterium burn under the extreme conditions considered by Eliezer et\nal. [Phys. Lett. A 243 (1998) 298] are shown to decrease the maximum burn\ntemperature from about 300 keV to only 100--150 keV. This decrease is such that\ntritium breeding by the DD --> T + p reaction is not sufficient to replace the\nsmall amount of tritium that is initially added to the deuterium plasma in\norder to trigger ignition at less than 10 keV.", "category": "physics_plasm-ph" }, { "text": "Half-and-half HFCG with double-end initiation: Design of helical flux compression generator with double-end initiation is\ndescribed. The developed design approach makes it possible to increase the\npower in the load by 60\\% and shorten the rise time of the pulse by 83\\% in\ncomparison with a conventional helical FCG that is made in the same dimensions\nbut with the single-end initiation of explosive. When using an\nelectro-explosive opening switch, the developed FCG allows one to increase the\nswitched current by 35\\%.", "category": "physics_plasm-ph" }, { "text": "SDR, EVC, and SDREVC: Limitations and Extensions: Methods for reducing the radius, temperature, and space charge of nonneutral\nplasma are usually reported for conditions which approximate an ideal Penning\nMalmberg trap. Here we show that (1) similar methods are still effective under\nsurprisingly adverse circumstances: we perform SDR and SDREVC in a strong\nmagnetic mirror field using only 3 out of 4 rotating wall petals. In addition,\nwe demonstrate (2) an alternative to SDREVC, using e-kick instead of EVC and\n(3) an upper limit for how much plasma can be cooled to T < 20 K using EVC.\nThis limit depends on the space charge, not on the number of particles or the\nplasma density.", "category": "physics_plasm-ph" }, { "text": "Magnetic field amplification in a laser-irradiated thin foil by return\n current electrons carrying orbital angular momentum: Magnetized high energy density physics offers new opportunities for observing\nmagnetic field-related physics for the first time in the laser-plasma context.\nWe focus on one such phenomenon, which is the ability of a laser-irradiated\nmagnetized plasma to amplify a seed magnetic field. We performed a series of\nfully kinetic 3D simulations of magnetic field amplification by a\npicosecond-scale relativistic laser pulse of intensity $4.2\\times 10^{18}$\nW/cm$^2$ incident on a thin foil. We observe axial magnetic field amplification\nfrom an initial 0.1 kT seed to 1.5 kT over a volume of several cubic microns,\npersisting hundreds of femtoseconds longer than the laser pulse duration. The\nmagnetic field amplification is driven by electrons in the return current\ngaining favorable orbital angular momentum from the seed magnetic field. This\nmechanism is robust to laser polarization and delivers order-of-magnitude\namplification over a range of simulation parameters.", "category": "physics_plasm-ph" }, { "text": "Single-Shot Electron Radiography Using a Laser-Plasma Accelerator: Contact and projection electron radiography of static targets was\ndemonstrated using a laser plasma accelerator driven by a kilojoule, picosecond\nclass laser as a source of relativistic electrons with an average energy of 20\nMeV. Objects with areal densities as high as 7.7 g/cm^2 were probed in\nmaterials ranging from plastic to tungsten, and radiographs with resolution as\ngood as 90 micrometers were produced. The effects of electric fields produced\nby the laser ablation of the radiography objects were observed and are well\ndescribed by an analytic expression relating imaging magnification change to\nelectric field strength.", "category": "physics_plasm-ph" }, { "text": "Spatial geometry of charged rotating and non-rotating rings in rotating\n and non-rotating frames: Spatial geometry of charged thin rotating and non-rotating rings in a\nrotating frame is investigated. It is shown, on an example of interaction\nbetween a charged probe and two positive charged non-rotating and negative\ncharged rotating rings that the spatial geometry of the rotating ring in the\nrotating frame has to be different to the spatial geometry of the rotating\nframe. In the absent of direct relation between the spatial geometry rotating\nframe and the spatial geometry of the rotating ring in that frame the\npossibility of a non-flat spatial geometry of rotating electron rings in\ntokamak plasma is discussed.", "category": "physics_plasm-ph" }, { "text": "Pauli blocking effects on pair creation in strong electric field: The process of electron-positron pair creation and oscillation in uniform\nelectric field is studied, taking into account Pauli exclusion principle.\nGenerally, we find that pair creation is suppressed, hence coherent\noscillations occur on longer time scales. Considering pair creation in already\nexisting electron-positron plasma we find that the dynamics depends on pair\ndistribution function. We considered Fermi-Dirac distribution of pairs and\nfound that for small temperatures pair creation is suppressed, while for small\nchemical potentials it increases: heating leads to enhancement of pair\ncreation.", "category": "physics_plasm-ph" }, { "text": "Recovering Gardner Restacking with Purely Diffusive Operations: The maximum particle kinetic energy that can be extracted from an initial\nsix-dimensional phase space distribution motivates the concept of free or\navailable energy. The free energy depends on the allowed operations that can be\nperformed. A key concept underlying the theoretical treatment of plasmas is the\nGardner free energy, where the exchange of the contents of equal phase volumes\nis allowed. A second free energy concept is the diffusive free energy, in which\nthe contents of volumes are instead averaged. For any finite discretization of\nphase space, the diffusive free energy is known to be less than the Gardner\nfree energy. However, despite the apparent fundamental differences between\nthese free energies, it is demonstrated here that the Gardner free energy may\nbe recovered from the continuous limit of the diffusive free energy, leading to\nthe surprise that macroscopic phase-space conservation can be achieved by\nostensibly entropy-producing microscopic operations.", "category": "physics_plasm-ph" }, { "text": "Suppression of turbulence and subcritical fluctuations in differentially\n rotating gyrokinetic plasmas: Differential rotation is known to suppress linear instabilities in fusion\nplasmas. However, even in the absence of growing eigenmodes, subcritical\nfluctuations that grow transiently can lead to sustained turbulence. Here\ntransient growth of electrostatic fluctuations driven by the parallel velocity\ngradient (PVG) and the ion temperature gradient (ITG) in the presence of a\nperpendicular ExB velocity shear is considered. The maximally simplified case\nof zero magnetic shear is treated in the framework of a local shearing box.\nThere are no linearly growing eigenmodes, so all excitations are transient. The\nmaximal amplification factor of initial perturbations and the corresponding\nwavenumbers are calculated as functions of q/\\epsilon (=safety factor/aspect\nratio), temperature gradient and velocity shear. Analytical results are\ncorroborated and supplemented by linear gyrokinetic numerical tests. For\nsufficiently low values of q/\\epsilon (<7 in our model), regimes with fully\nsuppressed ion-scale turbulence are possible. For cases when turbulence is not\nsuppressed, an elementary heuristic theory of subcritical PVG turbulence\nleading to a scaling of the associated ion heat flux with q, \\epsilon, velocity\nshear and temperature gradient is proposed; it is argued that the transport is\nmuch less stiff than in the ITG regime.", "category": "physics_plasm-ph" }, { "text": "Wave-particle interactions in a long traveling wave tube with upgraded\n helix: We investigate the interaction of electromagnetic waves and electron beams in\na 4 meters long traveling wave tube (TWT). The device is specially designed to\nsimulate beam-plasma experiments without appreciable noise. This TWT presents\nan upgraded slow wave structure (SWS) that results in more precise measurements\nand makes new experiments possible. We introduce a theoretical model describing\nwave propagation through the SWS and validated by the experimental dispersion\nrelation, impedance, phase and group velocities. We analyze nonlinear effects\narising from the beam-wave interaction, such as the modulation of the electron\nbeam and the wave growth and saturation process. When the beam current is low,\nthe wave growth coefficient and saturation amplitude follow the linear theory\npredictions. However, for high values of current, nonlinear space charge\neffects become important and these parameters deviate from the linear\npredictions, tending to a constant value. After saturation, we also observe\ntrapping of the beam electrons, which alters the wave amplitude along the TWT.", "category": "physics_plasm-ph" }, { "text": "Classification of head-on collisions of ion-acoustic solitary waves in a\n plasma with cold ions and Boltzmann elecrons: Head-on collisions of ion-acoustic solitary waves in a collisionless plasma\nconsisting of cold ions and Boltzmann electrons are studied using the\nparticle-in-cell simulation. It is shown that the collision of solitary waves\ncan occur under different scenarios. Solitary waves preserve or do not preserve\ntheir amplitudes and shapes after a collision, depending on their initial\namplitudes. The range of initial amplitudes, at which a solitary wave preserves\nits identity after collisions, is established. The use of a diagram of initial\namplitudes of colliding solitary waves to consider possible collision scenarios\nis discussed. The characteristic regions in the diagram of the initial\namplitudes corresponding to different collision scenarios are determined, and a\nclassification of head-on collisions of ion-acoustic solitary waves in a plasma\nis proposed.", "category": "physics_plasm-ph" }, { "text": "Thermomagnetic instability of plasma composition gradients: We show that, under Braginskii magneto-hydrodynamics, anti-parallel gradients\nin average ion charge state and electron temperature can be unstable to the\ngrowth of self-generated magnetic fields. The instability is analogous to the\nfield-generating thermomagnetic instability, although it is driven by the\ncollisional thermal force magnetic source term rather than the Biermann battery\nterm. The gradient in ion charge state causes a gradient in collisionality,\nwhich couples with temperature perturbations to create a self-generated\nmagnetic field. This magnetic field deflects the electron heat flux in a way\nthat reinforces the temperature perturbation. The derived linearized growth\nrate, typically on hydrodynamic timescales, includes the resistive and thermal\nsmoothing. It increases with large ion composition gradients and electron heat\nflux, conditions typical of the hohlraum walls or contaminant mix jets in\ninertial confinement fusion implosions. However, extended magneto-hydrodynamic\nsimulations indicate that the instability is usually dominated and stabilized\nby the nonlinear Nernst advection, in a similar manner to the standard\nthermomagnetic instability.", "category": "physics_plasm-ph" }, { "text": "Fast magnetic reconnection and the ideal evolution of a magnetic field: Regardless of how small non-ideal effects may be, phenomena associated with\nchanges in magnetic field line connections are frequently observed to occur on\nan Alfv\\'enic time scale. Since it is mathematically impossible for magnetic\nfield line connections to change when non-ideal effects are identically zero,\nan ideal evolution must naturally lead to states of unbounded sensitivity to\nnon-ideal effects. That such an evolution is natural is demonstrated using\nLagrangian coordinates based on the flow velocity of the magnetic field lines.\nThe Lagrangian representation of an evolving magnetic field is highly\nconstrained when neither the magnetic field strength nor the forces exerted by\nthe magnetic field increase exponentially with time. The development of a state\nof fast reconnection consistent with these constraints (1) requires a\nthree-dimensional evolution, (2) has an exponentially increasing sensitivity to\nnon-ideal effects, and (3) has a parallel current density, which lies in\nexponentially thinning but exponentially widening ribbons, with a magnitude\nthat is limited to a slow growth. The implication is that exponential growth in\nsensitivity is the cause of fast magnetic reconnection when non-ideal effects\nare sufficiently small. The growth of the non-ideal effect of the resistivity\nmultiplied by the parallel current density is far too slow to be competitive.", "category": "physics_plasm-ph" }, { "text": "A Dynamic Model of Streamer Coupling for High Pressure Discharges: A streamer coupling theory is developed to describe the formation of\nhomogenous emission, and the high moving speed of emission patterns in high\npressure discharges. By considering the effects of both electron diffusion and\nelectronic drift in the streamer head, the minimum required preionization level\n$n_{\\rm min}$ for the formation of streamer coupling is found to depend on\nelectric field strength, gas pressure and electron temperature. The homogeneity\nand moving speed of the emission pattern in streamer coupling head increase\nwith preionization level $n_0$, when $n_0 > n_{\\rm min}$. The predicted results\nfor atmospheric helium plasma indicate $n_{\\rm min} \\sim 10^5~{\\rm cm^{-3}}$\nand moving speed of $10^4 - 10^6$ m/s, in agreement with experiments.", "category": "physics_plasm-ph" }, { "text": "Gyrokinetic analysis and simulation of pedestals, to identify the\n culprits for energy losses using fingerprints: Fusion performance in tokamaks hinges critically on the efficacy of the Edge\nTransport Barrier (ETB) at suppressing energy losses. The new concept of\nfingerprints is introduced to identify the instabilities that cause the\ntransport losses in the ETB of many of today's experiments, from widely posited\ncandidates. Analysis of the Gyrokinetic-Maxwell equations, and gyrokinetic\nsimulations of experiments, find that each mode type produces characteristic\nratios of transport in the various channels: density, heat and impurities.\nThis, together with experimental observations of transport in some channel, or,\nof the relative size of the driving sources of channels, can identify or\ndetermine the dominant modes causing energy transport. In multiple ELMy H-mode\ncases that are examined, these fingerprints indicate that MHD-like modes are\napparently not the dominant agent of energy transport; rather, this role is\nplayed by Micro-Tearing Modes (MTM) and Electron Temperature Gradient (ETG)\nmodes, and in addition, possibly Ion Temperature Gradient (ITG)/Trapped\nElectron Modes (ITG/TEM) on JET. MHD-like modes may dominate the electron\nparticle losses. Fluctuation frequency can also be an important means of\nidentification, and is often closely related to the transport fingerprint. The\nanalytical arguments unify and explain previously disparate experimental\nobservations on multiple devices, including DIII-D, JET and ASDEX-U, and\ndetailed simulations of two DIII-D ETBs also demonstrate and corroborate this.", "category": "physics_plasm-ph" }, { "text": "On the influence of Langmuir wave spectra on the spectra of\n electromagnetic waves generated in solar plasma with double plasma frequency: In this paper, we consider the spectral dependences of transverse\nelectromagnetic waves generated in solar plasma at coalescence of Langmuir\nwaves. It is shown that different spectra of Langmuir waves lead to\ncharacteristic types of transversal electromagnetic wave spectra, what makes it\npossible to diagnose the features of the spectra of Langmuir waves generated in\nsolar plasma.", "category": "physics_plasm-ph" }, { "text": "Annihilation-Gamma-based Diagnostic Techniques for Magnetically Confined\n Electron-Positron Pair Plasma: Efforts are underway to magnetically confine electron--positron pair plasmas\nto study their unique behavior, which is characterized by significant changes\nin plasma time and length scales, supported waves, and unstable modes. However,\nuse of conventional plasma diagnostics presents challenges with these\nlow-density and annihilating matter-antimatter plasma. To address this problem,\nwe propose to develop techniques based on the distinct emission provided by\nannihilation. This emission exhibits two spatial correlations: the distance\nattenuation of isotropic sources and the back-to-back propagation of\nmomentum-preserving 2-$\\gamma$ annihilation. We present the results of our\nanalysis of the $\\gamma$ emission rate and the spatial profile of the\nannihilation in a magnetized pair plasma from direct pair collisions, from the\nformation and decay of positronium, as well as from transport processes. In\norder to demonstrate the effectiveness of annihilation-based techniques, we\ntested them on annular $\\gamma$ emission profiles produced by a $\\beta^+$\nradioisotope on a rotating turntable. Direct and positronium-mediated\nannihilation result in overlapping volumetric $\\gamma$ sources, and the\n2-$\\gamma$ emission from these volumetric sources can be tomographically\nreconstructed from coincident counts in multiple detectors. Transport processes\nresult in localized annihilation where field lines intersect walls, limiters,\nor internal magnets. These localized sources can be identified by the\nfractional $\\gamma$ counts on spatially distributed detectors.", "category": "physics_plasm-ph" }, { "text": "An ion species model for positive ion sources - part I description of\n the model: A one dimensional model of the magnetic multipole volume plasma source has\nbeen developed for use in intense ion/neutral atom beam injectors. The model\nuses plasma transport coefficients for particle and energy flow to create a\ndetailed description of the plasma parameters along an axis parallel to that of\nthe extracted beam. Primarily constructed for applications to neutral beam\ninjection systems on fusion devices, the model concentrates on the hydrogenic\nisotopes but can be extended to any gas by substitution of the relevant masses,\ncross sections and rate coefficients. The model considers the flow of fast\nionizing electrons that create the ratios of the three hydrogenic isotope ion\nspecies, H+, H2 +, H3 + (and similarly for deuterium and tritium) as they flow\ntowards the beam extraction electrode, together with the production of negative\nhydrogenic ions through volume processes. The use of detailed energy balance in\nthe discharge allows the determination of the fraction of the gas density that\nis in an atomic state and also the gas temperature as well as the electron\ntemperatures and plasma potential. Comparisons are made between the results of\nthe model and experimental measurements in deuterium from a number of different\nfilament driven sources used on beam heating facilities.", "category": "physics_plasm-ph" }, { "text": "Loading loss-cone distributions in particle simulations: Numerical procedures to generate random variates that follow loss-cone\nvelocity distributions in particle simulations are presented. We propose a\nsimple summation algorithm for the Ashour-Abdalla--Kennel-type loss-cone\ndistribution, also known as the subtracted Maxwellian. For the Dory-type\nloss-cone distribution, we use a random variate for the gamma distribution.\nExtending earlier algorithms for the kappa and Dory-type distributions, we\nconstruct a novel algorithm to generate a popular form of a kappa loss-cone\ndistribution. To better express the loss cone, we discuss another family of\nloss-cone distributions based on the pitch angle. In addition to the\nacceptance-rejection method, we propose two transformation algorithms that\nconvert an isotropic distribution into a loss-cone distribution. This allows us\nto generate loss-cone and kappa loss-cone distributions from the Maxwell and\nkappa distributions.", "category": "physics_plasm-ph" }, { "text": "Demonstration of tunability of HOFI waveguides via start-to-end\n simulations: In recent years, hydrodynamic optical-field-ionized (HOFI) channels have\nemerged as a promising technique to create laser waveguides suitable for\nguiding tightly-focused laser pulses in a plasma, as needed for laser-plasma\naccelerators. While experimental advances in HOFI channels continue to be made,\nthe underlying mechanisms and the roles of the main parameters remain largely\nunexplored. In this work, we propose a start-to-end simulation pipeline of the\nHOFI channel formation and the resulting guiding properties, and use it to\nexplore the underlying physics and the tunability of HOFI channels. This\napproach is benchmarked against experimental measurements. HOFI channels are\nshown to feature excellent guiding properties over a wide range of parameters,\nmaking them a promising and tunable waveguide option for laser-plasma\naccelerators.", "category": "physics_plasm-ph" }, { "text": "Hamilton--Jacobi theory for continuation of magnetic field across a\n toroidal surface supporting a plasma pressure discontinuity: The vanishing of the divergence of the total stress tensor (magnetic plus\nkinetic) in a neighborhood of an equilibrium plasma containing a toroidal\nsurface of discontinuity gives boundary and jump conditions that strongly\nconstrain allowable continuations of the magnetic field across the surface. The\nboundary conditions allow the magnetic fields on either side of the\ndiscontinuity surface to be described by surface magnetic potentials, reducing\nthe continuation problem to that of solving a Hamilton--Jacobi equation. The\ncharacteristics of this equation obey Hamiltonian equations of motion, and a\nnecessary condition for the existence of a continued field across a general\ntoroidal surface is that there exist invariant tori in the phase space of this\nHamiltonian system. It is argued from the Birkhoff theorem that existence of\nsuch an invariant torus is also, in general, sufficient for continuation to be\npossible. An important corollary is that the rotational transform of the\ncontinued field on a surface of discontinuity must, generically, be irrational.", "category": "physics_plasm-ph" }, { "text": "Dispersion Calibration for the National Ignition Facility Electron\n Positron Proton Spectrometers for Intense Laser Matter Interactions: Electron-positron pairs, produced in intense laser-solid interactions, are\ndiagnosed using magnetic spectrometers with image plates, such as the National\nIgnition Facility (NIF) Electron Positron Proton Spectrometers (EPPS). Although\nmodeling can help infer the quantitative value, the accuracy of the models\nneeds to be verified to ensure measurement quality. The dispersion of\nlow-energy electrons and positrons may be affected by fringe magnetic fields\nnear the entrance of the EPPS. We have calibrated the EPPS with six electron\nbeams from a Siemens Oncor linear accelerator (linac) ranging in energy from\n$2.7$--$15.2$ $\\mathrm{MeV}$ as they enter the spectrometer. A Geant4 TOPAS\nMonte-Carlo simulation was set up to match depth dose curves and lateral\nprofiles measured in water at $100$ $\\mathrm{cm}$ source-surface distance. An\naccurate relationship was established between the bending magnet current\nsetting and the energy of the electron beam at the exit window. The simulations\nand measurements were used to determine the energy distributions of the six\nelectron beams at the EPPS slit. Analysis of the scanned image plates together\nwith the determined energy distribution arriving in the spectrometer provide\nimproved dispersion curves for the EPPS.", "category": "physics_plasm-ph" }, { "text": "Diffusive time evolution of the Grad-Shafranov Equation for a Toroidal\n Plasma: We describe the evolution of a plasma equilibrium having a toroidal topology\nin the presence of constant electric resistivity. After outlining the main\nanalytical properties of the solution, we illustrate its physical implications\nby reproducing the essential features of a scenario for the upcoming Italian\nexperiment Divertor Tokamak Test Facility, with a good degree of accuracy.\nAlthough we find the resistive diffusion timescale to be of the order of\n$10^4\\,$s, we observe a macroscopic change in the plasma volume on a timescale\nof $10^2\\,$s, comparable to the foreseen duration of the plasma discharge by\ndesign. In the final part of the work, we compare our self-consistent solution\nto the more common Solov'ev one, and to a family of nonlinear configurations.", "category": "physics_plasm-ph" }, { "text": "Spectral control of high harmonics from relativistic plasmas using\n bicircular fields: We introduce two-color counterrotating circularly polarized laser fields as a\nnew way to spectrally control high harmonic generation (HHG) from relativistic\nplasma mirrors. Through particle-in-cell simulations, we show that only a\nselected group of harmonic orders can appear owing to the symmetry of the laser\nfields and the related conservation laws. By adjusting the intensity ratio of\nthe two driving field components, we demonstrate the overall HHG efficiency,\nthe relative intensity of allowed neighboring harmonic orders, and the\npolarization state of the harmonic source can be tuned. The HHG efficiency of\nthis scheme can be as high as that driven by a linearly polarized laser field.", "category": "physics_plasm-ph" }, { "text": "Plasma heating power dissipation in low temperature hydrogen plasmas: Theoretical framework for power dissipation in low temperature plasmas in\ncorona equilibrium is developed. The framework is based on fundamental\nconservation laws and reaction cross sections and is only weakly sensitive to\nplasma parameters, e.g. electron temperature and density. The theory is applied\nto low temperature atomic and molecular hydrogen laboratory plasmas for which\nthe plasma heating power dissipation to photon emission, ionization and\nchemical potential is calculated. The calculated photon emission is compared to\nrecent experimental results.", "category": "physics_plasm-ph" }, { "text": "Rotational modes of oscillation of rodlike dust grains in a plasma: Three dimensional rotatory modes of oscillations in a one-dimensional chain\nof rodlike charged particles or dust grains in a plasma are investigated. The\ndispersion characteristics of the modes are analyzed. The stability of\ndifferent equilibrium orientations of the rods, phase transitions between the\ndifferent equilibria, and a critical dependence on the relative strength of the\nconfining potential are analyzed. The relations of these processes with liquid\ncrystals, nanotubing, and plasma coating are discussed.", "category": "physics_plasm-ph" }, { "text": "Nonlinear dynamics of phase space zonal structures and energetic\n particle physics in fusion plasmas: A general theoretical framework for investigating nonlinear dynamics of phase\nspace zonal structures is presented in this work. It is then, more\nspecifically, applied to the limit where the nonlinear evolution time scale is\nsmaller or comparable to the wave-particle trapping period. In this limit, both\ntheoretical and numerical simulation studies show that non-adiabatic frequency\nchirping and phase locking could lead to secular resonant particle transport on\nmeso- or macro-scales. The interplay between mode structures and resonant\nparticles then provides the crucial ingredient to properly understand and\nanalyze the nonlinear dynamics of Alfv\\'en wave instabilities excited by\nnon-perturbative energetic particles in burning fusion plasmas. Analogies with\nautoresonance in nonlinear dynamics and with superradiance in free electron\nlasers are also briefly discussed.", "category": "physics_plasm-ph" }, { "text": "Dusty Plasma Experimental (DPEx) device for complex plasma experiments\n with flow: A versatile table-top dusty plasma experimental (DPEx) device to study flow\ninduced excitations of linear and nonlinear waves/structures in a complex\nplasma is presented. In this {$\\Pi$}-shaped apparatus a DC glow discharge\nplasma is produced between a disc shaped anode and a grounded long cathode\n{tray} by applying a high voltage DC in the background of a neutral gas\n{(Argon)} and subsequently a dusty plasma is created by introducing micron\nsized dust particles that get charged and levitated in the sheath region. A\nflow of the dust particles is induced in a controlled manner by adjusting the\npumping speed and the gas flow rate into the device. A full characterisation of\nthe plasma, using Langmuir and emissive probe data, and that of the dusty\nplasma using particle tracking data with the help of an idl based (super)\nParticle Identification and Tracking (sPIT) code is reported. Experimental\nresults on the variation of the dust flow velocity as a function of the neutral\npressure and the gas flow rate are given. {The neutral drag force acting on the\nparticles and the Epstein coefficient are estimated from the initial\nacceleration of the particles}. The potential experimental capabilities of the\ndevice for conducting fundamental studies of flow induced instabilities are\ndiscussed.", "category": "physics_plasm-ph" }, { "text": "A Robust Method for Handling Low Density Regions in Hybrid Simulations\n for Collisionless Plasmas: A robust method to handle vacuum and near vacuum regions in hybrid\nsimulations for space and astrophysical plasmas is presented. The conventional\nhybrid simulation model dealing with kinetic ions and a massless\ncharge-neutralizing electron fluid is known to be susceptible to numerical\ninstability due to divergence of the whistler-mode wave dispersion, as well as\ndivision-by-density operation in regions of low density. Consequently, a pure\nvacuum region is not allowed to exist in the simulation domain unless some ad\nhoc technique is used. To resolve this difficulty, an alternative way to\nintroduce finite electron inertia effect is proposed. Contrary to the\nconventional method, the proposed one introduces a correction to the electric\nfield rather than the magnetic field. It is shown that the generalized Ohm's\nlaw correctly reduces to Laplace's equation in a vacuum which therefore does\nnot involve any numerical problems. In addition, a variable ion-to-electron\nmass ratio is introduced to reduce the phase velocity of high frequency\nwhistler waves at low density regions so that the stability condition is always\nsatisfied. It is demonstrated that the proposed model is able to handle near\nvacuum regions generated as a result of nonlinear self-consistent development\nof the system, as well as pure vacuum regions set up at the initial condition,\nwithout losing the advantages of the standard hybrid code.", "category": "physics_plasm-ph" }, { "text": "Power Deposition on Tokamak Plasma-Facing Components: The SMARDDA software library is used to model plasma interaction with complex\nengineered surfaces. A simple flux-tube model of power deposition necessitates\nthe following of magnetic fieldlines until they meet geometry taken from a CAD\n(Computer Aided Design) database. Application is made to 1) models of ITER\ntokamak limiter geometry and 2) MASTU tokamak divertor designs, illustrating\nthe accuracy and effectiveness of SMARDDA, even in the presence of significant\nnonaxisymmetric ripple field. SMARDDA's ability to exchange data with CAD\ndatabases and its speed of execution also give it the potential for use\ndirectly in the design of tokamak plasma facing components.", "category": "physics_plasm-ph" }, { "text": "Polarization and magnetization in collisional and turbulent transport\n processes: Expressions of polarization and magnetization in magnetically confined\nplasmas are derived, which include full expansions in the gyroradius to treat\neffects of both equilibrium and microscopic electromagnetic turbulence. Using\nthe obtained expressions, densities and flows of particles are related to those\nof gyrocenters. To the first order in the normalized gyroradius expansion, the\nmean part of the particle flow is given by the sum of the gyrocenter flow and\nthe magnetization flow, which corresponds to the so-called magnetization law in\ndrift kinetics, while the turbulent part contains the polarization flow as\nwell. Collisions make an additional contribution to the second-order particle\nflow. The mean particle flux across the magnetic surface is of the second-order\nand it contains classical, neoclassical, and turbulent transport processes. The\nLagrangian variational principle is used to derive the gyrokinetic Poisson and\nAmp\\`{e}re equations which properly include mean and turbulent parts so as to\nbe useful for full-$f$ global electromagnetic gyrokinetic simulations. It is\nfound that the second-order Lagrangian term given by the inner product of the\nturbulent vector potential and the drift velocity consisting of the curvature\ndrift and the $\\nabla B$ drift should be retained in order for the derived\nAmp\\`{e}re equation to correctly include the diamagnetic current which is\nnecessary especially for the full-$f$ high-beta plasma simulations. The\nturbulent parts of these gyrokinetic Poisson and Amp\\`{e}re equations are\nconfirmed to agree with the results derived from the WKB representation in\nearlier works.", "category": "physics_plasm-ph" }, { "text": "Improved multispecies Dougherty collisions: The Dougherty model Fokker-Planck operator is extended to describe nonlinear\nfull-f collisions between multiple species in plasmas. Simple relations for\ncross-species interactions are developed which obey conservation laws, and\nreproduce familiar velocity and temperature relaxation rates. This treatment of\nmultispecies Dougherty collisions, valid for arbitrary mass ratios, satisfies\nthe H-Theorem unlike analogous Bhatnagar-Gross-Krook operators.", "category": "physics_plasm-ph" }, { "text": "A New Hybrid Scheme for Simulations of Highly Collisional RF-Driven\n Plasmas: This work describes a new 1D hybrid approach for modeling atmospheric\npressure discharges featuring complex chemistry. In this approach electrons are\ndescribed fully kinetically using Particle-In-Cell/Monte-Carlo (PIC/MCC)\nscheme, whereas the heavy species are modeled within a fluid description.\nValidity of the popular drift-diffusion approximation is verified against a\n\"full\" fluid model accounting for the ion inertia and a fully kinetic PIC/MCC\ncode for ions as well as electrons. The fluid models require knowledge of the\nmomentum exchange frequency and dependence of the ion mobilities on the\nelectric field when the ions are in equilibrium with the latter. To this end an\nauxiliary Monte-Carlo scheme is constructed. It is demonstrated that the\ndrift-diffusion approximation can overestimate ion transport in simulations of\nRF-driven discharges with heavy ion species operated in the $\\gamma$ mode at\nthe atmospheric pressure or in all discharge simulations for lower pressures.\nThis can lead to exaggerated plasma densities and incorrect profiles provided\nby the drift-diffusion models. Therefore, the hybrid code version featuring the\nfull ion fluid model should be favored against the more popular drfit-diffusion\nmodel, noting that the suggested numerical scheme for the former model implies\nonly a small additional computational cost.", "category": "physics_plasm-ph" }, { "text": "Nonlinear wave interaction and spin models in the MHD regime: Here we consider the influence on the electron spin in the MHD regime.\nRecently developed models which include spin-velocity correlations are taken as\na starting point. A theoretical argument is presented, suggesting that in the\nMHD regime a single fluid electron model with spin correlations is equivalent\nto a model with spin-up and spin-down electrons constituting different fluids,\nbut where the spin-velocity correlations are omitted. Three wave interaction of\n2 shear Alfven waves and a compressional Alfven wave is then taken as a model\nproblem to evaluate the asserted equivalence. The theoretical argument turns\nout to be supported, as the predictions of the two models agree completely.\nFurthermore, the three wave coupling coefficients obey the Manley-Rowe\nrelations, which give further support to the soundness of the models and the\nvalidity of the assumptions made in the derivation. Finally we point out that\nthe proposed two-fluid model can be incorporated in standard Particle-In-Cell\nschemes with only minor modifications.", "category": "physics_plasm-ph" }, { "text": "Self-organization of dissipative and coherent vortex structures in\n non-equilibrium magnetized two-dimensional plasmas: The properties of non-equilibrium magnetized plasmas confined in planar\ngeometry are studied on the basis of the first principle microscopic Langevin\ndynamics computer simulations. The non-equilibrium state of plasmas is\nmaintained due to the recombination and generation of charges.The intrinsic\nmicroscopic structure of non-equilibrium steady-state magnetized plasmas, in\nparticular, the inter-particle correlations and self-organization of vortex\nstructures are examined. The simulations have been performed for a wide range\nof parameters including strong plasma coupling, high charge recombination and\ngeneration rates, and intense magnetic field. As is shown in simulations, the\nnon-equilibrium recombination and generation processes trigger the formation of\nordered dissipative or coherent drift vortex states in 2D plasmas with\ndistinctly spatially separated components, which are far from thermal\nequilibrium. This is evident from the unusual properties of binary\ndistributions and behavior of the Coulomb energy of the system, which turn out\nto be quite different from the ones typical for the equilibrium state of\nplasmas under the same conditions.", "category": "physics_plasm-ph" }, { "text": "Accelerating self-modulated nonlinear waves in weakly and strongly\n magnetized relativistic plasmas: It is known that a nonlinear Schr\\\"odinger equation describes the\nself-modulation of a large amplitude circularly polarized wave in relativistic\nelectron-positron plasmas in the weakly and strongly magnetized limits. Here,\nwe show that such equation can be written as a modified second Painlev\\'e\nequation, producing accelerated propagating wave solutions for those nonlinear\nplasmas. This solution even allows the plasma wave to reverse its direction of\npropagation. The acceleration parameter depends on the plasma magnetization.\nThis accelerating solution is different to the usual soliton solution\npropagating at constant speed.", "category": "physics_plasm-ph" }, { "text": "Symmetry breaking in MAST plasma turbulence due to toroidal flow shear: The flow shear associated with the differential toroidal rotation of tokamak\nplasmas breaks an underlying symmetry of the turbulent fluctuations imposed by\nthe up-down symmetry of the magnetic equilibrium. Using experimental\nBeam-Emission-Spectroscopy (BES) measurements and gyrokinetic simulations, this\nsymmetry breaking in ion-scale turbulence in MAST is shown to manifest itself\nas a tilt of the spatial correlation function and a finite skew in the\ndistribution of the fluctuating density field. The tilt is a statistical\nexpression of the \"shearing\" of the turbulent structures by the mean flow. The\nskewness of the distribution is related to the emergence of long-lived density\nstructures in sheared, near-marginal plasma turbulence. The extent to which\nthese effects are pronounced is argued (with the aid of the simulations) to\ndepend on the distance from the nonlinear stability threshold. Away from the\nthreshold, the symmetry is effectively restored.", "category": "physics_plasm-ph" }, { "text": "Novel Gas-Doping Technique for Local Spectroscopic Measurements in\n Pulsed-Power Systems: A novel method for doping plasmas in pulsed-power experiments with gaseous\nelements has been developed. A fast gas valve, a nozzle, and a skimmer are used\nto generate an ultrasonic gas beam that is injected into a planar-geometry\nmicrosecond plasma-opening-switch (POS). An array of ionization probes with\nrelatively high spatial and temporal resolutions was developed for diagnosing\nthe absolute injected-gas density and its spatial profile. The properties of\nthe gas column were also studied using spectroscopy of line emission that\nresults from the interaction of the doped gas with the POS prefilled plasma.\nThe doped column is found to have a width of ~1 cm and a density of\n(0.8-1.7)*10^14 cm-3. Observations of characteristic emission lines from the\ndoped atoms and their ions allow for various spectroscopic measurements, such\nas the magnetic field from Zeeman splitting and the ion velocity distributions\nfrom Doppler shifts, that are local in three dimensions. It is shown that this\ngas doping technique can also be used to study proton-dominated plasmas that\ncannot be studied with simple emission spectroscopy due to the lack of light\nemitting ions. The variety of gases used with this method, together with the\nsmall valve dimensions and its fast opening, make it potentially useful for\nbroad diagnostics of various short-duration plasma experiments.", "category": "physics_plasm-ph" }, { "text": "Effect of radiation-reaction on charged particle dynamics in a focused\n electromagnetic wave: Effect of radiation-reaction force on the dynamics of a charged particle in\nan intense focused light wave is investigated using the physically appealing\nHartemann-Luhmann equation of motion. It is found that, irrespective of the\nchoice of initial conditions, radiation reaction force causes the charge\nparticle to cross the focal region, thereby enhancing the forward energy gained\nby the particle from the intense light wave. This result is in sharp contrast\nto the well known result, derived in the absence of radiation reaction forces,\nwhere for certain initial conditions the particle reflects from the high\nintensity region of the focused light wave, thereby losing forward energy.\nThese results, which are of relevance to the present day direct laser\nacceleration schemes of charge particle, also agrees with that obtained using\nthe well known Landau-Lifshitz equation of motion.", "category": "physics_plasm-ph" }, { "text": "Evolution of Electrical Resistivity, Thermal Conductivity, and\n Temperature of a solid under the action of Intense Ultrashort Laser pulse: The dynamical properties of Cu in a regime relevant to femtosecond micro\nmachining are obtained on picosecond time scales using pump-probe reflectivity\nstudy for 100fs, 1015 W cm-2 laser pulses. The electrical resistivity is\nobtained by solving Helmoltz equations. The dissipation mechanisms and scaling\nlaws are obtained in high and low temperature limits. The 'resistivity\nsaturation' effect in an unexplored regime intermediate to hot plasma and cold\nsolid is studied in detail. The temperature evolution and thermal conductivity\nis obtained in the temporal range 0 to 30ps after the interaction of laser\npulse with solid Cu.", "category": "physics_plasm-ph" }, { "text": "The gyrokinetic dispersion relation of microtearing modes in\n collisionless toroidal plasmas: We solve the linearized gyrokinetic equation, quasineutrality condition, and\nAmpere's law to obtain the dispersion relation of microtearing modes (MTMs) in\ncollisionless low-beta toroidal plasmas. Consistent with past studies, we find\nthat MTMs are driven unstable by the electron temperature gradient and that\nthis instability drive is mediated by magnetic drifts. The dispersion relation\nthat we derive can be evaluated numerically very quickly and may prove useful\nfor devising strategies to mitigate MTM instability in fusion devices.", "category": "physics_plasm-ph" }, { "text": "EP-Stability-WF: an IMAS-integrated workflow for energetic particle\n stability: The confinement of energetic particles (EPs) generated by fusion reactions\nand external heating methods is crucial for the performance of future fusion\ndevices. However, EP transport can occur due to their interaction with\nelectromagnetic perturbations, affecting heating efficiency and overall\nperformance. Robust reduced models are needed to analyze stability and\ntransport, but their development requires effort. This paper presents an\nautomated IMAS-based workflow for analyzing the time-dependent stability of\nEP-driven modes, focusing on the linear properties of Toroidal Alfven\nEigenmodes (TAEs) in general Tokamak geometry. The workflow utilizes efficient\ncomputational methods and reduced models to deliver fast and reproducible\nresults. A demonstration of the workflow's effectiveness was performed,\nidentifying key linear properties of TAEs in various projected ITER scenarios.\nThis approach represents a critical step towards developing tools for analyzing\nEP transport and optimizing the performance of future fusion reactors.", "category": "physics_plasm-ph" }, { "text": "Numerical investigations of the minimum-B effect in Electron Cyclotron\n Resonance Ion Source: The three-dimensional particle-in-cell model NAM-ECRIS is used for\ninvestigation of how the DECRIS-PM Electron Cyclotron Resonance Ion Source is\nreacting to changes in the source magnetic configuration. The accent is made on\nchanges in the magnetic field at the magnetic trap center, the minimum-B value.\nIt is calculated that the optimal normalized value of the field is ~0.8, close\nto the experimental observations. The reasons for existence of the optimum are\ndiscussed. It is observed that the electron energies are increasing with the\nincreased minimum-B values due to enhanced confinement of the energetic\nelectrons in the plasma. Bumps in energy spectra of the radially lost electrons\nare observed and explained to be due to nonadiabatic losses of electrons.", "category": "physics_plasm-ph" }, { "text": "Free energy cascade in gyrokinetic turbulence: In gyrokinetic theory, the quadratic nonlinearity is known to play an\nimportant role in the dynamics by redistributing (in a conservative fashion)\nthe free energy between the various active scales. In the present study, the\nfree energy transfer is analyzed for the case of ion temperature gradient\ndriven turbulence. It is shown that it shares many properties with the energy\ntransfer in fluid turbulence. In particular, one finds a forward (from large to\nsmall scales), extremely local, and self-similar cascade of free energy in the\nplane perpendicular to the background magnetic field. These findings shed light\non some fundamental properties of plasma turbulence, and encourage the\ndevelopment of large eddy simulation techniques for gyrokinetics.", "category": "physics_plasm-ph" }, { "text": "Quasi-localized charge approximation approach for the nonlinear\n structures in strongly coupled Yukawa systems: Strongly coupled systems occupying the transitional range between the Wigner\ncrystal and fluid phases are most dynamic constituents of the nature. Highly\nlocalized but strongly interacting elements in this phase posses enough thermal\nenergy to trigger the transition between a variety of short to long range order\nphases. Nonlinear excitations are often the carriers of proliferating\nstructural modifications in the strongly coupled Yukawa systems. Well\nrepresented by a laboratory dusty plasma, these systems show explicit\npropagation of nonlinear shocks and solitary structures both in experiments and\nin first principle simulations. The shorter scale length contributions remain\nabsent at strong screening in present approximate models which nevertheless\nprescribe nonlinear solitary solutions that consequently lose their coherence\nin a numerical evolution of the system under a special implementation of the\nquasi-localized charge approximation formulation. The stable coherent\nstructures self-consistently emerge following an initial transient in the\nnumerical evolution which adapts QLCA approach to spatiotemporal domain for\naccessing the nonlinear excitations in the strong screening limit. The present\nkappa ~ 1 limit of the existing Yukawa fluid models to show agreement with the\nexperiment and MD simulations has therefore been overcome and the coherent\nnonlinear excitaitons have become characterizable up to kappa ~ 2.7, before\nthey becoming computationally challenging in present implementation.", "category": "physics_plasm-ph" }, { "text": "Quantum Electrodynamics vacuum polarization solver: The self-consistent modeling of vacuum polarization due to virtual\nelectron-positron fluctuations is of relevance for many near term experiments\nassociated with high intensity radiation sources and represents a milestone in\ndescribing scenarios of extreme energy density. We present a generalized\nfinite-difference time-domain solver that can incorporate the modifications to\nMaxwell's equations due to vacuum polarization. Our multidimensional solver\nreproduced in one-dimensional configurations the results for which an analytic\ntreatment is possible, yielding vacuum harmonic generation and birefringence.\nThe solver has also been tested for two-dimensional scenarios where finite\nlaser beam spot sizes must be taken into account. We employ this solver to\nexplore different types of laser configurations that can be relevant for future\nplanned experiments aiming to detect quantum vacuum dynamics at ultra-high\nelectromagnetic field intensities.", "category": "physics_plasm-ph" }, { "text": "Strong Coulomb Coupling Influences Ion and Neutral Temperatures in\n Atmospheric Pressure Plasmas: Molecular dynamics simulations are used to model ion and neutral temperature\nevolution in partially-ionized atmospheric pressure plasma at different\nionization fractions. Results show that ion-ion interactions are strongly\ncoupled at ionization fractions as low as 10^-5 and that the temperature\nevolution is influenced by effects associated with the strong coupling.\nSpecifically, disorder-induced heating is found to rapidly heat ions on a\ntimescale of the ion plasma period (~10s ps) after an ionization pulse. This is\nfollowed by the collisional relaxation of ions and neutrals, which cools ions\nand heats neutrals on a longer (~ns) timescale. Slight heating then occurs over\na much longer (~ 100s ns) timescale due to ion-neutral three-body\nrecombination. An analytic model of the temperature evolution is developed that\nagrees with the simulation results. A conclusion is that strong coupling\neffects are important in atmospheric pressure plasmas.", "category": "physics_plasm-ph" }, { "text": "Gyrokinetic theory of slab universal modes and the non-existence of the\n Gradient Drift Coupling (GDC) instability: A gyrokinetic linear stability analysis of a collisionless slab geometry in\nthe local approximation is presented. We focus on $k_\\parallel=0$ universal (or\nentropy) modes driven by plasma gradients at small and large plasma $\\beta$.\nThese are small scale non-MHD instabilities with growth rates that typically\npeak near $k_\\perp\\rho_i\\sim 1$ and vanish in the long wavelength $k_\\perp\\to\n0$ limit. This work also discusses a mode known as the Gradient Drift Coupling\n(GDC) instability previously reported in the gyrokinetic literature, which has\na finite growth rate $\\gamma= \\sqrt{\\beta/[2(1+\\beta)]} C_s/|L_p|$ with\n$C_s^2=p_0/\\rho_0$ for $k_\\perp\\to 0$ and is universally unstable for\n$1/L_p\\neq 0$. We show the GDC instability is a spurious, unphysical artifact\nthat erroneously arises due to the failure to respect the total equilibrium\npressure balance $p_0+B_0^2/(8\\pi)=\\text{constant}$, which renders the\nassumption $B_0'=0$ inconsistent if $p_0'\\neq 0$.", "category": "physics_plasm-ph" }, { "text": "On the effect of beating during nonlinear frequency chirping: Spectral analyses of energetic particle (EP) driven bursts of MHD\nfluctuations in magnetically confined plasmas often exhibit multiple\nsimultaneous chirps. While the superposition of oscillations at multiple\nfrequencies necessarily causes beating in the signal acquired by a localized\nexternal probe, self-consistent hybrid simulations of chirping EP modes in a\nJT-60U tokamak plasma have demonstrated the possibility of global beating,\nwhere the electromagnetic field vanishes globally between beats and reappears\nwith opposite phase. This implies that there can be a single field mode that\noscillates at multiple frequencies simultaneously when resonantly driven by\nmultiple density waves in EP phase space. Conversely, this means that the EP\ndensity waves are mutually coupled and interfere with each other via the\njointly driven field, a mechanism ignored in some theories. In this treatise,\nwe study the role of field pulsations in general and beating in particular\nusing the Hamiltonian guiding center orbit-following code ORBIT with a reduced\nwave-particle interaction model in realistic geometry. Through amplitude\npulsations and phase jumps, beating is found to drive the evolution of EP phase\nspace structures. Observations: (1) Beating causes density wave fronts to\nadvance radially in pulses. The resulting chirps become staircase-like. (2) The\nbeats facilitate convective transfer of material between neighboring layers of\nphase space density waves. On the one hand, this may delay detachment of\nsolitary vortices. On the other hand, it facilitates the accumulation of hole\nand clump fragments into larger structures. (3) Long-range chirping occurs when\nmassive holes or clumps detach and drift away from the turbulent belt around\nthe seed resonance. The detached vortices can remain robust and, on average,\nmaintain their concentric nested layers while being perturbed by the field's\ncontinued beating.", "category": "physics_plasm-ph" }, { "text": "Robust avoidance of edge-localized modes alongside gradient formation in\n the negative triangularity tokamak edge: In a series of high performance diverted discharges on DIII-D, we demonstrate\nthat strong negative triangularity (NT) shaping robustly suppresses all\nedge-localized mode (ELM) activity over a wide range of plasma conditions:\n$\\langle n\\rangle=0.1-1.5\\times10^{20}$m$^{-3}$, $P_\\mathrm{aux}=0-15$MW and\n$|B_\\mathrm{t}|=1-2.2$T, corresponding to\n$P_\\mathrm{loss}/P_\\mathrm{LH08}\\sim8$. The full dataset is consistent with the\ntheoretical prediction that magnetic shear in the NT edge inhibits access to\nELMing H-mode regimes; all experimental pressure profiles are found to be at or\nbelow the infinite-$n$ ballooning stability limit. Importantly, we also report\nenhanced edge pressure gradients at strong NT that are significantly steeper\nthan in traditional ELM-free L-mode plasmas and provide significant promise for\nNT reactor integration.", "category": "physics_plasm-ph" }, { "text": "Development and Validation of a Tokamak Skin Effect Transformer model: A control oriented, lumped parameter model for the tokamak transformer\nincluding the slow flux penetration in the plasma (skin effect transformer\nmodel) is presented. The model does not require detailed or explicit\ninformation about plasma profiles or geometry. Instead, this information is\nlumped in system variables, parameters and inputs. The model has an exact\nmathematical structure built from energy and flux conservation theorems,\npredicting the evolution and non linear interaction of the plasma current and\ninternal inductance as functions of the primary coil currents, plasma\nresistance, non-inductive current drive and the loop voltage at a specific\nlocation inside the plasma (equilibrium loop voltage). Loop voltage profile in\nthe plasma is substituted by a three-point discretization, and ordinary\ndifferential equations are used to predict the equilibrium loop voltage as\nfunction of the boundary and resistive loop voltages. This provides a model for\nequilibrium loop voltage evolution, which is reminiscent of the skin effect.\nThe order and parameters of this differential equation are determined\nempirically using system identification techniques. Fast plasma current\nmodulation experiments with Random Binary Signals (RBS) have been conducted in\nthe TCV tokamak to generate the required data for the analysis. Plasma current\nwas modulated in Ohmic conditions between 200kA and 300kA with 30ms rise time,\nseveral times faster than its time constant L/R\\approx200ms. The model explains\nthe most salient features of the plasma current transients without requiring\ndetailed or explicit information about resistivity profiles. This proves that\nlumped parameter modeling approach can be used to predict the time evolution of\nbulk plasma properties such as plasma inductance or current with reasonable\naccuracy; at least in Ohmic conditions without external heating and current\ndrive sources.", "category": "physics_plasm-ph" }, { "text": "Kinetic temperature gradient driven modes in inhomogeneous plasmas: New unstable temperature gradient driven modes in an inhomogeneous plasma are\nidentified. These modes represent transient $\\omega \\simeq k_{\\Vert\n}v_{th}^{(e,i)}$ sound oscillations in magnetized plasma that are kinetically\ndestabilized via Landau interactions. Electron and ion sound branches are\nunstable for large values of the Larmor radius parameter $% k_{\\bot}\\rho\n_{e,i}\\gg 1,$ respectively. The instability occurs due to a specific plasma\nresponse that significantly deviates from Boltzmann distribution in the region\n$k_{\\bot}\\rho_{i,e}\\gg 1$ . Pacs: 52.35 Kt, 52.35 Qz", "category": "physics_plasm-ph" }, { "text": "Study of Gasdynamic Electron Cyclotron Resonance Plasma Vacuum\n Ultraviolet Emission to Optimize Negative Hydrogen Ion Production Efficiency: Negative hydrogen ion sources are used as injectors into accelerators and\ndrive the neutral beam heating in ITER. Certain processes in low-temperature\nhydrogen plasmas are accompanied by the emission of vacuum ultraviolet (VUV)\nemission. Studying the VUV radiation therefore provides volumetric rates of\nplasma-chemical processes and plasma parameters. In the past we have used\ngasdynamic ECR discharge for volumetric negative ion production and\ninvestigated the dependencies between the extracted H$^-$ current density and\nvarious ion source parameters. It was shown that it is possible to reach up to\n80 mA/cm$^2$ of negative ion current density with a two electrode extraction.\nWe report experimental studies on negative hydrogen ion production in a\nhigh-density gasdynamic ECR discharge plasma consisting of two simple mirror\ntraps together with the results of VUV emission measurements. The VUV-power was\nmeasured in three ranges -- Ly$_\\alpha$, Lyman band and molecular continuum --\nvarying the source control parameters near their optima for H$^-$ production.\nIt was shown that the molecular continuum emission VUV power is the highest in\nthe first chamber while Ly$_\\alpha$ emission prevails in the second one.\nModifications for the experimental scheme for further optimization of negative\nhydrogen ion production are suggested.", "category": "physics_plasm-ph" }, { "text": "Turbulence Generation by Shock Interaction with a Highly Non-Uniform\n Medium: An initially planar shock wave propagating into a medium of non-uniform\ndensity will be perturbed, leading to the generation of post-shock velocity\nperturbations. Using numerical simulations we study this phenomenon in the case\nof highly-non-uniform density (order-unity normalized variance,\n$\\sigma_{\\rho}/\\overline{\\rho} \\sim 1$) and strong shocks (shock Mach numbers\n$\\overline{M}_s \\gtrsim 10$). This leads to a highly disrupted shock and a\nturbulent post-shock flow. We simulate this interaction for a range of shock\ndrives and initial density configurations meant to mimic those which might be\npresently achieved in experiments. Theoretical considerations lead to scaling\nrelations, which are found to reasonably predict the post-shock turbulence\nproperties. The turbulent velocity dispersion and turbulent Mach number are\nfound to depend on the pre-shock density dispersion and shock speed in a manner\nconsistent with the linear Richtymer-Meshkov instability prediction. We also\nshow a dependence of the turbulence generation on the scale of density\nperturbations. The post-shock pressure and density, which can be substantially\nreduced relative to the unperturbed case, are found to be reasonably predicted\nby a simplified analysis that treats the extended shock transition region as a\nsingle normal shock.", "category": "physics_plasm-ph" }, { "text": "Dynamics of a 1-D model for the emergence of the plasma edge shear flow\n layer with momentum conserving Reynolds stress: A one-dimensional version of the second-order transition model based on the\nsheared flow amplification by Reynolds stress and turbulence supression by\nshearing is presented. The model discussed in this paper includes a form of the\nReynolds stress which explicitly conserves momentum. A linear stability\nanalysis of the critical point is performed. Then, it is shown that the\ndynamics of weakly unstable states is determined by a reduced equation for the\nshear flow. In the case in which the flow damping term is diffusive, the\nstationary solutions are those of the real Ginzburg-Landau equation.", "category": "physics_plasm-ph" }, { "text": "Non-linear neoclassical model for poloidal asymmetries in tokamak\n pedestals: diamagnetic and radial effects included: Stronger impurity density in-out poloidal asymmetries than predicted by the\nmost comprehensive neoclassical models have been measured in several tokamaks\naround the world during the last decade, calling into question the reduction of\nturbulence by sheared radial electric fields in H-mode tokamak pedestals.\nHowever, these pioneering theories neglect the impurity diamagnetic drift, or\nfail to retain it self-consistently; while recent measurements indicate that it\ncan be of the same order as the ExB drift. We have developed the first\nself-consistent theoretical model retaining the impurity diamagnetic flow and\nthe two-dimensional features it implies due to its associated non-negligible\nradial flow divergence. It successfully explains collisionally the experimental\nimpurity density, temperature and radial electric field in-out asymmetries;\nthus making them consistent with H-mode pedestal turbulence reduction.", "category": "physics_plasm-ph" }, { "text": "Particle-In-Cell observations of Brillouin scattering for laser\n interacting with magnetized overdense plasma: One dimensional Particle-in-cell simulations using OSIRIS-4.0 has been\nconducted to study the interaction of a laser electromagnetic pulse with an\noverdense magnetized plasma target. The external magnetic field has been chosen\nto be directed along the laser propagation direction. This geometry supports\nthe propagation of right (R) and left (L) circularly polarised electromagnetic\nwaves in the plasma. The laser pulse is allowed to propagate inside the plasma\nwhen its frequency falls in the pass band of the dispersion curves of L and/or\nR waves. The strength of the applied external magnetic field is chosen as a\nparameter to ensure that the laser frequency lies in the appropriate pass band.\nIt is demonstrated that for all possible polarization of the incident laser,\nparametric process involving a scattered Electromagnetic wave and an\nelectrostatic mode occur. The parametric process has been identified as that\ndue to the Brillouin back scattering process.", "category": "physics_plasm-ph" }, { "text": "Diagnostics of hot electrons leaving the ECR plasma sustained by the\n high-power gyrotron: Energy distribution of electrons in the plasma sustained by the\nelectron-cyclotron resonance (ECR) discharge has a complicated shape as a\nfunction of various parameters that still remains unknown. Meanwhile, it is an\nimportant plasma characteristic. Some methods and approaches give the\npossibility to estimate or measure the properties of the distributions of the\nelectrons lost from the plasma. One of them, similar to the ion mass\nspectrometry, was used in this work to obtain such distributions in the\nnon-classical continuous ECR ion source with high (up to $ 50-100$ W/cm$ ^ 3 $)\nenergy input for the first time along with bremsstrahlung spectra. For certain\nparameters, a threshold-like regime was discovered, which comprised of the\nbursts of energetic electrons, bremsstrahlung and, supposedly, the development\nof kinetic instabilities.", "category": "physics_plasm-ph" }, { "text": "Solution of the equations of electromagnetic self-consistency in a\n plasma: The system of equations of electromagnetic self-consistency in a plasma is\nanalytically solved for the case of a two-component homogeneous plasma in the\nnon-relativistic approximation.", "category": "physics_plasm-ph" }, { "text": "Low-shear three-dimensional equilibria and vacuum magnetic fields with\n flux surfaces: Stellarators are generically small current and low plasma beta ($\\beta=\np/B^2\\ll1$) devices. Often the construction of vacuum magnetic fields with good\nmagnetic surfaces is the starting point for an equilibrium calculation.\nAlthough in cases with some continuous spatial symmetry flux functions can\nalways be found for vacuum magnetic fields, an analogous function does not, in\ngeneral, exist in three dimensions. This work examines several simple\nequilibria and vacuum magnetic field problems with the intent of demonstrating\nthe possibilities and limitations in the construction of such states. Starting\nwith a simple vacuum magnetic field with closed field lines in a topological\ntorus (toroidal shell with a flat metric), we obtain a self-consistent formal\nperturbation series using the amplitude of the nonsymmetric vacuum fields as a\nsmall parameter. We show that systems possessing stellarator symmetry allow the\nconstruction order by order. We further indicate the significance of\nstellarator symmetry in the amplitude expansion of the full ideal\nmagnetohydrodynamics (MHD) problem as well. We then investigate the conditions\nthat guarantee neighboring flux surfaces given the data on one surface, by\nexpanding in the distance from that surface. We show that it is much more\ndifficult to find low shear vacuum fields with surfaces than force-free fields\nor ideal MHD equilibrium. Finally, we demonstrate the existence of a class of\nvacuum magnetic fields, analogous to `snakes' observed in tokamaks, which can\nbe expanded to all orders.", "category": "physics_plasm-ph" }, { "text": "Features of non-congruent phase transition in modified Coulomb model of\n the binary ionic mixture: Non-congruent gas-liquid phase transition (NCPT) have been studied in\nmodified Coulomb model of a binary ionic mixture C(+6) + O(+8) on a\n\\textit{uniformly compressible} ideal electronic background /BIM($\\sim$)/. The\nfeatures of NCPT in improved version of the BIM($\\sim$) model for the same\nmixture on background of \\textit{non-ideal} electronic Fermi-gas and comparison\nit with the previous calculations are the subject of present study. Analytical\nfits for Coulomb corrections to EoS of electronic and ionic subsystems were\nused in present calculations within the Gibbs--Guggenheim conditions of\nnon-congruent phase equilibrium.Parameters of critical point-line (CPL) were\ncalculated on the entire range of proportions of mixed ions $0\\ell,$ with smaller scales\neliminated, as in renormalization-group methodology. We prove that\nflux-conservation can be violated for an arbitrarily small length-scale $\\ell,$\nand in the absence of any non-ideality, but only if singular current sheets and\nvortex sheets both exist and intersect in sets of large enough dimension. This\nresult gives analytical support to and rigorous constraints on theories of fast\nturbulent reconnection. Mathematically, our theorem is analogous to Onsager's\nresult on energy dissipation anomaly in hydrodynamic turbulence. As a physical\nphenomenon, the breakdown of magnetic-flux conservation in ideal MHD is similar\nto the decay of magnetic flux through a narrow superconducting ring, by\nphase-slip of quantized flux lines. The effect should be observable both in\nnumerical MHD simulations and in laboratory plasma experiments at moderately\nhigh magnetic Reynolds numbers.", "category": "physics_plasm-ph" }, { "text": "A Femtosecond Neutron Source: The possibility to use the ultrashort ion bunches produced by circularly\npolarized laser pulses to drive a source of fusion neutrons with sub-optical\ncycle duration is discussed. A two-side irradiation of a thin foil deuterated\ntarget produces two countermoving ion bunches, whose collision leads to an\nultrashort neutron burst. Using particle-in-cell simulations and analytical\nmodeling, it is evaluated that, for intensities of a few $10^{19} W cm^{-2}$,\nmore than $10^3$ neutrons per Joule may be produced within a time shorter than\none femtosecond. Another scheme based on a layered deuterium-tritium target is\noutlined.", "category": "physics_plasm-ph" }, { "text": "Nonlinear Terms of MHD Equations for Homogeneous Magnetized Shear Flow: We have derived the full set of MHD equations for incompressible shear flow\nof a magnetized fluid and considered their solution in the wave-vector space.\nThe linearized equations give the famous amplification of slow magnetosonic\nwaves and describe the magnetorotational instability. The nonlinear terms in\nour analysis are responsible for the creation of turbulence and self-sustained\nspectral density of the MHD (Alfven and pseudo-Alfven) waves. Perspectives for\nnumerical simulations of weak turbulence and calculation of the effective\nviscosity of accretion disks are shortly discussed in k-space.", "category": "physics_plasm-ph" }, { "text": "Nonlinear electrodynamics at cylindrical \"cumulation\" fronts: Converging cylindrical electromagnetic fields in vacuum have been shown (E.\nI. Zababakhin, M. N. Nechaev, {\\it Soviet Physics JETP}, {\\bf 6}, 345 (1958))\nto exhibit amplitude \"cumulation\". It was found that the amplitude of\nself-similar waves increases without bounds at finite distances from the axis\non the front of the fields reflected from the cylindrical axis. In the present\npaper we propose to exploit this cylindrical cumulation process as a possible\nnew path towards the generation of ultra-strong electromagnetic fields where\nnonlinear quantum electrodynamics (QED) effects come into play. We show that\nthese effects, as described in the long wave-length limit within the framework\nof the Euler Heisenberg Lagrangian, induce a radius-dependent reduction of the\npropagation speed of the cumulation front. {Furthermore we compute the\n$e^+$-$e^-$ pair production rate at the cumulation front and show that the\ntotal number of pairs that are generated scales as the sixth power of the field\namplitude.", "category": "physics_plasm-ph" }, { "text": "Runaway Electron Generation by Decelerating Streamers in Inhomogeneous\n Atmosphere: The dynamics of positive and negative streamers is numerically simulated in\natmospheric pressure air. It is shown that positive and negative streamers\nbehave in radically different ways when decelerating and stopping. When the\nhead potential drops, the negative streamer transits to the mode in which the\npropagation is due to the forward electron drift. In this case, the radius of\nthe ionization wave front increases, whereas the electric field at the streamer\nhead decreases further and the streamer stops. The only advancement mechanism\nfor a positive streamer with decreasing head potential is a decrease in the\neffective radius of the ionization wave, leading to a local increase in the\nelectric field. This mechanism compensates for the decrease in the efficiency\nof gas photoionization at small head diameters. The local electric field at the\nstreamer head can exceed the runaway threshold when the head potential\ndecreases to ~1.2 kV in atmospheric pressure air. In this case, pulsed\ngeneration of a beam of runaway electrons directed into the channel of a\nstopping positive streamer can occur. The energy of the formed pulsed electron\nbeam can vary from 700 V (when increasing the photoionization rate by a factor\nof 10 with respect to the value in atmospheric pressure air) to 2.6 kV (in air\nwith an absorbing additive that reduces the photoionization rate by a factor of\n1000). It is possible that this behavior of decelerating positive streamers can\nexplain the observed bursts of X-ray radiation during the streamer propagation.", "category": "physics_plasm-ph" }, { "text": "An E & B Gyrokinetic Simulation Model for Kinetic Alfv\u00e9n Waves\n inTokamak Plasmas: The gyrokinetic particle simulation serves as a powerful tool for the studies\nof transport, nonlinear phenomenon, and energetic particle physics in tokamak\nplasmas. While most gyrokinetic simulations make use of the scalar and vector\npotentials, a new model (GK-E&B) has been developed by using the E and B field\nin a general and comprehensive form and has been implemented in simulating\nkinetic Alfv\\'en waves in uniform plasma [Chen et al, Science China Phys.\nMechanics & Astronomy 64 (2021)]. In our work, the Chen et al. GK-E&B model has\nbeen expressed in general tokamak geometry explicitly using specific\ncoordinates. Its reduction to the uniform plasma is verified and the numerical\nresults show good agreement with the work by Chen et al. The theoretical\ndispersion relation and numerical results in the local screw pinch model are in\nexcellent agreement. Numerical results show excellent performance in a\nrealistic parameter regime of burning plasmas in terms of high values of $\\beta\n/( M_e k_\\perp^2 \\rho_i^2)$, which is challenging for traditional methods due\nto the ``cancellation' problem. As one application, the GK-E&B model is\nimplemented with kinetic electrons in the local limit. With the matched\nITPA-TAE parameters, numerical results show the capability of the GK-E&B in\ntreating the parallel electron Landau damping for realistic tokamak plasma\nparameters. As another application, the global GK-E&B model is implemented with\nthe dominant electron contribution to $E_\\|$ in the cold electron limit. Its\ncapability in simulating the finite $E_\\|$ due to the finite electron mass is\ndemonstrated.", "category": "physics_plasm-ph" }, { "text": "Gyrokinetic investigation of the damping channels of Alfv\u00e9n modes in\n ASDEX Upgrade: The linear destabilization and nonlinear saturation of energetic-particle\ndriven Alfv\\'enic instabilities in tokamaks strongly depend on the damping\nchannels. In this work, the collisionless damping mechanisms of Alfv\\'enic\nmodes are investigated within a gyrokinetic framework, by means of global\nsimulations with the particle-in-cell code ORB5, and compared with the\neigenvalue code LIGKA and reduced models. In particular, the continuum damping\nand the Landau damping (of ions and electrons) are considered. The electron\nLandau damping is found to be dominant on the ion Landau damping for\nexperimentally relevant cases. As an application, the linear and nonlinear\ndynamics of toroidicity induced Alfv\\'en eigenmodes and energetic-particle\ndriven modes in ASDEX Upgrade is investigated theoretically and compared with\nexperimental measurements.", "category": "physics_plasm-ph" }, { "text": "Simulations of Non-Integer Upconversion in Resonant Six-Wave Scattering: Resonant upconversion through a sixth order relativistic nonlinearity\nresulting in a unique resonance was recently proposed [V. M. Malkin and N. J.\nFisch, Physical Review E 108, 045208 (2023)]. The high order resonance is a\nunique non-integer multiple of a driving pump frequency resulting in a\nfrequency upshift by a factor of $\\approx 3.73$. We demonstrate the presence,\nunique requirements, and growth of this mode numerically. Through tuning waves\nto high amplitude, in a mildly underdense plasma, the six-photon process may\ngrow more than other non-resonant, but lower order processes. The growth of the\nhigh frequency mode remains below the nonlinear growth regime. However,\nextending current numerical results to more strongly coupled resonances with\nlonger pulse propagation distances suggests a pathway to significant\nupconversion.", "category": "physics_plasm-ph" }, { "text": "Magnetic reconnection: from MHD to QED: The paper examines the prospects of using laser plasmas for studying novel\nregimes of the magnetic field line reconnection and charged particle\nacceleration. Basic features of plasma dynamics in the three-dimensional\nconfigurations relevant to the formation of current sheets in a plasma are\naddressed by analyzing exact self-similar solutions of the\nmagneto-hydrodynamics and electron magneto-hydrodynamics equations. Then the\nmagnetic field annihilation in the ultrarelativistic limit is considered, when\nthe opposite polarity magnetic field is generated in collisionless plasma by\nmultiple laser pulses, in the regime with a dominant contribution of the\ndisplacement current exciting a strong large-scale electric field. This field\nleads to the conversion of the magnetic energy into the kinetic energy of\naccelerated particles inside a thin current sheet. Charged particle\nacceleration during magnetic field reconnection is discussed when radiation\nfriction and quantum electrodynamics effects become dominant.", "category": "physics_plasm-ph" }, { "text": "Order of magnitude increase in laser-target coupling at\n near-relativistic intensities using compound parabolic concentrators: Achieving a high conversion efficiency into relativistic electrons is central\nto short-pulse laser application and fundamentally relies on creating\ninteraction regions with intensities ${\\gg}10^{18}$~W/cm$^2$. Small focal\nlength optics are typically employed to achieve this goal; however, this\nsolution is impractical for large kJ-class systems that are constrained by\nfacility geometry, debris concerns, and component costs. We fielded\ntarget-mounted compound parabolic concentrators to overcome these limitations\nand achieved nearly an order of magnitude increase to the conversion efficiency\nand more than tripled electron temperature compared to flat targets.\nParticle-in-cell simulations demonstrate that plasma confinement within the\ncone and formation of turbulent laser fields that develop from cone wall\nreflections are responsible for the improved laser-to-target coupling. {These\npassive target components can be used to improve the coupling efficiency for\nall high-intensity short-pulse laser applications, particularly at large\nfacilities with long focal length optics.", "category": "physics_plasm-ph" }, { "text": "Effects of collision enhanced charging on dust crystal: Numerical simulations of monolayer dust crystals in an RF complex plasma were\nperformed to examine the crystal structure and quantify the effects of\nincluding the collision enhanced ion current in the charging model. A GEC cell\nsimilar to a previous experimental work was modeled for a range of RF voltages,\nusing a continuum description for the plasma and a particle description for\ndust grains. The time history of each dust grain was monitored. The dust charge\nwas computed using both the OML and the collision enhanced charging (CEC) model\napplicable to the sheath region. The dust model accounted for the electric\nforce, ion drag force, neutral drag force, gravity, and the ion wake. The CEC\nmodel produced a lower charge and lower electric force which agreed better with\nthe experimental data. Then dust crystals composed of 40 to 100 grains were\nmodeled and the levitation height and inter-particle spacing of the resulting\ncrystals was examined. Including the collision enhanced current reduced the\ninter-particle spacing but only had a minor effect on the levitation height.", "category": "physics_plasm-ph" }, { "text": "Inverse design and experimental realization of plasma metamaterials: We apply inverse design methods to produce two-dimensional triangular-lattice\nplasma metamaterial (PMM) devices which are then constructed and demonstrated\nexperimentally. Finite difference frequency domain simulations are used along\nwith forward-mode automatic differentiation to optimize the plasma densities of\neach of the plasma elements in the PMM to perform beam steering and\ndemultiplexing under transverse magnetic polarization. The optimal device\nparameters are then used to assign plasma density values to elements that make\nup an experimental version of the device. Device performance is evaluated\nagainst both the simulated results and human-designed alternatives, showing the\nbenefits and disadvantages of in-silico inverse design and paving the way for\nfuture fully in-situ optimization.", "category": "physics_plasm-ph" }, { "text": "Nonlinear Two-dimensional potential plasma wake waves: The condition for potential description of the wake waves,generated by flat\nor cylindrical driving electron bunch in cold plasma is derived.\n The two-dimensional nonlinear equation for potential valid for small values\nof that is obtained and solved by the separation of variables. Solutions in the\nform of cnoidal waves,existing behind the moving bunch at small values of\nvertical coordinate,are obtained.In particular,at some boundary\nconditions,corresponding to blow-out regime in the underdense plasma,the\nsolution represents by a solitary nonlinear wave.\n Approximate solution is also obtained using the method of multiple sacales.\n The indications are obtained that the dependense of the amplitudes on\nlongitudinal coordinate determines essentially,even in the first\napproximation,by driving bunch charge distribution.The wake wave amplitude can\nincrease at some conditions along the longitudinal distance from the rear part\nof the bunch.", "category": "physics_plasm-ph" }, { "text": "Hydrodynamic and kinetic models for spin-1/2 electron-positron quantum\n plasmas: Annihilation interaction, helicity conservation, and wave dispersion\n in magnetized plasmas: We discuss complete theory of spin-1/2 electron-positron quantum plasmas,\nwhen electrons and positrons move with velocities mach smaller than the speed\nof light. We derive a set of two fluid quantum hydrodynamic equations\nconsisting of the continuity, Euler, spin (magnetic moment) evolution equations\nfor each species. We explicitly include the Coulomb, spin-spin, Darwin and\nannihilation interactions. The annihilation interaction is the main topic of\nthe paper. We consider contribution of the annihilation interaction in the\nquantum hydrodynamic equations and in spectrum of waves in magnetized\nelectron-positron plasmas. We consider propagation of waves parallel and\nperpendicular to an external magnetic field. We also consider oblique\npropagation of longitudinal waves. We derive set of quantum kinetic equations\nfor electron-positron plasmas with the Darwin and annihilation interactions. We\napply the kinetic theory for the linear wave behavior in absence of external\nfields. We calculate contribution of the Darwin and annihilation interactions\nin the Landau damping of the Langmuir waves. We should mention that the\nannihilation interaction does not change number of particles in the system. It\ndoes not related to annihilation itself, but it exists as a result of\ninteraction of an electron-positron pair via conversion of the pair into\nvirtual photon. A pair of the non-linear Schrodinger equations for\nelectron-positron plasmas including the Darwin and annihilation interactions.\nExistence of conserving helicity in electron-positron quantum plasmas of\nspinning particles with the Darwin and annihilation interactions is\ndemonstrated. We show that annihilation interaction plays an important role in\nquantum electron-positron plasmas giving contribution of the same magnitude as\nthe spin-spin interaction.", "category": "physics_plasm-ph" }, { "text": "Heat Transport in Confined Strongly Coupled 2D Dust Clusters: Dusty plasmas are a model system for studying strong correlation. The dust\ngrains' size of a few micro-meters and their characteristic oscillation\nfrequency of a few hertz allows for an investigation of many particle effects\non an atomic level. In this article, we model the heat transport through an\naxially confined 2D dust cluster from the center to the outside. The system\nbehaves particularly interesting since heat is not only conducted within the\ndust component but also transfered to the neutral gas. Fitting the analytical\nsolution to the obtained radial temperature profiles allows to determine the\nheat conductivity $\\kheat$. The heat conductivity is found to be constant over\na wide range of coupling strengths even including the phase transition from\nsolid to liquid here, as it was also found in extended systems by V. Nosenko et\nal. in 2008 \\cite{PhysRevLett.100.025003}", "category": "physics_plasm-ph" }, { "text": "The dispersion modification of electrostatic geodesic acoustic mode by\n electron geodesic drift current: The past studies treated the perturbed distribution of circulating electrons\nas adiabatic one when studying the dispersion relation of electrostatic\ngeodesic acoustic mode(GAM). In this paper, the flow of electron geodesic\ncurrent (FEGC) is added to modify this adiabatic distribution. Based on the\ndrift kinetic theory, it is found that FEGC obviously increases the magnitude\nof the standard GAM's frequency and reduces its damping rate. The increase of\nfrequency results from the contribution of FEGC to the radial flow. The reason\nfor the reduction of damping rate is that when the effect of FEGC counts, the\nnew resonant velocity becomes much larger than ions thermal velocity with\nequilibrium distribution obeying Maxwellian distribution, compared with\nunmodified Landau resonant velocity. Especially, FEGC changes the characters of\nthe frequency and damping rate of low-frequency GAM as functions of safety\nfactor $q$ .", "category": "physics_plasm-ph" }, { "text": "Compact formulas for bounce/transit averaging in axisymmetric tokamak\n geometry: Compact formulas for bounce and transit orbit averaging of the\nfluctuation-amplitude eikonal factor in axisymmetric tokamak geometry, which is\nfrequently encountered in bounce-gyrokinetic description of microturbulence,\nare given in terms of the Jacobi elliptic functions and elliptic integrals.\nThese formulas are readily applicable to the calculation of the neoclassical\nsusceptibility in the framework of modern bounce-gyrokinetic theory. In the\nlong-wavelength limit for axisymmetric electrostatic perturbations, we recover\nthe expression for the Rosenbluth-Hinton residual zonal flow [Rosenbluth and\nHinton, Phys.~Rev.~Lett.~{\\bf 80}, 724 (1998)] accurately.", "category": "physics_plasm-ph" }, { "text": "Derivation of Radiation from a Dipole immersed in a Homogenous\n Magnetized Plasma: We are considering a dipole antenna immersed in cold plasma and investigate\nits radiation patterns. We are solving Maxwell's equations using (1) analytic\nsolutions for the case when the antenna is parallel to the magnetic field $B_0$\nand (2) numerical solutions for any orientation and angle $\\theta$ of the\nantenna. We investigate various phase velocities regimes as well as the effect\nof varying the frequency and the plasma parameters on the electromagnetic waves\nare presented and discussed.", "category": "physics_plasm-ph" }, { "text": "Simultaneous generation and detection of energetic particle and\n radiation beams from relativistic plasma mirrors driven at kHz repetition\n rate: We report on the first simultaneous measurement of high-order harmonics,\nrelativistic electrons and low divergence proton beams generated from plasma\nmirrors driven at kHz repetition rate by relativistic-intensity\nmilliJoule-energy femtosecond laser pulses. This setup enables detailed\nparametric studies of the particle and radiation spatio-spectral beam\nproperties for a wide range of controlled interaction conditions, such as pulse\nduration and plasma density scale length. This versatile setup should aid in\nfurther understanding the collective laser absorption mechanisms at play during\nthe laser-plasma interaction and in optimizing the secondary beam properties\nfor potential applications.", "category": "physics_plasm-ph" }, { "text": "Optimal Control of Laser-Plasma Instabilities Using Spike Trains of\n Uneven Duration and Delay: STUD Pulses: Adaptive methods of laser irradiation of plasmas are proposed consisting of\ndeterministic, `on-off' amplitude modulations in time, and intermittently\nchanging speckle-patterns. These laser pulses consist of a series of picosecond\ntime-scale spikes in a spike train of uneven duration and delay (STUD pulses),\nin contrast to hydrodynamic-time-scale modulated, multi-nanosecond pulses for\nlaser fusion. Properly designed STUD pulses minimize backscatter and tame any\nabsorptive parametric instability for a given set of plasma conditions, by\nadjusting the modulation periods, duty cycles and spatial hot-spot-distribution\nscrambling-rates of the spikes. Traditional methods of beam conditioning are\nsubsumed or surpassed by STUD pulses. In addition, STUD pulses allow an advance\nin the control of instabilities driven by spatially overlapped laser beams by\nallowing the spikes of crossing beams to be temporally staggered. When the\nintensity peaks of one fall within the nulls of its crossing beam, it allows an\non-off switch or a dimmer for pairwise or multi-beam interactions.", "category": "physics_plasm-ph" }, { "text": "The effects of induced scattering of Alfven waves for the solar wind\n acceleratio: The analysis of energy balance of coronal holes gives that to accelerate the\nfast solar wind streams the energy flux of the order of 800 erg/cm$^2$ s is\nneeded. Axford and McKenzie suggested that the energy source, necessary to\naccelerate fast solar wind, is in the regions of strong magnetic field that\ndefine the boundaries of chromospheric supergranules. If the magnetic field is\nnot strictly unipolar in these regions, the necessary energy is released in the\nprocesses of impulsive reconnection events. For the plasma parameters\nappropriate for the coronal base such processes of impulsive reconnection are\naccompanied by the generation of Alfven and fast magnetosonic waves with the\nperiod of the order of 1 s. On the basis of kinetic equation the model for the\nevolution of Alfven waves in solar wind is suggested. The induced scattering of\nthese waves by plasma ions is considered as the dominant mechanism of\ndissipation of Alfven waves. The description of the evolution of the wave\nspectra in the process in the processes of their propagation till the distance\nof 187 solar radii was found.", "category": "physics_plasm-ph" }, { "text": "Confinement of passing and trapped runaway electrons in the simulation\n of an ITER current quench: Runaway electrons (REs) present a high-priority issue for ITER but little is\nknown about the extent to which RE generation is affected by the stochastic\nfield intrinsic to disrupting plasmas. RE generation can be modelled with\nreduced kinetic models and there has been recent progress in involving losses\ndue to field stochasticity, either via a loss-time parameter or radial\ntransport coefficients which can be estimated by tracing test electrons in 3D\nfields. We evaluate these terms in ITER using a recent JOREK 3D MHD simulation\nof plasma disruption to provide the stochastic magnetic fields where RE markers\nare traced with the built-in particle tracing module. While the MHD simulation\nmodelled only the current quench phase, the case is MHD unstable and exhibits\nsimilar relaxation as would be expected during the thermal quench. Therefore,\nthe RE simulations can be considered beginning right after the thermal quench\nbut before the MHD relaxation is complete. The plasma is found to become fully\nstochastic for 8 ms and the resulting transport is sufficient to overcome RE\navalanche before flux surfaces are reformed. We also study transport mechanisms\nfor trapped REs and find those to be deconfined as well during this phase.\nWhile the results presented here are not sufficient to assess the magnitude of\nthe formed RE beam, we show that significant RE losses could be expected to\narise due to field stochasticity.", "category": "physics_plasm-ph" }, { "text": "Improved Numerical Cherenkov Instability Suppression in the Generalized\n PSTD PIC Algorithm: The family of generalized Pseudo-Spectral Time Domain (including the\nPseudo-Spectral Analytical Time Domain) Particle-in-Cell algorithms offers\nsubstantial versatility for simulating particle beams and plasmas, and well\nwritten codes using these algorithms run reasonably fast. When simulating\nrelativistic beams and streaming plasmas in multiple dimensions, they are,\nhowever, subject to the numerical Cherenkov instability. Previous studies have\nshown that instability growth rates can be reduced substantially by modifying\nslightly the transverse fields as seen by the streaming particles . Here, we\noffer an approach which completely eliminates the fundamental mode of the\nnumerical Cherenkov instability while minimizing the transverse field\ncorrections. The procedure, numerically computed residual growth rates (from\nweaker, higher order instability aliases), and comparisons with WARP\nsimulations are presented. In some instances, there are no numerical\ninstabilities whatsoever, at least in the linear regime.", "category": "physics_plasm-ph" }, { "text": "Supermagnetosonic jets behind a collisionless quasi-parallel shock: The downstream region of a collisionless quasi-parallel shock is structured\ncontaining bulk flows with high kinetic energy density from a previously\nunidentified source. We present Cluster multi-spacecraft measurements of this\ntype of supermagnetosonic jet as well as of a weak secondary shock front within\nthe sheath, that allow us to propose the following generation mechanism for the\njets: The local curvature variations inherent to quasi-parallel shocks can\ncreate fast, deflected jets accompanied by density variations in the downstream\nregion. If the speed of the jet is super(magneto)sonic in the reference frame\nof the obstacle, a second shock front forms in the sheath closer to the\nobstacle. Our results can be applied to collisionless quasi-parallel shocks in\nmany plasma environments.", "category": "physics_plasm-ph" }, { "text": "Ethanol reforming in non-equilibrium plasma of glow discharge: The results of a detailed kinetic study of the main plasma chemical processes\nin non-equilibrium ethanol/argon plasma are presented. It is shown that at the\nbeginning of the discharge the molecular hydrogen is mainly generated in the\nreaction of ethanol H-abstraction. Later hydrogen is formed from active H,\nCH2OH and CH3CHOH and formaldehyde. Comparison with experimental data has shown\nthat the used kinetic mechanism predicts well the concentrations of main\nspecies at the reactor outlet.", "category": "physics_plasm-ph" }, { "text": "Kubo-Greenwood approach to conductivity in dense plasmas with average\n atom models: A new formulation of the Kubo-Greenwood conductivity for average atom models\nis given. The new formulation improves upon previous by explicitly including\nthe ionic-structure factor. Calculations based on this new expression lead to\nmuch improved agreement with ab initio results for DC conductivity of warm\ndense hydrogen and beryllium, and for thermal conductivity of hydrogen. We also\ngive and test a slightly modified Ziman-Evans formula for the resistivity that\nincludes a non-free electron density of states, thus removing an ambiguity in\nthe original Ziman-Evans formula. Again results based on this expression are in\ngood agreement with ab initio simulations for warm dense beryllium and\nhydrogen. However, for both these expressions, calculations of the electrical\nconductivity of warm dense aluminum lead to poor agreement at low temperatures\ncompared to ab initio simulations.", "category": "physics_plasm-ph" }, { "text": "On ponderomotive metallization of magnetospheric plasma: The problem of ponderomotive separation of ions with different charge-to-mass\nratios under the influence of Alfven waves, which permanently exist in the\nmagnetosphere in the form of geomagnetic pulsations, is posed. Formulas are\nderived for partial ponderomotive forces acting on light and heavy (metal)\nions. In the quasi-hydrodynamic approximation, it was found that the Clarke\nnumber, which characterizes the metallicity of the plasma, is maximum at the\nminimum of the magnetic field on the field line along which the Alfv\\'en wave\npropagates. Key words: ponderomotive forces, Alfv\\'en waves, heavy ions, Clarke\nnumber.", "category": "physics_plasm-ph" }, { "text": "Energetic bounds on gyrokinetic instabilities. Part III. Generalized\n free energy: Free energy, widely used as a measure of turbulence intensity in weakly\ncollisional plasmas, has been recently found to be a suitable basis to describe\nboth linear and nonlinear growth in a wide class gyrokinetic systems. The\nsimplicity afforded by this approach is accompanied by some drawbacks, notably\nthe lack of any explicit treatment of wave-particle effects, which makes the\ntheory unable to describe things like stability thresholds or dependence on the\ngeometry of the background magnetic field. As a step toward overcoming these\nlimitations, we propose an extension of the theory based on a generalization of\nfree energy. With this it is demonstrated that resonance effects are recovered,\nand the bounds on growth are significantly reduced. The simplicity and\nefficient computation of the associated \"optimal\" growth rates makes the theory\npotentially applicable to stellarator optimization.", "category": "physics_plasm-ph" }, { "text": "Research Opportunities in Plasma Astrophysics: Major scientific questions and research opportunities are described on 10\nunprioritized plasma astrophysics topics: (1) magnetic reconnection, (2)\ncollisionless shocks and particle acceleration, (3) waves and turbulence, (4)\nmagnetic dynamos, (5) interface and shear instabilities, (6) angular momentum\ntransport, (7) dusty plasmas, (8) radiative hydrodynamics, (9) relativistic,\npair-dominated and strongly magnetized plasmas, (10) jets and outflows. Note\nthat this is a conference report from a Workshop on Opportunities in Plasma\nAstrophysics (WOPA, https://w3.pppl.gov/conferences/2010/WOPA/) in January\n2010, that attracted broad representation from the community and was supported\nby the U.S. Department of Energy, National Aeronautics and Space\nAdministration, National Science Foundation, American Physical Society's\nTopical Group for Plasma Astrophysics and Division of Plasma Physics, and\nCenter for Magnetic Self-Organization in Laboratory and Astrophysical Plasmas.\nAlthough there has been much planning and many developments in both science and\ninfrastructure since the report was written, most of the motivation,\npriorities, problems and technical challenges discussed therein remain\nunaddressed and are relevant at the time of posting.", "category": "physics_plasm-ph" }, { "text": "Dust as probe for horizontal field distribution in low pressure gas\n discharges: Using dust grains as probes in gas discharge plasma is a very promising, but\nat the same time very challenging method, as the individual external control of\ndust grains has to be solved. We propose and demonstrate the applicability of\nthe RotoDust experiment, where the well controlled centrifugal force is\nbalanced by the horizontal confinement field in plane electrode argon radio\nfrequency gas discharges. We have reached a resolution of 0.1 V/cm for the\nelectric field. This technique is used to verify numerical simulations and to\nmap symmetry properties of the confinement in dusty plasma experiments using a\nglass box.", "category": "physics_plasm-ph" }, { "text": "Self-organizing Knotted Magnetic Structures in Plasma: We perform full-MHD simulations on various initially helical configurations\nand show that they reconfigure into a state where the magnetic field lines span\nnested toroidal surfaces. This relaxed configuration is not a Taylor state, as\nis often assumed for relaxing plasma, but a state where the Lorentz force is\nbalanced by the hydrostatic pressure, which is lowest on the central ring of\nthe nested tori. Furthermore, the structure is characterized by a spatially\nslowly varying rotational transform, which leads to the formation of a few\nmagnetic islands at rational surfaces. We then obtain analytic expressions that\napproximate the global structure of the quasi-stable linked and knotted plasma\nconfigurations that emerge, using maps from $S^3$ to $S^2$ of which the Hopf\nfibration is a special case. The knotted plasma configurations have a highly\nlocalized magnetic energy density and retain their structure on time scales\nmuch longer than the Alfvenic time scale.", "category": "physics_plasm-ph" }, { "text": "Effect of magnetic field on transports of charged particles in the\n weakly ionized plasma with power-law q-distributions in nonextensive\n statistics: By using the generalized Boltzmann equation of transport in nonextensive\nstatistics, we study transport properties of the diffusion flux and the heat\nflux of charged particles in the weakly ionized plasma with the power-law\nq-distributions under the magnetic field. We derive the tensor expressions of\ndiffusion coefficient, thermal diffusion coefficient, mobility and thermal\nconductivity of electrons and ions in the q-distributed plasma under magnetic\nfield. We show that the tensors of the diffusion coefficient, the thermal\ndiffusion coefficient and the thermal conductivity are strongly depend on the\nq-parameters in nonextensive statistics, and so they are generally not the same\nas those in the magnetized plasma with a Maxwell distribution.", "category": "physics_plasm-ph" }, { "text": "In-situ, variable thickness, liquid crystal film target inserter for\n moderate repetition rate intense laser applications: Liquid crystal films have recently been demonstrated as variable thickness,\nplanar targets for ultra-intense laser matter experiments and applications such\nas ion acceleration. By controlling the parameters of film formation, including\nliquid crystal temperature and volume, their thickness can be varied on-demand\nfrom 10 $nm$ to above 10 $\\mu m$. This thickness range enables for the first\ntime real-time selection and optimization of various ion acceleration\nmechanisms using low cost, high quality targets. Our previous work employed\nthese targets in single shot configuration, requiring chamber cycling after the\npre-made films were expended. Presented here is a film formation device capable\nof drawing films from a bulk liquid crystal source volume to any thickness in\nthe aforementioned range. This device will form films under vacuum within 2\n$\\mu m$ of the same location each time, well within the Rayleigh range of even\ntight $F/ \\#$ systems. The repetition rate of the device exceeds 0.1 $Hz$ for\nsub-100 $nm$ films, enabling inexpensive, moderate repetition rate plasma\ntarget insertion for state of the art lasers currently in use or under\ndevelopment. Characterization tests of the device performed at the Scarlet\nlaser facility at Ohio State will be presented.", "category": "physics_plasm-ph" }, { "text": "Basic microscopic plasma physics from N-body mechanics: Computing is not understanding. This is exemplified by the multiple and\ndiscordant interpretations of Landau damping still present after seventy years.\nFor long deemed impossible, the mechanical N-body description of this damping,\nnot only enables its rigorous and simple calculation, but makes unequivocal and\nintuitive its interpretation as the synchronization of almost resonant passing\nparticles. This synchronization justifies mechanically why a single formula\napplies to both Landau growth and damping. As to the electrostatic potential,\nthe phase mixing of many beam modes produces Landau damping, but it is\nunexpectedly essential for Landau growth too. Moreover, collisions play an\nessential role in collisionless plasmas. In particular, Debye shielding results\nfrom a cooperative dynamical self-organization process, where \"collisional\"\ndeflections due to a given electron diminish the apparent number of charges\nabout it. The finite value of exponentiation rates due to collisions is crucial\nfor the equivalent of the van Kampen phase mixing to occur in the N-body\nsystem. The N-body approach incorporates spontaneous emission naturally, whose\ncompound effect with Landau damping drives a thermalization of Langmuir waves.\nO'Neil's damping with trapping typical of initially large enough Langmuir waves\nresults from a phase transition. As to collisional transport, there is a smooth\nconnection between impact parameters where the two-body Rutherford picture is\ncorrect, and those where a collective description is mandatory. The N-body\napproach reveals two important features of the Vlasovian limit: it is singular\nand it corresponds to a renormalized description of the actual N-body dynamics.", "category": "physics_plasm-ph" }, { "text": "Flexible, integrated modeling of tokamak stability, transport,\n equilibrium, and pedestal physics: The STEP (Stability, Transport, Equilibrium, and Pedestal)\nintegrated-modeling tool has been developed in OMFIT to predict stable, tokamak\nequilibria self-consistently with core-transport and pedestal calculations.\nSTEP couples theory-based codes to integrate a variety of physics, including\nMHD stability, transport, equilibrium, pedestal formation, and current-drive,\nheating, and fueling. The input/output of each code is interfaced with a\ncentralized ITER-IMAS data structure, allowing codes to be run in any order and\nenabling open-loop, feedback, and optimization workflows. This paradigm\nsimplifies the integration of new codes, making STEP highly extensible. STEP\nhas been verified against a published benchmark of six different integrated\nmodels. Core-pedestal calculations with STEP have been successfully validated\nagainst individual DIII-D H-mode discharges and across more than 500 discharges\nof the $H_{98,y2}$ database, with a mean error in confinement time from\nexperiment less than 19%. STEP has also reproduced results in less conventional\nDIII-D scenarios, including negative-central-shear and negative-triangularity\nplasmas. Predictive STEP modeling has been used to assess performance in\nseveral tokamak reactors. Simulations of a high-field, large-aspect-ratio\nreactor show significantly lower fusion power than predicted by a\nzero-dimensional study, demonstrating the limitations of scaling-law\nextrapolations. STEP predictions have found promising EXCITE scenarios,\nincluding a high-pressure, 80%-bootstrap-fraction plasma. ITER modeling with\nSTEP has shown that pellet fueling enhances fusion gain in both the baseline\nand advanced-inductive scenarios. Finally, STEP predictions for the SPARC\nbaseline scenario are in good agreement with published results from the physics\nbasis.", "category": "physics_plasm-ph" }, { "text": "Incorporating Kinetic Effects on Nernst Advection in Inertial Fusion\n Simulations: We present a simple method to incorporate nonlocal effects on the Nernst\nadvection of magnetic fields down steep temperature gradients, and demonstrate\nits effectiveness in a number of inertial fusion scenarios. This is based on\nassuming that the relationship between the Nernst velocity and the heat flow\nvelocity is unaffected by nonlocality. The validity of this assumption is\nconfirmed over a wide range of plasma conditions by comparing\nVlasov-Fokker-Planck and flux-limited classical transport simulations.\nAdditionally, we observe that the Righi-Leduc heat flow is more severely\naffected by nonlocality due to its dependence on high velocity moments of the\nelectron distribution function, but are unable to suggest a reliable method of\naccounting for this in fluid simulations.", "category": "physics_plasm-ph" }, { "text": "Self-organization phenomena in cold atmospheric pressure plasma slit jet: The RF plasma slit jet, which produces 150 mm wide streaming plasma outside\nthe jet body, exhibits exciting self-organization phenomena that resemble the\nself-organized patterns of dielectric barrier discharge (DBD) filaments.\nSimilarly, as in DBD, the filaments are surrounded by an inhibition zone that\ndoes not allow two filaments to come closer to each other. With fast camera\nimaging, we observed the filamentary character of the discharge in all the\nstudied gas feeds (Ar, Ar/N$_2$, and Ar/O$_2$). Still, the visual appearance of\nthe filaments in the plasma and their interaction with a dielectric surface\ndepended significantly on the gas feed. As the breakdown voltage in pure Ar is\nrelatively low compared to the applied one, new filaments form frequently. Such\nnewly created filaments disrupted the characteristic inter-filament distance,\nforcing the system to rearrange. The frequent ignition and decay processes in\nAr led to short filament lifetimes (0.020-0.035 s) and their high jitter speed\n(0.9-1.7 m/s), as determined with an image processing custom code based on\nGwyddion libraries. The number of filaments was lower in the Ar/O$_2$ and\nAr/N$_2$ mixtures. It was attributed to a loss of energy in the excitation of\nrotational and vibrational levels and oxygen electronegativity. Since the\nprobability of low-current side discharges transitioning into the full plasma\nfilaments was limited in the gas mixtures, the self-organized pattern was\nseldom disrupted, leading to lesser movement and longer lifetimes. Unlike in Ar\nor Ar/O$_2$, the constricted filaments in Ar/N$_2$ were surrounded by diffuse\nplasma plumes, likely connected to the presence of long-lived nitrogen species.\nWe demonstrated in the polypropylene treatment that the self-organization\nphenomena affected the treatment uniformity.", "category": "physics_plasm-ph" }, { "text": "Impurity intrusion in radio-frequency micro-plasma jets operated in\n ambient air: Space and time resolved concentrations of helium metastable atoms in an\natmospheric pressure radio-frequency micro-plasma jet were measured using\ntunable diode laser absorption spectroscopy. Spatial profiles as well as\nlifetime measurements show significant influences of air entering the discharge\nfrom the front nozzle and of impurities originating from the gas supply system.\nQuenching of metastables was used to deduce quantitative concentrations of\nintruding impurities. The impurity profile along the jet axis was determined\nfrom optical emission spectroscopy as well as their dependance on the feed gas\nflow through the jet.", "category": "physics_plasm-ph" }, { "text": "Melting of 2D Coulomb clusters in dusty plasmas: The melting of 2D dust clusters caused by one additional particle in the\nlower layer has experimentally been observed to undergo a two-step transition,\nwhich divides the phase of the cluster into three stages. The first transition\nis a jump of the dust kinetic energy due to the onset of an instability of the\nlower-layer particle, shifting the cluster from an ordinary to a hot\ncrystalline state. The second transition is the actual phase transition into a\nliquid state, which occurs at a decisively lower gas pressure. The detailed\ndynamical properties of the system during the transition were determined in\nterms of the normal mode analysis.", "category": "physics_plasm-ph" }, { "text": "Dust-ion-acoustic shock waves in magnetized plasma having super-thermal\n electrons: The propagation of dust-ion-acoustic shock waves (DIASHWs) in a\nthree-component magnetized plasma having inertialess super-thermal electrons,\ninertial warm positive ions and negative dust grains has been investigated. A\nBurgers' equation is derived by employing the reductive perturbation method.\nUnder consideration of inertial warm positive ions and negative dust grains,\nboth positive and negative shock structures are numerically observed in the\npresence of super-thermal electrons. The effects of oblique angle ($\\delta$),\nspectral index ($\\kappa$), kinematic viscosity ($\\eta$), number density and\ncharge state of the plasma species on the formation of the DIASHWs are\nexamined. It is found that the positive and negative shock wave potentials\nincrease with the oblique angle. It is also observed that the magnitude of the\namplitude of positive and negative shock waves is not affected by the variation\nof the kinematic viscosity of plasma species but the steepness of the positive\nand negative shock waves decreases with kinematic viscosity of plasma species.\nThe implications of our findings in space and laboratory plasmas are briefly\ndiscussed.", "category": "physics_plasm-ph" }, { "text": "Electron slingshot acceleration in relativistic preturbulent shocks\n explored via emitted photon polarization: Transient electron dynamics near the interface of counterstreaming plasmas at\nthe onset of a relativistic collisionless shock (RCS) is investigated using\nparticle-in-cell simulations. We identify a slingshot-like injection process\ninduced by the drifting electric field sustained by the flowing focus of\nbackwards-moving electrons, which is distinct from the well-known stochastic\nacceleration. The flowing focus signifies the plasma kinetic transition from a\npreturbulent laminar motion to a chaotic turbulence. We find a characteristic\ncorrelation between the electron dynamics in the slingshot acceleration and the\nphoton emission features. In particular, the integrated radiation from the RCS\nexhibits a counterintuitive non-monotonic dependence of the photon polarization\ndegree on the photon energy, which originates from a polarization degradation\nof relatively high-energy photons emitted by the slingshot-injected electrons.\nOur results demonstrate the potential of photon polarization as an essential\ninformation source in exploring intricate transient dynamics in RCSs with\nrelevance for earth-based plasma and astrophysical scenarios.", "category": "physics_plasm-ph" }, { "text": "Observation of Radially Inward Turbulent Particle Flux in ETG dominated\n Plasma of LVPD: Radially inward turbulent particle flux is observed in the core region of\ntarget plasma of Large Volume Plasma Device(LVPD). The region satisfy\nconditions for ETG turbulence, i.e. threshold condition, $ \\eta_e = L_{n_e} /\nL_{T_e} > 2/3 $ , where density scale length, $ L_{n_e} \\sim 300 cm $ and\ntemerature scale length, $ L_{T_e} \\sim 50cm $[S.K. Mattoo et al., Phys. Rev.\nLett., 108, 255007(2012)\\cite{Mattoo_PRL}]. The measured flux is dominantly\nelectrostatic ($\\Gamma_{es} \\approx 10^{5} \\Gamma_{em}$) although the nature of\nthe measured turbulence is electromagnetic($\\beta \\approx 0.6 $). The\nturbulence has been established as a consequence of electron temperature\ngradient (ETG) driven modes. Experimental observations of phase angle between\ndensity ($ n_e $) and potential ($\\phi $) fluctuations, $ \\theta_{\\tilde{n}_e,\n\\tilde{\\phi}} $ and electrostatic particle flux, $ \\Gamma_{es} $ shows good\nagreement with the corresponding theoretical estimates for ETG turbulence.", "category": "physics_plasm-ph" }, { "text": "Hierarchical foliation of one-dimensional Vlasov-Poisson system: We elucidate the intermediate of the macroscopic fluid model and the\nmicroscopic kinetic model by studying the Poisson algebraic structure of the\none-dimensional Vlasov-Poisson system. The water-bag model helps formulating\nthe hierarchy of sub-algebras, which interpolates the gap between the fluid and\nkinetic models. By analyzing the embedding of the sub-manifold of an\nintermediate hierarchy in a more microscopic hierarchy, we characterize the\nmicroscopic effect as the symmetry breaking pertinent to a macroscopic\ninvariant.", "category": "physics_plasm-ph" }, { "text": "Effects of ion mobility and positron fraction on solitary waves in weak\n relativistic electron-positron-ion plasma: Effects of ion mobility and positron fraction on solitary waves of envelop of\nlaser field and potential of electrostatic field in weak relativistic\nelectron-positron-ion plasma are investigated. The parameter region for the\nexistence of solitary waves is obtained analytically, and the reasonable choice\nof parameters is clarified. Both cases of mobile and immobile ions are\nconsidered. It is found that the amplitudes of solitary waves in the former\ncase are larger compared to the latter case. For small plasma density, the\nlocalized solitary wave solutions in terms of approximate perturbation\nanalytical method are consistent well with that by exact numerical\ncalculations. However as the plasma density increases the analytical method\nloses its validity more and more. The influence of the positron fraction on the\namplitudes of solitary waves shows a monotonous increasing relation.\nImplication of our results to the particle acceleration is also discussed\nbriefly.", "category": "physics_plasm-ph" }, { "text": "Estimate of convection-diffusion coefficients from modulated\n perturbative experiments as an inverse problem: The estimate of coefficients of the Convection-Diffusion Equation (CDE) from\nexperimental measurements belongs in the category of inverse problems, which\nare known to come with issues of ill-conditioning or singularity. Here we\nconcentrate on a particular class that can be reduced to a linear algebraic\nproblem, with explicit solution. Ill-conditioning of the problem corresponds to\nthe vanishing of one eigenvalue of the matrix to be inverted. The comparison\nwith algorithms based upon matching experimental data against numerical\nintegration of the CDE sheds light on the accuracy of the parameter estimation\nprocedures, and suggests a path for a more precise assessment of the profiles\nand of the related uncertainty. Several instances of the implementation of the\nalgorithm to real data are presented.", "category": "physics_plasm-ph" }, { "text": "Comment on 'Evidence for Stratification of Deuterium-Tritium Fuel in\n Inertial Confinement Fusion Implosions': Recent implosion experiments performed at the OMEGA laser facility reported\nby Casey et al.[1], displayed an anomalously low dd proton yield and a high tt\nneutron yield as compared to dt fusion reactions, explained as a stratification\nof the fuel in the implosion core. We suggest that in the com- pression stage\nthe fuel is out of equilibrium. Ions are inward accelerated to a velocity v0\nindependent on the particle type. Yield ratios are simply given by the ratios\nof fusion cross-sections obtained at the same velocity. A 'Hubble' type model\ngives also a reasonable description of the data. These considerations might be\nrelevant for implosion experiments at the National Ignition Facility as well.", "category": "physics_plasm-ph" }, { "text": "Filamentation instability in a quantum plasma: The growth rate of the filamentation instability triggered when a diluted\ncold electron beam passes through a cold plasma is evaluated using the quantum\nhydrodynamic equations. Compared with a cold fluid model, quantum effects\nreduce both the unstable wave vector domain and the maximum growth rate.\nStabilization of large wave vector modes is always achieved, but significant\nreduction of the maximum growth rate depends on a dimensionless parameter that\nis provided. Although calculations are extended to the relativistic regime,\nthey are mostly relevant to the non-relativistic one.", "category": "physics_plasm-ph" }, { "text": "Hydrodynamic Scaling Analysis of Nuclear Fusion driven by ultra-intense\n laser-plasma interactions: We discuss scaling laws of fusion yields generated by laser-plasma\ninteractions. The yields are found to scale as a function of the laser power.\nThe origin of the scaling law in the laser driven fusion yield is derived in\nterms of hydrodynamic scaling. We point out that the scaling properties can be\nattributed to the laser power dependence of three terms: the reaction rate, the\ndensity of the plasma and the projected range of the plasma particle in the\ntarget medium. The resulting scaling relations have a predictive power that\nenables estimating the fusion yield for a nuclear reaction which has not been\ninvestigated by means of the laser accelerated ion beams.", "category": "physics_plasm-ph" }, { "text": "The reflectivity of relativistic ultra-thin electron layers: The coherent reflectivity of a dense, relativistic, ultra-thin electron layer\nis derived analytically for an obliquely incident probe beam. Results are\nobtained by two-fold Lorentz transformation. For the analytical treatment, a\nplane uniform electron layer is considered. All electrons move with uniform\nvelocity under an angle to the normal direction of the plane; such electron\nmotion corresponds to laser acceleration by direct action of the laser fields,\nas it is described in a companion paper. Electron density is chosen high enough\nto ensure that many electrons reside in a volume \\lambda_R^3, where \\lambda_R\nis the wavelength of the reflected light in the rest frame of the layer. Under\nthese conditions, the probe light is back-scattered coherently and is directed\nclose to the layer normal rather than the direction of electron velocity. An\nimportant consequence is that the Doppler shift is governed by\n\\gamma_x=(1-(V_x/c)^2)^{-1/2} derived from the electron velocity component V_x\nin normal direction rather than the full \\gamma-factor of the layer electrons.", "category": "physics_plasm-ph" }, { "text": "Extraordinary Drift Wave in Space and Cylindrically Bounded Heavy Ion\n Plasmas: It is pointed out that electrostatic and electromagnetic waves propagating in\nperpendicular direction to the density gradient $\\nabla n_0=\\hat{x}\n|\\frac{dn_0}{dx}|$ and external magnetic field $\\vec{B}_0=B_0 \\hat{z}$ in\nhybrid frequency range $\\Omega_i \\ll \\omega \\ll \\Omega_e$ (where\n$\\Omega_j=\\frac{eB_0}{m_j c}$ and $j=e,i$) have several applications in space\nand laboratory plasmas. The electron plasma waves are important for the future\nexperiments on heavier ion plasmas as well as the results are applicable to the\nexperiments performed to create pure pair ion fullerene plasmas. The\nelectrostatic modes are shown to exist in upper F-region of terrestrial\nionosphere and electromagnetic waves are applied to cylindrically bounded heavy\nion plasmas.", "category": "physics_plasm-ph" }, { "text": "Radiofrequency and Mechanical Tests of Silver Coated CuCrZr Contacts for\n the ITER Ion Cyclotron Antenna: The ITER Ion Cyclotron Resonance Heating (ICRH) system is designed to couple\nto the plasma 20 MW of RF power from two antennas in the 40-55 MHz frequency\nrange during long pulses of up to 3600 s and under various plasma conditions\nwith Edge Localized Modes. Radio-Frequency (RF) contacts are integrated within\nthe ITER ICRH launcher in order to ensure the RF current continuity and ease\nthe mechanical assembly by allowing the free thermal expansion of the Removable\nVacuum Transmission Line coaxial conductors during RF operations or during\n250{\\textdegree}C baking phases. A material study has been carried out to\ndetermine which materials and associated coatings are relevant for RF contacts\napplication in ITER. In parallel, RF tests have been performed with a new\nprototype of Multi-Contact (r) LA-CUT/0,25/0 contacts made of silver-coated\nCuCrZr louvers. During these tests on a RF vacuum resonator, currents between\n1.2 kA and 1.3 kA peak have been reached a few tens of times in steady-state\nconditions without any visible damage on the louvers. A final 62MHz pulse\nending in a 300s flat top at 1.9kA resulted in severe damage to the contact. In\naddition, a test bed which performs sliding test cycles has been built in order\nto reproduce the wear of the contact prototype after 30 000 sliding cycles on a\n3 mm stroke at 175{\\textdegree}C under vacuum. The silver coating of the\nlouvers is removed after approximately a hundred cycles whilst, to the\ncontrary, damage to the CuCrZr louvers is relatively low.", "category": "physics_plasm-ph" }, { "text": "Electron power absorption dynamics in a low pressure radio frequency\n driven capacitively coupled discharge in oxygen: We use the one-dimensional object-oriented particle-in-cell Monte Carlo\ncollision code oopd1 to explore the properties and the origins of both the\nelectric field and electron power absorption within the plasma bulk for a\ncapacitively coupled oxygen discharge operated at 10 and 100 mTorr for 45 mm of\ngap distance. The properties of the electric field at three different time\nslices as well as time averaged have been explored considering the moments of\nthe Boltzmann equation. The electron power absorption is distinctly different\nat these operating pressures. The most relevant contributions to the electric\nfield at different time steps come from the pressure terms, the ambipolar and\nthe electron temperature gradient terms, along with the ohmic term. The same\napplies for the electron power absorption. At both 10 and 100 mTorr the\nrelative ohmic contribution to the electron power absorption remains roughly\nthe same, while the ambipolar term contributes to power absorption and the\ntemperature gradient term to electron cooling at 100 mTorr, and the opposite\napplies at 10 mTorr. At 100 mTorr the discharge is weekly electronegative, and\nelectron power absorption is mainly due to sheath expansion while at 10 mTorr\nit is strongly electronegative, and the electron power absorption occurs mainly\nwithin the electronegative core and drift-ambipolar mode dominates. The\nagreement between the calculated values and the simulations is good for both\nthe electric field and the electron power absorption within the plasma bulk and\nin the collapsed sheath region for all the cases considered.", "category": "physics_plasm-ph" }, { "text": "Kinetic simulations of magnetic reconnection in presence of a background\n O+ population: Particle-in-Cell simulations of magnetic reconnection with an H+ current\nsheet and a mixed background plasma of H+ and O+ ions are completed using\nphysical mass ratios. Four main results are shown. First, the O+ presence\nslightly decreases the reconnection rate and the magnetic reconnection\nevolution depends mainly on the lighter H+ ion species in the presented\nsimulations. Second, the Hall magnetic field is characterized by a two-scale\nstructure in presence of O+ ions: it reaches sharp peak values in a small area\nin proximity of the neutral line, and then decreases slowly over a large\nregion. Third, the two background species initially separate in the outflow\nregion because H+ and O+ ions are accelerated by different mechanisms occurring\non different time scales and with different strengths. Fourth, the effect of a\nguide field on the O+ dynamics is studied: the O+ presence does not change the\nreconnected flux and all the characteristic features of guide field magnetic\nreconnection are still present. Moreover, the guide field introduces an O+\ncirculation pattern between separatrices that enhances high O+ density areas\nand depletes low O+ density regions in proximity of the reconnection fronts.\nThe importance and the validity of these results are finally discussed.", "category": "physics_plasm-ph" }, { "text": "Ion energy measurements on MAST using a midplane RFEA: Ion energy measurements have been made in the scrape off layer of the Mega\nAmp Spherical Tokamak (MAST) using a midplane retarding field energy analyser\n(RFEA) in H-mode plasmas during the inter-edge localised mode (ELM) period and\nduring type I and type III ELMs. During the inter-ELM period at distances of 3\nto 8 cm from the last closed flux surface (LCFS), ion temperatures of 20 to 70\neV have been measured giving an ion to electron temperature ratio of 2 to 7\nwith a mean of 4. During type III ELMs, an ion temperature of 50 eV has been\nmeasured 3 to 6 cm from the LCFS which decreases to 30 eV at distances 11 to 16\ncm from the LCFS. During type I ELMs, an ion temperature of 40 eV has been\nmeasured at a distance of 10 to 15 cm from the LCFS.", "category": "physics_plasm-ph" }, { "text": "Renormalized theory of the ion cyclotron turbulence in magnetic\n field--aligned plasma shear flow: The analytical treatment of nonlinear evolution of the shear-flow-modified\ncurrent driven ion cyclotron instability and shear-flow-driven ion cyclotron\nkinetic instabilities of magnetic field--aligned plasma shear flow is\npresented. Analysis is performed on the base of the nonlinear dispersion\nequation, which accounts for a new combined effect of plasma turbulence and\nshear flow. It consists in turbulent scattering of ions across the shear flow\nwith their convection by shear flow and results in enhanced nonlinear\nbroadening of ion cyclotron resonances. This effect is found to lead to the\nsaturation of ion cyclotron instabilities as well as to the development of\nnonlinear shear flow driven ion cyclotron instability. 52.35.Ra", "category": "physics_plasm-ph" }, { "text": "Energy absorption in the laser-QED regime: A theoretical and numerical investigation of non-ponderomotive absorption at\nlaser intensities relevant to quantum electrodynamics is presented. It is\npredicted that there is a regime change in the dependence of fast electron\nenergy on incident laser energy that coincides with the onset of pair\nproduction via the Breit-Wheeler process. This prediction is numerically\nverified via an extensive campaign of QED-inclusive particle-in-cell\nsimulations. The dramatic nature of the power law shift leads to the conclusion\nthat this process is a candidate for an unambiguous signature that future\nexperiments on multi-petawatt laser facilities have truly entered the QED\nregime.", "category": "physics_plasm-ph" }, { "text": "Anisotropic spectra of acoustic type turbulence: We consider the problem of spectra for acoustic type of turbulence generated\nby shocks being randomly distributed in space. We show that for turbulence with\na weak anisotropy such spectra have the same dependence in $k$-space as the\nKadomtsev-Petviashvili (KP) spectrum: $E(k)\\sim k^{-2}$. However, the frequency\nspectrum has always the falling $\\sim \\omega^{-2}$, independently on\nanisotropy. In the strong anisotropic case the energy distribution relative to\nwave vectors takes anisotropic dependence forming in the large $% k $ region\nthe spectra of the jet type.", "category": "physics_plasm-ph" }, { "text": "A Relativistic Plasma Polarizer: Impact of Temperature Anisotropy on\n Relativistic Transparency: 3D particle-in-cell simulations demonstrate that the enhanced transparency of\na relativistically hot plasma is sensitive to how the energy is partitioned\nbetween different degrees of freedom. For an anisotropic electron distribution,\npropagation characteristics, like the critical density, will depend on the\npolarization of the electromagnetic wave. Despite the onset of the Weibel\ninstability in such plasmas, the anisotropy can persist long enough to affect\nlaser propagation. This plasma can then function as a polarizer or a waveplate\nto dramatically alter the pulse polarization.", "category": "physics_plasm-ph" }, { "text": "Characterization of dielectric barrier discharge in air applying current\n measurement, numerical simulation and emission spectroscopy: Dielectric barrier discharge (DBD) in air is characterized applying current\nmeasurement, numerical simulation and optical emission spectroscopy (OES). For\nOES, a non-calibrated spectrometer is used. This diagnostic method is\napplicable when cross-sectional area of the active plasma volume and current\ndensity can be determined. The nitrogen emission in the spectral range of 380\nnm- 406 nm is used for OES diagnostics. Electric field in the active plasma\nvolume is determined applying the measured spectrum, well-known Frank-Condon\nfactors for nitrogen transitions and numerically- simulated electron\ndistribution functions. The measured electric current density is used for\ndetermination of electron density in plasma. Using the determined plasma\nparameters, the dissociation rate of nitrogen and oxygen in active plasma\nvolume are calculated, which can be used by simulation of the chemical\nkinetics.", "category": "physics_plasm-ph" }, { "text": "High quality electron bunch generation with CO2-laser plasma accelerator: CO2 laser-driven electron acceleration is demonstrated with particle-in-cell\nsimulation in low-density plasma. An intense CO2 laser pulse with long\nwavelength excites wakefield. The bubble behind it has a broad space to sustain\na large amount of electrons before reaching its charge saturation limit. A\ntransversely propagating inject pulse is used to induce and control the ambient\nelectron injection. The accelerated electron bunch with total charge up to 10\nnC and the average charge per energy interval of more than 0.6 nC/MeV are\nobtained. Plasma-based electron acceleration driven by intense CO2 laser\nprovides a new potential way to generate high-charge electron bunch with low\nenergy spread, which has broad applications, especially for X-ray generation by\ntable-top FEL and bremsstrahlung.", "category": "physics_plasm-ph" }, { "text": "A Kinetic Model for Electron-Ion Transport in Warm Dense Matter: We present a model for electron-ion transport in Warm Dense Matter that\nincorporates Coulomb coupling effects into the quantum Boltzmann equation of\nUehling and Uhlenbeck through the use of a statistical potential of mean force.\nAlthough this model has been derived rigorously in the classical limit [S.D.\nBaalrud and J. Daligault, Physics of Plasmas 26, 8, 082106 (2019)], its quantum\ngeneralization is complicated by the uncertainty principle. Here we apply an\nexisting model for the potential of mean force based on the quantum\nOrnstein-Zernike equation coupled with an average-atom model [C. E. Starrett,\nHigh Energy Density Phys. 25, 8 (2017)]. This potential contains correlations\ndue to both Coulomb coupling and exchange, and the collision kernel of the\nkinetic theory enforces Pauli blocking while allowing for electron diffraction\nand large-angle collisions. By solving the Uehling-Uhlenbeck equation for\nelectron-ion relaxation rates, we predict the momentum and temperature\nrelaxation time and electrical conductivity of solid density aluminum plasma\nbased on electron-ion collisions. We present results for density and\ntemperature conditions that span the transition from classical weakly-coupled\nplasma to degenerate moderately-coupled plasma. Our findings agree well with\nrecent quantum molecular dynamics simulations.", "category": "physics_plasm-ph" }, { "text": "Phase-space structures in quantum-plasma wave turbulence: The quasilinear theory of the Wigner-Poisson system in one spatial dimension\nis examined. Conservation laws and properties of the stationary solutions are\ndetermined. Quantum effects are shown to manifest themselves in transient\nperiodic oscillations of the averaged Wigner function in velocity space. The\nquantum quasilinear theory is checked against numerical simulations of the\nbump-on-tail and the two-stream instabilities. The predicted wavelength of the\noscillations in velocity space agrees well with the numerical results.", "category": "physics_plasm-ph" }, { "text": "Unlimited Energy Gain in the Laser-Driven Radiation Pressure Dominant\n Acceleration of Ions: The energy of the ions accelerated by an intense electromagnetic wave in the\nradiation pressure dominated regime can be greatly enhanced due to a transverse\nexpansion of a thin target. The expansion decreases the number of accelerated\nions in the irradiated region increasing the energy and the longitudinal\nvelocity of remaining ions. In the relativistic limit, the ions become\nphase-locked with respect to the electromagnetic wave resulting in the\nunlimited ion energy gain. This effect and the use of optimal laser pulse shape\nprovide a new approach for great enhancing the energy of laser accelerated\nions.", "category": "physics_plasm-ph" }, { "text": "Kinetic Model Evaluation of Dynamical Properties of Nanaorod Antennas\n Embedded in a Polymer Carrying the Nuclei of Fusion Fuel: Recently laser induced fusion with simultaneous volume ignition, a spin-off\nfrom relativistic heavy ion collisions, was proposed, where implanted\nnanoantennas regulated and amplified the light absorption in the fusion target.\nStudies of resilience of the nanoantennas was published recently in vacuum.\nThese studies are extended to nanoantennas embedded into a polymer, which\nmodifies the nanoantenna's lifetime and absorption properties.", "category": "physics_plasm-ph" }, { "text": "Dispersion properties of electrostatic oscillations in quantum plasmas: We present a derivation of the dispersion relation for electrostatic\noscillations (ESOs) in a zero temperature quantum plasma. In the latter,\ndegenerate electrons are governed by the Wigner equation, while non-degenerate\nions follow the classical fluid equations. The Poisson equation determines the\nelectrostatic wave potential. We consider parameters ranging from semiconductor\nplasmas to metallic plasmas and electron densities of compressed matter such as\nin laser-compression schemes and dense astrophysical objects. Due to the wave\ndiffraction caused by overlapping electron wave function due to the Heisenberg\nuncertainty principle in dense plasmas, we have possibility of Landau damping\nof the high-frequency electron plasma oscillations (EPOs) at large enough\nwavenumbers. The exact dispersion relations for the EPOs are solved numerically\nand compared to the ones obtained by using approximate formulas for the\nelectron susceptibility in the high- and low-frequency cases.", "category": "physics_plasm-ph" }, { "text": "Machine learning methods for Schlieren imaging of a plasma channel in\n tenuous atomic vapor: We investigate the usage of a Schlieren imaging setup to measure the\ngeometrical dimensions of a plasma channel in atomic vapor. Near resonant probe\nlight is used to image the plasma channel in a tenuous vapor and machine\nlearning techniques are tested for extracting quantitative information from the\nimages. By building a database of simulated signals with a range of plasma\nparameters for training Deep Neural Networks, we demonstrate that they can\nextract from the Schlieren images reliably and with high accuracy the location,\nthe radius and the maximum ionization fraction of the plasma channel as well as\nthe width of the transition region between the core of the plasma channel and\nthe unionized vapor. We test several different neural network architectures\nwith supervised learning and show that the parameter estimations supplied by\nthe networks are resilient with respect to slight changes of the experimental\nparameters that may occur in the course of a measurement.", "category": "physics_plasm-ph" }, { "text": "Three dimensional filamentary structures of a relativistic electron beam\n in Fast Ignition plasmas: The filamentary structures and associated electromagnetic fields of a\nrelativistic electron beam have been studied by three dimensional\nparticle-in-cell (PIC) simulations in the context of Fast Ignition fusion. The\nsimulations explicitly include collisions in return plasma current and\ndistinctly examine the effects of beam temperature and collisions on the growth\nof filamentary structures generated.", "category": "physics_plasm-ph" }, { "text": "Topology optimization of permanent magnets for stellarators: We introduce a topology optimization method to design permanent magnets for\nadvanced stellarators. Recent researches show that permanent magnets have great\npotentials to simplify stellarator coils. We adopt state-of-the-art numerical\ntechniques to determine the presence of magnets in the entire designing space.\nThe FAMUS code is developed and it can design engineering-feasible permanent\nmagnets for general stellarators satisfying the constraints of the maximum\nmaterial magnetization and explicitly forbidden regions. FAMUS has been\nsuccessfully verified against the previously proposed linear method. Three\ndifferent permanent magnet designs together with planar TF coils for a\nhalf-Tesla NCSX configuration have been obtained for demonstrations. The\ndesigns have good accuracy in generating the desired equilibrium and offer\nconsiderably large plasma access on the outboard side. The results show that\nFAMUS is a flexible, advanced numerical tool for future permanent magnet\nstellarator designs.", "category": "physics_plasm-ph" }, { "text": "Collisionless plasma interpenetration in a strong magnetic field for\n laboratory astrophysics experiments: A theoretical analysis for astrophysics-oriented laser-matter interaction\nexperiments in the presence of a strong ambient magnetic field is presented. It\nis shown that the plasma collision in the ambient magnetic field implies\nsignificant perturbations in the electron density and magnetic field\ndistribution. This transient stage is difficult to observe in astrophysical\nphenomena, but it could be investigated in laboratory experiments. Analytic\nmodels are presented, which are supported by particle-in-cell simulations.", "category": "physics_plasm-ph" }, { "text": "Discharge-produced plasma extreme ultraviolet (EUV) source and ultra\n high vacuum chamber for studying EUV-induced processes: An experimental setup that directly reproduces Extreme UV-lithography\nrelevant conditions for detailed component exposure tests is described. The EUV\nsetup includes a pulsed plasma radiation source, operating at 13.5 nm; a debris\nmitigation system; collection and filtering optics; and an UHV experimental\nchamber, equipped with optical and plasma diagnostics. The first results,\nidentifying the physical parameters and evolution of EUV-induced plasmas are\npresented. Finally, the applicability and accuracy of the in situ diagnostics\nis briefly discussed.", "category": "physics_plasm-ph" }, { "text": "The case of the trapped singularities: A case study in bifurcation and stability analysis is presented, in which\nreduced dynamical system modelling yields substantial new global and predictive\ninformation about the behaviour of a complex system. The first smooth pathway,\nfree of pathological and persistent degenerate singularities, is surveyed\nthrough the parameter space of a nonlinear dynamical model for a\ngradient-driven, turbulence-shear flow energetics in magnetized fusion plasmas.\nAlong the route various obstacles and features are identified and treated\nappropriately. An organizing centre of low codimension is shown to be robust,\nseveral trapped singularities are found and released, and domains of\nhysteresis, threefold stable equilibria, and limit cycles are mapped.\nCharacterization of this rich dynamical landscape achieves unification of\nprevious disparate models for plasma confinement transitions, supplies valuable\nintelligence on the big issue of shear flow suppression of turbulence, and\nsuggests targeted experimental design, control and optimization strategies.", "category": "physics_plasm-ph" }, { "text": "Nucleus Driven Electronic Pulsation: We derive and solve by the spectral method the equations for a neutral system\nof ultra-relativistic electrons that are compressed to the radius of the\nnucleus and subject to a driving force. This driving force can be thought of as\noriginating from a nuclear breathing mode, a possibility we discuss in detail.", "category": "physics_plasm-ph" }, { "text": "Bistable Intrinsic Charge Fluctuations of a Dust Grain Subject to\n Secondary Electron Emission in a Plasma: A master equation was formulated to study intrinsic charge fluctuations of a\ngrain in a plasma as ions and primary electrons are attached to the grain\nthrough collisional collection, and secondary electrons are emitted from the\ngrain. Two different plasmas with Maxwellian and non-Maxwellian distributions\nwere considered. The fluctuations could be bistable in either plasma when the\nsecondary electron emission is present, as two stable macrostates, associated\nwith two stable roots of the charge net current, may exist. Metastablity of\nfluctuations, manifested by the passage of the grain charge between two\nmacrostates, was shown to be possible.", "category": "physics_plasm-ph" }, { "text": "First principles calculation of the effect of Coulomb collisions in\n partially ionized gases: Coulomb collisions, at appreciable ratios (\\eta) of the electron to the\nneutral particle density, influence significantly the electron kinetics in\nparticle swarms and in plasmas of gas discharges. This paper introduces a\ncombination of Molecular Dynamics and Monte Carlo simulation techniques, to\nprovide a novel, approximation free, first principles calculation method for\nthe velocity distribution function of electrons, and related swarm\ncharacteristics, at arbitrary \\eta. Simulation results are presented for\nelectrons in argon gas, for density ratios between zero and 0.1, representing\nthe limits of a negligible electron density and an almost complete\nMaxwellization of the velocity distribution function, respectively.", "category": "physics_plasm-ph" }, { "text": "Fast magnetization in counterstreaming plasmas with temperature\n anisotropies: Counterstreaming plasmas exhibits an electromagnetic unstable mode of\nfilamentation type, which is responsible for the magnetization of plasma\nsystem. It is shown that filamentation instability becomes significantly faster\nwhen plasma is hotter in the streaming direction. This is relevant for\nastrophysical sources, where strong magnetic fields are expected to exist and\nexplain the nothermal emission observed.", "category": "physics_plasm-ph" }, { "text": "Idealized Slab Plasma approach for the study of Warm Dense Matter: Recently, warm dense matter (WDM) has emerged as an interdisciplinary field\nthat draws increasing interest in plasma physics, condensed matter physics,\nhigh pressure science, astrophysics, inertial confinement fusion, as well as\nmaterials science under extreme conditions. To allow the study of well-defined\nWDM states, we have introduced the concept of idealized-slab plasmas that can\nbe realized in the laboratory via (i) the isochoric heating of a solid and (ii)\nthe propagation of a shock wave in a solid. The application of this concept\nprovides new means for probing the dynamic conductivity, equation of state,\nionization and opacity. These approaches are presented here using results\nderived from first-principles (density-functional type) theory, Thomas-Fermi\ntype theory, and numerical simulations.", "category": "physics_plasm-ph" }, { "text": "Monte Carlo method and High Performance Computing for solving\n Fokker-Planck equation of minority plasma particles: This paper explains how to obtain the distribution function of minority ions\nin tokamak plasmas using the Monte Carlo method. Since the emphasis is on\nenergetic ions, the guiding-center transformation is outlined, including also\nthe transformation of the collision operator. Even within the guiding-center\nformalism, the fast particle simulations can still be very CPU intensive and,\ntherefore, we introduce the reader also to the world of high-performance\ncomputing. The paper is concluded with a few examples where the presented\nmethod has been applied.", "category": "physics_plasm-ph" }, { "text": "Cooling of relativistic electron beams in chirped laser pulses: The next few years will see next-generation high-power laser facilities (such\nas the Extreme Light Infrastructure) become operational, for which it is\nimportant to understand how interaction with intense laser pulses affects the\nbulk properties of a relativistic electron beam. At such high field\nintensities, we expect both radiation reaction and quantum effects to play a\nsignificant role in the beam dynamics. The resulting reduction in relative\nenergy spread (beam cooling) at the expense of mean beam energy predicted by\nclassical theories of radiation reaction depends only on the energy of the\nlaser pulse. Quantum effects suppress this cooling, with the dynamics\nadditionally sensitive to the distribution of energy within the pulse. Since\nchirps occur in both the production of high-intensity pulses (CPA) and the\npropagation of pulses in media, the effect of using chirps to modify the pulse\nshape has been investigated using a semi-classical extension to the\nLandau--Lifshitz theory. Results indicate that even large chirps introduce a\nsignificantly smaller change to final state predictions than going from a\nclassical to quantum model for radiation reaction, the nature of which can be\nintuitively understood.", "category": "physics_plasm-ph" }, { "text": "Bose-Einstein condensation in relativistic plasma: The phenomenon of Bose-Einstein condensation is traditionally associated with\nand experimentally verified for low temperatures: either of nano-Kelvin scale\nfor alkali atoms [1-3] or room temperatures for quasi-particles [4,5] or\nphotons in two dimensions [6]. Here we demonstrate out of first principles that\nfor certain initial conditions non-equilibrium plasma at relativistic\ntemperatures of billions of Kelvin undergoes condensation, predicted by\nZeldovich and Levich in their seminal work [7]. We determine the necessary\nconditions for the onset of condensation and discuss the possibilities to\nobserve such a phenomenon in laboratory and astrophysical conditions.", "category": "physics_plasm-ph" }, { "text": "Dynamic Phase Alignment in Inertial Alfven Turbulence: In weakly-collisional plasma environments with sufficiently low electron\nbeta, Alfv\\'enic turbulence transforms into inertial Alfv\\'enic turbulence at\nscales below the electron skin-depth, $k_\\perp d_e\\gtrsim 1$. We argue that, in\ninertial Alfv\\'enic turbulence, both energy and generalized kinetic helicity\nexhibit direct cascades. We demonstrate that the two cascades are compatible\ndue to the existence of a strong scale-dependence of the phase alignment angle\nbetween velocity and magnetic field fluctuations, with the phase alignment\nangle scaling as $\\cos\\alpha_k\\propto k_{\\perp}^{-1}$. The kinetic and magnetic\nenergy spectra scale as $\\propto k_{\\perp}^{-5/3}$ and $\\propto\nk_{\\perp}^{-11/3}$, respectively. As a result of the dual direct cascade, the\ngeneralized-helicity spectrum scales as $\\propto k^{-5/3}_{\\perp}$, implying\nprogressive balancing of the turbulence as the cascade proceeds to smaller\nscales in the $k_{\\perp} d_e \\gg 1$ range. Turbulent eddies exhibit a\nphase-space anisotropy $k_{\\parallel} \\propto k_{\\perp}^{5/3}$, consistent with\ncritically-balanced inertial Alfv\\'en fluctuations. Our results may be\napplicable to a variety of geophysical, space, and astrophysical environments,\nincluding the Earth's magnetosheath and ionosphere, solar corona,\nnon-relativistic pair plasmas, as well as to strongly rotating non-ionized\nfluids.", "category": "physics_plasm-ph" }, { "text": "Relativistic Vlasov-Maxwell modelling using finite volumes and adaptive\n mesh refinement: The dynamics of collisionless plasmas can be modelled by the Vlasov-Maxwell\nsystem of equations. An Eulerian approach is needed to accurately describe\nprocesses that are governed by high energy tails in the distribution function,\nbut is of limited efficiency for high dimensional problems. The use of an\nadaptive mesh can reduce the scaling of the computational cost with the\ndimension of the problem. Here, we present a relativistic Eulerian\nVlasov-Maxwell solver with block-structured adaptive mesh refinement in one\nspatial and one momentum dimension. The discretization of the Vlasov equation\nis based on a high-order finite volume method. A flux corrected transport\nalgorithm is applied to limit spurious oscillations and ensure the physical\ncharacter of the distribution function. We demonstrate a speed-up by a factor\nof 7x in a typical scenario involving laser-plasma interaction with an\nunderdense plasma due to the use of an adaptive mesh.", "category": "physics_plasm-ph" }, { "text": "Characteristics of the edge temperature ring oscillation during\n stationary improved confnement mode in EAST: I-mode is a natural ELMy-free regime with H-mode like improved energy\nconfnement and L-mode like particle confnement, making it an attractive\nscenario for future tokamak based fusion reactors. A kind of low frequency\noscillation was widely found and appeared to be unique in I-mode, with the\nfrequency between stationary zonal flow and geodesic-acoustic mode (GAM) zonal\nflow. In EAST, 90 percent I-mode shots have such mode, called edge temperature\nring oscillation (ETRO). The mode probably plays an important role during\nI-mode development and sustainment, while investigations are needed to clarify\nthe differences between ETRO and the similar mode named as low frequency edge\noscillation (LFEO) in AUG and C-Mod, especially whether it is still GAM. In the\npaper, the ETRO characteristics in EAST were investigated in detail and most do\nnot agree with GAM, including that 1) during L-I transition with edge Te and Ti\nboth increasing, ETRO has a smaller frequency than GAM; 2) ETRO has distinct\nharmonics in various diagnostics; 3) The magnetic component of ETRO is\ndominated by m = 1 structure; 4) ETRO is accompanied by turbulence transition\nbetween electron-scale and ion-scale; 5) As I-mode approaching to H-mode, ETRO\nfrequency would decrease rapidly with Te increasing. These features imply that\nETRO is probably caused by the stationary zonal flow with fnite frequency.\nMoreover, other damping mechanisms need to be involved besides collision in the\nImode edge region. It was found that modest fueling could decrease the ETRO\nintensity with the I-mode confnement sustaining, suggesting that supersonic\nmolecular beam injection (SMBI) could be used as an effective tool to control\nETRO.", "category": "physics_plasm-ph" }, { "text": "On the breaking of a plasma wave in a thermal plasma: II.\n Electromagnetic wave interaction with the breaking plasma wave: The structure of the density singularity formed in a relativistically large\namplitude plasma wave close to the wavebreaking limit leads to a refraction\ncoefficient which has a coordinate dependence with discontinuous derivatives.\nThis results in a non-exponentially small above-barrier reflection of an\nelectromagnetic wave interacting with the nonlinear plasma wave.", "category": "physics_plasm-ph" }, { "text": "Linear Stability of an Impulsively Accelerated Density Interface in an\n Ideal Two-Fluid Plasma: We investigate the linear evolution of Richtmyer-Meshkov (RM) instability in\nthe framework of an ideal two-fluid plasma model. The two-fluid plasma\nequations of motion are separated into a base state and a set of linearized\nequations governing the evolution of the perturbations. Different coupling\nregimes between the charged species are distinguished based on a\nnon-dimensional Debye length parameter $d_{D,0}$. When $d_{D,0}$ is large, the\ncoupling between ions and electrons is sufficiently small that the induced\nLorentz force is very weak and the two species evolve as two separate fluids.\nWhen $d_{D,0}$ is small, the coupling is strong and the induced Lorentz force\nis strong enough that the difference between state of ions and electrons is\nrapidly decreased by the force. As a consequence, the ions and electrons are\ntightly coupled and evolve like one fluid. The temporal dynamics is divided\ninto two phases: an early phase wherein electron precursor waves are prevalent,\nand a post ion shock-interface interaction phase during which the RM\ninstability manifests itself. We also examine the effect of an initially\napplied magnetic field in the streamwise direction characterized by the\nnon-dimensional parameter $\\beta_0$. For a short duration after the ion\nshock-interface interaction, the growth rate is similar for different initial\nmagnetic field strengths. As time progresses the suppression of the instability\ndue to the magnetic field is observed. The growth rate shows oscillations with\na frequency that is related to the ion or electron cyclotron frequency.\n The instability is suppressed due to the vorticity being transported away\nfrom the interface.", "category": "physics_plasm-ph" }, { "text": "Comparative merits of the memory function and dynamic local field\n correction of the classical one-component plasma: The complementarity of the liquid and plasma descriptions of the classical\none-component plasma (OCP) is explored by studying wavevector and frequency\ndependent dynamical quantities: the dynamical structure factor (DSF), and the\ndynamic local field correction (LFC). Accurate Molecular Dynamics (MD)\nsimulations are used to validate/test models of the DSF and LFC. Our\nsimulations, which span the entire fluid regime ($\\Gamma = 0.1 - 175$), show\nthat the DSF is very well represented by a simple and well known memory\nfunction model of generalized hydrodynamics. On the other hand, the LFC, which\nwe have computed using MD for the first time, is not well described by existing\nmodels.", "category": "physics_plasm-ph" }, { "text": "Tuning the electron energy by controlling the density perturbation\n position in laser plasma accelerators: A density perturbation produced in an underdense plasma was used to improve\nthe quality of electron bunches produced in the laser-plasma wakefield\nacceleration scheme. Quasi-monoenergetic electrons were generated by controlled\ninjection in the longitudinal density gradients of the density perturbation. By\ntuning the position of the density perturbation along the laser propagation\naxis, a fine control of the electron energy from a mean value of 60 MeV to 120\nMeV has been demonstrated with a relative energy-spread of 15 +/- 3.6%,\ndivergence of 4 +/- 0.8 mrad and charge of 6 +/- 1.8 pC.", "category": "physics_plasm-ph" }, { "text": "Analytical edge power loss at the lower hybrid resonance: comparison\n with ANTITER IV and application to ICRH systems: In non-inverted heating scenarios, a lower hybrid (LH) resonance can appear\nin the plasma edge of tokamaks. This resonance can lead to large edge power\ndeposition when heating in the ion cyclotron resonance frequency (ICRF) range.\nIn this paper, the edge power loss associated with this LH resonance is\nanalytically computed for a cold plasma description using an asymptotic\napproach and analytical continuation. This power loss can be directly linked to\nthe local radial electric field and is then compared to the corresponding power\nloss computed with the semi-analytical code ANTITER IV. This method offers the\npossibility to check the precision of the numerical integration made in ANTITER\nIV and gives insights in the physics underlying the edge power absorption.\nFinally, solutions to minimize this edge power absorption are investigated and\napplied to the case of ITER's ion cyclotron resonance heating (ICRH) launcher.\nThis study is also of direct relevance to DEMO.", "category": "physics_plasm-ph" }, { "text": "Ignition and propagation of nanosecond pulsed discharges in distilled\n water -- negative vs. positive polarity applied to a pin electrode: Nanosecond plasmas in liquids are being used for water treatment,\nelectrolysis or biomedical applications. The exact nature of these very dynamic\nplasmas and most important their ignition physics are strongly debated. The\nignition itself may be explained by two competing hypothesis: (i) ignition via\nfield effects or (ii) via electron multiplication in nanovoids. Both hypothesis\nare supported by theory, but experimental data are very sparse due to the\ndifficulty to monitor the very fast processes in space and time. In this paper,\nwe are using fast camera measurements and fast emission spectroscopy of\nnanosecond plasmas in water applying a positive and a negative polarity to a\nsharp tungsten electrode. It is shown that plasma ignition is dominated by\nfield effects at the electrode-liquid interface either as field ionization for\npositive polarity or as field emission for negative polarity. This leads to a\nhot tungsten surface at a temperature of 7000 K for positive polarity, whereas\nthe surface temperature is much lower for the negative polarity. At ignition,\nthe electron density reaches 4 $\\cdot$ 10$^{25}$ m$^{-3}$ for positive and only\n2 $\\cdot$ 10$^{25}$ m$^{-3}$ for the negative polarity. At the same time, the\nemission of the \\Ha~light for the positive polarity is 4 times higher than that\nfor the negative polarity. During plasma propagation, the electron densities\nare almost identical of the order of a 1 to 2 $\\cdot$ 10$^{25}$ m$^{-3}$ and\ndecay after the end of the pulse over 15 ns. It is concluded that plasma\npropagation is governed by field effects in a low density region that is\ncreated either by nanovoids or by density fluctuation in super critical water\nsurrounding the electrode that is created by the pressure at the moment of\nplasma ignition.", "category": "physics_plasm-ph" }, { "text": "Comment on \"Adiabatic Expansion of Electron Gas in a Magnetic Nozzle\" by\n Kazunori Takahashi, Christine Charles, Rod Roswell, and Akira Ando, Phys.\n Rev. Lett. Vol. 120, 045001 (2018): Heat capacities at fixed volume and pressure as a function of the degree of\nionization are graphically depicted as functions of reciprocal temperature and\nionization degree. The polytropic index is calculated as a function of the same\nvariables; as it is not constant a partially ionized plasma can be only\napproximately polytropic fluid. In parallel, it is launched the idea that\nAlfv\\'en waves can be used to heat the plasma in a propulsion jet with magnetic\nnozzle.", "category": "physics_plasm-ph" }, { "text": "Reciprocating probe measurements in the test divertor operation phase of\n Wendelstein 7-X: Reciprocating probes are a classic and widespread tool for the investigation\nof the edge and Scrape-Off Layer of magnetic fusion plasmas. In the Wendelstein\n7-X (W7-X) stellarator, the Multi-Purpose Manipulator serves as a multi-user\nplatform for probe measurements of various kinds. This paper presents a review\non reciprocating probe operation during the first operation phase of W7-X with\na test divertor (2017-2018). It gives an overview of the diverse zoo of probe\nheads and presents lessons learned about probe operation in complex magnetic\ngeometries, operation safety, and probe head design. A few examples of probe\nmeasurements with a focus on unexpected observations are presented.", "category": "physics_plasm-ph" }, { "text": "The Generation of Random Variates From a Relativistic Maxwellian\n Distribution: A procedure for generating random variates from a relativistic Maxwellian\ndistribution with arbitrary temperature and drift velocity is presented. The\nalgorithm is based on the rejection method and can be used to initialize\nparticle velocities in kinetic simulations of plasmas and gases.", "category": "physics_plasm-ph" }, { "text": "Corrections to linear mixing in binary ionic mixtures and plasma\n screening at zero separation: Using the results of extensive Monte Carlo simulations we discuss corrections\nto the linear mixing rule in strongly coupled binary ionic mixtures. We analyze\nthe plasma screening function at zero separation, H_{jk}(0), for two ions (of\ntypes j=1,2 and k=1,2) in a strongly coupled binary mixture. The function\nH_{jk}(0) is estimated by two methods: (1) from the difference of Helmholtz\nCoulomb free energies at large and zero separations; (2) by fitting the Widom\nexpansion of H_{jk}(x) in powers of interionic distance x to Monte Carlo data\non the radial pair distribution function g_{jk}(x). These methods are shown to\nbe in good agreement. For illustration, we analyze the plasma screening\nenhancement of nuclear burning rates in dense stellar matter.", "category": "physics_plasm-ph" }, { "text": "Electron-acoustic solitons in an electron-beam plasma system with\n kappa-distributed electrons: We investigate the existence conditions and propagation properties of\nelectron-acoustic solitary waves in a plasma consisting of an electron beam\nfluid, a cold electron fluid, and a hot suprathermal electron component modeled\nby a $\\kappa$-distribution function. The Sagdeev pseudopotential method was\nused to investigate the occurrence of stationary-profile solitary waves. We\nhave determined how the soliton characteristics depend on the electron beam\nparameters. It is found that the existence domain for solitons becomes narrower\nwith an increase in the suprathermality of hot electrons, increasing the beam\nspeed, and decreasing the beam-to-cold electron population ratio.", "category": "physics_plasm-ph" }, { "text": "Anisotropic Inverse Cascade toward Zonal Flow in Magnetically Confined\n Plasmas: We propose a new mechanism for the generation of zonal flows in magnetically\nconfined plasmas, complementing previous theories based on a modulational\ninstability. We derive a new conservation law that operates in the regime of\nweakly nonlinear dynamics, and show that it serves to focus the inverse cascade\nof turbulent drift wave energy into zonal flows. This mechanism continues to\noperate in the absence of the separation of dynamical scales typically assumed\nin instability calculations.", "category": "physics_plasm-ph" }, { "text": "The environment effect on operation of in-vessel mirrors for plasma\n diagnostics in fusion devices: First mirrors will be the plasma facing components of optical diagnostic\nsystems in ITER. Mirror surfaces will undergo modification caused by erosion\nand re-deposition processes [1,2]. As a consequence, the mirror performance may\nbe changed and may deteriorate [3,4]. In the divertor region it may also be\nobscured by deposition [5-7]. The limited access to in-vessel components of\nITER calls for testing the mirror materials in present day devices in order to\ngather information on the material damage and degradation of the mirror\nperformance, i.e. reflectivity. A dedicated experimental programme, First\nMirror Test (FMT), has been initiated at the JET tokamak within the framework\nTritium Retention Studies (TRS).", "category": "physics_plasm-ph" }, { "text": "Contribution of the Hall effect to radial electric field and\n spontaneous/intrinsic rotation in tokamak core plasmas: The Hall effect, defined as the separation of electric charges of opposite\nsign when they move in a magnetic field, is suggested to contribute\nsubstantially to the observed negative radial electric field Er in the core\nplasma in tokamaks and, respectively, to the spontaneous/intrinsic rotation of\nplasma. A simple way to evaluate the Hall effect contribution to the Er value,\nusing the independently measured space distributions of magnetic field and\nplasma rotation velocity, is suggested. The estimates of the effect for\nexperimental data from the TM-4 and T-10 tokamaks suggest that the above\nphenomena in tokamaks should be described in the framework of the two-fluid\nmagnetohydrodynamics.", "category": "physics_plasm-ph" }, { "text": "Intrinsic rotation with gyrokinetic models: The generation of intrinsic rotation by turbulence and neoclassical effects\nin tokamaks is considered. To obtain the complex dependences observed in\nexperiments, it is necessary to have a model of the radial flux of momentum\nthat redistributes the momentum within the tokamak in the absence of a\npreexisting velocity. When the lowest order gyrokinetic formulation is used, a\nsymmetry of the model precludes this possibility, making small effects in the\ngyroradius over scale length expansion necessary. These effects that are\nusually small become important for momentum transport because the symmetry of\nthe lowest order gyrokinetic formulation leads to the cancellation of the\nlowest order momentum flux. The accuracy to which the gyrokinetic equation\nneeds to be obtained to retain all the physically relevant effects is\ndiscussed.", "category": "physics_plasm-ph" }, { "text": "On the late phase of relaxation of two-dimensional fluids: turbulence of\n unitons: The two-dimensional ideal fluid and the plasma confined by a strong magnetic\nfield exhibit an intrinsic tendency to organization due to the inverse spectral\ncascade. In the asymptotic states reached at relaxation the turbulence has\nvanished and there are only coherent vortical structures. We are interested in\nthe regime that precedes these ordered flow patterns, in which there still is\nturbulence and imperfect but robust structures have emerged. To develop an\nanalytical description we propose to start from the stationary coherent states\nand (in the direction opposite to relaxation) explore the space of\nconfigurations before the extremum of the functional that defines the\nstructures has been reached. We find necessary to assemble different but\nrelated models: point-like vortices, its field theoretical formulation as\ninteracting matter and gauge fields, chiral model and surfaces with constant\nmean curvature. These models are connected by the similar ability to describe\nrandomly interacting coherent structures. They derive exactly the same equation\nfor the asymptotic state (sinh-Poisson equation, confirmed by numerical\ncalculation of fluid flows). The chiral model, to which one can arrive from\nself-duality equation of the field theoretical model for fluid and from\nconstant mean curvature surface equations, appears to be the suitable\nanalytical framework. Its solutions, the unitons, aquire dynamics when the\nsystem is not at the extremum of the action. In the present work we provide\narguments that the underlying common nature of these models can be used to\ndevelop an approach to fluid and plasma states of turbulence interacting with\nstructures.", "category": "physics_plasm-ph" }, { "text": "Strong collisionless damping of the low-velocity branch of\n electromagnetic wave in plasmas with Maxwellian-like electron velocity\n distribution function: After approximate replacing of Maxwellian distribution exponent with the\nrational polynomial fraction we have obtained precise analytical expression for\nand calculated the principal value of logarithmically divergent integral in the\nelectron wave dispersion equation. At the same time our calculations have shown\nthe presence of strong collisionless damping of the electromagnetic\nlow-velocity (electron) wave in plasmas with Maxwellian-like electron velocity\ndistribution function at some small, of the order of several per cents,\ndifferences from Maxwellian distribution in the main region of large electron\ndensities, however due to the differences in the distribution tail, where\nelectron density itself is negligibly small.", "category": "physics_plasm-ph" }, { "text": "Spectral self-action of THz emission from ionizing two-color laser\n pulses in gases: The spectrum of terahertz (THz) emission in gases via ionizing two-color\nfemtosecond pulses is analyzed by means of a semi-analytic model and\nfinite-difference-time-domain simulations in 1D and 2D geometries. We show that\nproduced THz signals interact with free electron trajectories and thus\ninfluence significantly further THz generation upon propagation, i.e., make the\nprocess inherently nonlocal. This self-action plays a key role in the observed\nstrong spectral broadening of the generated THz field. Diffraction limits the\nachievable THz bandwidth by efficiently depleting the low frequency amplitudes\nin the propagating field.", "category": "physics_plasm-ph" }, { "text": "The dispersion and propagation of topological Langmuir-cyclotron waves\n in cold magnetized plasmas: Topological Langmuir-Cyclotron Wave (TLCW) is a recently identified\ntopological surface excitation in magnetized plasmas. We show that TLCW\noriginates from the topological phase transition at the Langmuir wave-cyclotron\nwave resonance. By isofrequency surface analysis and 2D and 3D time-dependent\nsimulations, we demonstrate that the TLCW can propagate robustly along complex\nphase transition interfaces in a unidirectional manner and without scattering.\nBecause of these desirable features, the TLCW could be explored as an effective\nmechanism to drive current and flow in magnetized plasmas. The analysis also\nestablishes a close connection between the newly instituted topological phase\nclassification of plasmas and the classical CMA diagram of plasma waves.", "category": "physics_plasm-ph" }, { "text": "Ultrafast target charging due to polarization triggered by\n laser-accelerated electrons: A significant step has been made towards understanding the physics of the\ntransient surface current triggered by ejected electrons during the interaction\nof a short intense laser pulse with a high-conductivity target. Unlike the\ncommonly discussed hypothesis of neutralization current generation as a result\nof the fast loss of hot electrons to the vacuum, the proposed mechanism is\nassociated with excitation of the fast current by electric polarization due to\ntransition radiation triggered by ejected electrons. We present a corresponding\ntheoretical model and compare it with two simulation models using the FDTD\n(finite-difference time-domain) and PIC (particle-in-cell) methods. Distinctive\nfeatures of the proposed theory are clearly manifested in both of these models.", "category": "physics_plasm-ph" }, { "text": "On the Identification of Coherent Structures in Space Plasmas: the\n Magnetic Helicity-PVI Method: Plasma turbulence can be viewed as a magnetic landscape populated by large\nand small scale coherent structures. In this complex network, large helical\nmagnetic tubes might be separated by small scale magnetic reconnection events\n(current sheets). However, the identification of these magnetic structures in a\ncontinuous stream of data has always been a challenging task. Here we present a\nmethod that is able to characterize both the large and small scale structures\nof the turbulent solar wind, based on the combined use of a filtered magnetic\nhelicity ($H_m$) and the Partial Variance of Increments (PVI). This simple,\nsingle-spacecraft technique, has been validated first via direct numerical\nsimulations of plasma turbulence and then applied to data from the Parker Solar\nProbe (PSP) mission. This novel analysis, combining $H_m$&PVI methods, reveals\nthat a large number of flux tubes populate the solar wind and continuously\nmerge in contact regions where magnetic reconnection and particle acceleration\nmay occur.", "category": "physics_plasm-ph" }, { "text": "Magnetic dynamics of simple collective modes in a two-sphere plasma\n model: A plasma blob is modeled as consisting of two homogeneous spheres of equal\nradius and equal but opposite charge densities that can move relative to each\nother. Relative translational and rotational motion are considered separately.\nMagnetic effects from the current density caused by the relative motion are\nincluded. Magnetic interaction is seen to cause an inductive inertia. In the\nrelative translation case the Coulomb attraction, approximately a linear force\nfor small amplitudes, causes an oscillation. For a large number of particles\nthe corresponding oscillation frequency will not be the Langmuir plasma\nfrequency, because of the large inductive inertia. For rotation an external\nmagnetic field is included and the energy and diamagnetism of the plasma in the\nmodel is calculated. Finally it is noted how the neglect of resistivity is\nmotivated by the results.", "category": "physics_plasm-ph" }, { "text": "Generation of powerful terahertz emission in a beam-driven strong plasma\n turbulence: Generation of terahertz electromagnetic radiation due to coalescence of\nupper-hybrid waves in the long-wavelength region of strong plasma turbulence\ndriven by a high-current relativistic electron beam in a magnetized plasma is\ninvestigated. The width of frequency spectrum as well as angular\ncharacteristics of this radiation for various values of plasma density and\nturbulence energy are calculated using the simple theoretical model adequately\ndescribing beam-plasma experiments at mirror traps. It is shown that the power\ndensity of electromagnetic emission at the second harmonic of plasma frequency\nin the terahertz range for these laboratory experiments can reach the level of\n1 ${MW/cm}^3$ with 1% conversion efficiency of beam energy losses to\nelectromagnetic emission.", "category": "physics_plasm-ph" }, { "text": "Linear and nonlinear properties of Rao-dust-Alfv\u00e9n waves in magnetized\n plasmas: The linear and nonlinear properties of the Rao-dust-magnetohydrodynamic\n(R-D-MHD) waves in a dusty magnetoplasma are studied. By employing the\ninertialess electron equation of motion, inertial ion equation of motion,\nAmp\\`ere's law, Faraday's law, and the continuity equation in a plasma with\nimmobile charged dust grains, the linear and nonlinear propagation of\ntwo-dimensional R-D-MHD waves are investigated. In the linear regime, the\nexistence of immobile dust grains produces the Rao cutoff frequency, which is\nproportional to the dust charge density and the ion gyrofrequency. On the other\nhand, the dynamics of an amplitude modulated R-D-MHD waves is governed by the\ncubic nonlinear Schroedinger equation. The latter has been derived by using the\nreductive perturbation technique and the two-timescale analysis which accounts\nfor the harmonic generation nonlinearity in plasmas. The stability of the\nmodulated wave envelope against non-resonant perturbations is studied. Finally,\nthe possibility of localized envelope excitations is discussed.", "category": "physics_plasm-ph" }, { "text": "Lower Hybrid antennas for nuclear fusion experiments: The nuclear fusion research goal is to demonstrate the feasibility of fusion\npower for peaceful purposes. In order to achieve the conditions similar to\nthose expected in an electricity-generating fusion power plant, plasmas with a\ntemperature of several hundreds of millions of degrees must be generated and\nsustained for long periods. For this purpose, RF antennas delivering\nmulti-megawatts of power to magnetized confined plasma are commonly used in\nexperimental tokamaks. In the gigahertz range of frequencies, high power phased\narrays known as \"Lower Hybrid\" (LH) antennas are used to extend the plasma\nduration. This paper reviews some of the technological aspects of the LH\nantennas used in the Tore Supra tokamak and presents the current design of a\nproposed 20 MW LH system for the international experiment ITER.", "category": "physics_plasm-ph" }, { "text": "14-moment maximum-entropy modelling of collisionless ions for Hall\n thruster discharges: Ions in Hall effect thrusters are often characterized by a low\ncollisionality. In the presence of acceleration fields and azimuthal electric\nfield waves, this results in strong deviations from thermodynamic equilibrium,\nintroducing kinetic effects. This work investigates the application of the\n14-moment maximum-entropy model to this problem. This method consists in a set\nof 14 PDEs for the density, momentum, pressure tensor components, heat flux\nvector and fourth-order moment associated to the particle velocity distribution\nfunction. The model is applied to the study of collisionless ion dynamics in a\nHall thruster-like configuration, and its accuracy is assessed against\ndifferent models, including the Vlasov kinetic equation. Three test cases are\nconsidered: a purely axial acceleration problem, the problem of ion-wave\ntrapping and finally the evolution of ions in the axial-azimuthal plane.\n Most of this work considers ions only, and the coupling with electrons is\nremoved by prescribing reasonable values of the electric field. This allows us\nto obtain a direct comparison among different ion models. However, the\npossibility to run self-consistent plasma simulations is also briefly\ndiscussed, considering quasi-neutral or multi-fluid models. The maximum-entropy\nsystem appears to be a robust and accurate option for the considered test\ncases. The accuracy is improved over the simpler pressureless gas model (cold\nions) and the Euler equations for gas dynamics, while the computational cost\nshows to remain much lower than direct kinetic simulations.", "category": "physics_plasm-ph" }, { "text": "A Hybrid Gyrokinetic Ion and Isothermal Electron Fluid Code for\n Astrophysical Plasma: This paper describes a new code for simulating astrophysical plasmas that\nsolves a hybrid model composed of gyrokinetic ions (GKI) and an isothermal\nelectron fluid (ITEF) [A. Schekochihin et al., Astrophys. J. Suppl.\n\\textbf{182}, 310 (2009)]. This model captures ion kinetic effects that are\nimportant near the ion gyro-radius scale while electron kinetic effects are\nordered out by an electron-ion mass ratio expansion. The code is developed by\nincorporating the ITEF approximation into {\\tt AstroGK}, an Eulerian $\\delta f$\ngyrokinetics code specialized to a slab geometry [R. Numata et al., J. Compute.\nPays. \\textbf{229}, 9347 (2010)]. The new code treats the linear terms in the\nITEF equations implicitly while the nonlinear terms are treated explicitly. We\nshow linear and nonlinear benchmark tests to prove the validity and\napplicability of the simulation code. Since the fast electron timescale is\neliminated by the mass ratio expansion, the Courant--Friedrichs--Lewy condition\nis much less restrictive than in full gyrokinetic codes; the present hybrid\ncode runs $\\sim 2\\sqrt{m_\\mathrm{i}/m_\\mathrm{e}} \\sim 100$ times faster than\n{\\tt AstroGK}\\ with a single ion species and kinetic electrons where\n$m_\\mathrm{i}/m_\\mathrm{e}$ is the ion-electron mass ratio. The improvement of\nthe computational time makes it feasible to execute ion scale gyrokinetic\nsimulations with a high velocity space resolution and to run multiple\nsimulations to determine the dependence of turbulent dynamics on parameters\nsuch as electron--ion temperature ratio and plasma beta.", "category": "physics_plasm-ph" }, { "text": "A model for the non-universal power-law of the solar wind sub-ion scale\n magnetic spectrum: A phenomenological turbulence model for kinetic Alfv\\'en waves in a\nmagnetizedcollisionless plasma, able to reproduce the non-universalpower-law\nspectra observed at the sub-ion scales in the solar wind and the terrestrial\nmagnetosphere, is presented.The process of temperature homogenization along\ndistortedmagnetic field lines, induced by Landau damping,affects the\nturbulencetransfer time and results in a steepening of the sub-ion power-law\nspectrumof critically-balanced turbulence, whose exponent is sensitive to the\nratio between the Alfv\\'en wave period and the nonlinear timescale. Transition\nfrom large-scaleweak turbulence to smaller scale strong turbulence is\ncapturedand non local interactions, relevant in the case of steep spectra, are\naccounted for.", "category": "physics_plasm-ph" }, { "text": "Global anomalous transport of ICRH- and NBI-heated fast ions: By taking advantage of the trace approximation, one can gain an enormous\ncomputational advantage when solving for the global turbulent transport of\nimpurities. In particular, this makes feasible the study of non-Maxwellian\ntransport coupled in radius and energy, allowing collisions and transport to be\naccounted for on similar time scales, as occurs for fast ions. In this work, we\nstudy the fully-nonlinear ITG-driven trace turbulent transport of locally\nheated and injected fast ions. Previous results indicated the existence of\nMeV-range minorities heated by cyclotron resonance, and an associated density\npinch effect. Here, we build upon this result using the t3core code to solve\nfor the distribution of these minorities, consistently including the effects of\ncollisions, gyrokinetic turbulence, and heating. Using the same tool to study\nthe transport of injected fast ions, we contrast the qualitative features of\ntheir transport with that of the heated minorities. Our results indicate that\nheated minorities are more strongly affected by microturbulence than injected\nfast ions. The physical interpretation of this difference provides a possible\nexplanation for the observed synergy when NBI heating is combined with ICRH.\nFurthermore, we move beyond the trace approximation to develop a model which\nallows one to easily account for the reduction of anomalous transport due to\nthe presence of fast ions in electrostatic turbulence.", "category": "physics_plasm-ph" }, { "text": "Quasimonoenergetic and low emittance ion bunch generation from ultrathin\n targets by counterpropagating laser pulses of ultrarelativistic intensities: A new method for generation of quasimonoenergetic and low emittance fast\nion/nuclei bunches of solid densities from nanotargets by two\ncounterpropagating laser pulses of ultrarelativistic intensities is proposed,\nbased on the threshold phenomenon of particles \"reflection\" due to induced\nnonlinear Compton scattering. Particularly, a setup is considered which\nprovides generation of ion bunches with parameters that are required in hadron\ntherapy.", "category": "physics_plasm-ph" }, { "text": "Photon polarization effects in polarized electron-positron pair\n production in a strong laser field: Deep understanding of photon polarization impact on pair production is\nessential for the efficient creation of laser driven polarized positron beams,\nand demands a complete description of polarization effects in strong-field QED\nprocesses. We investigate, employing fully polarization resolved Monte Carlo\nsimulations, the correlated photon and electron (positron) polarization effects\nin multiphoton Breit-Wheeler pair production process during the interaction of\nan ultrarelativistic electron beam with a counterpropagating elliptically\npolarized laser pulse. We showed that the polarization of e^-e^+ pairs is\ndegraded by 35\\%, when the polarization of the intermediate photon is resolved,\naccompanied with an approximately 13\\% decrease of the pair yield. Moreover,\nthe polarization direction of energetic positrons in small angle region is\nreversed, which originates from the pair production of hard photons with\npolarization parallel with electric field.", "category": "physics_plasm-ph" }, { "text": "Ion heating and magnetic flux pile-up in a magnetic reconnection\n experiment with super-Alfvenic plasma inflows: This work presents a magnetic reconnection experiment in which the kinetic,\nmagnetic and thermal properties of the plasma each play an important role in\nthe overall energy balance and structure of the generated reconnection layer.\nMagnetic reconnection occurs during the interaction of continuous and steady\nflows of super-Alfvenic, magnetized, aluminum plasma, which collide in a\ngeometry with two-dimensional symmetry, producing a stable and long-lasting\nreconnection layer. Optical Thomson scattering measurements show that when the\nlayer forms, ions inside the layer are more strongly heated than electrons,\nreaching temperatures of Ti~ZTe>300 eV - much greater than can be expected from\nstrong shock and viscous heating alone. Later in time, as the plasma density in\nthe layer increases, the electron and ion temperatures are found to\nequilibrate, and a constant plasma temperature is achieved through a balance of\nthe heating mechanisms and radiative losses of the plasma. Measurements from\nFaraday rotation polarimetry also indicate the presence of significant magnetic\nfield pile-up occurring at the boundary of the reconnection region, which is\nconsistent with the super-Alfvenic velocity of the inflows.", "category": "physics_plasm-ph" }, { "text": "A Parallel Low-Rank Solver for the Six-Dimensional Vlasov-Maxwell\n Equations: Continuum Vlasov simulations can be utilized for highly accurate modelling of\nfully kinetic plasmas. Great progress has been made recently regarding the\napplicability of the method in realistic plasma configurations. However, a\nreduction of the high computational cost that is inherent to fully kinetic\nsimulations would be desirable, especially at high velocity space resolutions.\nFor this purpose, low-rank approximations can be employed. The so far available\nlow-rank solvers are restricted to either electrostatic systems or low\ndimensionality and can therefore not be applied to most space, astrophysical\nand fusion plasmas. In this paper we present a new parallel low-rank solver for\nthe full six-dimensional electromagnetic Vlasov-Maxwell equations with a\ncompression of the particle distribution function in velocity space. Special\nattention is paid to mass conservation and Gauss's law. The low-rank Vlasov\nsolver is applied to standard benchmark problems of plasma turbulence and\nmagnetic reconnection and compared to the full grid method. It yields accurate\nresults at significantly reduced computational cost.", "category": "physics_plasm-ph" }, { "text": "High-energy-density electron-positron pair plasma production and its\n dynamics in the relativistic transparency regime: High-energy-density electron-positron pair plasma production and its dynamics\nin a thin foil illuminated by two counter-propagating laser pulses are\ninvestigated through multi-dimensional particle-in-cell simulations. We compare\nthe production of electron-positron pairs and gamma-photons via quantum\nelectrodynamics processes in the relativistic transparent and opaque regimes,\nand find that the target transparency can significantly enhance the\nelectron-positron pair production due to the formation of stable standing wave\n(SW). An optimum foil density of 200 - 280 n_c (n_c is the laser critical\ndensity) is found for enhancing electron-positron pair production when laser\nintensity reaches a few 10e23 W/cm2. At such foil density, laser energy\nconversion to electron-positron pairs is approximately four times higher than\nat foil density of 710n_c, whereas laser energy conversion to gamma-photons\nkeeps almost the same. Consequently, high dense electron-positron plasma with a\nmaximum intensity above 10e20 W/cm2 is produced. Modulation dynamics of created\npair plasmas is further observed when target foil becomes transparent. It is\nshown that stable SWs formed directly by two counter-propagating lasers, not\nonly trap the created electron-positron pairs to their nodes, but also modulate\nperiodically average energy and phase-space and angular distributions of\ntrapped particles. However, similar trapping and modulation effects become\nobscure in the opaque regime due to the absence of stable SW field.", "category": "physics_plasm-ph" }, { "text": "Lowering the reactor breakeven requirements for proton-Boron 11 fusion: Recently, it has been shown that altering the natural collisional power flow\nof the proton-Boron 11 (pB11) fusion reaction can significantly reduce the\nLawson product of ion density and confinement time required to achieve\nignition. However, these products are still onerous - on the order of $7 \\times\n10^{15}$ cm$^{-3}$s under the most optimistic scenarios. Fortunately, a\nbreakeven fusion power plant does not require an igniting plasma, but rather a\nreactor that produces more electrical power than it consumes. Here, we extend\nthe existing 0D power balance analysis to check the conditions on power plant\nbreakeven. We find that even for the base thermonuclear reaction, modern\nhigh-efficiency thermal engines should reduce the Lawson product to $1.2 \\times\n10^{15}$ cm$^{-3}$s. We then explore the impact of several potential\nimprovements, including fast proton heating, alpha power capture, direct\nconversion, and efficient heating. We find that such improvements could reduce\nthe required Lawson product by a further order of magnitude, bringing\naneutronic fusion to within target ITER design parameters.", "category": "physics_plasm-ph" }, { "text": "Undamped electrostatic plasma waves: Electrostatic waves in a collision-free unmagnetized plasma of electrons with\nfixed ions are investigated for electron equilibrium velocity distribution\nfunctions that deviate slightly from Maxwellian. Of interest are undamped waves\nthat are the small amplitude limit of nonlinear excitations, such as electron\nacoustic waves (EAWs). A deviation consisting of a small plateau, a region with\nzero velocity derivative over a width that is a very small fraction of the\nelectron thermal speed, is shown to give rise to new undamped modes, which here\nare named {\\it corner modes}. The presence of the plateau turns off Landau\ndamping and allows oscillations with phase speeds within the plateau. These\nundamped waves are obtained in a wide region of the $(k,\\omega_{_R})$ plane\n($\\omega_{_R}$ being the real part of the wave frequency and $k$ the\nwavenumber), away from the well-known `thumb curve' for Langmuir waves and EAWs\nbased on the Maxwellian. Results of nonlinear Vlasov-Poisson simulations that\ncorroborate the existence of these modes are described. It is also shown that\ndeviations caused by fattening the tail of the distribution shift roots off of\nthe thumb curve toward lower $k$-values and chopping the tail shifts them\ntoward higher $k$-values. In addition, a rule of thumb is obtained for\nassessing how the existence of a plateau shifts roots off of the thumb curve.\nSuggestions are made for interpreting experimental observations of\nelectrostatic waves, such as recent ones in nonneutral plasmas.", "category": "physics_plasm-ph" }, { "text": "Effect of beam emittance on self-modulation of long beams in plasma\n wakefield accelerators: The initial beam emittance determines the maximum wakefield amplitude that\ncan be reached as a result of beam self-modulation in the plasma. The wakefield\nexcited by the fully self-modulated beam decreases linearly with the increase\nof the beam emittance. There is a value of initial emittance beyond which the\nself-modulation does not develop even if the instability is initiated by a\nstrong seed perturbation. The emittance scale at which the wakefield is twice\nsuppressed with respect to the zero-emittance case (the so called critical\nemittance) is determined by inability of the excited wave to confine beam\nparticles radially and is related to beam and plasma parameters by a simple\nformula. The effect of beam emittance can be observed in several discussed\nself-modulation experiments.", "category": "physics_plasm-ph" }, { "text": "Targeting realistic geometry in Tokamak code Gysela: In magnetically confined plasmas used in Tokamak, turbulence is responsible\nfor specific transport that limits the performance of this kind of reactors.\nGyrokinetic simulations are able to capture ion and electron turbulence that\ngive rise to heat losses, but require also state-of-the-art HPC techniques to\nhandle computation costs. Such simulations are a major tool to establish good\noperating regime in Tokamak such as ITER, which is currently being built. Some\nof the key issues to address more realistic gyrokinetic simulations are:\nefficient and robust numerical schemes, accurate geometric description, good\nparallelization algorithms. The framework of this work is the Semi-Lagrangian\nsetting for solving the gyrokinetic Vlasov equation and the Gyseka code. In\nthis paper, a new variant for the interpolation method is proposed that can\nhandle the mesh singularity in the poloidal plane at r=0 (polar system is used\nfor the moment in Gysela). A non-uniform meshing of the poloidal plane is\nproposed instead of uniform one in order to save memory and computations. The\ninterpolation method, the gyroaverage operator, and the Poisson solver are\nrevised in order to cope with non-uniform meshes. A mapping that establish a\nbijection from polar coordinates to more realistic plasma shape is used to\nimprove realism. Convergence studies are provided to establish the validity and\nrobustness of our new approach.", "category": "physics_plasm-ph" }, { "text": "Localized waves in plasmas at varying magnetic field: By considering the continuity, Navier-Stoks and Poisson's equations in a\nnon-relativistic frame work for plasmas, we study the behavior of small\namplitude ion acoustic solitary waves in plasmas under the influence of a\nvarying magnetic field. The result is a nonlinear wave equation which complies\nwith the KdV-Burgers (KdVB) equation, surprisingly in the absence of thermal\npressure or any dissipative effects. We show that the complete set of\nequations, by considering the varying magnetic field, create solitary waves\nwhich radiate energy during their traveling in the medium. An interesting\nresult is the existence of small amplitude localized shock profiles beside the\nsolitary waves. Properties of this solitaire solution is studied by considering\ndifferent values for the environmental characters.", "category": "physics_plasm-ph" }, { "text": "Accounting for speckle scale beam-bending in classical ray tracing\n schemes for propagating realistic pulses in indirect drive ignition\n conditions: We propose a semi-analytical modeling of smoothed laser beams deviation\ninduced by plasma flows. Based on a Gaussian description of speckles, the model\nincludes spatial, temporal and polarization smoothing techniques,through fits\nissued from hydrodynamic simulations with a paraxial description of\nelectromagnetic waves. This beam bending model is then included in a ray\ntracing algorithm, and carefully validated. When applied as a post-process to\nthe propagation of the inner cone in a full-scale simulation of a NIF\nexperiment,the beam bending along the path of the laser affects the refraction\nconditions inside the hohlraum and the energy deposition, and could explain the\nanomalous refraction measurements, the so-called glint observed in some NIF\nexperiments.", "category": "physics_plasm-ph" }, { "text": "Microfield Fluctuations and Spectral Line Shapes in Strongly Coupled\n Two-Component Plasmas: The spectral line shapes for hydrogen-like heavy ion emitters embedded in\nstrongly correlated two-component electron-ion plasmas are investigated with\nnumerical simulations. For that purpose the microfield fluctuations are\ncalculated by molecular dynamics simulations where short range quantum effects\nare taken into account by using a regularized Coulomb potential for the\nelectron-ion interaction. The microfield fluctuations are used as input in a\nnumerical solution of the time-dependent Schroedinger equation for the\nradiating electron. In distinction to the standard impact and quasistatic\napproximations the method presented here allows to account for the correlations\nbetween plasma ions and electrons. The shapes of the Ly-alpha line in Al are\ninvestigated in the intermediate regime. The calculations are in good agreement\nwith experiments on the Ly-alpha line in laser generated plasmas.", "category": "physics_plasm-ph" }, { "text": "Electron polarization in ultrarelativistic plasma current filamentation\n instabilities: Plasma current filamentation of an ultrarelativistic electron beam impinging\non an overdense plasma is investigated, with emphasis on radiation-induced\nelectron polarization. Particle-in-cell simulations provide the classification\nand in-depth analysis of three different regimes of the current filaments,\nnamely, the normal filament, abnormal filament, and quenching regimes. We show\nthat electron radiative polarization emerges during the instability along the\nazimuthal direction in the momentum space, which significantly varies across\nthe regimes. We put forward an intuitive Hamiltonian model to trace the origin\nof the electron polarization dynamics. In particular, we discern the role of\nnonlinear transverse motion of plasma filaments, which induces asymmetry in\nradiative spin flips, yielding an accumulation of electron polarization. Our\nresults break the conventional perception that quasi-symmetric fields are\ninefficient for generating radiative spin-polarized beams, suggesting the\npotential of electron polarization as a source of new information on laboratory\nand astrophysical plasma instabilities.", "category": "physics_plasm-ph" }, { "text": "Vacuum laser acceleration of relativistic electrons using plasma mirror\n injectors: Accelerating particles to relativistic energies over very short distances\nusing lasers has been a long standing goal in physics. Among the various\nschemes proposed for electrons, vacuum laser acceleration has attracted\nconsiderable interest and has been extensively studied theoretically because of\nits appealing simplicity: electrons interact with an intense laser field in\nvacuum and can be continuously accelerated, provided they remain at a given\nphase of the field until they escape the laser beam. But demonstrating this\neffect experimentally has proved extremely challenging, as it imposes stringent\nrequirements on the conditions of injection of electrons in the laser field.\nHere, we solve this long-standing experimental problem for the first time by\nusing a plasma mirror to inject electrons in an ultraintense laser field, and\nobtain clear evidence of vacuum laser acceleration. With the advent of PetaWatt\nclass lasers, this scheme could provide a competitive source of very high\ncharge (nC) and ultrashort relativistic electron beams.", "category": "physics_plasm-ph" }, { "text": "Multidimensional Iterative Filtering: a new approach for investigating\n plasma turbulence in numerical simulations: Turbulent space and astrophysical plasmas exhibit a complex dynamics, which\ninvolves nonlinear coupling across different temporal and spatial scales. There\nis growing evidence that impulsive events, such as magnetic reconnection\ninstabilities, lead to a spatially localized enhancement of energy dissipation,\nthus speeding up the energy transfer at small scales. Capturing such a diverse\ndynamics is challenging. Here, we employ the Multidimensional Iterative\nFiltering (MIF) method, a novel technique for the analysis of nonstationary\nmultidimensional signals. Unlike other traditional methods (e.g., based on\nFourier or wavelet decomposition), MIF does not require any previous assumption\non the functional form of the signal to be identified. Using MIF, we carry out\na multiscale analysis of Hall-magnetohydrodynamic (HMHD) and hybrid\nparticle-in-cell (HPIC) numerical simulations of decaying plasma turbulence.\nThe results assess the ability of MIF to spatially identify and separate the\ndifferent scales (the MHD inertial range, the sub-ion kinetic, and the\ndissipation scales) of the plasma dynamics. Furthermore, MIF decomposition\nallows to detect localized current structures and to characterize their\ncontribution to the statistical and spectral properties of turbulence. Overall,\nMIF arises as a very promising technique for the study of turbulent plasma\nenvironments.", "category": "physics_plasm-ph" }, { "text": "Beam quality requirements for the Ion-Channel Laser: In this paper, we determine the electron beam quality requirements to obtain\nexponential radiation amplification in the ion-channel laser, where a\nrelativistic electron beam wiggles in a focusing ion-channel that can be\ncreated in a wakefield accelerator. The beam energy and wiggler parameter\nspreads should be limited. Those spread limits are functions of the Pierce\nparameter, which is calculated here without neglecting the radiation\ndiffraction. Two dimensional and three dimensional simulations of the\nself-consistent ion-channel laser confirm our theoretical predictions.", "category": "physics_plasm-ph" }, { "text": "AWECS: A Linear Gyrokinetic Delta-f Particle-in-Cell Simulation Code for\n the Study of Alfvenic Instabilities in High-Beta Tokamak Plasmas: A 1-D linear gyrokinetic code called AWECS is developed to study the kinetic\nexcitation of Alfvenic instabilities in a high-beta tokamak plasma, with beta\nbeing the ratio of thermal to magnetic pressure. It is designed to describe\nphysics associated with a broad range of frequencies and wavelengths. For\nexample, AWECS is capable of simulating kinetic ballooning modes, Alfvenic\nion-temperature-gradient-driven modes, as well as Alfven instabilities due to\nenergetic particles. In addition, AWECS may be used to study drift-Alfven\ninstabilities in the low-beta regime. Here, the layout of the code and the\nnumerical methods used are described. AWECS is benchmarked against other codes\nand a convergence study is carried out.", "category": "physics_plasm-ph" }, { "text": "Neutral dissociation of methane by electron impact and a complete and\n consistent cross section set: We present cross sections for the neutral dissociation of methane, in a large\npart obtained through analytical approximations. With these cross sections the\nwork of Song $\\textit{et al.}$ [J. Phys. Chem. Ref. Data, $\\textbf{44}$,\n023101, (2015)] can be extended which results in a complete and consistent set\nof cross sections for the collision of electrons with up to 100 eV energy with\nmethane molecules. Notably, the resulting cross section set does not require\nany data fitting to produce bulk swarm parameters that match with experiments.\nTherefore consistency can be considered an inherent trait of the set, since\nswarm parameters are used exclusively for validation of the cross sections.\nNeutral dissociation of methane is essential to include (1) because it is a\ncrucial electron energy sink in methane plasma, and (2) because it largely\ncontributes to the production of hydrogen radicals that can be vital for\nplasma-chemical processes. Finally, we compare the production rates of hydrogen\nspecies for a swarm-fitted data set with ours. The two consistent cross section\nsets predict different production rates, with differences of $45\\%$ (at $100$\nTd) and $125\\%$ (at $50$ Td) for production of H$_2$ and a similar trend for\nproduction of H. With this comparison we underline that the swarm-fitting\nprocedure, used to ensure consistency of the electron swarm parameters, can\npossibly deteriorate the accuracy with which chemical production rates are\nestimated. This is of particular importance for applications with an emphasis\non plasma-chemical activation of the gas.", "category": "physics_plasm-ph" }, { "text": "Non-Linear Ablative Rayleigh-Taylor Instability: Increased Growth due to\n Self-Generated Magnetic Fields: The growth rate of the non-linear ablative Rayleigh-Taylor (RT) instability\nis enhanced by magnetic fields self-generated by the Biermann battery\nmechanism; a scaling for this effect with perturbation height and wavelength is\nproposed and validated with extended-magnetohydrodynamic simulations. The\nmagnetic flux generation rate around a single RT spike is found to scale with\nthe spike height. The Hall Parameter, which quantifies electron magnetization,\nis found to be strongly enhanced for short wavelength spikes due to Nernst\ncompression of the magnetic field at the spike tip. The impact of the magnetic\nfield on spike growth is through both the suppressed thermal conduction into\nthe unstable spike and the Righi-Leduc heat-flow deflecting heat from the spike\ntip to the base. Righi-Leduc is found to be the dominant effect for small Hall\nParameters, while suppressed thermal conduction dominates for large Hall\nParameters. These results demonstrate the importance of considering magnetic\nfields in all perturbed inertial confinement fusion hot-spots.", "category": "physics_plasm-ph" }, { "text": "Generation of a strong reverse shock wave in the interaction of a\n high-contrast high-intensity femtosecond laser pulse with a silicon target: We present ultrafast pump-probe reflectivity and Doppler spectrometry of a\nsilicon target at relativistic laser intensity. We observe an unexpected rise\nin reflectivity to a peak approximately $\\sim$9 ps after the main pulse\ninteraction with the target. This occurs after the reflectivity has fallen off\nfrom the initially high \"plasma-mirror\" phase. Simultaneously measured\ntime-dependent Doppler shift data show an increase in blue shift at the same\ntime. Numerical simulations show that the aforementioned trends in the\nexperimental measurements correspond to a strong shock wave propagating back\ntowards the laser. The relativistic laser-plasma interaction indirectly heats\nthe cool-dense ($n_{e}\\geq10^{23} cm^{-3}$ and $T_{e} \\sim 10 eV$) target\nmaterial adjacent to the corona, by hot electron induced return current\nheating, raising its temperature to around 150eV and causing it to explode\nviolently. The increase in reflectivity is caused by the transient steepening\nof the plasma density gradient at the probe critical surface due to this\nexplosive behaviour.", "category": "physics_plasm-ph" }, { "text": "Light emission from particle beam induced plasma - An overview: Experiments to study the light emission from plasma produced by particle\nbeams are presented. Fundamental aspects in comparison with discharge plasma\nformation are discussed. It is shown that the formation of excimer molecules is\nan important process. This paper summarizes various studies of particle beam\ninduced light emission and presents first results of a direct comparison of\nlight emission induced by electron- and ion beam excitation. Both high energy\nheavy ion beam and low energy electron beam experiments are described and an\noverview over applications in the form of light sources, lasers, and ionization\ndevices is given.", "category": "physics_plasm-ph" }, { "text": "picFoam: An OpenFOAM based electrostatic Particle-in-Cell solver: picFoam is a fully kinetic electrostatic Particle-in-Cell(PIC) solver,\nincluding Monte Carlo Collisions(MCC), for non-equilibrium plasma research in\nthe open-source framework of OpenFOAM. The solver's modular design, based on\nthe same principles used in OpenFOAM, makes it highly flexible, by allowing the\nuser to choose different methods at run time, and extendable, by building upon\ntemplated modular classes. The implementation of the PIC method employing the\nfinite volume method, allows it to simulate on arbitrary geometries in one to\nthree dimensions. OpenFOAM's barycentric particle tracking is used effectively\nto performe charge and field weighting from the Lagrangian particle based\ndescription to the Eulerian field description and backwards without\ncomputational expensive particle searching algorithm. picFoam also includes\nopen and general circuit boundary models for the description of real plasma\ndevices.", "category": "physics_plasm-ph" }, { "text": "Extreme high field plasmonics: electron acceleration and XUV harmonic\n generation from ultrashort surface plasmons: Experiments on the excitation of propagating surface plasmons (SPs) by\nultrashort, high intensity laser interaction with \"grating\" targets are\nreviewed. At intensities exceeding $10^{19}~\\mbox{W cm}^{-2}$ on target, i.e.\nin the strongly relativistic regime of electron dynamics, multi-MeV electrons\nare accelerated by the SP field as dense bunches collimated in a near-tangent\ndirection. By the use of a suitable blazed grating, the bunch charge can be\nincreased up to $\\simeq $660 picoCoulomb. Intense XUV high harmonics (HH)\ndiffracted by the grating are observed when a plasma with sub-micrometer scale\nis produced at the target surface by a controlled prepulse. When the SP is\nexcited, the HH are strongly enhanced in a direction quasi-parallel to the\nelectrons. Simulations show that the HH are boosted by nanobunching in the SP\nfield of the electrons which scatter the laser field. Besides the static and\ndynamic tailoring of the target density profile, further control of electron\nand HH emission might be achieved by changing the SP duration using a laser\npulse with rotating wavefront. This latter technique may be capable to produce\nnearly single-cycle SPs.", "category": "physics_plasm-ph" }, { "text": "The characteristic shape of emission profiles of plasma spokes in\n HiPIMS: the role of secondary electrons: A time resolved analysis of the emission of HiPIMS plasmas reveals\ninhomogeneities in the form of rotating spokes. The shape of these spokes is\nvery characteristic depending on the target material. The localized enhanced\nlight emission has been correlated with the ion production. Based on these\ndata, the peculiar shape of the emission profiles can be explained by the\nlocalized generation of secondary electrons, resulting in an energetic electron\npressure exceeding the magnetic pressure. This general picture is able to\nexplain the observed emission profile for different target materials including\ngas rarefaction and second ionization potential of the sputtered elements.", "category": "physics_plasm-ph" }, { "text": "Generation of cold magnetized relativistic plasmas at the rear of thin\n foils irradiated by ultra-high-intensity laser pulses: A scheme to generate magnetized relativistic plasmas in laboratory is\nproposed. It is based on interaction of ultra-high-intensity sub-picosecond\nlaser pulses with few-micron thick foils or films. By means of Particle-In-Cell\nsimulations it is shown that energetic electrons produced by the laser and\nevacuated at the rear of the target trigger an expansion of the target and\nbuilds up a strong azimuthal magnetic field. It is shown that in the expanding\nplasma sheath a ratio of the magnetic pressure and the electron rest-mass\nenergy density exceeds unity whereas a the plasma pressure is lower than the\nmagnetic pressure and the electron gyroradius is lower than the plasma\ndimension. This scheme can be utilised to study astrophysical extreme phenomena\nsuch as relativistic magnetic reconnection in laboratory.", "category": "physics_plasm-ph" }, { "text": "Two-dimensional turbulence in magnetised plasmas: In an inhomogeneous magnetised plasma the transport of energy and particles\nperpendicular to the magnetic field is in general mainly caused by quasi\ntwo-dimensional turbulent fluid mixing. The physics of turbulence and structure\nformation is of ubiquitous importance to every magnetically confined laboratory\nplasma for experimental or industrial application. Specifically, high\ntemperature plasmas for fusion energy research are also dominated by the\nproperties of this turbulent transport. Self-organisation of turbulent vortices\nto mesoscopic structures like zonal flows is related to the formation of\ntransport barriers that can significantly enhance the confinement of a fusion\nplasma. This subject of great importance in research is rarely touched on in\nintroductory plasma physics or continuum dynamics courses. Here a brief\ntutorial on 2D fluid and plasma turbulence is presented as an introduction to\nthe field, appropriate for inclusion in undergraduate and graduate courses.", "category": "physics_plasm-ph" }, { "text": "L-H transition dynamics in fluid turbulence simulations with\n neoclassical force balance: Spontaneous transport barrier generation at the edge of a magnetically\nconfined plasma is investigated. To this end, a model of electrostatic\nturbulence in three-dimensional geometry is extended to account for the impact\nof friction between trapped and passing particles on the radial electric field.\nNon-linear flux-driven simulations are carried out, and it is shown that\nconsidering the radial and temporal variations of the neoclassical friction\ncoefficients allows for a transport barrier to be generated above a threshold\nof the input power.", "category": "physics_plasm-ph" }, { "text": "A Lagrangian perspective on the stability of ideal MHD equilibria with\n flow: We take a careful look at two approaches to deriving stability criteria for\nideal MHD equilibria. One is based on a tedious analysis of the linearized\nequations of motion, while the other examines the second variation of the MHD\nHamiltonian computed with proper variational constraints. For equilibria\nwithout flow, the two approaches are known to be fully consistent. However, for\nequilibria with flow, the stability criterion obtained from the constrained\nvariation approach was claimed to be stronger than that derived using the\nlinearized equations of motion. We show this claim is incorrect by deriving and\ncomparing both criteria within the same framework. It turns out that the\ncriterion obtained from the constrained variation approach has stricter\nrequirements on the initial perturbations than the other. Such requirements\nnaturally emerge in our new treatment of the constrained variation approach\nusing the Euler-Poincar\\'e structure of ideal MHD, which is more direct and\nsimple than the previous derivation from the Poisson perspective.", "category": "physics_plasm-ph" }, { "text": "PoPe (Projection on Proper elements) for code control: verification,\n numerical convergence and reduced models. Application to plasma turbulence\n simulations: The Projection on Proper elements (PoPe) is a novel method of code control\ndedicated to 1) checking the correct implementation of models, 2) determining\nthe convergence of numerical methods and 3) characterizing the residual errors\nof any given solution at very low cost. The basic idea is to establish a\nbijection between a simulation and a set of equations that generate it.\nRecovering equations is direct and relies on a statistical measure of the\nweight of the various operators. This method can be used in any dimensions and\nany regime, including chaotic ones. This method also provides a procedure to\ndesign reduced models and quantify the ratio costs to benefits. PoPe is applied\nto a kinetic and a fluid code of plasma turbulence.", "category": "physics_plasm-ph" }, { "text": "Rarefaction Shock Waves in Collisionless Plasma with Electronic Beam: We show that an electronic beam passing through the collisionless plasma of\nthe \"cold\" ions and the \"hot\" Boltzmann electrons can give rise to the\npropagation of the supersonic ion-acoustic rarefaction shock waves. These waves\nare analogous to those predicted by Zeldovich [5] in gasodynamics and\ncomplementary to the ion-acoustic compression shock waves in collisionless\nplasma described by Sagdeev [3].", "category": "physics_plasm-ph" }, { "text": "Geometric phase in Brillouin flows: A geometric phase is found to arise from the cyclic adiabatic variation of\nthe crossed magnetic and electric fields which sustain the Brillouin rotation\nof a plasma column. The expression of the gauge field associated with this\ngeometric phase accumulation is explicited. The physical origin of this phase\nis shown to be the uncompensated inductive electric field drift that stems from\nmagnetic field cyclic variations. Building on this result, the effect of a\nweak, periodic and adiabatic modulation of the axial magnetic field on the\nparticle guiding center drift motion is demonstrated to be equivalent to that\nof a perpendicular electric field, allowing to study the gauge induced\nBrillouin flow through a geometrically equivalent linear radial electric field.\nThis finding opens new perspectives to drive plasma rotation and hints at\npossible applications of this basic effect.", "category": "physics_plasm-ph" }, { "text": "Comment on \"Ion velocity analysis of rotating structures in a magnetic\n linear plasma device\" [Phys. Plasmas 25, 061203 (2018)]: In a recent paper (Phys. Plasmas 25, 061203, 2018), the authors have\npresented the analysis of the electric ion drift velocity experienced by heavy\nions created in a plasma submitted to a low magnetic field. Unfortunately, they\nhave used the classical ExB drift formula that is valid only in slab geometry.\nThe authors have not taken into account that the cylindrical geometry induces a\nslow electric drift of the ions around the axis of the column. Moreover, the\nlow magnetization of the ions induces a Larmor radius that is larger than the\ndiameter of the plasma column. The movement of the ions immediately after their\ncreation is parallel to the local electric field, not perpendicular as\nindicated by the authors. Most often the ions are neutralized before\nexperiencing the electric drift calculated along the classical guiding center\ntheory. This has not been taken into account carefully by the authors so that\nthe theoretical analysis of the Laser Induced Fluorescence measurements\npresented in this paper is clearly invalid.", "category": "physics_plasm-ph" }, { "text": "Electron dynamics inside a vacuum tube diode through linear differential\n equations: In this paper we analyze the motion of charged particles in a vacuum tube\ndiode by solving linear differential equations. Our analysis is based on\nexpressing the volume charge density as a function of the current density and\ncoordinates only, while in the usual scheme the volume charge density is\nexpressed as a function of the current density and electrostatic potential. Our\napproach gives the well known behavior of the classical current density\nproportional to the three-halves power of the bias potential and inversely\nproportional to the square of the gap distance between the electrodes, and does\nnot require the solution of the nonlinear differential equation normally\nassociated with the Child-Langmuir formulation.", "category": "physics_plasm-ph" }, { "text": "Positron acceleration via laser-augmented blowouts in two-column plasma\n structures: We propose a setup for positron acceleration consisting of an electron driver\nand a laser pulse creating a two-fold plasma column structure. The resulting\nwakefield is capable of accelerating positron bunches over long distances even\nwhen evolution of the driver is considered. The scheme is studied by means of\nparticle-in-cell simulations. Further, the analytical expression for the\naccelerating and focusing fields are obtained, showing the equilibrium lines\nalong which the witness bunch is accelerated.", "category": "physics_plasm-ph" }, { "text": "Diffusion regime of electron-electron collisions in weakly ionized\n plasmas: We consider weakly ionized plasma where frequent elastic scattering of\nelectrons on neutrals change the individual acts and the rate of\nelectron-electron collisions significantly. In this case, the kinetics of\nelectron thermalization is very different from that in fully ionized plasma.\nThe colliding electrons diffuse because of fast scattering on neutrals. We\ndemonstrate how a proper account of this diffusion enables one to estimate the\ncharacteristic time of electron thermalization. We also present a rigorous\nderivation of the kinetic equation for electrons by using Bogolyubov method\nbased on Liouville equations for multi-particle distribution functions.", "category": "physics_plasm-ph" }, { "text": "Hierarchy of instabilities for two counter-streaming magnetized pair\n beams: influence of field obliquity: The hierarchy of unstable modes when two counter-streaming pair plasmas\ninteract over a flow-aligned magnetic field has been recently investigated [PoP\n\\textbf{23}, 062122 (2016)]. The analysis is here extended to the case of an\narbitrarily tilted magnetic field. The two plasma shells are initially cold and\nidentical. For any angle $\\theta \\in [0,\\pi/2]$ between the field and the\ninitial flow, the hierarchy of unstable modes is numerically determined in\nterms of the initial Lorentz factor of the shells $\\gamma_0$, and the field\nstrength as measured by a parameter denoted $\\sigma$. For $\\theta=0$, four\ndifferent kinds of mode are likely to lead the linear phase. The hierarchy\nsimplifies for larger $\\theta$'s, partly because the Weibel instability can no\nlonger be cancelled in this regime. For $\\theta>0.78$ (44$^\\circ$) and in the\nrelativistic regime, the Weibel instability always govern the interaction. In\nthe non-relativistic regime, the hierarchy becomes $\\theta$-independent because\nthe interaction turns to be field-independent. As a result, the two-stream\ninstability becomes the dominant one, regardless of the field obliquity.", "category": "physics_plasm-ph" }, { "text": "Simulation of neutral beam current drive on EAST tokamak: Neutral beam current drive (NBCD) on the EAST tokamak is studied by using\nMonte-Carlo test particle code TGCO. Phase-space structure of the steady-state\nfast ion distribution is examined and visualized. We find that trapped ions\ncarry co-current current near the edge and counter-current current near the\ncore. However, the magnitude of the trapped ion current is one order smaller\nthan that of the passing ions. Therefore their contribution to the fast ion\ncurrent is negligible (1% of the fast ion current). We examine the dependence\nof the fast ion current on two basic plasma parameters: the plasma current I_p\nand plasma density n_e. The results indicate that the dependence of fast ion\ncurrent on I_p is not monotonic: with I_p increasing, the fast ion current\nfirst increases and then decreases. This dependence can be explained by the\nchange of trapped fraction and drift-orbit width with I_p. The fast ion current\ndecreases with the increase of plasma density n_e. This dependence is related\nto the variation of the slowing-down time with n_e, which is already well known\nand is confirmed in our specific situation. The electron shielding effect to\nthe fast ion current is taken into account by using a fitting formula\napplicable to general tokamak equilibria and arbitrary collisionality regime.\nThe dependence of the net current on the plasma current and density follows the\nsame trend as that of the fast ion current.", "category": "physics_plasm-ph" }, { "text": "Evolution of field line helicity during magnetic reconnection: We investigate the evolution of field line helicity for magnetic fields that\nconnect two boundaries without null points, with emphasis on localized finite-B\nmagnetic reconnection. Total (relative) magnetic helicity is already recognized\nas an important topological constraint on magnetohydrodynamic processes. Field\nline helicity offers further advantages because it preserves all topological\ninformation and can distinguish between different magnetic fields with the same\ntotal helicity. Magnetic reconnection changes field connectivity and field line\nhelicity reflects these changes; the goal of this paper is to characterize that\nevolution. We start by deriving the evolution equation for field line helicity\nand examining its terms, also obtaining a simplified form for cases where\ndynamics are localized within the domain. The main result, which we support\nusing kinematic examples, is that during localized reconnection in a complex\nmagnetic field, the evolution of field line helicity is dominated by a\nwork-like term that is evaluated at the field line endpoints, namely the scalar\nproduct of the generalized field line velocity and the vector potential.\nFurthermore, the flux integral of this term over certain areas is very small\ncompared to the integral of the unsigned quantity, which indicates that changes\nof field line helicity happen in a well-organized pairwise manner. It follows\nthat reconnection is very efficient at redistributing helicity in complex\nmagnetic fields despite having little effect on the total helicity.", "category": "physics_plasm-ph" }, { "text": "One-dimensional radiation-hydrodynamic scaling studies of imploding\n spherical plasma liners: One-dimensional radiation-hydrodynamic simulations are performed to develop\ninsight into the scaling of stagnation pressure with initial conditions of an\nimploding spherical plasma shell or \"liner.\" Simulations reveal the evolution\nof high-Mach-number (M), annular, spherical plasma flows during convergence,\nstagnation, shock formation, and disassembly, and indicate that cm- and\n{\\mu}s-scale plasmas with peak pressures near 1 Mbar can be generated by liners\nwith initial kinetic energy of several hundred kilo-joules. It is shown that\nradiation transport and thermal conduction must be included to avoid\nnon-physical plasma temperatures at the origin which artificially limit liner\nconvergence and thus the peak stagnation pressure. Scalings of the stagnated\nplasma lifetime ({\\tau}stag) and average stagnation pressure (Pstag, the\npressure at the origin, averaged over {\\tau}stag) are determined by evaluating\na wide range of liner initial conditions. For high-M flows, {\\tau}stag L0/v0,\nwhere L0 and v0 are the initial liner thickness and velocity, respectively.\nFurthermore, for argon liners, Pstag scales approximately as v0^(15/4) over a\nwide range of initial densities (n0), and as n0^(1/2) over a wide range of v0.\nThe approximate scaling Pstag ~ M 3/2 is also found for a wide range of\nliner-plasma initial conditions.", "category": "physics_plasm-ph" }, { "text": "Multitoroidal configurations as equilibrium flow eigenstates: Equilibrium eigenstates of an axisymmetric magnetically confined plasma with\ntoroidal flow are investigated by means of exact solutions of the ideal\nmagnetohydrodynamic equations. The study includes \"compressible\" flows with\nconstant temperature, but varying density on magnetic surfaces and\nincompressible ones with constant density, but varying temperature thereon. In\nboth cases eigenfunctions of the form Psi_{nl} = Z_l(z)R_n(R) (l, n=1,2,...)\ndescribe configurations with lxn magnetic axes. By varying the flow parameters\na change in magnetic topology is possible. In addition, the effects of the flow\nand the aspect ratio on the Shafranov shift are evaluated along with the\nvariations of density and temperature on magnetic surfaces.", "category": "physics_plasm-ph" }, { "text": "Macroscopic electromagnetic stress tensor for ionized media: Following the arguments presented by Mansuripur [Opt. Express 16, 14821-14835\n(2008)], we suggest a form for the macroscopic electromagnetic stress tensor\nappropriate for ionized media. The generalized Lorentz force includes the\neffects of polarization forces as well as those on the free charge and current\ndensities. The resulting tensor is written in terms of the fields D, B, E, and\nH. Its expression for a fully ionized medium subject to an external\nelectromagnetic field is discussed, as are the plasma conservation equations.\nAn apparatus is suggested for its experimental discrimination.", "category": "physics_plasm-ph" }, { "text": "Theory of the Laser Wake-Field Accelerator Revisited: Wake Overtaking,\n Localized Spectrum and Ponderomotive Acceleration: The electron and positron acceleration in the first cycle of a laser-driven\nwakefield is investigated. Separatrices between different types of the particle\nmotion (confined, reflected by the wakefield or ponderomotive potential and\ntransient) are demonstrated. The ponderomotive acceleration is negligible for\nelectrons but is substantial for positrons. An electron bunch, injected as\nquasi-monoenergetic, acquires a localized energy spectrum with a cut-off at the\nmaximum energy.", "category": "physics_plasm-ph" }, { "text": "Fluid-like dissipation of magnetic turbulence at electron scales in the\n solar wind: The turbulent spectrum of magnetic fluctuations in the solar wind displays a\nspectral break at ion characteristic scales. At electron scales the spectral\nshape is not yet completely established. Here, we perform a statistical study\nof 102 spectra at plasma kinetic scales, measured by the Cluster/STAFF\ninstrument in the free solar wind. We show that the magnetic spectrum in the\nhigh frequency range, [1,400] Hz, has a form similar to what is found in\nhydrodynamics in the dissipation range ~Ak^(-\\alpha)exp(-kl_d). The dissipation\nscale l_d is found to be correlated with the electron Larmor radius \\rho_e. The\nspectral index \\alpha varies in the range [2.2,2.9] and is anti-correlated with\nl_d, as expected in the case of the balance between the energy injection and\nthe energy dissipation. The coefficient A is found to be proportional to the\nion temperature anisotropy, suggesting that local ion instabilities may play\nsome role for the solar wind turbulence at plasma kinetic scales. The\nexponential spectral shape found here indicates that the effective dissipation\nof magnetic fluctuations in the solar wind has a wave number dependence similar\nto that of the resistive term in collisional fluids ~k^2\\delta B.", "category": "physics_plasm-ph" }, { "text": "Drive Asymmetry and the Origin of Turbulence in an ICF Implosion: 2D and 3D numerical simulations with the adaptive mesh refinement Eulerian\nradiation-hydrocode RAGE at unprecedented spatial resolution are used to\ninvestigate the connection between drive asymmetry and the generation of\nturbulence in the DT fuel in a simplified inertial-confinement fusion (ICF)\nimplosion. Long-wavelength deviations from spherical symmetry in the pressure\ndrive lead to the generation of coherent vortical structures in the DT gas and\nit is the three-dimensional instability of these structures that in turn leads\nto turbulence and mix. The simulations sug-gest that this mechanism may be an\nadditional important source of mix in ICF implosions. Applications to target\nignition at the National Ignition Facility are briefly discussed.", "category": "physics_plasm-ph" }, { "text": "Acoustic waves in the Jovian dusty magnetosphere: A brief review and\n meta-analysis: The omnipresence of dust particulates in space and astrophysical plasmas has\nbeen attracting numerous researchers to study the collective excitation and\npropagation dynamics of different eigen-mode structures in diversifed\nastrocosmic circumstances for years. It includes planetary rings,\ninterplanetary space, cometary tails, asteroid zones, planetary atmospheres,\netc. The ubiquitous charged dust particulates possess collective degrees of\ndynamic freedom resulting in the excitation of relatively low-frequency modes,\nsuch as dust-ion-acoustic waves (DIAWs), dust-acoustic waves (DAWs),\ndust-Coulomb waves (DCWs), and so forth. An interesting prevalency of dusty\nplasma stability research lies in the Jovian magnetosphere (i.e., Jovian\nplasmas), embedded inside the supersonic solar wind. A brief review of the\nupdated research works on dust-acoustic waves and related collective\ninstability dynamics in the presence of trapped plasma particles is presented\nherein. The key aim of the proposed explorative meta-analysis is rooted in\noutlining concisely the main up-to-date investigations on such collective\ninstability processes chronologically. An especial attention is given primarily\nto the thermostatistical distribution laws of the constitutive lighter\nelectrons and ions against the heavier positively charged dust grains\n(microspheres). The trapping mechanism of both the lighter species\n(electrons+ions) is another additive feature revisited here properly. Finally,\nwe clearly extrapolate a number of futuristic directions in light of sensible\nnovelties with a wider scope, both horizontally as well as vertically.", "category": "physics_plasm-ph" }, { "text": "Angular momentum evolution in laser-plasma accelerators: The transverse properties of an electron beam are characterized by two\nquantities, the emittance which indicates the electron beam extend in the phase\nspace and the angular momentum which allows for non-planar electron\ntrajectories. Whereas the emittance of electron beams produced in laser- plasma\naccelerator has been measured in several experiments, their angular momentum\nhas been scarcely studied. It was demonstrated that electrons in laser-plasma\naccelerator carry some angular momentum, but its origin was not established.\nHere we identify one source of angular momentum growth and we present\nexperimental results showing that the angular momentum content evolves during\nthe acceleration.", "category": "physics_plasm-ph" }, { "text": "Observation of Plasma Bubble Structures in a GeV Laser-Plasma\n Accelerator: We measure characteristics of plasma bubbles in GeV-class laser-plasma\naccelerators (LPAs) using Faraday rotation diagnostics. We extend these\ntechniques, previously demonstrated for LPAs in atmospheric density plasmas\n(electron density $n_e >10^{19}$ cm$^{-3}$), to LPAs in low-density plasmas\n($n_e \\approx 5\\times10^{17}$ cm$^{-3}$), in which plasma bubbles are $\\sim 5$\ntimes larger, and correspondingly easier to visualize in detail. The signals\nshow $\\approx 0.5^\\circ$ rotation streaks of opposite sign separated by\n$\\sim50$ $\\mu$m, consistent with bubble diameter; no on-axis rotation; streaks\nlength consistent with transverse probe pulse duration ($180$ $\\mu$m for $500$\nfs pulse length, and $600$ $\\mu$m for $2$ ps pulse length). We utilized an\nanamorphic imaging system to obtain a wide longitudinal field of view ($>1$ cm)\nand a high transverse resolution ($<9$ $\\mu$m). We also demonstrated that\nFaraday rotation signals are sensitive to the stages of acceleration processes\nusing extended 2D Finite Difference Time Domain (FDTD) simulation.", "category": "physics_plasm-ph" }, { "text": "Tunable, all-optical quasi-monochromatic Thomson X-ray source: Brilliant X-ray sources are of great interest for many research fields from\nbiology via medicine to material research. The quest for a cost-effective,\nbrilliant source with unprecedented temporal resolution has led to the recent\nrealization of various high-intensity-laser-driven X-ray beam sources. Here we\ndemonstrate the first all-laser-driven, energy-tunable and quasi-monochromatic\nX-ray source based on Thomson backscattering. This is a decisive step beyond\nprevious results, where the emitted radiation exhibited an uncontrolled broad\nenergy distribution. In the experiment, one part of the laser beam was used to\ndrive a few-fs bunch of quasi-monoenergetic electrons from a Laser-Wakefield\nAccelerator (LWFA), while the remainder was scattered off the bunch in a\nnear-counter-propagating geometry. When the electron energy was tuned from\n10-50 MeV, narrow-bandwidth X-ray spectra peaking at 5-35keV were directly\nmeasured, limited in photon energy by the sensitivity curve of our X-ray\ndetector. Due to the ultrashort LWFA electron bunches, these beams exhibit\nfew-fs pulse duration.", "category": "physics_plasm-ph" }, { "text": "Quasilinear diffusion for the chaotic motion of a particle in a set of\n longitudinal waves: The rigorous analytical calculation of the diffusion coefficient is performed\nfor the chaotic motion of a particle in a set of longitudinal waves with random\nphases and large amplitudes (~ A). A first step proves the existence of a\nquasilinear diffusion on a time scale ~ A^{-2/3} \\ln A. A second step uses this\nproperty to extend the result to asymptotic times by introducing the\nconditional probability distribution of position and velocity of an orbit at a\ngiven time when they are known at a previous time.", "category": "physics_plasm-ph" }, { "text": "Automated Fluid Model Generation and Numerical Analysis of Dielectric\n Barrier Discharges Using Comsol: MCPlas is introduced as a powerful tool for automated fluid model generation\nwith application to the analysis of dielectric barrier discharges operating in\ndifferent regimes. MCPlas consists of a number of\nMATLAB\\textsuperscript{\\textregistered} scripts and uses the COMSOL\nMultiphysics\\textsuperscript{\\textregistered} module\nLiveLink\\textsuperscript{\\texttrademark} for\nMATLAB\\textsuperscript{\\textregistered} to build up equation-based COMSOL\nMultiphysics\\textsuperscript{\\textregistered} models from scratch. The present\ncontribution highlights how MCPlas is used to implement time-dependent models\nfor non-thermal plasmas in spatially one-dimensional and axisymmetric\ntwo-dimensional geometries and stresses out the benefit of automation of the\nmodelling procedure. The modelling codes generated by MCPlas are used to study\ndiffuse and filamentary dielectric barrier discharges in argon at\nsub-atmospheric and atmospheric pressure, respectively. The seamless transition\nbetween different levels of model complexity with respect to the considered\nmodel geometry is demonstrated. The presented investigation of a\nsingle-filament dielectric barrier discharge interacting with a dielectric\nsurface shows that complex phenomena of high technological relevance can be\ntackled by using plasma models implemented in COMSOL\nMultiphysics\\textsuperscript{\\textregistered} via MCPlas.", "category": "physics_plasm-ph" }, { "text": "Real time reconstruction of the fast electron spectrum from high\n intensity laser plasma interaction using gamma counting technique: X-ray and gamma fluxes from the high intensity laser-plasma interaction are\nextremely short, well beyond temporal resolution of any detectors. If laser\npulses come repetitively, the single photon counting technique allows to\naccumulate the photon spectra, however, its relation to the spectrum of the\ninitial fast electron population in plasma is not straightforward. We present\nefficient and fast approach based on the Geant4 package that significantly\nreduces computer time needed to re-construct the high energy tail of electron\nspectrum from experimental data accounting for the pileup effect. Here, we\nfirst tabulate gamma spectrum from monoenergetic electron bunches of different\nenergy for a given experimental setup, and then compose the simulated spectrum.\nTo account for the pileups, we derive analytical formula to reverse the data.\nWe also consider errors coming from the approximation of the initial electron\nspectrum by the sum of monoenergetic impacts, the finite range of the electron\nspectrum, etc. and give estimates on how to choose modelling parameters to\nminimize the approximation errors. Finally, we present an example of the\nexperimental data treatment for the case of laser-solid interaction using 50 fs\nlaser pulse with intensity above 1018 W/cm2.", "category": "physics_plasm-ph" }, { "text": "Radiation Reaction Effects on Electron Nonlinear Dynamics and Ion\n Acceleration in Laser-solid Interaction: Radiation Reaction (RR) effects in the interaction of an ultra-intense laser\npulse with a thin plasma foil are investigated analytically and by\ntwo-dimensional (2D3P) Particle-In-Cell (PIC) simulations. It is found that the\nradiation reaction force leads to a significant electron cooling and to an\nincreased spatial bunching of both electrons and ions. A fully relativistic\nkinetic equation including RR effects is discussed and it is shown that RR\nleads to a contraction of the available phase space volume. The results of our\nPIC simulations are in qualitative agreement with the predictions of the\nkinetic theory.", "category": "physics_plasm-ph" }, { "text": "Derivation via free energy conservation constraints of gyrofluid\n equations with finite-gyroradius electromagnetic nonlinearities: The derivation of electromagnetic gyrofluid equations is made systematic by\nusing the Hermite polynomial form of the underlying delta-f gyrokinetic\ndistribution function. The gyrokinetic free-energy functional is explicitly\nused to set up the model. The gyrofluid free energy follows directly. The\ninteraction term in the gyrokinetic Lagrangian is used to obtain the gyrofluid\ncounterpart, from which the polarisation equation follows. One closure rule is\ndecided for taking moments over the kinetic gyroaveraging operator. These steps\nfix the rest of the derivation of the conservative part of the gyrofluid\nequations. Dissipation is then added in a form to obtain positive definite\ndissipation and to obtain the collisional fluid equations in their appropriate\nlimit. Existing results are recovered, with the addition of a completely\nconsistent model for finite gyroradius effects in the nonlinearities\nresponsible for magnetic reconnection.", "category": "physics_plasm-ph" }, { "text": "Planar and nonplanar nucleus-acoustic solitary waves in thermally\n degenerate multi-nucleus plasma systems: The novel thermally degenerate plasma model (based on a system containing\nrelativistically and thermally degenerate inertial-less electron species,\nnon-relativistically and thermally degenerate inertial light nucleus species,\nand stationary heavy nucleus species) is considered. The basic features of\nplanar and nonplanar solitary structures associated with the thermally\ndegenerate pressure driven nucleus-acoustic waves propagating in such a\nthermally degenerate plasma system has been investigated. The reductive\nperturbation method, which is valid for small amplitude solitary waves, is\nused. It is found that the effects of nonplanar cylindrical and spherical\ngeometries, non and ultra-relativistically degenerate electron species, thermal\nand degenerate pressures of electron and light nucleus species, and number\ndensities of light and heavy nucleus species significantly modify the basic\nfeatures (viz. speed, amplitude, and width) of the solitary potential\nstructures associated with thermally degenerate pressure driven\nnucleus-acoustic waves. The degenerate plasma model under consideration is so\ngeneral and realistic that it is applicable not only in astrophysical compact\nobjects like hot white dwarfs, but also in space plasma systems like\nmesospheres containing positively charged heavy particles in addition to\nelectron and ion plasma species.", "category": "physics_plasm-ph" }, { "text": "Simulation of non-resonant stellarator divertor: An efficient numerical method of studying nonresonant stellarator divertors\nwas introduced in Boozer and Punjabi [Phys. Plasmas 25, 092505 (2018)]. This\nmethod is used in this paper to study a different magnetic field model of a\nnonresonant divertor. The most novel and interesting finding of this study is\nthat diffusive magnetic field lines can be distinguished from lines that exit\nthrough the primary and the secondary turnstile, and that below some diffusive\nvelocity, all lines exit through only the primary turnstile. The footprints of\neach family are stellarator symmetric and have a fixed location on the wall for\nall velocities. The probability exponent of the primary turnstile is d1 = 9/4\nand that of the secondary turnstile is d2 = 3/2. This study also addresses the\nissues of an inadequate separation of the chamber walls from the outermost\nconfining magnetic surface and a marginal step size of the numerical\nintegrations that could compromise the interpretation of the earlier results\n[Boozer and Punjabi, Phys. Plasmas 25, 092505 (2018)]. The previous value of d1\n= 2 is within the error bar of d1 = 9/4 estimated here.", "category": "physics_plasm-ph" }, { "text": "Investigating the one-photon annihilation channel in an e-e+ plasma\n created from vacuum in strong laser fields: It is well known that in the presence of strong external electromagnetic\nfields many processes forbidden in standard QED become possible. One example is\nthe one-photon annihilation process considered recently by the present authors\nin the framework of a kinetic approach to the quasiparticle e-e+ gamma plasma\ncreated from vacuum in the focal spot of two counter-propagating laser beams.\nIn these works the domain of large values of the adiabaticity parameter gamma\n>> 1 (corresponding to multiphoton processes) was considered. In the present\nwork we estimate the intensity of the radiation stemming from photon\nannihilation in the framework of the effective mass model where gamma < 1,\ncorresponding to large electric fields E < E_c=m^2/e and high laser field\nfrequencies (the domain characteristic for X-ray lasers of the next\ngeneration). Under such limiting conditions the resulting effect is\nsufficiently large to be accessible to experimental observation.", "category": "physics_plasm-ph" }, { "text": "Toroidal and slab ETG instability dominance in the linear spectrum of\n JET-ILW pedestals: Local linear gyrokinetic simulations show that electron temperature gradient\n(ETG) instabilities are the fastest growing modes for $k_y \\rho_i \\gtrsim 0.1$\nin the steep gradient region for a JET pedestal discharge (92174) where the\nelectron temperature gradient is steeper than the ion temperature gradient.\nHere, $k_y$ is the wavenumber in the direction perpendicular to both the\nmagnetic field and the radial direction, and $\\rho_i$ is the ion gyroradius. At\n$k_y \\rho_i \\gtrsim 1$, the fastest growing mode is often a novel type of\ntoroidal ETG instability. This toroidal ETG mode is driven at scales as large\nas $k_y \\rho_i \\sim (\\rho_i/\\rho_e) L_{Te} / R_0 \\sim 1$ and at a sufficiently\nlarge radial wavenumber that electron finite Larmor radius effects become\nimportant; that is, $K_x \\rho_e \\sim 1$, where $K_x$ is the effective radial\nwavenumber. Here, $\\rho_e$ is the electron gyroradius, $R_0$ is the major\nradius of the last closed flux surface, and $1/L_{Te}$ is an inverse length\nproportional to the logarithmic gradient of the equilibrium electron\ntemperature. The fastest growing toroidal ETG modes are often driven far away\nfrom the outboard midplane. In this equilibrium, ion temperature gradient\ninstability is subdominant at all scales and kinetic ballooning modes are shown\nto be suppressed by $\\mathbf{ E} \\times \\mathbf{ B} $ shear. ETG modes are very\nresilient to $\\mathbf{ E} \\times \\mathbf{ B}$ shear. Heuristic quasilinear\narguments suggest that the novel toroidal ETG instability is important for\ntransport.", "category": "physics_plasm-ph" }, { "text": "On The Origin of Super-Hot Electrons from Intense Laser Interactions\n with Solid Targets having Moderate Scale Length Preformed Plasmas: We use PIC modeling to identify the acceleration mechanism responsible for\nthe observed generation of super-hot electrons in ultra-intense laser-plasma\ninteractions with solid targets with pre-formed plasma. We identify several\nfeatures of direct laser acceleration (DLA) that drive the generation of\nsuper-hot electrons. We find that, in this regime, electrons that become\nsuper-hot are primarily injected by a looping mechanism that we call\nloop-injected direct acceleration (LIDA).", "category": "physics_plasm-ph" }, { "text": "Modelling cathode spots in glow discharges in the cathode boundary layer\n geometry: Self-organized patterns of cathode spots in glow discharges are computed in\nthe cathode boundary layer geometry, which is the one employed in most of the\nexperiments reported in the literature. The model comprises conservation and\ntransport equations of electrons and a single ion species, written in the\ndrift-diffusion and local-field approximations, and Poisson's equation.\nMultiple solutions existing for the same value of the discharge current and\ndescribing modes with different configurations of cathode spots are computed by\nmeans of a stationary solver. The computed solutions are compared to their\ncounterparts for plane-parallel electrodes, and experiments. All of the\ncomputed spot patterns have been observed in the experiment.", "category": "physics_plasm-ph" }, { "text": "Beam-driven ECH waves: A parametric study: Electron cyclotron harmonic (ECH) waves play a significant role in driving\nthe diffuse aurora, which constitutes more than 75% of the particle energy\ninput into the ionosphere. ECH waves in magnetospheric plasmas have long been\nthought to be excited predominantly by the loss cone anisotropy (velocity-space\ngradients) that arises naturally in a planetary dipole field. Recent THEMIS\nobservations, however, indicate that an electron beam can also excite such\nwaves in Earth's magnetotail. The ambient and beam plasma conditions under\nwhich electron beam excitation can take place are unknown. Knowledge of such\nconditions would allow us to further explore the relative contribution of this\nexcitation mechanism to ECH wave scattering of magnetospheric electrons at\nEarth and the outer planets. Using the hot plasma dispersion relation, we\naddress the nature of beam-driven ECH waves and conduct a comprehensive\nparametric survey of this instability. We find that growth is provided by beam\nelectron cyclotron resonances of both first and higher orders. We also find\nthat these waves are unstable under a wide range of plasma conditions. The\ngrowth rate increases with beam density, beam velocity, and hot electron\ntemperature; it decreases with increasing beam temperature and beam temperature\nanisotropy, hot electron density, and cold electron density and temperature.\nSuch conditions abound in Earth's magnetotail, where magnetospheric electrons\nheated by earthward convection and magnetic reconnection coexist with colder\nionospheric electrons.", "category": "physics_plasm-ph" }, { "text": "Focusability in the multi-pump Raman amplification of short laser pulses: Spatially combining multiple strong laser beams is a promising concept for\nachieving ultrahigh laser intensities. Proof-of-principle experiments have been\nconducted at the National Ignition Facility to report a combination of up to\ntwenty pulses with high energy conversion efficiency. However, the combination\nprocess might damage the seed focusability due to mismatch of the seed and pump\nwavefronts. Here, we investigate the effect of the finite pump beam size on the\nfocusability of the seed pulse. We propose an approach to retain and even\nimprove the seed focusability by specifically arranging multiple pump beams.\nThe results are demonstrated by numerical solution of coupled nonlinear\nSchr\\\"{o}dinger equations.", "category": "physics_plasm-ph" }, { "text": "Normal solution and transport coefficients to the Enskog-Landau kinetic\n equation for a two-component system of charged hard spheres. The\n Chapman-Enskog method: An Enskog-Landau kinetic equation for a many-component system of charged hard\nspheres is proposed. It has been obtained from the Liouville equation with\nmodified boundary conditions by the method of nonequilibrium statistical\noperator. On the basis of this equation the normal solutions and transport\ncoefficients such as bulk kappa and shear eta viscosities, thermal conductivity\nlambda, mutual diffusion D^{\\alpha\\beta} and thermal diffusion D_T^\\alpha have\nbeen obtained for a binary mixture in the first approximation using the\nChapman-Enskog method. Numerical calculations of all transport coefficients for\nmixtures Ar-Kr, Ar-Xe, Kr-Xe with different concentrations of compounds have\nbeen evaluated for the cases of absence and presence of long-range Coulomb\ninteractions. The results are compared with those obtained from other theories\nand experiment.", "category": "physics_plasm-ph" }, { "text": "Direct Acceleration of Ions With Variable-frequency Lasers: A method is proposed for producing monoergetic, high-quality ion beams in\nvacuum, via direct acceleration by the electromagnetic field of two\ncounterpropagating, variable-frequency lasers: ions are trapped and accelerated\nby a beat-wave structure with variable phase velocity, allowing for fine\ncontrol over the energy and the charge of the beam via tuning of the frequency\nvariation. The physical mechanism is described with a one-dimensional theory,\nproviding the general conditions for trapping and scaling laws for the relevant\nfeatures of the ion beam. Two-dimensional, electromagnetic particle-in-cell\nsimulations, in which hydrogen gas is considered as an ion source, confirm the\nvalidity and the robustness of the method.", "category": "physics_plasm-ph" }, { "text": "Enhanced target normal sheath acceleration with a grooved hydrocarbon\n target: The interaction of a high-intensity ultrashort laser pulse with a few\nmicrons-thick hydrocarbon target is known to accelerate protons/ions to\nmulti-MeV, on the rear side of the target, via the mechanism of target normal\nsheath acceleration. Micro-structuring the target front is one of the promising\napproaches to enhance the cut-off energy as well as to reduce the divergence of\naccelerated protons/ions. In this paper, the interaction of a normally incident\nintense laser pulse with targets having single micron-sized grooves, at their\nfront side, of semi-circular, triangular, and rectangular shapes has been\nstudied by using two-dimensional Particle-In-Cell (PIC) simulations. It is\nobserved that as compared to a flat target for targets with a rectangular\ngroove at the front side the focused hot electron beam at the rear side results\nin an approximately four-fold increase in the cut-off energy of accelerated\nprotons. For triangular and semi-circular groove targets, the cut-off energy\nremains comparatively lower (higher than the flat target though). The angular\ndivergence of the accelerated protons/ions is also found to be relatively much\nlower in the case of a rectangular groove.", "category": "physics_plasm-ph" }, { "text": "Kinetic Ballooning Instability of the Near-Earth Magnetotail in Voigt\n Equilibrium: For a long time, ballooning instabilities have been believed to be a possible\ntriggering mechanism for the onset of substorm and current disruption\ninitiation in the near-Earth magnetotail. Yet the stability of the kinetic\nballooning mode (KBM) in a global and realistic magnetotail configuration has\nnot been well examined. In this paper, stability of the KBM is evaluated for\nthe two-dimensional Voigt equilibrium of the near-Earth magnetotail based on an\nanalytical kinetic theory of ballooning instability in the framework of kinetic\nmagnetohydrodynamic (MHD) model, where the kinetic effects such as the finite\ngyroradius effect, wave-particle resonances, particle drifts motions are\nincluded usually through kinetic closures. The growth rate of the KBM strongly\ndepends on the magnetic field line stiffening factor $S$, which is in turn\ndetermined by the effects of the trapped electrons, the finite ion gyroradius,\nand the magnetic drift motion of charged particles. The KBM is unstable in a\nfinite intermediate range of equatorial $\\beta_{eq}$ and only marginally\nunstable at higher $\\beta_{eq}$ regime for higher $T_e/T_i$ values. The finite\nion gyroradius and the trapped electron fraction enhance the stiffening factor\nthat tends to stabilize the KBM in the magnetotail far away from Earth. On the\nother hand, the current sheet thinning destabilizes KBM in the lower\n$\\beta_{eq}$ regime and stabilizes KBM in the higher $\\beta_{eq}$ regime.", "category": "physics_plasm-ph" }, { "text": "Vibrational Modes and Instabilities of a Dust Particle Pair in a Complex\n Plasma: Vibrational modes and instabilities of a dust particle pair in a terrestrial\nlaboratory complex plasma are investigated employing an analytical method\nwhereby the plasma wakefield induced by an external electric field is modeled\nusing an image charge method. It is found that for both horizontally and\nvertically aligned dust particle pairs in equilibrium, four normal modes exist.\nVariations of the confinement parameters cause a single type of instability in\nthe horizontal pair and two types of instabilities in the vertical pair.", "category": "physics_plasm-ph" }, { "text": "A New Optimized Quasihelically SymmetricStellarator: A new optimized quasihelically symmetric configuration is described that has\nthe desir-able properties of improved energetic particle confinement, reduced\nturbulent transportby 3D shaping, and non-resonant divertor capabilities. The\nconfiguration presented in thispaper is explicitly optimized for quasihelical\nsymmetry, energetic particle confinement,neoclassical confinement, and\nstability near the axis. Post optimization, the configurationwas evaluated for\nits performance with regard to energetic particle transport,\nidealmagnetohydrodynamic (MHD) stability at various values of plasma pressure,\nand iontemperature gradient instability induced turbulent transport. The effect\nof discrete coilson various confinement figures of merit, including energetic\nparticle confinement, aredetermined by generating single-filament coils for the\nconfiguration. Preliminary divertoranalysis shows that coils can be created\nthat do not interfere with expansion of thevessel volume near the regions of\noutgoing heat flux, thus demonstrating the possibilityof operating a\nnon-resonant divertor.", "category": "physics_plasm-ph" }, { "text": "Finite gyro-radius multidimensional electron hole equilibria: Finite electron gyro-radius influences on the trapping and charge density\ndistribution of electron holes of limited transverse extent are calculated\nanalytically and explored by numerical orbit integration in low to moderate\nmagnetic fields. Parallel trapping is shown to depend upon the gyro-averaged\npotential energy and to give rise to gyro-averaged charge deficit. Both types\nof average are expressible as convolutions with perpendicular Gaussians of\nwidth equal to the thermal gyro-radius. Orbit-following confirms these\nphenomena but also confirms for the first time in self-consistent potential\nprofiles the importance of gyro-bounce-resonance detrapping and consequent\nvelocity diffusion on stochastic orbits. The averaging strongly reduces the\ntrapped electron deficit that can be sustained by any potential profile whose\ntransverse width is comparable to the gyro-radius $r_g$. It effectively\nprevents equilibrium widths smaller than $\\sim r_g$ for times longer than a\nquarter parallel-bounce-period. Avoiding gyro-bounce resonance detrapping is\neven more restrictive, except for very small potential amplitudes, but it takes\nmultiple bounce-periods to act. Quantitative criteria are given for both types\nof orbit loss.", "category": "physics_plasm-ph" }, { "text": "Hamiltonian description for magnetic field lines: a tutorial: Under certain circumstances, the equations for the magnetic field lines can\nbe recast in a canonical form, after defining a suitable field line\nHamiltonian. This analogy is extremely useful for dealing with a variety of\nproblems involving magnetically confined plasmas, like in tokamaks and other\ntoroidal devices, where there is usually one symmetric coordinate which plays\nthe role of time in the canonical equations. In this tutorial paper we review\nthe basics of the Hamiltonian description for magnetic field lines, emphasizing\nthe role of a variational principle and gauge invariance. We present\nrepresentative applications of the formalism, using cylindrical and magnetic\nflux coordinates in tokamak plasmas.", "category": "physics_plasm-ph" }, { "text": "Analytical solution for Klein-Gordon equation and action function of the\n solution for Dirac equation in counter-propagating laser waves: Nonperturbative calculation of QED processes participated by a strong\nelectromagnetic field, especially provided by strong laser facilities at\npresent and in the near future, generally resorts to the Furry picture with the\nusage of analytical solutions of the particle dynamical equation, such as the\nKlein-Gordon equation and Dirac equation. However only for limited field\nconfigurations such as a plane-wave field could the equations be solved\nanalytically. Studies have shown significant interests in QED processes in a\nstrong field composed of two counter-propagating laser waves, but the exact\nsolutions in such a field is out of reach. In this paper, inspired by the\nobservation of the structure of the solutions in a plane-wave field, we develop\na new method and obtain the analytical solution for the Klein-Gordon equation\nand equivalently the action function of the solution for the Dirac equation in\nthis field, under a largest dynamical parameter condition that there exists an\ninertial frame in which the particle free momentum is far larger than the other\nfield dynamical parameters. The applicable range of the new solution is\ndemonstrated and its validity is proven clearly. The result has the advantage\nof Lorentz covariance, clear structure and close similarity to the solution in\na plane-wave field, and thus favors convenient application.", "category": "physics_plasm-ph" }, { "text": "Workshop Report: Brightest Light Initiative (March 27-29 2019, OSA\n Headquarters, Washington, D.C.): This Brightest Light Initiative (BLI) Workshop Report captures the important\nresearch ideas and recommendations for enabling that work developed by over 100\nleading scientists at a community-initiated workshop held March 27-29, 2019 in\nWashington, DC. Workshop attendees developed an understanding of key\nopportunities, as well as gaps in current technologies and capabilities, for\nscience enabled by the highest-intensity lasers.", "category": "physics_plasm-ph" }, { "text": "Scaling laws of resistive magnetohydrodynamic reconnection in the\n high-Lundquist-number, plasmoid-unstable regime: The Sweet-Parker layer in a system that exceeds a critical value of the\nLundquist number ($S$) is unstable to the plasmoid instability. In this paper,\na numerical scaling study has been done with an island coalescing system driven\nby a low level of random noise. In the early stage, a primary Sweet-Parker\nlayer forms between the two coalescing islands. The primary Sweet-Parker layer\nbreaks into multiple plasmoids and even thinner current sheets through multiple\nlevels of cascading if the Lundquist number is greater than a critical value\n$S_{c}\\simeq4\\times10^{4}$. As a result of the plasmoid instability, the system\nrealizes a fast nonlinear reconnection rate that is nearly independent of $S$,\nand is only weakly dependent on the level of noise. The number of plasmoids in\nthe linear regime is found to scales as $S^{3/8}$, as predicted by an earlier\nasymptotic analysis (Loureiro \\emph{et al.}, Phys. Plasmas \\textbf{14}, 100703\n(2007)). In the nonlinear regime, the number of plasmoids follows a steeper\nscaling, and is proportional to $S$. The thickness and length of current sheets\nare found to scale as $S^{-1}$, and the local current densities of current\nsheets scale as $S^{-1}$. Heuristic arguments are given in support of theses\nscaling relations.", "category": "physics_plasm-ph" }, { "text": "Laboratory model of magnetosphere created by strong plasma perturbation\n with frozen-in magnetic field: Transient interaction of magnetic dipole with plasma flow carrying southward\nmagnetic field is studied in laboratory experiment. The flow with transverse\nfrozen-in field is generated by means of laser-produced plasma cross-field\nexpansion into background plasma which fills vacuum chamber along externally\napplied magnetic field prior to interaction. Probe measurements showed that at\nrealized plasma parameters effective collisionless Larmor coupling takes place\nresulting in formation of strong compressive perturbation which propagates in\nbackground with super-Alfvenic velocity and generates in laboratory frame\ntransverse electric field comparable in value to expected induction one.\nCompression pulse with southward field after short propagation interacts with\ndipole and creates well defined magnetosphere. Comparison of magnetospheres\ncreated by laser-produced plasma expanding in vacuum field and in magnetized\nbackground revealed fundamental differences in structure and behavior of\nelectric potential in plasma. In presence of frozen-in southward field direct\n{\\guillemotleft}sub-solar{\\guillemotright} penetration of outside electric\npotential deep inside of magnetosphere was observed taking place with velocity\nclose to upstream Alfven speed.", "category": "physics_plasm-ph" }, { "text": "Current sheet bifurcation and collapse in electron magnetohydrodynamics: Inertial effects in nonlinear magnetic reconnection are studied within the\ncontext of 2D electron magnetohydrodynamics (EMHD) with resistive and viscous\ndissipation. Families of nonlinear solutions for relevant current sheet\nparameters are predicted and confirmed numerically in all regimes of interest.\nElectron inertia becomes important for current sheet thicknesses $\\delta$ below\nthe inertial length $d_{e}$. In this case, in the absence of electron\nviscosity, the sheet thickness experiences a nonlinear collapse. Viscosity\nregularizes solutions at small scales. Transition from resistive to viscous\nregimes shows a nontrivial dependence on resistivity and viscosity, featuring a\nhysteresis bifurcation. In all accessible regimes, the nonlinear reconnection\nrate is found to be explicitly independent of the electron inertia and\ndissipation coefficients.", "category": "physics_plasm-ph" }, { "text": "Initial Fulcher band observations from high resolution spectroscopy in\n the MAST-U divertor: High resolution Fulcher band spectroscopy was used in the MAST-U divertors\nduring Super-X and elongated conventional divertor density ramps with\n$\\text{D}_{2}$ fuelling from the mid-plane high-field side. In the Super-X case\n(density ramp from Greenwald fraction 0.12 to 0.24), the upper divertor showed\nground state rotational temperatures of the $\\text{D}_{2}$ molecules increasing\nfrom $\\sim$6000 K, starting at the detachment onset, to $\\sim$9000 K during\ndeepening detachment. This was correlated with the movement of the Fulcher\nemission region, which is correlated with the ionisation source. The increase\nin rotational temperature did not occur near the divertor entrance, where the\nplasma was still ionising. Qualitative agreement was obtained between the lower\nand upper divertor. Similar rotational temperatures were obtained in the\nelongated divertor before the detachment onset, although the increase in\nrotational temperature during detachment was less clearly observed as less deep\ndetachment was obtained. %In the elongated conventional divertor there was some\nqualitative agreement of this effect impeded by low signal.\n The measured vibrational distribution of the upper Fulcher state (first four\nbands) does not agree with a ground state Boltzmann distribution but shows a\ndifferent characteristic with an elevated population especially in the $\\nu =\n2$ and $\\nu = 3$ bands. The populations of the $\\nu = 2$ and $\\nu = 3$ band\nrelative to the $\\nu = 0$ band are roughly proportional to the\n$\\textit{rotational}$ temperature.", "category": "physics_plasm-ph" }, { "text": "Fluid Modes of a Spherically Confined Yukawa Plasma: The normal modes of a three-dimensional Yukawa plasma in an isotropic,\nharmonic confinement are investigated by solving the linearized cold fluid\nequations. The eigenmodes are found analytically and expressed in terms of\nhypergeometric functions. It is found that the mode frequencies solely depend\non the dimensionless plasma parameter $\\xi=\\kappa R$, where $R$ is the plasma\nradius and $\\kappa$ the inverse screening length. The eigenfrequencies increase\nmonotonically with $\\xi$ and saturate in the limit $\\xi\\to\\infty$. Compared\nwith the results in the Coulomb limit~[D. H. E. Dubin, Phys. Rev. Lett.\n\\textbf{66}, 2076 (1991)], we find a new class of modes characterized by the\nnumber $n$ which determines the number of radial nodes in the perturbed\npotential. These modes originate from the degenerate bulk modes of the Coulomb\nsystem. Analytical formulas for the eigenfrequencies are derived for limiting\ncases.", "category": "physics_plasm-ph" }, { "text": "Collisionless Magnetic Reconnection via Alfven Eigenmodes: We propose an analytic approach to the problem of collisionless magnetic\nreconnection formulated as a process of Alfven eigenmodes' generation and\ndissipation. Alfven eigenmodes are confined by the current sheet in the same\nway that quantum mechanical waves are confined by the tanh^2 potential. The\ndynamical time scale of reconnection is the system scale divided by the\neigenvalue propagation velocity of the n=1 mode. The prediction of the n=1 mode\nshows good agreement with the in situ measurement of the\nreconnection-associated Hall fields.", "category": "physics_plasm-ph" }, { "text": "Proton deflectometry analysis in magnetized plasmas: magnetic field\n reconstruction in one dimension: Proton deflectometry is increasingly used in magnetized high-energy-density\nplasmas to observe electromagnetic fields. We describe a reconstruction\nalgorithm to recover the electromagnetic fields from proton fluence data in\n1-D. The algorithm is verified against analytic solutions and applied to\nexample data. The virtue of a 1-D algorithm is that it is fast and can be\nincorporated into higher-level analysis routines and workflows, for example to\nscan parameters and conduct uncertainty analysis. Furthermore, working through\nthe 1-D algorithm exposes the fundamental importance of boundary conditions and\nthe initial proton fluence profile for an accurate reconstruction. From these\nconsiderations we propose a hybrid mesh-fluence reconstruction technique where\nfields are reconstructed from fluence data in an interior region with boundary\nconditions supplied by direct mesh measurements at the boundary.", "category": "physics_plasm-ph" }, { "text": "Gyrokinetic investigation of the nonlinear interaction of Alfv\u00e9n\n instabilities and energetic-particle driven geodesic acoustic modes: This paper presents a study of the interaction between Alfv\\'en modes and\nzonal structures, considering a realistic ASDEX Upgrade equilibrium. The\nresults of gyrokinetic simulations with the global, electromagnetic,\nparticle-in-cell code ORB5 are presented, where the modes are driven unstable\nby energetic particles with a bump-on-tail equilibrium distribution function,\nwith radial density gradient. Two regimes have been observed: at low energetic\nparticles concentration, the Alfv\\'en mode saturates at much higher level in\npresence of zonal structures; on the other hand at high energetic particles\nconcentration the difference is less pronounced. The former regime is\ncharacterized by the zonal structure (identified as an energetic particle\ndriven geodesic acoustic mode), being more unstable than the Alfv\\'en mode. In\nthe latter regime the Alfv\\'en mode is more unstable than the zonal structure.\nThe theoretical explanation is given in terms of a 3-wave coupling of the\nenergetic particle driven geodesic acoustic mode and Alfv\\'en mode, mediated by\nthe curvature-pressure coupling term of the energetic particles.", "category": "physics_plasm-ph" }, { "text": "Magnetic reconnection in the era of exascale computing and multiscale\n experiments: Astrophysical plasmas have the remarkable ability to preserve magnetic\ntopology, which inevitably gives rise to the accumulation of magnetic energy\nwithin stressed regions including current sheets. This stored energy is often\nreleased explosively through the process of magnetic reconnection, which\nproduces a reconfiguration of the magnetic field, along with high-speed flows,\nthermal heating, and nonthermal particle acceleration. Either collisional or\nkinetic dissipation mechanisms are required to overcome the topological\nconstraints, both of which have been predicted by theory and validated with in\nsitu spacecraft observations or laboratory experiments. However, major\nchallenges remain in understanding magnetic reconnection in large systems, such\nas the solar corona, where the collisionality is weak and the kinetic scales\nare vanishingly small in comparison to macroscopic scales. The plasmoid\ninstability or formation of multiple plasmoids in long reconnecting current\nsheets is one possible multiscale solution for bridging this vast range of\nscales, and new laboratory experiments are poised to study these regimes. In\nconjunction with these efforts, we anticipate that the coming era of exascale\ncomputing, together with the next generation of observational capabilities,\nwill enable new progress on a range of challenging problems, including the\nenergy build-up and onset of reconnection, partially ionized regimes, the\ninfluence of magnetic turbulence, and particle acceleration.", "category": "physics_plasm-ph" }, { "text": "Direct path from microscopic mechanics to Debye shielding, Landau\n damping, and wave-particle interaction: The derivation of Debye shielding and Landau damping from the $N$-body\ndescription of plasmas is performed directly by using Newton's second law for\nthe $N$-body system. This is done in a few steps with elementary calculations\nusing standard tools of calculus, and no probabilistic setting. Unexpectedly,\nDebye shielding is encountered together with Landau damping. This approach is\nshown to be justified in the one-dimensional case when the number of particles\nin a Debye sphere becomes large. The theory is extended to accommodate a\ncorrect description of trapping and chaos due to Langmuir waves. Shielding and\ncollisional transport are found to be two related aspects of the repulsive\ndeflections of electrons, in such a way that each particle is shielded by all\nother ones while keeping in uninterrupted motion.", "category": "physics_plasm-ph" }, { "text": "Transport, flow topology and Lagrangian conditional statistics in edge\n plasma turbulence: Lagrangian statistics and particle transport in edge plasma turbulence are\ninvestigated using the Hasegawa-Wakatani model and its modified version. The\nlatter shows the emergence of pronounced zonal flows. Different values of the\nadiabaticity parameter are considered. The main goal is to characterize the\nrole of coherent structures, i.e., vortices and zonal flows, and their impact\non the Lagrangian statistics of particles. Computationally intensive long time\nsimulations following ensembles of test particles over hundreds of eddy\nturnover times are considered in statistically stationary turbulent flows. The\nflow topology is characterized using the Lagrangian Okubo-Weiss criterion, and\nthe flow can thus be split into topologically different domains. In elliptic\nand hyperbolic regions, the probability density functions (pdfs) of the\nresidence time have self-similar algebraic decaying tails. However, in the\nintermediate regions the pdfs do exhibit exponentially decaying tails.\nTopologically conditioned pdfs of the Lagrangian velocity, and acceleration and\ndensity fluctuations are likewise computed. The differences between the\nclassical Hasegawa-Wakatani system and its modified version are assessed and\nthe role of zonal flows is highlighted. The density flux spectrum which\ncharacterizes the contributions of different length scales is studied and its\ninertial scaling is found to be in agreement with predictions based on\ndimensional arguments. Analyzing the angular change of particle tracers at\ndifferent time scales, corresponding to coarse grained curvature, completes the\nstudy and the multiscale geometric statistics quantify the directional\nproperties of the particle motion in the different flow regimes.", "category": "physics_plasm-ph" }, { "text": "Topological phase in plasma physics: Recent discoveries have demonstrated that matter can be distinguished on the\nbasis of topological considerations, giving rise to the concept of topological\nphase. Introduced originally in condensed matter physics, the physics of\ntopological phase can also be fruitfully applied to plasmas. Here, the theory\nof topological phase is introduced, including a discussion of Berry phase,\nBerry connection, Berry curvature, and Chern number. One of the clear physical\nmanifestations of topological phase is the bulk-boundary correspondence, the\nexistence of localized unidirectional modes at the interface between\ntopologically distinct phases. These concepts are illustrated through examples,\nincluding the simple magnetized cold plasma. An outlook is provided for future\ntheoretical developments and possible applications.", "category": "physics_plasm-ph" }, { "text": "Fine-sorting One-dimensional Particle-In-Cell Algorithm with Monte-Carlo\n Collisions on a Graphics Processing Unit: Particle-in-cell (PIC) simulations with Monte-Carlo collisions are used in\nplasma science to explore a variety of kinetic effects. One major problem is\nthe long run-time of such simulations. Even on modern computer systems, PIC\ncodes take a considerable amount of time for convergence. Most of the\ncomputations can be massively parallelized, since particles behave\nindependently of each other within one time step. Current graphics processing\nunits (GPUs) offer an attractive means for execution of the parallelized code.\nIn this contribution we show a one-dimensional PIC code running on Nvidia GPUs\nusing the CUDA environment. A distinctive feature of the code is that size of\nthe cells that the code uses to sort the particles with respect to their\ncoordinates is comparable to size of the grid cells used for discretization of\nthe electric field. Hence, we call the corresponding algorithm \"fine-sorting\".\nImplementation details and optimization of the code are discussed and the\nspeed-up compared to classical CPU approaches is computed.", "category": "physics_plasm-ph" }, { "text": "Effects of resonant magnetic perturbations on neutral beam heating in a\n tokamak: Effects of resonant magnetic perturbations (RMPs) on tangential neutral beam\nheating in the EAST tokamak are studied numerically. RMPs with linear resistive\nmagnetohydrodynamics response are used in the modeling. A variety of\nrepresenting configurations of RMP coil currents are examined and their effects\non the NBI heating efficiency are compared, in order to find a parameter window\nwhere deleterious effects of RMPs on NBI heating efficiency are minimized. It\nis found that the internal redistribution of fast ions by RMPs induces local\naccumulation of fast ions, resulting in higher local fast ion pressure than the\ncase without RMPs. It is also found that the toroidal phasing of the RMP with\nrespect to the fast ion source has slight effects on the steady-state radial\nprofile of fast ions. The dependence of fast ion loss fraction on the RMP\nup-down phase difference shows similar behavior as the dependence of the radial\nwidth of chaotic magnetic field on the phase difference. A statistical method\nof identifying resonances between RMPs and lost fast ions is proposed and the\nresults indicate that some resonances between RMPs and lost passing particles\nmay be of non-integer fractional order, rather than the usual integer order.", "category": "physics_plasm-ph" }, { "text": "Dynamo theories: These lecture notes are based on a tutorial given in 2017 at a plasma physics\nwinter school in Les Houches. Their aim is to provide a self-contained\ngraduate-student level introduction to the theory and modelling of the dynamo\neffect in turbulent fluids and plasmas, blended with a review of current\nresearch in the field. The primary focus is on the physical and mathematical\nconcepts underlying different (turbulent) branches of dynamo theory, with some\nastrophysical, geophysical and experimental context disseminated throughout the\ndocument. The text begins with an introduction to the rationale, observational\nand historical roots of the subject, and to the basic concepts of\nmagnetohydrodynamics relevant to dynamo theory. The next two sections discuss\nthe fundamental phenomenological and mathematical aspects of (linear and\nnonlinear) small- and large-scale MHD dynamos. These sections are complemented\nby an overview of a selection of current active research topics in the field,\nincluding the numerical modelling of the geo- and solar dynamos, shear dynamos\ndriven by turbulence with zero net helicity, and MHD-instability-driven dynamos\nsuch as the magnetorotational dynamo. The difficult problem of a unified,\nself-consistent statistical treatment of small and large-scale dynamos at large\nmagnetic Reynolds numbers is also discussed throughout the text. Finally, an\nexcursion is made into the relatively new but increasingly popular realm of\nmagnetic-field generation in weakly-collisional plasmas. A short discussion of\nthe outlook and challenges for the future of the field concludes the\npresentation.", "category": "physics_plasm-ph" }, { "text": "Well-posedness and generalized plane waves simulations of a 2D mode\n conversion model: Certain types of electro-magnetic waves propagating in a plasma can undergo a\nmode conversion process. In magnetic confinement fusion, this phenomenon is\nvery useful to heat the plasma, since it permits to transfer the heat at or\nnear the plasma center. This work focuses on a mathematical model of wave\npropagation around the mode conversion region, from both theoretical and\nnumerical points of view. It aims at developing, for a well-posed equation,\nspecific basis functions to study a wave mode conversion process. These basis\nfunctions, called generalized plane waves, are intrinsically based on variable\ncoefficients. As such, they are particularly adapted to the mode conversion\nproblem. The design of generalized plane waves for the proposed model is\ndescribed in detail. Their implementation within a discontinuous Galerkin\nmethod then provides numerical simulations of the process. These first 2D\nsimulations for this model agree with qualitative aspects studied in previous\nworks.", "category": "physics_plasm-ph" }, { "text": "Probing Ultrafast Magnetic-Field Generation by Current Filamentation\n Instability in Femtosecond Relativistic Laser-Matter Interactions: We present experimental measurements of the femtosecond time-scale generation\nof strong magnetic-field fluctuations during the interaction of ultrashort,\nmoderately relativistic laser pulses with solid targets. These fields were\nprobed using low-emittance, highly relativistic electron bunches from a laser\nwakefield accelerator, and a line-integrated $B$-field of $2.70 \\pm 0.39\\,\\rm\nkT\\,\\mu m$ was measured. Three-dimensional, fully relativistic particle-in-cell\nsimulations indicate that such fluctuations originate from a Weibel-type\ncurrent filamentation instability developing at submicron scales around the\nirradiated target surface, and that they grow to amplitudes strong enough to\nbroaden the angular distribution of the probe electron bunch a few tens of\nfemtoseconds after the laser pulse maximum. Our results highlight the potential\nof wakefield-accelerated electron beams for ultrafast probing of relativistic\nlaser-driven phenomena.", "category": "physics_plasm-ph" }, { "text": "Path Integral Monte Carlo Simulation of the Low-Density Hydrogen Plasma: Restricted path integral Monte Carlo simulations are used to calculate the\nequilibrium properties of hydrogen in the density and temperature range of\n$9.83 \\times 10^{-4}\\rm \\leq \\rho \\leq 0.153 \\rm gcm^{-3}$ and $5000 \\leq T\n\\leq 250 000 \\rm K$. We test the accuracy of the pair density matrix and\nanalyze the dependence on the system size, on the time step of the path\nintegral and on the type of nodal surface. We calculate the equation of state\nand compare with other models for hydrogen valid in this regime. Further, we\ncharacterize the state of hydrogen and describe the changes from a plasma to an\natomic and molecular liquid by analyzing the pair correlation functions and\nestimating the number of atoms and molecules present.", "category": "physics_plasm-ph" }, { "text": "DREAM: a fluid-kinetic framework for tokamak disruption runaway electron\n simulations: Avoidance of the harmful effects of runaway electrons (REs) in\nplasma-terminating disruptions is pivotal in the design of safety systems for\nmagnetic fusion devices. Here, we describe a computationally efficient\nnumerical tool, that allows for self-consistent simulations of plasma cooling\nand associated RE dynamics during disruptions. It solves flux-surface averaged\ntransport equations for the plasma density, temperature and poloidal flux,\nusing a bounce-averaged kinetic equation to self-consistently provide the\nelectron current, heat, density and RE evolution, as well as the electron\ndistribution function. As an example, we consider disruption scenarios with\nmaterial injection and compare the electron dynamics resolved with different\nlevels of complexity, from fully kinetic to fluid modes.", "category": "physics_plasm-ph" }, { "text": "Theoretical Plasma Physics: These lecture notes were presented by Allan N. Kaufman in his graduate plasma\ntheory course and a follow-on special topics course (Physics 242A, B, C and\nPhysics 250 at the University of California Berkeley). The notes follow the\norder of the lectures. The equations and derivations are as Kaufman presented,\nbut the text is a reconstruction of Kaufman's discussion and commentary. The\nnotes were transcribed by Bruce I. Cohen in 1971 and 1972, and word-processed,\nedited, and illustrations added by Cohen in 2017 and 2018. The series of\nlectures are divided into four major parts: (1) collisionless Vlasov plasmas\n(linear theory of waves and instabilities with and without an applied magnetic\nfield, Vlasov-Poisson and Vlasov-Maxwell systems, WKBJ eikonal theory of wave\npropagation); (2) nonlinear Vlasov plasmas and miscellaneous topics (the plasma\ndispersion function, singular solutions of the Vlasov-Poisson system,\npulse-response solutions for initial-value problems, Gardiner's stability\ntheorem, gyroresonant effects, nonlinear waves, particle trapping in waves,\nquasi-linear theory, nonlinear three-wave interactions); (3) plasma collisional\nand discreteness phenomena (test-particle theory of dynamic friction and wave\nemission, classical resistivity, extension of test-particle theory to\nmany-particle phenomena and the derivation of the Boltzmann and Lenard-Balescu\nequations, the Fokker-Planck collision operator, a general scattering theory,\nnonlinear Landau damping, radiation transport, and Dupree's theory of clumps);\n(4) nonuniform plasmas (adiabatic invariance, guiding center drifts,\nhydromagnetic theory, introduction to drift-wave stability theory).", "category": "physics_plasm-ph" }, { "text": "Continuous wavelet transform based time-scale and multi-fractal analysis\n of the nonlinear oscillations in a hollow cathode glow discharge plasma: Continuous wavelet transform (CWT) based time-scale and multi-fractal\nanalyses have been carried out on the anode glow related nonlinear floating\npotential fluctuations in a hollow cathode glow discharge plasma. CWT has been\nused to obtain the contour and ridge plots. Scale shift (or inversely frequency\nshift) which is a typical nonlinear behaviour, has been detected from the\nundulating contours. From the ridge plots, we have identified the presence of\nnonlinearity and degree of chaoticity. Using the wavelet transform modulus\nmaxima technique we have obtained the multi-fractal spectrum for the\nfluctuations at different discharge voltages and the spectrum was observed to\nbecome a monofractal for periodic signals. These multi-fractal spectra were\nalso used to estimate different quantities like the correlation and fractal\ndimension, degree of multi-fractality and complexity parameters. These\nestimations have been found to be consistent with the nonlinear time series\nanalysis.", "category": "physics_plasm-ph" }, { "text": "Fully Kinetic Simulation of 3D Kinetic Alfven Turbulence: We present results from a three-dimensional particle-in-cell simulation of\nplasma turbulence, resembling the plasma conditions found at kinetic scales of\nthe solar wind. The spectral properties of the turbulence in the subion range\nare consistent with theoretical expectations for kinetic Alfv\\' en waves.\nFurthermore, we calculate the local anisotropy, defined by the relation\n$k_{\\parallel}(k_{\\perp})$, where $k_{\\parallel}$ is a characteristic wave\nnumber along the local mean magnetic field at perpendicular scale\n$l_{\\perp}\\sim 1/k_{\\perp}$. The subion range anisotropy is scale dependent\nwith $k_{\\parallel}