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2016-05-09 | Storage-ring Electron Cooler for Relativistic Ion Beams | Application of electron cooling at ion energies above a few GeV has been
limited due to reduction of electron cooling efficiency with energy and
difficulty in producing and accelerating a high-current high-quality electron
beam. A high-current storage-ring electron cooler offers a solution to both of
these problems by maintaining high cooling beam quality through
naturally-occurring synchrotron radiation damping of the electron beam.
However, the range of ion energies where storage-ring electron cooling can be
used has been limited by low electron beam damping rates at low ion energies
and high equilibrium electron energy spread at high ion energies. This paper
reports a development of a storage ring based cooler consisting of two sections
with significantly different energies: the cooling and damping sections. The
electron energy and other parameters in the cooling section are adjusted for
optimum cooling of a stored ion beam. The beam parameters in the damping
section are adjusted for optimum damping of the electron beam. The necessary
energy difference is provided by an energy recovering SRF structure. A
prototype linear optics of such storage-ring cooler and initial tracking
simulations are presented and some potential issues such as coherent
synchrotron radiation and beam break up are discussed. | 1605.02594v1 |
2016-07-06 | Measuring Collisionless Damping in Heliospheric Plasmas using Field-Particle Correlations | An innovative field-particle correlation technique is proposed that uses
single-point measurements of the electromagnetic fields and particle velocity
distribution functions to investigate the net transfer of energy from fields to
particles associated with the collisionless damping of turbulent fluctuations
in weakly collisional plasmas, such as the solar wind. In addition to providing
a direct estimate of the local rate of energy transfer between fields and
particles, it provides vital new information about the distribution of that
energy transfer in velocity space. This velocity-space signature can
potentially be used to identify the dominant collisionless mechanism
responsible for the damping of turbulent fluctuations in the solar wind. The
application of this novel field-particle correlation technique is illustrated
using the simplified case of the Landau damping of Langmuir waves in an
electrostatic 1D-1V Vlasov-Poisson plasma, showing that the procedure both
estimates the local rate of energy transfer from the electrostatic field to the
electrons and indicates the resonant nature of this interaction. Modifications
of the technique to enable single-point spacecraft measurements of fields and
particles to diagnose the collisionless damping of turbulent fluctuations in
the solar wind are discussed, yielding a method with the potential to transform
our ability to maximize the scientific return from current and upcoming
spacecraft missions, such as the Magnetospheric Multiscale (MMS) and Solar
Probe Plus missions. | 1607.01738v1 |
2016-07-22 | Excitation of nonlinear ion acoustic waves in CH plasmas | Excitation of nonlinear ion acoustic wave (IAW) by an external electric field
is demonstrated by Vlasov simulation. The frequency calculated by the
dispersion relation with no damping is verified much closer to the resonance
frequency of the small-amplitude nonlinear IAW than that calculated by the
linear dispersion relation. When the wave number $ k\lambda_{De} $ increases,
the linear Landau damping of the fast mode (its phase velocity is greater than
any ion's thermal velocity) increases obviously in the region of $ T_i/T_e <
0.2 $ in which the fast mode is weakly damped mode. As a result, the deviation
between the frequency calculated by the linear dispersion relation and that by
the dispersion relation with no damping becomes larger with $k\lambda_{De}$
increasing. When $k\lambda_{De}$ is not large, such as $k\lambda_{De}=0.1, 0.3,
0.5$, the nonlinear IAW can be excited by the driver with the linear frequency
of the modes. However, when $k\lambda_{De}$ is large, such as
$k\lambda_{De}=0.7$, the linear frequency can not be applied to exciting the
nonlinear IAW, while the frequency calculated by the dispersion relation with
no damping can be applied to exciting the nonlinear IAW. | 1607.06598v1 |
2016-11-17 | A stable partitioned FSI algorithm for rigid bodies and incompressible flow. Part II: General formulation | A stable partitioned algorithm is developed for fluid-structure interaction
(FSI) problems involving viscous incompressible flow and rigid bodies. This
{\em added-mass partitioned} (AMP) algorithm remains stable, without
sub-iterations, for light and even zero mass rigid bodies when added-mass and
viscous added-damping effects are large. The scheme is based on a generalized
Robin interface condition for the fluid pressure that includes terms involving
the linear acceleration and angular acceleration of the rigid body. Added mass
effects are handled in the Robin condition by inclusion of a boundary integral
term that depends on the pressure. Added-damping effects due to the viscous
shear forces on the body are treated by inclusion of added-damping tensors that
are derived through a linearization of the integrals defining the force and
torque. Added-damping effects may be important at low Reynolds number, or, for
example, in the case of a rotating cylinder or rotating sphere when the
rotational moments of inertia are small. In this second part of a two-part
series, the general formulation of the AMP scheme is presented including the
form of the AMP interface conditions and added-damping tensors for general
geometries. A fully second-order accurate implementation of the AMP scheme is
developed in two dimensions based on a fractional-step method for the
incompressible Navier-Stokes equations using finite difference methods and
overlapping grids to handle the moving geometry. The numerical scheme is
verified on a number of difficult benchmark problems. | 1611.05703v2 |
2017-03-01 | The Plastic Scintillator Detector at DAMPE | he DArk Matter Particle Explorer (DAMPE) is a general purposed
satellite-borne high energy $\gamma-$ray and cosmic ray detector, and among the
scientific objectives of DAMPE are the searches for the origin of cosmic rays
and an understanding of Dark Matter particles. As one of the four detectors in
DAMPE, the Plastic Scintillator Detector (PSD) plays an important role in the
particle charge measurement and the photons/electrons separation. The PSD has
82 modules, each consists of a long organic plastic scintillator bar and two
PMTs at both ends for readout, in two layers and covers an overall active area
larger than 82 cm $\times$ 82 cm. It can identify the charge states for
relativistic ions from H to Fe, and the detector efficiency for Z=1 particles
can reach 0.9999. The PSD has been successfully launched with DAMPE on Dec. 17,
2015. In this paper, the design, the assembly, the qualification tests of the
PSD and some of the performance measured on the ground have been described in
detail. | 1703.00098v1 |
2017-03-22 | Observation of a strong coupling effect on electron-ion collisions in ultracold plasmas | Ultracold plasmas (UCP) provide a well-controlled system for studying
multiple aspects in plasma physics that include collisions and strong coupling
effects. By applying a short electric field pulse to a UCP, a plasma electron
center-of-mass (CM) oscillation can be initiated. In accessible parameter
ranges, the damping rate of this oscillation is determined by the electron-ion
collision rate. We performed measurements of the oscillation damping rate with
such parameters and compared the measured rates to both a molecular dynamic
(MD) simulation that includes strong coupling effects and to Monte-Carlo
collisional operator simulation designed to predict the damping rate including
only weak coupling considerations. We found agreement between experimentally
measured damping rate and the MD result. This agreement did require including
the influence of a previously unreported UCP heating mechanism whereby the
presence of a DC electric field during ionization increased the electron
temperature, but estimations and simulations indicate that such a heating
mechanism should be present for our parameters. The measured damping rate at
our coldest electron temperature conditions was much faster than the weak
coupling prediction obtained from the Monte-Carlo operator simulation, which
indicates the presence of significant strong coupling influence. The density
averaged electron strong coupling parameter $\Gamma$ measured at our coldest
electron temperature conditions was 0.35. | 1703.07852v2 |
2017-04-18 | Critical pairing fluctuations in the normal state of a superconductor: pseudogap and quasi-particle damping | We study the effect of critical pairing fluctuations on the electronic
properties in the normal state of a clean superconductor in three dimensions.
Using a functional renormalization group approach to take the non-Gaussian
nature of critical fluctuations into account, we show microscopically that in
the BCS regime, where the inverse coherence length is much smaller than the
Fermi wavevector, critical pairing fluctuations give rise to a non-analytic
contribution to the quasi-particle damping of order $ T_c \sqrt{Gi} \ln ( 80 /
Gi )$, where the Ginzburg-Levanyuk number $Gi$ is a dimensionless measure for
the width of the critical region. As a consequence, there is a temperature
window above $T_c$ where the quasiparticle damping due to critical pairing
fluctuations can be larger than the usual $T^2$-Fermi liquid damping due to
non-critical scattering processes. On the other hand, in the strong coupling
regime where $Gi$ is of order unity, we find that the quasiparticle damping due
to critical pairing fluctuations is proportional to the temperature. Moreover,
we show that in the vicinity of the critical temperature $T_c$ the electronic
density of states exhibits a fluctuation-induced pseudogap. We also use
functional renormalization group methods to derive and classify various types
of processes induced by the pairing interaction in Fermi systems close to the
superconducting instability. | 1704.05282v2 |
2017-05-29 | Probing decoherence in plasmonic waveguides in the quantum regime | We experimentally investigate the decoherence of single surface plasmon
polaritons in metal stripe waveguides. In our study we use a Mach-Zehnder
configuration previously considered for measuring decoherence in atomic,
electronic and photonic systems. By placing waveguides of different length in
one arm we are able to measure the amplitude damping time T_1 = 1.90 +/- 0.01 x
10^-14 s, pure phase damping time T_2^* = 11.19 +/- 4.89 x 10^-14 s and total
phase damping time T_2 = 2.83 +/- 0.32 x 10^-14 s. We find that decoherence is
mainly due to amplitude damping and thus loss arising from inelastic electron
and photon scattering plays the most important role in the decoherence of
plasmonic waveguides in the quantum regime. However, pure phase damping is not
completely negligible. The results will be useful in the design of plasmonic
waveguide systems for carrying out phase-sensitive quantum applications, such
as quantum sensing. The probing techniques developed may also be applied to
other plasmonic nanostructures, such as those used as nanoantennas, as unit
cells in metamaterials and as nanotraps for cold atoms. | 1705.10344v2 |
2017-07-21 | Spatially Localized Particle Energization by Landau Damping in Current Sheets Produced by Strong Alfven Wave Collisions | Understanding the removal of energy from turbulent fluctuations in a
magnetized plasma and the consequent energization of the constituent plasma
particles is a major goal of heliophysics and astrophysics. Previous work has
shown that nonlinear interactions among counterpropagating Alfven waves---or
Alfven wave collisions---are the fundamental building block of astrophysical
plasma turbulence and naturally generate current sheets in the strongly
nonlinear limit. A nonlinear gyrokinetic simulation of a strong Alfven wave
collision is used to examine the damping of the electromagnetic fluctuations
and the associated energization of particles that occurs in self-consistently
generated current sheets. A simple model explains the flow of energy due to the
collisionless damping and the associated particle energization, as well as the
subsequent thermalization of the particle energy by collisions. The net
particle energization by the parallel electric field is shown to be spatially
intermittent, and the nonlinear evolution is essential in enabling that spatial
non-uniformity. Using the recently developed field-particle correlation
technique, we show that particles resonant with the Alfven waves in the
simulation dominate the energy transfer, demonstrating conclusively that Landau
damping plays a key role in the spatially intermittent damping of the
electromagnetic fluctuations and consequent energization of the particles in
this strongly nonlinear simulation. | 1708.00757v1 |
2017-10-30 | Enhancement of intrinsic magnetic damping in defect-free epitaxial Fe3O4 thin films | We have investigated the magnetic damping of precessional spin dynamics in
defect-controlled epitaxial grown Fe$_3$O$_4$(111)/Yttria-stabilized Zirconia
(YSZ) nanoscale films by all-optical pump-probe measurements. The intrinsic
damping constant of the defect-free Fe$_3$O$_4$ film is found to be strikingly
larger than that of the as-grown Fe$_3$O$_4$ film with structural defects. We
demonstrate that the population of the first-order perpendicular standing spin
wave (PSSW) mode, which is exclusively observed in the defect-free film under
sufficiently high external magnetic fields, leads to the enhancement of the
magnetic damping of the uniform precession (Kittel) mode. We propose a physical
picture in which the PSSW mode acts as an additional channel for the extra
energy dissipation of the Kittel mode. The energy transfer from Kittel mode to
PSSW mode increases as in-plane magnetization precession becomes more uniform,
resulting in the unique intrinsic magnetic damping enhancement in the
defect-free Fe$_3$O$_4$ film. | 1710.10938v2 |
2018-05-26 | Critical collapse of ultra-relativistic fluids: damping or growth of aspherical deformations | We perform fully nonlinear numerical simulations to study aspherical
deformations of the critical self-similar solution in the gravitational
collapse of ultra-relativistic fluids. Adopting a perturbative calculation,
Gundlach predicted that these perturbations behave like damped or growing
oscillations, with the frequency and damping (or growth) rates depending on the
equation of state. We consider a number of different equations of state and
degrees of asphericity and find very good agreement with the findings of
Gundlach for polar $\ell = 2$ modes. For sufficiently soft equations of state,
the modes are damped, meaning that, in the limit of perfect fine-tuning, the
spherically symmetric critical solution is recovered. We find that the degree
of asphericity has at most a small effect on the frequency and damping
parameter, or on the critical exponents in the power-law scalings. Our findings
also confirm, for the first time, Gundlach's prediction that the $\ell = 2$
modes become unstable for sufficiently stiff equations of state. In this regime
the spherically symmetric self-similar solution can no longer be recovered by
fine-tuning to the black-hole threshold, and one can no longer expect power-law
scaling to hold to arbitrarily small scales. | 1805.10442v1 |
2018-06-19 | Non-linear Relaxation of Interacting Bosons Coherently Driven on a Narrow Optical Transition | We study the dynamics of a two-component Bose-Einstein condensate (BEC) of
$^{174}$Yb atoms coherently driven on a narrow optical transition. The
excitation transfers the BEC to a superposition of states with different
internal and momentum quantum numbers. We observe a crossover with decreasing
driving strength between a regime of damped oscillations, where coherent
driving prevails, and an incoherent regime, where relaxation takes over.
Several relaxation mechanisms are involved: inelastic losses involving two
excited atoms, leading to a non-exponential decay of populations; Doppler
broadening due to the finite momentum width of the BEC and inhomogeneous
elastic interactions, both leading to dephasing and to damping of the
oscillations. We compare our observations to a two-component Gross-Pitaevskii
(GP) model that fully includes these effects. For small or moderate densities,
the damping of the oscillations is mostly due to Doppler broadening. In this
regime, we find excellent agreement between the model and the experimental
results. For higher densities, the role of interactions increases and so does
the damping rate of the oscillations. The damping in the GP model is less
pronounced than in the experiment, possibly a hint for many-body effects not
captured by the mean-field description. | 1806.07210v2 |
2018-10-16 | The Solution to the Differential Equation with Linear Damping describing a Physical Systems governed by a Cubic Energy Potential | An analytical solution to the nonlinear differential equation describing the
equation of motion of a particle moving in an unforced physical system with
linear damping, governed by a cubic potential well, is presented in terms of
the Jacobi elliptic functions. In the attractive region of the potential the
system becomes an anharmonic damped oscillator, however with asymmetric
displacement. An expression for the period of oscillation is derived, which for
a nonlinear damped system is time dependent, and in particular it contains a
quartic root of an exponentially decaying term in the denominator. Initially
the period is longer as compared to that of a linear oscillator, however
gradually it decreases to that of a linear damped oscillator.
Transforming the undamped nonlinear differential equation into the
differential equation describing orbital motion of planets, the perihelion
advance of Mercury can be estimated to 42.98 arcseconds/century, close to
present day observations of 43.1 arcseconds/century.
Some familiarity with the Jacobi elliptic functions is required, in
particular with respect to the differential behavior of these functions,
however, they are standard functions of advanced mathematical computer algebra
tools. The expression derived for the solution to the nonlinear physical
system, and in particular the expression for the period of oscillation, is
useful for an accurate evaluation of experiments in introductory and advanced
physics labs, but also of interest for specialists working with nonlinear
phenomena governed by the cubic potential well. | 1810.10336v1 |
2019-01-10 | Damping and softening of transverse acoustic phonons in colossal magnetoresistive La$_{0.7}$Ca$_{0.3}$MnO$_3$ and La$_{0.7}$Sr$_{0.3}$MnO$_3$ | Neutron spectroscopy is used to probe transverse acoustic phonons near the
(2, 2, 0) Bragg position in colossal magnetoresistive La0.7Ca0.3MnO3 and
La0.7Sr0.3MnO3. Upon warming to temperatures near Tc = 257 K the phonon peaks
in La0.7Ca0.3MnO3 soften and damp significantly with the phonon half width at
half maximum approaching 2.5 meV for phonons at a reduced wave vector of q =
(0.2, 0.2, 0). Concurrently a quasielastic component develops that dominates
the spectrum near the polaron position at high temperatures. This quasielastic
scattering is ~5 times more intense near Tc than in La0.7Sr0.3MnO3 despite
comparable structural distortions in the two. The damping becomes more
significant near the polaron position with a temperature dependence similar to
that of polaron structural distortions. An applied magnetic field of 9.5 T only
partially reverses the damping and quasielastic component, despite smaller
fields being sufficient to drive the colossal magnetoresistive effect. The
phonon energy, on the other hand, is unaffected by field. The damping in
La0.7Sr0.3MnO3 near Tc at a reduced wave vector of q = (0.25, 0.25, 0) is
significantly smaller but displays a similar trend with an applied magnetic
field. | 1901.03394v1 |
2019-03-13 | Inference of magnetic field strength and density from damped transverse coronal waves | A classic application of coronal seismology uses transverse oscillations of
waveguides to obtain estimates of the magnetic field strength. The procedure
requires information on the density of the structures. Often, it ignores the
damping of the oscillations. We computed marginal posteriors for parameters
such as the waveguide density; the density contrast; the transverse
inhomogeneity length-scale; and the magnetic field strength, under the
assumption that the oscillations can be modelled as standing
magnetohydrodynamic (MHD) kink modes damped by resonant absorption. Our results
show that the magnetic field strength can be properly inferred, even if the
densities inside and outside the structure are largely unknown. Incorporating
observational estimates of plasma density further constrains the obtained
posteriors. The amount of information one is willing to include (a priori) for
the density and the density contrast influences their corresponding posteriors,
but very little the inferred magnetic field strength. The decision to include
or leave out the information on the damping and the damping time-scales have a
minimal impact on the obtained magnetic field strength. In contrast to the
classic method which provides with numerical estimates with error bars or
possible ranges of variation for the magnetic field strength, Bayesian methods
offer the full distribution of plausibility over the considered range of
possible values. The methods are applied to available datasets of observed
transverse loop oscillations, can be extended to prominence fine structures or
chromospheric spicules and implemented to propagating waves in addition to
standing oscillations. | 1903.05437v1 |
2019-03-14 | A metal-poor damped Ly-alpha system at redshift 6.4 | We identify a strong Ly-alpha damping wing profile in the spectrum of the
quasar P183+05 at z=6.4386. Given the detection of several narrow metal
absorption lines at z=6.40392, the most likely explanation for the absorption
profile is that it is due to a damped Ly-alpha system. However, in order to
match the data a contribution of an intergalactic medium 5-38% neutral or
additional weaker absorbers near the quasar is also required. The absorption
system presented here is the most distant damped Ly-alpha system currently
known. We estimate an HI column density ($10^{20.68\pm0.25}\,$cm$^{-2}$),
metallicity ([O/H]$=-2.92\pm 0.32$), and relative chemical abundances of a
system consistent with a low-mass galaxy during the first Gyr of the universe.
This object is among the most metal-poor damped Ly-alpha systems known and,
even though it is observed only ~850 Myr after the big bang, its relative
abundances do not show signatures of chemical enrichment by Population III
stars. | 1903.06186v2 |
2019-04-30 | DmpIRFs and DmpST: DAMPE Instrument Response Functions and Science Tools for Gamma-Ray Data Analysis | GeV gamma ray is an important observation target of DArk Matter Particle
Explorer (DAMPE) for indirect dark matter searching and high energy
astrophysics. We present in this work a set of accurate instrument response
functions of DAMPE (DmpIRFs) including the effective area, point-spread
function and energy dispersion that are crucial for the gamma-ray data analysis
based on the high statistics simulation data. A dedicated software named DmpST
is developed to facilitate the scientific analyses of DAMPE gamma-ray data.
Considering the limited number of photons and the angular resolution of DAMPE,
the maximum likelihood method is adopted in the DmpST to better disentangle
different source components. The basic mathematics and the framework regarding
this software are also introduced in this paper. | 1904.13098v1 |
2019-05-14 | Fractional damping through restricted calculus of variations | We deliver a novel approach towards the variational description of Lagrangian
mechanical systems subject to fractional damping by establishing a restricted
Hamilton's principle. Fractional damping is a particular instance of non-local
(in time) damping, which is ubiquitous in mechanical engineering applications.
The restricted Hamilton's principle relies on including fractional derivatives
to the state space, the doubling of curves (which implies an extra mirror
system) and the restriction of the class of varied curves. We will obtain the
correct dynamics, and will show rigorously that the extra mirror dynamics is
nothing but the main one in reversed time; thus, the restricted Hamilton's
principle is not adding extra physics to the original system. The price to pay,
on the other hand, is that the fractional damped dynamics is only a sufficient
condition for the extremals of the action. In addition, we proceed to
discretise the new principle. This discretisation provides a set of numerical
integrators for the continuous dynamics that we denote Fractional Variational
Integrators (FVIs). The discrete dynamics is obtained upon the same
ingredients, say doubling of discrete curves and restriction of the discrete
variations. We display the performance of the FVIs, which have local truncation
order 1, in two examples. As other integrators with variational origin, for
instance those generated by the discrete Lagrange-d'Alembert principle, they
show a superior performance tracking the dissipative energy, in opposition to
direct (order 1) discretisations of the dissipative equations, such as explicit
and implicit Euler schemes. | 1905.05608v1 |
2019-07-16 | Damping of slow magnetoacoustic oscillations by the misbalance between heating and cooling processes in the solar corona | Rapidly decaying slow magnetoacoustic waves are regularly observed in the
solar coronal structures, offering a promising tool for a seismological
diagnostics of the coronal plasma, including its thermodynamical properties.
The effect of damping of standing slow magnetoacoustic oscillations in the
solar coronal loops is investigated accounting for the field-aligned thermal
conductivity and a wave-induced misbalance between radiative cooling and some
unspecified heating rates. The non-adiabatic terms were allowed to be
arbitrarily large, corresponding to the observed values. The thermal
conductivity was taken in its classical form, and a power-law dependence of the
heating function on the density and temperature was assumed. The analysis was
conducted in the linear regime and in the infinite magnetic field
approximation. The wave dynamics is found to be highly sensitive to the
characteristic time scales of the thermal misbalance. Depending on certain
values of the misbalance time scales three regimes of the wave evolution were
identified, namely the regime of a suppressed damping, enhanced damping where
the damping rate drops down to the observational values, and acoustic
over-stability. The specific regime is determined by the dependences of the
radiative cooling and heating functions on thermodynamical parameters of the
plasma in the vicinity of the perturbed thermal equilibrium. The comparison of
the observed and theoretically derived decay times and oscillation periods
allows us to constrain the coronal heating function. For typical coronal
parameters, the observed properties of standing slow magnetoacoustic
oscillations could be readily reproduced with a reasonable choice of the
heating function. | 1907.07051v1 |
2019-08-22 | Influence of structure and cation distribution on magnetic anisotropy and damping in Zn/Al doped nickel ferrites | An in-depth analysis of Zn/Al doped nickel ferrites grown by reactive
magnetron sputtering is relevant due to their promising characteristics for
applications in spintronics. The material is insulating and ferromagnetic at
room temperature with an additional low magnetic damping. By studying the
complex interplay between strain and cation distribution their impact on the
magnetic properties, i.e. anisotropy, damping and g-factor is unravelled. In
particular, a strong influence of the lattice site occupation of
Ni$^{2+}_{\text{Td}}$ and cation coordination of Fe$^{2+}_{\text{Oh}}$ on the
intrinsic damping is found. Furthermore, the critical role of the incorporation
of Zn$^{2+}$ and Al$^{3+}$ is evidenced by comparison with a sample of altered
composition. Especially, the dopant Zn$^{2+}$ is evidenced as a tuning factor
for Ni$^{2+}_{\text{Td}}$ and therefore unquenched orbital moments directly
controlling the g-factor. A strain-independent reduction of the magnetic
anisotropy and damping by adapting the cation distribution is demonstrated. | 1908.08257v3 |
2019-09-11 | Critical corrections to formulations of nonlinear energy dissipation of ultrasonically excited bubbles and a unifying parameter to asses and enhance bubble activity in applications | Nonlinear oscillations of bubbles can significantly increase the attenuation
of the host media. Optimization of bubble related applications needs a
realistic estimation of the medium attenuation and bubble activity. A correct
estimation of the wave attenuation in bubbly media requires an accurate
estimation of the power dissipated by nonlinear oscillations of bubbles.
Pioneering work of Louisnard \cite{1} meticulously derived the nonlinear energy
terms for viscous and thermal damping; however, radiation damping arising from
the compressibility of the liquid was neglected. Jamshidi $\&$ Brenner \cite{2}
have considered the effects of the compressibility of the liquid and showed
that damping due to radiation becomes the most significant factor at pressures
above the blake threshold. Despite the improvement in their formulation;
however, the radiation damping term estimates non-physical values for some
frequency and pressure regions including near resonance oscillations. Thus, the
new terms arising from the compressibility of the liquid needs critical
assessment. In this work, we provide critical corrections to the present
formulations. Importance of the new corrections are highlighted by the
scattering to damping ratio (STDR). We then introduce a unifying parameter to
assess the efficacy of applications; this parameter is defined as the
multiplication of maximum scattered pressure by STDR. | 1909.04864v1 |
2019-09-14 | Measurement-Based Wide-Area Damping of Inter-Area Oscillations based on MIMO Identification | Interconnected power grid exhibits oscillatory response after a disturbance
in the system. One such type of oscillations, the inter-area oscillations has
the oscillation frequency in the range of 0.1 to 1 Hz. The damping of
inter-area oscillations is difficult with local controllers, but it can be
achieved using a Wide Area Damping Controller (WADC). For effective control,
the input to the WADC should be the most observable signal and the WADC output
should be sent to the most controllable generator. This paper presents a
measurement-based novel algorithm for multi-input-multi-output (MIMO) transfer
function identification of the power system based on optimization to estimate
such oscillation frequencies. Based on the MIMO transfer function the optimal
control loop for WADC is estimated. The WADC design is based on the discrete
linear quadratic regulator (DLQR) and Kalman filtering for damping of
inter-area oscillations. Since the MIMO identification is based on actual
measurements, the proposed method can accurately monitor changes in the power
grid whereas the conventional methods are based on small-signal analysis of a
linearized model which does not consider changing operating conditions. The
overall algorithm is implemented and validated on a RTDS/RSCAD and MATLAB
real-time co-simulation platform using two-area and IEEE 39 bus power system
models. | 1909.06687v1 |
2020-03-16 | Spin-orbit torques originating from bulk and interface in Pt-based structures | We investigated spin-orbit torques in prototypical Pt-based spintronic
devices. We found that, in Pt/Ni and Pt/Fe bilayers, the damping-like torque
efficiency depends on the thickness of the Pt layer. We also found that the
damping-like torque efficiency is almost identical in the Pt/Ni and Pt/Fe
bilayers despite the stronger spin memory loss at the Pt/Fe interface. These
results suggest that although the dominant source of the damping-like torque is
the bulk spin Hall effect in the Pt layer, a sizable damping-like torque is
generated by the interface in the Pt/Fe bilayer due to the stronger interfacial
spin-orbit coupling. In contrast to the damping-like torque, whose magnitude
and sign are almost identical in the Pt/Ni and Pt/Fe bilayers, the field-like
torque strongly depends on the choice of the ferromagnetic layer. The sign of
the field-like torque originating from the bulk spin Hall effect in the Pt
layer is opposite between the Pt/Ni and Pt/Fe bilayers, which can be attributed
to the opposite sign of the imaginary part of the spin-mixing conductance.
These results demonstrate that the spin-orbit torques are quite sensitive to
the electronic structure of the FM layer. | 2003.07271v2 |
2020-03-23 | Heat-like and wave-like lifespan estimates for solutions of semilinear damped wave equations via a Kato's type lemma | In this paper we study several semilinear damped wave equations with
"subcritical" nonlinearities, focusing on demonstrating lifespan estimates for
energy solutions. Our main concern is on equations with scale-invariant damping
and mass. Under different assumptions imposed on the initial data, lifespan
estimates from above are clearly showed. The key fact is that we find
"transition surfaces", which distinguish lifespan estimates between "wave-like"
and "heat-like" behaviours. Moreover we conjecture that the lifespan estimates
on the "transition surfaces" can be logarithmically improved. As direct
consequences, we reorganize the blow-up results and lifespan estimates for the
massless case in which the "transition surfaces" degenerate to "transition
curves". Furthermore, we obtain improved lifespan estimates in one space
dimension, comparing to the known results. We also study semilinear wave
equations with the scattering damping and negative mass term, and find that if
the decay rate of the mass term equals to 2, the lifespan estimate is the same
as one special case of the equations with the scale-invariant damping and
positive mass. The main strategy of the proof consists of a Kato's type lemma
in integral form, which is established by iteration argument. | 2003.10578v1 |
2020-09-03 | Dynamics of magnetic collective modes in the square and triangular lattice Mott insulators at finite temperature | We study the equilibrium dynamics of magnetic moments in the Mott insulating
phase of the Hubbard model on the square and triangular lattice. We rewrite the
Hubbard interaction in terms of an auxiliary vector field and use a recently
developed Langevin scheme to study its dynamics. A thermal `noise', derivable
approximately from the Keldysh formalism, allows us to study the effect of
finite temperature. At strong coupling, $U \gg t$, where $U$ is the local
repulsion and $t$ the nearest neighbour hopping, our results reproduce the well
known dynamics of the nearest neighbour Heisenberg model with exchange $J \sim
{\cal O}(t^2/U)$. These include crossover from weakly damped dispersive modes
at temperature $T \ll J$ to strong damping at $T \sim {\cal O}(J)$, and
diffusive dynamics at $T \gg J$. The crossover temperatures are naturally
proportional to $J$. To highlight the progressive deviation from Heisenberg
physics as $U/t$ reduces we compute an effective exchange scale $J_{eff}(U)$
from the low temperature spin wave velocity. We discover two features in the
dynamical behaviour with decreasing $U/t$: (i)~the low temperature dispersion
deviates from the Heisenberg result, as expected, due to longer range and
multispin interactions, and (ii)~the crossovers between weak damping, strong
damping, and diffusion take place at noticeably lower values of $T/J_{eff}$. We
relate this to enhanced mode coupling, in particular to thermal amplitude
fluctuations, at weaker $U/t$. A comparison of the square and triangular
lattice reveals the additional effect of geometric frustration on damping. | 2009.01833v2 |
2020-09-15 | Classification of the mechanisms of wave energy dissipation in the nonlinear oscillations of coated and uncoated bubbles | Acoustic waves are dissipated when they pass through bubbly media.
Dissipation by bubbles takes place through thermal damping (Td), radiation
damping (Rd) and damping due to the friction of the liquid (Ld) and friction of
the coating (Cd). Knowledge of the contributions of the Td, Rd, Ld and Cd
during nonlinear bubble oscillations will help in optimizing bubble and
ultrasound exposure parameters for the relevant applications by maximizing a
desirable parameter. In this work we investigate the mechanisms of dissipation
in bubble oscillations and their contribution to the total damping (Wtotal) in
various nonlinear regimes. By using bifurcation analysis, we have classified
nonlinear dynamics of bubbles that are sonicated with their 3rd superharmonic
(SuH) and 2nd SuH resonance frequency (fr), pressure dependent resonance
frequency (PDfr), fr, subharmonic (SH) resonance (fsh=2fr), pressure dependent
SH resonance (PDfsh) and 1/3 order SH resonance. The corresponding Td, Rd, Ld,
Cd, Wtotal, scattering to dissipation ratio (STDR), maximum wall velocity and
maximum back-scattered pressure from non-destructive oscillations of bubbles
were calculated and analyzed using the bifurcation diagrams. We classified
different regimes of dissipation and provided parameter regions in which a
particular parameter of interest (e.g. Rd) can be enhanced. Afterwards enhanced
bubble activity is linked to some relevant applications in ultrasound. This
paper represents the first comprehensive analysis of the nonlinear oscillations
regimes and the corresponding damping mechanisms. | 2009.07380v1 |
2020-11-18 | The effect of redshift degeneracy and the damping effect of viscous medium on the information extracted from gravitational wave signals | Considering the cosmological redshift $z_c$ , the mass of GW source extracted
from GW signal is $1+z_c$ times larger than its intrinsic value, and distance
between detector and GW source should be regarded as luminosity distance.
However, besides cosmological redshift, there are other kinds of redshifts
should be considered, which is actually ignored, in the analysis of GW data,
such as Doppler redshift and gravitational redshift, so the parameters
extracted from GW may deviate from their intrinsic values. Another factor that
may affect GW is the viscous medium in propagation path of GW, which may damp
the GW with a damping rate of $16{\pi}G{\eta}$. Some studies indicate dark
matter may interact with each other, thus dark matter may be the origin of
viscosity of cosmic medium. Then the GW may be rapidly damped by the viscous
medium that is made of dark matter, such as dark matter "mini-spike" around
intermediate mass black hole. In this article, we mainly discuss how Doppler
and gravitational redshift, together with the damping effect of viscous medium,
affect the information, such as the mass and redshift of GW source, extracted
from GW signals. | 2011.09169v2 |
2020-12-28 | On the Efficacy of Friction Damping in the Presence of Nonlinear Modal Interactions | This work addresses friction-induced modal interactions in jointed
structures, and their effects on the passive mitigation of vibrations by means
of friction damping. Under the condition of (nearly) commensurable natural
frequencies, the nonlinear character of friction can cause so-called nonlinear
modal interactions. If harmonic forcing near the natural frequency of a
specific mode is applied, for instance, another mode may be excited due to
nonlinear energy transfer and thus contribute considerably to the vibration
response. We investigate how this phenomenon affects the performance of
friction damping. To this end, we study the steady-state, periodic forced
vibrations of a system of two beams connected via a local mechanical friction
joint. The system can be tuned to continuously adjust the ratio between the
first two natural frequencies in the range around the $1:3$ internal resonance,
in order to trigger or suppress the emergence of modal interactions. Due to the
re-distribution of the vibration energy, the vibration level can in fact be
reduced in certain situations. However, in other situations, the multi-harmonic
character of the vibration has detrimental effects on the effective damping
provided by the friction joint. The resulting response level can be
significantly larger than in the absence of modal interactions. Moreover, it is
shown that the vibration behavior is highly sensitive in the neighborhood of
internal resonances. It is thus concluded that the condition of internal
resonance should be avoided in the design of friction-damped systems. | 2101.03232v1 |
2021-06-30 | Origin of Nonlinear Damping due to Mode Coupling in Auto-Oscillatory Modes Strongly Driven by Spin-Orbit Torque | We investigate the physical origin of nonlinear damping due to mode coupling
between several auto-oscillatory modes driven by spin-orbit torque in
constricted Py/Pt heterostructures by examining the dependence of
auto-oscillation on temperature and applied field orientation. We observe a
transition in the nonlinear damping of the auto-oscillation modes extracted
from the total oscillation power as a function of drive current, which
coincides with the onset of power redistribution amongst several modes and the
crossover from linewidth narrowing to linewidth broadening in all individual
modes. This indicates the activation of another relaxation process by nonlinear
magnon-magnon scattering within the modes. We also find that both nonlinear
damping and threshold current in the mode-interaction damping regime at high
drive current after transition are temperature independent, suggesting that the
mode coupling occurs dominantly through a non-thermal magnon scattering process
via a dipole or exchange interaction rather than thermally excited
magnon-mediated scattering. This finding presents a promising pathway to
overcome the current limitations of efficiently controlling the interaction
between two highly nonlinear magnetic oscillators to prevent mode crosstalk or
inter-mode energy transfer and deepens understanding of complex nonlinear spin
dynamics in multimode spin wave systems. | 2107.00150v2 |
2021-07-15 | On the long-time behavior for a damped Navier-Stokes-Bardina model | In this paper, we consider a damped Navier-Stokes-Bardina model posed on the
whole three-dimensional. These equations have an important physical motivation
and they arise from some oceanic model. From the mathematical point of view,
they write down as the well-know Navier-Stokes equations with an additional
nonlocal operator in their nonlinear transport term, and moreover, with an
additional damping term depending of a parameter $\beta>0$. We study first the
existence and uniqueness of global in time weak solutions in the energy space.
Thereafter, our main objective is to describe the long time behavior of these
solutions. For this, we use some tools in the theory of dynamical systems to
prove the existence of a global attractor, which is a compact subset in the
energy space attracting all the weak solutions when the time goes to infinity.
Moreover, we derive an upper bound for the fractal dimension of the global
attractor associated to these equations.
Finally, we find a range of values for the damping parameter $\beta>0$, where
we are able to give an acutely description of the internal structure of the
global attractor. More precisely, we prove that the global attractor only
contains the stationary (time-independing) solution of the damped
Navier-Stokes-Bardina equations. | 2107.07070v2 |
2021-07-17 | Plasmon-Exciton Coupling Effect on Plasmon Damping | Plasmon decay via the surface or interface is a critical process for
practical energy conversion and plasmonic catalysis. However, the relationship
between plasmon damping and the coupling between the plasmon and 2D materials
is still unclear. The spectral splitting due to plasmon-exciton interaction
impedes the conventional single-particle method to evaluate the plasmon damping
rate by the spectral linewidth directly. Here, we investigated the interaction
between a single gold nanorod (GNR) and 2D materials using the single-particle
spectroscopy method assisted with in situ nanomanipulation technique by
comparing scattering intensity and linewidth together. Our approach allows us
to indisputably identify that the plasmon-exciton coupling in the GNR-WSe2
hybrid would induce plasmon damping. We can also isolate the contribution
between the charge transfer channel and resonant energy transfer channel for
the plasmon decay in the GNR-graphene hybrid by comparing that with thin hBN
layers as an intermediate medium to block the charge transfer. We find out that
the contact layer between the GNR and 2D materials contributes most of the
interfacial plasmon damping. These findings contribute to a deep understanding
of interfacial excitonic effects on the plasmon and 2D materials hybrid. | 2107.08230v1 |
2021-10-12 | Outflows in the presence of cosmic rays and waves with cooling | Plasma outflow from a gravitational potential well with cosmic rays and
self-excited Alfv\'en waves with cooling and wave damping is studied in the
hydrodynamics regime. We study outflows in the presence of cosmic ray and
Alfv\'en waves including the effect of cooling and wave damping. We seek
physically allowable steady-state subsonic-supersonic transonic solutions. We
adopted a multi-fluid hydrodynamical model for the cosmic ray plasma system.
Thermal plasma, cosmic rays, and self-excited Alfv\'en waves are treated as
fluids. Interactions such as cosmic-ray streaming instability, cooling, and
wave damping were fully taken into account. We considered one-dimensional
geometry and explored steady-state solutions. The model is reduced to a set of
ordinary differential equations, which we solved for subsonic-supersonic
transonic solutions with given boundary conditions at the base of the
gravitational potential well. We find that physically allowable
subsonic-supersonic transonic solutions exist for a wide range of parameters.
We studied the three-fluid system (considering only forward-propagating
Alfv\'en waves) in detail. We examined the cases with and without cosmic ray
diffusion separately. Comparisons of solutions with and without cooling and
with and without wave damping for the same set of boundary conditions (on
density, pressures of thermal gas, cosmic rays and waves) are presented. We
also present the interesting case of a four-fluid system (both forward- and
backward-propagating Alfv\'en waves are included), highlighting the intriguing
relation between different components. | 2110.06170v1 |
2021-11-19 | Finite time extinction for a class of damped Schr{ö}dinger equations with a singular saturated nonlinearity | We present some sharper finite extinction time results for solutions of a
class of damped nonlinear Schr{\"o}dinger equations when the nonlinear damping
term corresponds to the limit cases of some ``saturating non-Kerr law''
$F(|u|^2)u=\frac{a}{\varepsilon+(|u|^2)^\alpha}u,$ with $a\in\mathbb{C},$
$\varepsilon\geqslant0,$ $2\alpha=(1-m)$ and $m\in[0,1).$ To carry out the
improvement of previous results in the literature we present in this paper a
careful revision of the existence and regularity of weak solutions under very
general assumptions on the data. We prove that the problem can be solved in the
very general framework of the maximal monotone operators theory, even under a
lack of regularity of the damping term. This allows us to consider, among other
things, the singular case $m=0.$ We replace the above approximation of the
damping term by a different one which keeps the monotonicity for any
$\varepsilon\geqslant0$. We prove that, when $m=0,$ the finite extinction time
of the solution arises for merely bounded right hand side data $f(t,x).$ This
is specially useful in the applications in which the Schr{\"o}dinger equation
is coupled with some other functions satisfying some additional equations. | 2111.10136v2 |
2022-01-26 | Effect of Chiral Damping on the dynamics of chiral domain walls and skyrmions | Friction plays an essential role in most physical processes that we
experience in our everyday life. Examples range from our ability to walk or
swim, to setting boundaries of speed and fuel efficiency of moving vehicles. In
magnetic systems, the displacement of chiral domain walls (DW) and skyrmions
(SK) by Spin Orbit Torques (SOT), is also prone to friction. Chiral damping,
the dissipative counterpart of the Dzyaloshinskii Moriya Interaction (DMI),
plays a central role in these dynamics. Despite experimental observation, and
numerous theoretical studies confirming its existence, the influence of chiral
damping on DW and SK dynamics has remained elusive due to the difficulty of
discriminating from DMI. Here we unveil the effect that chiral damping has on
the flow motion of DWs and SKs driven by current and magnetic field. We use a
static in-plane field to lift the chiral degeneracy. As the in-plane field is
increased, the chiral asymmetry changes sign. When considered separately,
neither DMI nor chiral damping can explain the sign reversal of the asymmetry,
which we prove to be the result of their competing effects. Finally, numerical
modelling unveils the non-linear nature of chiral dissipation and its critical
role for the stabilization of moving SKs. | 2201.10742v1 |
2022-01-27 | A Study on Monte Carlo simulation of the radiation environment above GeV at the DAMPE orbit | The Dark Matter Particle Explorer (DAMPE) has been undergoing a stable
on-orbit operation for more than 6 years and acquired observation of over 11
billion events. And a better understanding of the overall radiation environment
on the DAMPE orbit is crucial for both simulation data production and flight
data analysis. In this work, we study the radiation environment at the low
Earth orbit and develop a simulation software package using the framework of
ATMNC3, in which state-of-the-art full 3D models of the Earth's atmospheric and
magnetic-field configurations is integrated. We consider in our Monte Carlo
procedure event-by-event propagation of the cosmic rays in the geomagnetic
field and their interaction with the Earth's atmosphere, focusing on the
particles above GeV that are able to trigger the DAMPE data acquisition system.
We compare the simulation results with the cosmic-ray electrons and positrons
(CREs) flux measurements made by DAMPE. The overall agreement on both the
spectral and angular distribution of the CREs flux demonstrates that our
simulation is well established. Our software package could be of more general
usage for the simulation of the radiation environment at the low Earth orbit of
various altitudes. | 2201.11364v1 |
2022-06-08 | Motion control with optimal nonlinear damping: from theory to experiment | Optimal nonlinear damping control was recently introduced for the
second-order SISO systems, showing some advantages over a classical PD feedback
controller. This paper summarizes the main theoretical developments and
properties of the optimal nonlinear damping controller and demonstrates, for
the first time, its practical experimental evaluation. An extended analysis and
application to more realistic (than solely the double-integrator) motion
systems are also given in the theoretical part of the paper. As comparative
linear feedback controller, a PD one is taken, with the single tunable gain and
direct compensation of the plant time constant. The second, namely
experimental, part of the paper includes the voice-coil drive system with
relatively high level of the process and measurement noise, for which the
standard linear model is first identified in frequency domain. The linear
approximation by two-parameters model forms the basis for designing the PD
reference controller, which fixed feedback gain is the same as for the optimal
nonlinear damping control. A robust sliding-mode based differentiator is used
in both controllers for a reliable velocity estimation required for the
feedback. The reference PD and the proposed optimal nonlinear damping
controller, both with the same single design parameter, are compared
experimentally with respect to trajectory tracking and disturbance rejection. | 2206.03802v2 |
2022-09-22 | Neutrino Fast Flavor Pendulum. Part 2: Collisional Damping | In compact astrophysical objects, the neutrino density can be so high that
neutrino-neutrino refraction can lead to fast flavor conversion of the kind
$\nu_e \bar\nu_e \leftrightarrow \nu_x \bar\nu_x$ with $x=\mu,\tau$, depending
on the neutrino angle distribution. Previously, we have shown that in a
homogeneous, axisymmetric two-flavor system, these collective solutions evolve
in analogy to a gyroscopic pendulum. In flavor space, its deviation from the
weak-interaction direction is quantified by a variable $\cos\vartheta$ that
moves between $+1$ and $\cos\vartheta_{\rm min}$, the latter following from a
linear mode analysis. As a next step, we include collisional damping of flavor
coherence, assuming a common damping rate $\Gamma$ for all modes. Empirically
we find that the damped pendular motion reaches an asymptotic level of pair
conversion $f=A+(1-A)\cos\vartheta_{\rm min}$ (numerically $A\simeq 0.370$)
that does not depend on details of the angular distribution (except for fixing
$\cos\vartheta_{\rm min}$), the initial seed, nor $\Gamma$. On the other hand,
even a small asymmetry between the neutrino and antineutrino damping rates
strongly changes this picture and can even enable flavor instabilities in
otherwise stable systems. | 2209.11235v3 |
2022-10-12 | Second order two-species systems with nonlocal interactions: existence and large damping limits | We study the mathematical theory of second order systems with two species,
arising in the dynamics of interacting particles subject to linear damping, to
nonlocal forces and to external ones, and resulting into a nonlocal version of
the compressible Euler system with linear damping. Our results are limited to
the $1$ space dimensional case but allow for initial data taken in a
Wasserstein space of probability measures. We first consider the case of smooth
nonlocal interaction potentials, not subject to any symmetry condition, and
prove existence and uniqueness. The concept of solutions relies on a stickiness
condition in case of collisions, in the spirit of previous works in the
literature. The result uses concepts from classical Hilbert space theory of
gradient flows (cf. Brezis [7]) and a trick used in [4]. We then consider a
large-time and large-damping scaled version of our system and prove convergence
to solutions to the corresponding first order system. Finally, we consider the
case of Newtonian potentials -- subject to symmetry of the cross-interaction
potentials -- and external convex potentials. After showing existence in the
sticky particles framework in the spirit of [4], we prove convergence for large
times towards Dirac delta solutions for the two densities. All the results
share a common technical framework in that solutions are considered in a
Lagrangian framework, which allows to estimate the behavior of solutions via
$L^2$ estimates of the pseudo-inverse variables corresponding to the two
densities. In particular, due to this technique, the large-damping result holds
under a rather weak condition on the initial data, which does not require
well-prepared initial velocities. We complement the results with numerical
simulations. | 2210.06162v1 |
2022-10-12 | Stability of the Néel quantum critical point in the presence of Dirac fermions | We investigate the stability of the N\'eel quantum critical point of
two-dimensional quantum antiferromagnets, described by a non-linear $\sigma$
model (NL$\sigma$M), in the presence of a Kondo coupling to $N_f$ flavours of
two-component Dirac fermion fields. The long-wavelength order parameter
fluctuations are subject to Landau damping by electronic particle-hole
fluctuations. Using momentum-shell RG, we demonstrate that the Landau damping
is weakly irrelevant at the N\'eel quantum critical point, despite the fact
that the corresponding self-energy correction dominates over the quadratic
gradient terms in the IR limit. In the ordered phase, the Landau damping
increases under the RG, indicative of damped spin-wave excitations. Although
the Kondo coupling is weakly relevant, sufficiently strong Landau damping
renders the N\'eel quantum critical point quasi-stable for $N_f\ge 4$ and
thermodynamically stable for $N_f<4$. In the latter case, we identify a new
multi-critical point which describes the transition between the N\'eel critical
and Kondo run-away regimes. The symmetry breaking at this fixed point results
in the opening of a gap in the Dirac fermion spectrum. Approaching the
multi-critical point from the disordered phase, the fermionic quasiparticle
residue vanishes, giving rise to non-Fermi-liquid behavior. | 2210.06577v3 |
2022-11-13 | Damping analysis of Floating Offshore Wind Turbine (FOWT): a new control strategy reducing the platform vibrations | In this paper, the coupled dynamics of the floating platform and the WTG
rotor is analysed. In particular, the damping is explicitly derived from the
coupled equations of rotor and floating platform. The analysis of the damping
leads to the study of the instability phenomena and it derives the explicit
conditions that lead to the Non Minimum Phase Zero (NMPZ). Two NMPZs, one
related to the rotor dynamics and the other one to the platform pitch dynamics,
are analysed. The latter is a novelty and it is analysed in this work,
providing the community of an explicit condition for its verification. The
domain of the instability of the platform is explicitly derived from the
coupled system of equations. In the second part of the paper, from the analysis
of the damping of the floating platform, a new strategy for the control of
FOWTs is proposed. This strategy allows one to impose to the controller an
explicit level of damping in the platform pitch motion without changing the
period of platform pitching. Finally the new strategy is compared to the one
without compensation by performing aero-hydro-servo-elastic numerical
simulations of the UMaine IEA15MW FOWT. Generated power, movements, blade pitch
and tower base fatigue are compared showing that the new control strategy can
reduce fatigue in the structure without affecting the power production. | 2211.10362v1 |
2022-11-22 | Universal Dynamics of Damped-Driven Systems: The Logistic Map as a Normal Form for Energy Balance | Damped-driven systems are ubiquitous in engineering and science. Despite the
diversity of physical processes observed in a broad range of applications, the
underlying instabilities observed in practice have a universal characterization
which is determined by the overall gain and loss curves of a given system. The
universal behavior of damped-driven systems can be understood from a
geometrical description of the energy balance with a minimal number of
assumptions. The assumptions on the energy dynamics are as follows: the energy
increases monotonically as a function of increasing gain, and the losses become
increasingly larger with increasing energy, i.e. there are many routes for
dissipation in the system for large input energy. The intersection of the gain
and loss curves define an energy balanced solution. By constructing an
iterative map between the loss and gain curves, the dynamics can be shown to be
homeomorphic to the logistic map, which exhibits a period doubling cascade to
chaos. Indeed, the loss and gain curves allow for a geometrical description of
the dynamics through a simple Verhulst diagram (cobweb plot). Thus irrespective
of the physics and its complexities, this simple geometrical description
dictates the universal set of logistic map instabilities that arise in complex
damped-driven systems. More broadly, damped-driven systems are a class of
non-equilibrium pattern forming systems which have a canonical set of
instabilities that are manifest in practice. | 2211.11748v1 |
2023-01-23 | Optimal Inter-area Oscillation Damping Control: A Transfer Deep Reinforcement Learning Approach with Switching Control Strategy | Wide-area damping control for inter-area oscillation (IAO) is critical to
modern power systems. The recent breakthroughs in deep learning and the broad
deployment of phasor measurement units (PMU) promote the development of
datadriven IAO damping controllers. In this paper, the damping control of IAOs
is modeled as a Markov Decision Process (MDP) and solved by the proposed Deep
Deterministic Policy Gradient (DDPG) based deep reinforcement learning (DRL)
approach. The proposed approach optimizes the eigenvalue distribution of the
system, which determines the IAO modes in nature. The eigenvalues are evaluated
by the data-driven method called dynamic mode decomposition. For a given power
system, only a subset of generators selected by participation factors needs to
be controlled, alleviating the control and computing burdens. A Switching
Control Strategy (SCS) is introduced to improve the transient response of IAOs.
Numerical simulations of the IEEE-39 New England power grid model validate the
effectiveness and advanced performance of the proposed approach as well as its
robustness against communication delays. In addition, we demonstrate the
transfer ability of the DRL model trained on the linearized power grid model to
provide effective IAO damping control in the non-linear power grid model
environment. | 2301.09321v1 |
2023-04-13 | Centralised Multimode Power Oscillation Damping Controller for Photovoltaic Plants with Communication Delay Compensation | Low-frequency oscillations are an inherent phenomena in transmission networks
and renewable energy plants should be configured to damp them. Commonly, a
centralised controller is used in PV plants to coordinate PV generators via
communication channels. However, the communication systems of PV plants
introduce delays of a stochastic nature that degrade the performance of
centralised control algorithms. Therefore, controllers for oscillation damping
may not operate correctly unless the communication channel characteristics are
not considered and compensated. In this paper, a centralised controller is
proposed for the oscillation damping that uses a PV plant with all the
realistic effects of communication channels taken into consideration. The
communication channels are modelled based on measurements taken in a laboratory
environment. The controller is designed to damp several modes of oscillation by
using the open-loop phase shift compensation. Theoretical developments were
validated in a laboratory using four converters acting as two PV inverters, a
battery and a STATCOM. A real-time processing platform was used to implement
the centralised controller and to deploy the communication infrastructure.
Experimental results show the communication channels impose severe restrictions
on the performance of centralised POD controllers, highlighting the importance
of their accurate modelling and consideration during the controller design
stage. | 2304.06415v1 |
2023-05-09 | Glassy heat capacity from overdamped phasons and a hypothetical phason-induced superconductivity in incommensurate structures | Phasons are collective low-energy modes that appear in disparate condensed
matter systems such as quasicrystals, incommensurate structures, fluctuating
charge density waves, and Moir\'e superlattices. They share several
similarities with acoustic phonon modes, but they are not protected by any
exact translational symmetry. As a consequence, they are subject to a
wavevector independent damping, and they develop a finite pinning frequency,
which destroy their acoustic linearly propagating dispersion. Under a few and
simple well-motivated assumptions, we compute the phason density of states, and
we derive the phason heat capacity as a function of the temperature. Finally,
imagining a hypothetical s-wave pairing channel with electrons, we compute the
critical temperature $T_c$ of the corresponding superconducting state as a
function of phason damping using the Eliashberg formalism. We find that for
large phason damping, the heat capacity is linear in temperature, showing a
distinctive glass-like behavior. Additionally, we observe that the phason
damping can strongly enhance the effective Eliashberg coupling, and we reveal a
sharp non-monotonic dependence of the superconducting temperature $T_c$ on the
phason damping, with a maximum located at the underdamped to overdamped
crossover scale. Our simple computations confirm the potential role of
overdamped modes in explaining the glassy properties of incommensurate
structures, but also in possibly inducing strongly-coupled superconductivity
therein, and enhancing the corresponding $T_c$. | 2305.05407v2 |
2023-08-03 | Flavor-wave theory with quasiparticle damping at finite temperatures: Application to chiral edge modes in the Kitaev model | We propose a theoretical framework to investigate elementary excitations at
finite temperatures within a localized electron model that describes the
interactions between multiple degrees of freedom, such as quantum spin models
and Kugel-Khomskii models. Thus far, their excitation structures have been
mainly examined using the linear flavor-wave theory, an SU($N$) generalization
of the linear spin-wave theory. These techniques introduce noninteracting
bosonic quasiparticles as elementary excitations from the ground state, thereby
elucidating numerous physical phenomena, including excitation spectra and
transport properties characterized by topologically nontrivial band structures.
Nevertheless, the interactions between quasiparticles cannot be ignored in
systems exemplified by $S=1/2$ quantum spin models, where strong quantum
fluctuations are present. Recent studies have investigated the effects of
quasiparticle damping at zero temperature in such models. In our study,
extending this approach to the flavor-wave theory for general localized
electron models, we construct a comprehensive method to calculate excitation
spectra with the quasiparticle damping at finite temperatures. We apply our
method to the Kitaev model under magnetic fields, a typical example of models
with topologically nontrivial magnon bands. Our calculations reveal that chiral
edge modes undergo significant damping in weak magnetic fields, amplifying the
damping rate by the temperature increase. This effect is caused by collisions
with thermally excited quasiparticles. Since our approach starts from a general
Hamiltonian, it will be widely applicable to other localized systems, such as
spin-orbital coupled systems derived from multi-orbital Hubbard models in the
strong correlation limit. | 2308.01711v1 |
2024-02-13 | Investigating the Effect of Noise on the Training Performance of Hybrid Quantum Neural Networks | In this paper, we conduct a comprehensively analyze the influence of
different quantum noise gates, including Phase Flip, Bit Flip, Phase Damping,
Amplitude Damping, and the Depolarizing Channel, on the performance of HyQNNs.
Our results reveal distinct and significant effects on HyQNNs training and
validation accuracies across different probabilities of noise. For instance,
the Phase Flip gate introduces phase errors, and we observe that HyQNNs exhibit
resilience at higher probability (p = 1.0), adapting effectively to consistent
noise patterns, whereas at intermediate probabilities, the performance
declines. Bit Flip errors, represented by the PauliX gate, impact HyQNNs in a
similar way to that Phase Flip error gate. The HyQNNs, can adapt such kind of
errors at maximum probability (p = 1.0). Unlike Phase and Bit Flip error gates,
Phase Damping and Amplitude Damping gates disrupt quantum information, with
HyQNNs demonstrating resilience at lower probabilities but facing challenges at
higher probabilities. Amplitude Damping error gate, in particular, poses
efficiency and accuracy issues at higher probabilities however with lowest
probability (p = 0.1),it has the least effect and the HyQNNs, however not very
effectively, but still tends to learn. The Depolarizing Channel proves most
detrimental to HyQNNs performance, with limited or no training improvements.
There was no training potential observed regardless of the probability of this
noise gate. These findings underscore the critical need for advanced quantum
error mitigation and resilience strategies in the design and training of
HyQNNs, especially in environments prone to depolarizing noise. This paper
quantitatively investigate that understanding the impact of quantum noise gates
is essential for harnessing the full potential of quantum computing in
practical applications. | 2402.08523v1 |
2024-03-03 | Magnonic $\varphi$ Josephson Junctions and Synchronized Precession | There has been a growing interest in non-Hermitian physics. One of its main
goals is to engineer dissipation and to explore ensuing functionality. In
magnonics, the effect of dissipation due to local damping on magnon transport
has been explored. However, the effects of non-local damping on the magnonic
analog of the Josephson effect remain missing, despite that non-local damping
is inevitable and has been playing a central role in magnonics. Here, we
uncover theoretically that a surprisingly rich dynamics can emerge in magnetic
junctions due to intrinsic non-local damping, using analytical and numerical
methods. In particular, under microwave pumping, we show that coherent spin
precession in the right and left insulating ferromagnet (FM) of the junction
becomes synchronized by non-local damping and thereby a magnonic analog of the
$\varphi$ Josephson junction emerges, where $\varphi$ stands here for the
relative precession phase of right and left FM in the stationary limit.
Remarkably, $\varphi$ decreases monotonically from $ \pi$ to $\pi/2$ as the
magnon-magnon interaction, arising from spin anisotropies, increases. Moreover,
we also find a magnonic diode effect giving rise to rectification of magnon
currents. Our predictions are readily testable with current device and
measurement technologies at room temperatures. | 2403.01625v1 |
2003-03-03 | Sur une generalisation des coefficients binomiaux | We prove a recent conjecture of Lassalle about positivity and integrality of
coefficients in some polynomial expansions. We also give a combinatorial
interpretation of those numbers. Finally, we show that this question is closely
related to the fundamental problem of calculating the linearization
coefficients for binomial coefficients. | 0303025v1 |
2013-06-18 | Estimating Certain Non-Zero Littlewood-Richardson Coefficients | Littlewood Richardson coefficients are structure constants appearing in the
representation theory of the general linear groups ($GL_n$). The main results
of this paper are: 1. A strongly polynomial randomized approximation scheme for
certain Littlewood-Richardson coefficients. 2. A proof of approximate
log-concavity of certain Littlewood-Richardson coefficients. | 1306.4060v1 |
2014-10-29 | Rectangular symmetries for coefficients of symmetric functions | We show that some of the main structural constants for symmetric functions
(Littlewood-Richardson coefficients, Kronecker coefficients, plethysm
coefficients, and the Kostka--Foulkes polynomials) share symmetries related to
the operations of taking complements with respect to rectangles and adding
rectangles. | 1410.8017v1 |
2020-02-03 | The Computational Complexity of Plethysm Coefficients | In two papers, B\"urgisser and Ikenmeyer (STOC 2011, STOC 2013) used an
adaption of the geometric complexity theory (GCT) approach by Mulmuley and
Sohoni (Siam J Comput 2001, 2008) to prove lower bounds on the border rank of
the matrix multiplication tensor. A key ingredient was information about
certain Kronecker coefficients. While tensors are an interesting test bed for
GCT ideas, the far-away goal is the separation of algebraic complexity classes.
The role of the Kronecker coefficients in that setting is taken by the
so-called plethysm coefficients: These are the multiplicities in the coordinate
rings of spaces of polynomials. Even though several hardness results for
Kronecker coefficients are known, there are almost no results about the
complexity of computing the plethysm coefficients or even deciding their
positivity.
In this paper we show that deciding positivity of plethysm coefficients is
NP-hard, and that computing plethysm coefficients is #P-hard. In fact, both
problems remain hard even if the inner parameter of the plethysm coefficient is
fixed. In this way we obtain an inner versus outer contrast: If the outer
parameter of the plethysm coefficient is fixed, then the plethysm coefficient
can be computed in polynomial time.
Moreover, we derive new lower and upper bounds and in special cases even
combinatorial descriptions for plethysm coefficients, which we consider to be
of independent interest. Our technique uses discrete tomography in a more
refined way than the recent work on Kronecker coefficients by Ikenmeyer,
Mulmuley, and Walter (Comput Compl 2017). This makes our work the first to
apply techniques from discrete tomography to the study of plethysm
coefficients. Quite surprisingly, that interpretation also leads to new
equalities between certain plethysm coefficients and Kronecker coefficients. | 2002.00788v1 |
2007-06-04 | Bounds for Hilbert coefficients | We compute the Hilbert coefficients of a graded module with pure resolution
and discuss lower and upper bounds for these coefficients for arbitrary graded
modules. | 0706.0400v1 |
2020-06-23 | Recurrence relations of Li coefficients | One of equivalents of the Riemann hypothesis is Li's criterion that all Li
coefficients are positive. We study recurrence relations of Li coefficients in
this note. | 2006.13103v1 |
2007-05-02 | The Truth About Ballistic Coefficients | The ballistic coefficient of a bullet describes how it slows in flight due to
air resistance. This article presents experimental determinations of ballistic
coefficients showing that the majority of bullets tested have their previously
published ballistic coefficients exaggerated from 5-25% by the bullet
manufacturers. These exaggerated ballistic coefficients lead to inaccurate
predictions of long range bullet drop, retained energy and wind drift. | 0705.0389v1 |
2008-12-11 | Lorentz Violation and Alpha-Decay | Relating the effective Lorentz violation coefficients for composite particles
to the coefficients for their constituent fields is a challenging problem. We
calculate the Lorentz violation coefficients relevant to the dynamics of an
alpha-particle in terms of proton and neutron coefficients. The alpha-particle
coefficients would lead to anisotropies in the alpha-decays of nuclei, and
because the decay process involves quantum tunneling, the effects of any
Lorentz violations could be exponentially enhanced. | 0812.2236v1 |
2010-09-28 | Absolute continuity under flows generated by SDE with measurable drift coefficient | We consider the It\^{o} SDE with non-degenerate diffusion coefficient and
measurable drift coefficient. Under the condition that the gradient of the
diffusion coefficient and the divergences of the diffusion and drift
coefficients are exponentially integrable with respect to the Gaussian measure,
we show that the stochastic flow leaves the reference measure absolutely
continuous. | 1009.5525v1 |
2016-01-03 | BCJ Relations for One-Loop QCD Integral Coefficients | We present a set of one-loop integral coefficient relations in QCD. The
unitarity method is useful for exposing one-loop amplitudes in terms of tree
amplitudes. The coefficient relations are induced by tree-level BCJ amplitude
relations. We provide examples for box, triangle, and bubble coefficients.
These relations reduce the total number of independent coefficients needed to
calculate one-loop QCD amplitudes. | 1601.00235v1 |
2017-03-10 | On Coefficient problem for bi-univalent analytic functions | Estimates for initial coefficients of Taylor-Maclaurin series of bi-univalent
functions belonging to certain classes defined by subordination are obtained.
Our estimates improve upon the earlier known estimates for second and third
coefficient. The bound for the fourth coefficient is new. In addition, bound
for the fifth coefficient is obtained for bi-starlike and strongly bi-starlike
functions of order $\rho$ and $\beta$ respectively. | 1703.03598v1 |
2023-05-09 | Saturation for Flagged Skew Littlewood-Richardson Coefficients | We define and study a generalization of the Littlewood-Richardson (LR)
coefficients, which we call the flagged skew LR coefficients. These subsume
several previously studied extensions of the LR coefficients. We establish the
saturation property for these coefficients, generalizing work of Knutson-Tao
and Kushwaha-Raghavan-Viswanath. | 2305.05195v2 |
2023-06-11 | Extreme coefficients of multiplicity Tutte polynomials | The multiplicity Tutte polynomial, which includes the arithmetic Tutte
polynomial, is a generalization of the classical Tutte polynomial of matroids.
In this paper, we obtain an expression of the general coefficient and the
expressions of six extreme coefficients of multiplicity Tutte polynomials. In
particular, an expression of the general coefficient and the expressions of
corresponding extreme coefficients of classical Tutte polynomial of matroids
are deduced. | 2306.06568v2 |
2023-09-10 | Testing for Stationary or Persistent Coefficient Randomness in Predictive Regressions | This study considers tests for coefficient randomness in predictive
regressions. Our focus is on how tests for coefficient randomness are
influenced by the persistence of random coefficient. We find that when the
random coefficient is stationary, or I(0), Nyblom's (1989) LM test loses its
optimality (in terms of power), which is established against the alternative of
integrated, or I(1), random coefficient. We demonstrate this by constructing
tests that are more powerful than the LM test when random coefficient is
stationary, although these tests are dominated in terms of power by the LM test
when random coefficient is integrated. This implies that the best test for
coefficient randomness differs from context to context, and practitioners
should take into account the persistence of potentially random coefficient and
choose from several tests accordingly. We apply tests for coefficient constancy
to real data. The results mostly reverse the conclusion of an earlier empirical
study. | 2309.04926v3 |
2004-10-24 | Note on the generalized Hansen and Laplace coefficients | Recently, Breiter et al (2004) reported the computation of Hansen
coefficients $X_k^{\gamma,m}$ for non integer values of $\gamma$. In fact, the
Hansen coefficients are closely related to the Laplace $b_{s}^{(m)}$, and
generalized Laplace coefficients $b_{s,r}^{(m)}$ (Laskar and Robutel, 1995)
that do not require $s,r$ to be integers. In particular, the coefficients
$X_0^{\g,m}$ have very simple expressions in terms of the usual Laplace
coefficients $b_{\g+2}^{(m)}$, and all their properties derive easily from the
known properties of the Laplace coefficients. | 0410557v2 |
2010-10-19 | Coefficient convexity of divisors of x^n-1 | We say a polynomial f having integer coefficients is strongly coefficient
convex if the set of coefficients of f consists of consecutive integers only.
We establish various results suggesting that the divisors of x^n-1 with integer
coefficients have the tendency to be strongly coefficient convex and have small
coefficients. The case where n=p^2*q with p and q primes is studied in detail. | 1010.3938v2 |
2013-10-15 | Measuring correlations between non-stationary series with DCCA coefficient | In this short report, we investigate the ability of the DCCA coefficient to
measure correlation level between non-stationary series. Based on a wide Monte
Carlo simulation study, we show that the DCCA coefficient can estimate the
correlation coefficient accurately regardless the strength of non-stationarity
(measured by the fractional differencing parameter $d$). For a comparison, we
also report the results for the standard Pearson's correlation coefficient. The
DCCA coefficient dominates the Pearson's coefficient for non-stationary series. | 1310.3984v1 |
2019-07-18 | Horn inequalities for nonzero Kronecker coefficients | The Kronecker coefficients and the Littlewood-Richardson coefficients are
nonnegative integers depending on three partitions. By definition, these
coefficients are the multiplicities of the tensor product decomposition of two
irreducible representations of symmetric groups (resp. linear groups). By a
classical Littlewood-Murnaghan's result the Kronecker coefficients extend the
Littlewood-Richardson ones.The nonvanishing of a Littlewood-Richardson
coefficient implies linear inequalities on the triple of partitions, called
Horn inequalities. In thispaper, we extend the essential Horn inequalities to
the triples of partitions corresponding to a nonzero Kronecker coefficient. | 1907.07931v1 |
2022-12-13 | Interaction phenomenon for variable coefficient Kadomtsev-Petviashvili equation by utilizing variable coefficient bilinear neural network method | In this paper, a variable coefficient Bilinear neural network method is
proposed to deal with the analytical solutions of variable coefficient
nonlinear partial differential equations. As an example, a
Kadomstev-Petviashvili equation with variable coefficients is investigated by
using the variable coefficient Bilinear neural network method. By establishing
"3-2-2-1" and "3-3-2-1" models respectively, rich analytical solutions of the
variable coefficient Kadomstev-Petviashvili equation are obtained. By choosing
some special values of the parameters, the dynamics properties are demonstrated
in some three-dimensional and density graphics. | 2301.10069v1 |
2023-08-22 | Sharp bounds on Coefficient functionals of certain Sakaguchi functions | We determine sharp bounds on some Hankel determinants involving initial
coefficients, inverse coefficients, and logarithmic inverse coefficients for
two subclasses of Sakaguchi functions which are associated with the right half
of the lemniscate of Bernoulli and the exponential function. Further, we
compute sharp bounds on the second Hermitian-Toeplitz determinants involving
logarithmic coefficients and logarithmic inverse coefficients. We also discuss
invariant property for the obtained estimates with respect to various
coefficients. | 2308.11352v1 |
2023-12-22 | Asymptotic coefficients of multiple zeta functions at the origin and generalized Gregory coefficients | Due to their singularities, multiple zeta functions behave sensitively at
non-positive integer points. In this article, we focus on the asymptotic
behavior at the origin $(0,\dots, 0)$ and unveil the generating series of the
asymptotic coefficients as a generalization of the classical Gregory
coefficients. This enables us to reveal the underlying symmetry of the
asymptotic coefficients. Additionally, we extend the relationship between the
asymptotic coefficients and the Gregory coefficients to include Hurwitz
multiple zeta functions. | 2312.14475v1 |
1997-01-17 | Evidence for Rotation in the Galaxy at z=3.15 Responsible for a Damped Lyman-alpha Absorption System in the Spectrum of Q2233+1310 | Proof of the existence of a significant population of normal disk galaxies at
redshift z>2 would have profound implications for theories of structure
formation and evolution. We present evidence based on Keck HIRES observations
that the damped Lyman-alpha absorber at z=3.15 toward the quasar Q2233+1310 may
well be such an example. Djorgovski et al have recently detected the
Lyman-alpha emission from the absorber, which we assume is at the systemic
redshift of the absorbing galaxy. By examining the profiles of the metal
absorption lines arising from the absorbing galaxy in relation to its systemic
redshift, we find strong kinematical evidence for rotation. Therefore the
absorber is likely to be a disk galaxy. The inferred circular velocity for the
galaxy is >200 km/s. With a separation of ~17 kpc between the galaxy and the
quasar sightline, the implied dynamic mass for the galaxy is >1.6x10(11) solar
mass. The metallicity of the galaxy is found to be [Fe/H]=-1.4, typical of
damped Lyman-alpha galaxies at such redshifts. However, in another damped
galactic rotation is evident. In the latter case, the damped Lyman-alpha
absorber occurs near the background quasar in redshift so its properties may be
influenced by the background quasar. These represent the only two cases at
present for which the technique used here may be applied. Future applications
of the same technique to a large sample of damped Lyman-alpha galaxies may
allow us to determine if a significant population of disk galaxies already
existed only a few billion years after the Big Bang. | 9701116v2 |
1997-04-11 | The Metallicity of High Redshift Galaxies: The Abundance of Zinc in 34 Damped Lyman Alpha Systems from z = 0.7 to 3.4 | We report new observations of ZnII and CrII absorption lines in 10 damped
\lya systems (DLAs), mostly at redshift $z_{abs} \simgt 2.5$ . By combining
these results with those from our earlier survey (Pettini et al. 1994) and
other recent data, we construct a sample of 34 measurements (or upper limits)
of the Zn abundance relative to hydrogen [Zn/H]; the sample includes more than
one third of the total number of DLAs known.
The plot of the abundance of Zn as a function of redshift reinforces the two
main findings of our previous study. (1) Damped \lya systems are mostly
metal-poor, at all redshifts sampled; the column density weighted mean for the
whole data set is [Zn/H] $= -1.13 \pm 0.38$ (on a logarithmic scale), or
approximately 1/13 of solar. (2) There is a large spread, by up to two orders
of magnitude, in the metallicities we measure at essentially the same
redshifts. We propose that damped \lya systems are drawn from a varied
population of galaxies of different morphological types and at different stages
of chemical evolution, supporting the idea of a protracted epoch of galaxy
formation.
At redshifts $z \simgt 2$ the typical metallicity of the damped \lya systems
is in agreement with expectations based on the consumption of HI gas implied by
the recent measurements of $\Omega_{DLA}$ by Storrie-Lombardi et al. (1996a),
and with the metal ejection rates in the universe at these epochs deduced by
Madau (1996) from the ultraviolet luminosities of high redshift galaxies
revealed by deep imaging surveys. There are indications in our data for an
increase in the mean metallicity of the damped \lya systems from $z > 3$ to
$\approx 2$, consistent with the rise in the comoving star formation rate
indicated by the relative numbers of $U$ and $B$ drop-outs in the Hubble Deep
Field. Although such comparisons are still tentative, it appears that these
different avenues for exploring the early evolution of galaxies give a broadly
consistent picture. | 9704102v1 |
1997-04-17 | On the Kinematics of the Damped Lyman Alpha Protogalaxies | We present the first results of an ongoing program to investigate the
kinematic characteristics of high redshift damped lya systems. Because damped
lya systems are widely believed to be the progenitors of current massive
galaxies, an analysis of their kinematic history allows a direct test of galaxy
formation scenarios.
We have collected a kinematically unbiased sample of 17 high S/N ratio, high
resolution damped lya spectra taken with HIRES on the 10m W.M. Keck Telescope.
Our study focuses on the unsaturated, low-ion transitions of these systems
which reveal their kinematic traits. The profiles exhibit a nearly uniform
distribution of velocity widths ranging from 20 - 200 km/s and a relatively
high degree of asymmetry. In an attempt to explain these characteristics, we
introduce several physical models, which have previously been attributed to
damped lya systems, including rapidly rotating cold disks, slowly rotating hot
disks, massive isothermal halos, and a hydrodynamic spherical accretion model.
Using standard Monte Carlo techniques, we run sightlines through these model
systems to derive simulated low-ion profiles. Comparing statistical measures of
the simulated profiles with the observed profiles, we determine that the
rapidly rotating cold disk model is the only tested model consistent with the
data at high confidence levels. A Relative Likelihood Test of the rapidly
rotating cold disk model indicates the disks must have large rotation speeds; v
> 180 km/s at the 99% c.l. In turn, we demonstrate that the Cold Dark Matter
Model, as developed by Kauffmann (1996), is inconsistent with the damped lya
data at very high c.l. This is because the CDM Model does not predict a large
enough fraction of rapidly rotating disks at z approx 2.5. | 9704169v2 |
2000-11-20 | H-alpha Imaging with HST+NICMOS of An Elusive Damped Ly-alpha Cloud at z=0.6 | Despite previous intensive ground-based imaging and spectroscopic campaigns
and wide-band HST imaging of the z=0.927 QSO 3C336 field, the galaxy that hosts
the damped Ly-alpha system along this line-of-sight has eluded detection. We
present a deep narrow-band H-alpha image of the field of this z=0.656 damped
Ly-alpha absorber, obtained through the F108N filter of NICMOS 1 onboard the
Hubble Space Telescope. The goal of this project was to detect any H-alpha
emission 10 times closer than previous studies to unveil the damped absorber.
We do not detect H-alpha emission between 0.05'' and 6'' (0.24 and 30 $h^{-1}$
kpc) from the QSO, with a 3-sigma flux limit of $3.70 \times 10^{-17} h^{-2}$
erg/s/cm^2 for an unresolved source, corresponding to a star formation rate
(SFR) of $0.3 h^{-2}$ M_sun/yr. This leads to a 3-sigma upper limit of 0.15
M_sun/yr/kpc^2 on the SFR density, or a maximum SFR of 1.87 M_sun/yr assuming a
disk of 4 kpc in diameter. This result adds to the number of low redshift
damped Ly-alpha absorbers that are not associated with the central regions of
Milky-Way-like disks. Damped Ly-alpha absorption can arise from high density
concentrations in a variety of galactic environments including some that,
despite their high local HI densities, are not conducive to widespread star
formation. | 0011374v2 |
2005-08-17 | The SDSS Damped Lya Survey: Data Release 3 | We present the results from a damped Lya survey of the Sloan Digital Sky
Survey, Data Release 3 based on over 500 new damped Lya systems at z>2.2. We
measure the HI column density distribution f(N) and its zeroth and first
moments (the incidence l(X) and gas mass-density O_dla of damped Lya systems,
respectively) as a function of redshift. The key results include: (1) the f(N)
distribution is well fit by a Gamma-function with `break' column density log
N_g=10^21.5 and `faint-end' slope alpha=-1.8; (2) the shape of the f(N)
distributions do not show evolution with redshift; (3) l(X) and O_dla decrease
by 35% and 50% during ~1Gyr between redshift z=[3.,3.5] to z=[2.2,2.5]; and (4)
l(X) and O_dla in the lowest SDSS redshift bin (z=2.2) are consistent with the
current values. We investigate systematic errors in damped Lya analysis and
identify only one important effect: we measure 40 +/- 20% higher O_dla values
toward a subset of brighter quasars than toward a faint subset. This effect
runs contrary to the bias associated with dust obscuration and suggests that
gravitational lensing may be important. Comparing the results against models of
galaxy formation, we find all of the models significantly underpredict l(X) at
z=3 and only SPH models with significant feedback may reproduce O_dla at high
redshift. We argue that the Lyman limit systems contribute ~1/3 of the
universe's HI atoms at all redshifts z=2 to 5 and that the f(N) distribution
for N(HI)<10^20 has an inflection with slope >-1. We advocate a new mass
density definition -- the mass density of predominantly neutral gas O_neut --
to be contrasted with the mass density of gas associated with HI atoms. We
contend the damped Lya systems contribute >80% of O_neut at all redshifts and
therefore are the main reservoirs for star formation. [abridged] | 0508361v1 |
2011-09-07 | Weakly collisional Landau damping and three-dimensional Bernstein-Greene-Kruskal modes: New results on old problems | Landau damping and Bernstein-Greene-Kruskal (BGK) modes are among the most
fundamental concepts in plasma physics. While the former describes the
surprising damping of linear plasma waves in a collisionless plasma, the latter
describes exact undamped nonlinear solutions of the Vlasov equation. There does
exist a relationship between the two: Landau damping can be described as the
phase-mixing of undamped eigenmodes, the so-called Case-Van Kampen modes, which
can be viewed as BGK modes in the linear limit. While these concepts have been
around for a long time, unexpected new results are still being discovered. For
Landau damping, we show that the textbook picture of phase-mixing is altered
profoundly in the presence of collision. In particular, the continuous spectrum
of Case-Van Kampen modes is eliminated and replaced by a discrete spectrum,
even in the limit of zero collision. Furthermore, we show that these discrete
eigenmodes form a complete set of solutions. Landau-damped solutions are then
recovered as true eigenmodes (which they are not in the collisionless theory).
For BGK modes, our interest is motivated by recent discoveries of electrostatic
solitary waves in magnetospheric plasmas. While one-dimensional BGK theory is
quite mature, there appear to be no exact three-dimensional solutions in the
literature (except for the limiting case when the magnetic field is
sufficiently strong so that one can apply the guiding-center approximation). We
show, in fact, that two- and three-dimensional solutions that depend only on
energy do not exist. However, if solutions depend on both energy and angular
momentum, we can construct exact three-dimensional solutions for the
unmagnetized case, and two-dimensional solutions for the case with a finite
magnetic field. The latter are shown to be exact, fully electromagnetic
solutions of the steady-state Vlasov-Poisson-Amp\`ere system. | 1109.1353v1 |
2013-05-16 | Application of vibration-transit theory to distinct dynamic response for a monatomic liquid | We examine the distinct part of the density autocorrelation function Fd(q,t),
also called the intermediate scattering function, from the point of view of the
vibration-transit (V-T) theory of monatomic liquid dynamics. A similar study
has been reported for the self part, and we study the self and distinct parts
separately because their damping processes are not simply related. We begin
with the perfect vibrational system, which provides precise definitions of the
liquid correlations, and provides the vibrational approximation Fdvib(q,t) at
all q and t. Two independent liquid correlations are defined, motional and
structural, and these are decorrelated sequentially, with a crossover time
tc(q). This is done by two independent decorrelation processes: the first,
vibrational dephasing, is naturally present in Fdvib(q,t) and operates to damp
the motional correlation; the second, transit-induced decorrelation, is invoked
to enhance the damping of motional correlation, and then to damp the structural
correlation. A microscopic model is made for the "transit drift", the averaged
transit motion that damps motional correlation on 0 < t < tc(q). Following the
previously developed self-decorrelation theory, a microscopic model is also
made for the "transit random walk," which damps the structural correlation on t
> tc(q). The complete model incorporates a property common to both self and
distinct decorrelation: simple exponential decay following a delay period,
where the delay is tc(q, the time required for the random walk to emerge from
the drift. Our final result is an accurate expression for Fd(q,t) for all q
through the first peak in Sd(q). The theory is calibrated and tested using
molecular dynamics (MD) calculations for liquid Na at 395K; however, the theory
itself does not depend on MD, and we consider other means for calibrating it. | 1305.3954v2 |
2013-09-16 | Two-atom system as a nano-antenna for mode switching and light routing | We determine how a system composed of two nonidentical two-level atoms with
different resonance frequencies and different damping rates could work as a
nano-antenna for controlled mode switching and light routing. We calculate the
angular distribution of the emitted field detected in a far-field zone of the
system including the direct interatomic interactions and arbitrary linear
dimensions of the system. The calculation is carried out in terms of the
symmetric and antisymmetric modes of the two atom system. We find that as long
as the atoms are identical, the emission cannot be switched between the
symmetric and antisymmetric modes. The switching may occur when the atoms are
non-identical and the emission can then be routed to different modes by
changing the relative ratio of the atomic frequencies, or damping rates or by a
proper tuning of the laser frequency to the atomic resonance frequencies. It is
shown that in the case of atoms of different resonance frequencies but equal
damping rates, the light routing is independent of the frequency of the driving
laser field. It depends only on the sign of the detuning between the atomic
resonance frequencies. In the case of atoms of different damping rates, the
emission can be switched between different modes by changing the laser
frequency from the blue to red detuned from the atomic resonance. The effect of
the interatomic interactions is also considered and it is found that in the
case of unequal resonance frequencies of the atoms, the interactions slightly
modify the visibility of the intensity pattern. The case of unequal damping
rates of the atoms is affected rather more drastically, the light routing
becoming asymmetric under the dipole-dipole interaction with the enhanced
intensities of the modes turned towards the atom of smaller damping rate. | 1309.3924v1 |
2015-04-01 | Landau damping of Gardner solitons in a dusty bi-ion plasma | The effects of linear Landau damping on the nonlinear propagation of
dust-acoustic solitary waves (DASWs) are studied in a collisionless
unmagnetized dusty plasma with two species of positive ions. The extremely
massive, micron-seized, cold and negatively charged dust particles are
described by fluid equations, whereas the two species of positive ions, namely
the cold (heavy) and hot (light) ions are described by the kinetic Vlasov
equations. Following Ott and Sudan [Phys. Fluids {\bf 12}, 2388 (1969)], and by
considering lower and higher-order perturbations, the evolution of DASWs with
Landau damping is shown to be governed by Korteweg-de Vries (KdV), modified KdV
(mKdV) or Gardner (KdV-mKdV)-like equations. The properties of the phase
velocity and the Landau damping rate of DASWs are studied for different values
of the ratios of the temperatures $(\sigma)$ and the number densities $(\mu)$
of hot and cold ions as well the cold to hot ion mass ratio $m$. The
distinctive features of the decay rates of the amplitudes of the KdV, mKdV and
Gardner solitons with a small effect of Landau damping are also studied in
different parameter regimes. It is found that the Gardner soliton points to
lower wave amplitudes than the KdV and mKdV solitons. The results may be useful
for understanding the localization of solitary pulses and associated wave
damping (collisionless) in laboratory and space plasmas (e.g., the F-ring of
Saturn) in which the number density of free electrons is much smaller than that
of ions and the heavy, micron seized dust grains are highly charged. | 1504.00089v2 |
2017-07-18 | Explanations of the DAMPE high energy electron/positron spectrum in the dark matter annihilation and pulsar scenarios | Many studies have shown that either the nearby astrophysical source or dark
matter (DM) annihilation/decay is required to explain the origin of high energy
cosmic ray (CR) $e^\pm$, which are measured by many experiments, such as PAMELA
and AMS-02. Recently, the Dark Matter Particle Explorer (DAMPE) collaboration
has reported its first result of the total CR $e^\pm$ spectrum from $25
\,\mathrm{GeV}$ to $4.6 \,\mathrm{TeV}$ with high precision. In this work, we
study the DM annihilation and pulsar interpretations of the DAMPE high energy
$e^\pm$ spectrum. In the DM scenario, the leptonic annihilation channels to
$\tau^+\tau^-$, $4\mu$, $4\tau$, and mixed charged lepton final states can well
fit the DAMPE result, while the $\mu^+\mu^-$ channel has been excluded. In
addition, we find that the mixed charged leptons channel would lead to a sharp
drop at $\sim$ $\mathrm{TeV}$. However, these DM explanations are almost
excluded by the observations of gamma-ray and CMB, unless some complicated DM
models are introduced. In the pulsar scenario, we analyze 21 nearby known
pulsars and assume that one of them is the primary source of high energy CR
$e^\pm$.Considering the constraint from the Fermi-LAT observation of the
$e^\pm$ anisotropy, we find that two pulsars are possible to explain the DAMPE
data. Our results show that it is difficult to distinguish between the DM
annihilation and single pulsar explanations of high energy $e^\pm$ with the
current DAMPE result. | 1707.05664v2 |
2018-06-27 | In-flight performance of the DAMPE silicon tracker | DAMPE (DArk Matter Particle Explorer) is a spaceborne high-energy cosmic ray
and gamma-ray detector, successfully launched in December 2015. It is designed
to probe astroparticle physics in the broad energy range from few GeV to 100
TeV. The scientific goals of DAMPE include the identification of possible
signatures of Dark Matter annihilation or decay, the study of the origin and
propagation mechanisms of cosmic-ray particles, and gamma-ray astronomy. DAMPE
consists of four sub-detectors: a plastic scintillator strip detector, a
Silicon-Tungsten tracKer-converter (STK), a BGO calorimeter and a neutron
detector. The STK is composed of six double layers of single-sided silicon
micro-strip detectors interleaved with three layers of tungsten for photon
conversions into electron-positron pairs. The STK is a crucial component of
DAMPE, allowing to determine the direction of incoming photons, to reconstruct
tracks of cosmic rays and to estimate their absolute charge (Z). We present the
in-flight performance of the STK based on two years of in-flight DAMPE data,
which includes the noise behavior, signal response, thermal and mechanical
stability, alignment and position resolution. | 1806.10355v1 |
2019-03-28 | Improving convergence of volume penalised fluid-solid interactions | We analyse and improve the volume-penalty method, a simple and versatile way
to model objects in fluid flows. The volume-penalty method is a kind of
fictitious-domain method that approximates no-slip boundary conditions with
rapid linear damping inside the object. The method can then simulate complex,
moving objects in general numerical solvers without specialised algorithms or
boundary-conforming grids. Volume penalisation pays for this simplicity by
introducing an equation-level error, the $\textit{model error}$, that is
related to the damping time $\eta \ll 1$. While the model error has been proven
to vanish as the damping time tends to zero, previous work suggests convergence
at a slow rate of $\mathcal{O}(\eta^{1/2})$. The stiffness of the damping
implies conventional volume penalisation only achieves first order numerical
accuracy. We analyse the volume-penalty method using multiple-scales
matched-asymptotics with a signed-distance coordinate system valid for
arbitrary smooth geometries. We show the dominant model error stems from a
displacement length that is proportional to a Reynolds number $\text{Re}$
dependent boundary layer of size $\mathcal{O}(\eta^{1/2}\text{Re}^{-1/2})$. The
relative size of the displacement length and damping time leads to multiple
error regimes. Our key finding derives a simple smoothing prescription for the
damping that eliminates the displacement length and reduces the model error to
$\mathcal{O}(\eta)$ in all regimes. This translates to second order numerical
accuracy. We validate our findings in several comprehensive benchmark problems
and finally combine Richardson extrapolation of the model error with our
correction to further improve convergence to $\mathcal{O}(\eta^{2})$. | 1903.11914v4 |
2019-06-12 | Study of Alfven Eigenmodes stability in plasma with multiple NBI driven energetic particle specie | The aim of this study is to analyze the destabilization of Alfven Eigenmodes
(AE) by multiple energetic particles (EP) species in DIII-D and LHD discharges.
We use the reduced MHD equations to describe the linear evolution of the
poloidal flux and the toroidal component of the vorticity in a full 3D system,
coupled with equations of density and parallel velocity moments for the
energetic particles species, including the effect of the acoustic modes,
diamagnetic currents and helical couplings. We add the Landau damping and
resonant destabilization effects using a closure relation. The simulations with
multiple NBI lines show three different regimes: the non damped regime where
the multi beam AEs growth rate is larger compared to the growth rate of the AEs
destabilized by the individual NBI lines, the interaction regime where the
multi beam AEs growth rate is smaller than the single NBI AEs and the damped
regime where the AEs are suppressed. Operations in the damped regime requires
EP species with different density profile flatness or gradient locations. In
addition, the AEs growth rate in the interaction regime is further reduced if
the combined NBI lines have similar beam temperatures and the beta of the NBI
line with flatter EP density profile increases. Then, optimization trends are
identified in DIII-D high poloidal beta and LHD low density / magnetic field
discharges with multiple NBI lines as well as the configuration requirements to
operate in the damped and interaction regimes. DIII-D simulations show a
decrease of the n=2 to 6 AEs growth rate and n=1 AE are stabilized in the LHD
case. The helical coupling effects in LHD simulations lead to a transition from
the interaction to the damped regime of the n=2,-8,12 helical family. | 1906.05701v1 |
2019-11-05 | Observation of Nanoscale Opto-Mechanical Molecular Damping; Origin of Spectroscopic Contrast in Photo Induced Force Microscopy | We experimentally investigated the contrast mechanism of infrared
photoinduced force microscopy (PiFM) for recording vibrational resonances.
Extensive experiments have demonstrated that spectroscopic contrast in PiFM is
mediated by opto-mechanical damping of the cantilever oscillation as the
optical wavelength is scanned through optical resonance. To our knowledge, this
is the first time opto-mechanical damping has been observed in the AFM. We
hypothesize that this damping force is a consequence of the dissipative
interaction between the sample and the vibrating tip; the modulated light
source in PiFM modulates the effective damping constant of the 2nd eigenmode of
the cantilever which in turn generate side-band signals producing the PiFM
signal at the 1st eigenmode. A series of experiments have eliminated other
mechanisms of contrast. By tracking the frequency shift of the PiFM signal at
the 1st cantilever eigenmode as the excitation wavenumber is tuned through a
mid-infrared absorption band, we showed that the near-field optical interaction
is attractive. By using a vibrating piezoelectric crystal to mimic sample
thermal expansion in a PiFM operating in mixing mode, we determined that the
minimum thermal expansion our system can detect is 30 pm limited by system
noise. We have confirmed that van der Waal mediated thermal-expansion forces
have negligible effect on PiFM signals by detecting the resonant response of a
4-methylbenzenethiol mono molecular layer deposited on template-stripped gold,
where thermal expansion was expected to be < 3 pm, i.e., 10 times lower than
our system noise level. Finally, the basic theory for dissipative tip-sample
interactions was introduced to model the photoinduced opto-mechanical damping.
Theoretical simulations are in excellent agreement with experiment. | 1911.05190v1 |
2020-06-08 | Stochastic re-acceleration and magnetic-field damping in Tycho's supernova remnant | A number of studies suggest that shock acceleration with particle feedback
and very efficient magnetic-field amplification combined with Alfv\'{e}nic
drift are needed to explain the rather soft radio spectrum and the narrow rims
observed for Tycho's SNR. We show that the broadband spectrum of Tycho's SNR
can alternatively be well explained when accounting for stochastic acceleration
as a secondary process. The re-acceleration of particles in the turbulent
region immediately downstream of the shock should be efficient enough to impact
particle spectra over several decades in energy. The so-called Alfv\'{e}nic
drift and particle feedback on the shock structure are not required in this
scenario. Additionally, we investigate whether synchrotron losses or
magnetic-field damping play a more profound role in the formation of the
non-thermal filaments. We solve the full particle transport equation in
test-particle mode using hydrodynamic simulations of the SNR plasma flow. The
background magnetic field is either computed from the induction equation or
follows analytic profiles, depending on the model considered. Fast-mode waves
in the downstream region provide the diffusion of particles in momentum space.
We show that the broadband spectrum of Tycho can be well explained if
magnetic-field damping and stochastic re-acceleration of particles are taken
into account. Although not as efficient as standard DSA, stochastic
acceleration leaves its imprint on the particle spectra, which is especially
notable in the emission at radio wavelengths. We find a lower limit for the
post-shock magnetic-field strength $\sim330\,\mathrm{\mu G}$, implying
efficient amplification even for the magnetic-field damping scenario. For the
formation of the filaments in the radio range magnetic-field damping is
necessary, while the X-ray filaments are shaped by both the synchrotron losses
and magnetic-field damping. | 2006.04832v1 |
2021-02-23 | Influence of Ion-Neutral Damping on the Cosmic-Ray Streaming Instability: Magnetohydrodynamic Particle-in-cell Simulations | We explore the physics of the gyro-resonant cosmic ray streaming instability
(CRSI) including the effects of ion-neutral (IN) damping. This is the main
damping mechanism in (partially-ionized) atomic and molecular gas, which are
the primary components of the interstellar medium (ISM) by mass. Limitation of
CRSI by IN damping is important in setting the amplitude of Alfv\'en waves that
scatter cosmic rays and control galactic-scale transport. Our study employs the
MHD-PIC hybrid fluid-kinetic numerical technique to follow linear growth as
well as post-linear and saturation phases. During the linear phase of the
instability -- where simulations and analytical theory are in good agreement --
IN damping prevents wave growth at small and large wavelengths, with the
unstable bandwidth lower for higher ion-neutral collision rate $\nu_{\rm in}$.
Purely MHD effects during the post-linear phase extend the wave spectrum
towards larger $k$. In the saturated state, the cosmic ray distribution evolves
toward greater isotropy (lower streaming velocity) by scattering off of Alv\'en
waves excited by the instability. In the absence of low-$k$ waves, CRs with
sufficiently high momentum are not isotropized. The maximum wave amplitude and
rate of isotropization of the distribution function decreases at higher
$\nu_{\rm in}$. When the IN damping rate approaches the maximum growth rate of
CSRI, wave growth and isotropization is suppressed. Implications of our results
for CR transport in partially ionized ISM phases are discussed. | 2102.11878v3 |
2022-06-17 | Quantum Dynamics of Magnetic Skyrmions: Consistent Path Integral Formulation | We present a path integral formalism for the intrinsic quantum dynamics of
magnetic skyrmions coupled to a thermal background of magnetic fluctuations.
Upon promoting the skyrmion's collective coordinate $\boldsymbol{R}$ to a
dynamic variable and integrating out the magnonic heat bath, we derive the
generalized equation of motion for $\boldsymbol{R}$ with a non-local damping
term that describes a steady-state skyrmion dynamics at finite temperatures.
Being essentially temperature dependent, the intrinsic damping is shown to
originate from the coupling of thermally activated magnon modes to the
adiabatic potential driven by a rigid skyrmion motion, which can be regarded as
another manifestation of emergent electrodynamics inherent to topological
magnetic textures. We further argue that the diagonal components of the damping
term act as the source of dissipation and inertia, while its off-diagonal
components modify the gyrotropic motion of a magnetic skyrmion. By means of
numerical calculations for the lattice spin model of chiral ferromagnets, we
study the temperature behavior of the intrinsic damping as a function of
magnetic field in periodic and confined geometries. The intrinsic damping is
demonstrated to be highly non-local, revealing its quantum-mechanical nature,
that becomes more pronounced with increasing temperature. At high temperatures
when the magnon occupation factors are large, the intrinsic damping is shown to
yield a modified Thiele's equation with the additional non-local dissipative
and mass terms that exhibit an almost linear temperature behavior. Our results
provide a microscopic background for semiclassical magnetization dynamics and
establish a framework for understanding spin caloritronics effects in
topological magnetic textures. | 2206.08532v2 |
2024-02-05 | Revisiting the role of cosmic-ray driven Alfvén waves in pre-existing magnetohydrodynamic turbulence. I. Turbulent damping rates and feedback on background fluctuations | Alfv\'en waves (AWs) excited by the cosmic-ray (CR) streaming instability
(CRSI) are a fundamental ingredient for CR confinement. The effectiveness of
self-confinement relies on a balance between CRSI growth rate and damping
mechanisms acting on quasi-parallel AWs excited by CRs. One relevant mechanism
is the so-called turbulent damping, in which an AW packet injected in
pre-existing turbulence undergoes a cascade process due to its nonlinear
interaction with fluctuations of the background. The turbulent damping of an AW
packet in pre-existing magnetohydrodynamic turbulence is re-examined, revised,
and extended to include most-recent theories of MHD turbulence that account for
dynamic alignment and reconnection-mediated regime. The case in which the role
of feedback of CR-driven AWs on pre-existing turbulence is important will also
be discussed. Particular attention is given to the nonlinearity parameter
$\chi^w$ that estimates the strength of nonlinear interaction between CR-driven
AWs and background fluctuations. We point out the difference between $\chi^w$
and $\chi^z$ that instead describes the strength of nonlinear interactions
between pre-existing fluctuations. When $\chi^w$ is properly taken into
account, one finds that (i) the turbulent damping rate of quasi-parallel AWs in
anisotropic turbulence depends on the background-fluctuations' amplitude to the
third power, hence is strongly suppressed, and (ii) the dependence on the AW's
wavelength (and thus on the CR gyro-radius from which it is excited) is
different from what has been previously obtained. Finally, (iii) when dynamic
alignment of cascading fluctuations and the possibility of a
reconnection-mediated range is included in the picture, the turbulent damping
rate exhibits novel regimes and breaks. Finally, a criterion for CR-feedback is
derived and simple phenomenological models of CR-modified turbulent scaling are
provided. | 2402.02901v1 |
2006-10-24 | Logical contradictions of Landau damping | Landau damping/growing at boundary condition of excitation of a harmonic wave
in collisionless ion-electron-neutrals plasma contradicts to the law of energy
conservation of a wave damping/growing in space. There is also no criterion of
a choice either damping or growing solution in difference from always
non-damping in the direction of propagation Vlasov waves. Variety of other
incongruities as consequence of Landau damping is specified also. Absence of
explicit positivity and finiteness of wave solutions for electron distribution
function near singularity point leads to need of imposing additional cutting
off constraints with resulting positivity and finiteness of the electron
distribution function at the singularity points and finiteness of the complex
dispersion integral. Landau damping as a real physical phenomenon of
collisionless damping does not exist. A relation is established for the real
dispersion equation with real waves (see Appendices 2,4) between the averaged
over period wave damping decrement and the collisional energy-exchange term of
kinetic equation. Collisionless Vlasov-Landau damping is explained finally by
the usual wrong use of nonlinearly complex wave functions leading to complex
dispersion equation. All used solution of the complex dispersion equation for
the simultaneously existing collisionless both exponentially damping and
growing nonlinear complex waves is entirely, quantitatively and in its logical
sense, different from the solution of initially real dispersion equation for
real either damping or growing waves and should be discarded (see Appendices
2,4,5,6). Collisionless damping is caused by unreasonable use of wave functions
with complex frequency or complex wave number leading to complex dispersion
relation with unphysical binomial virtual complex roots. Thus finding roots of
the complex dispersion equation has only abstract mathematical interest. | 0610220v67 |
1999-12-17 | Infrared Spectroscopy of a Massive Obscured Star Cluster in the Antennae Galaxies (NGC 4038/4039) with NIRSPEC | We present infrared spectroscopy of the Antennae Galaxies (NGC 4038/4039)
with NIRSPEC at the W. M. Keck Observatory. We imaged the star clusters in the
vicinity of the southern nucleus (NGC 4039) in 0.39" seeing in K-band using
NIRSPEC's slit-viewing camera. The brightest star cluster revealed in the
near-IR (M_K(0) = -17.9) is insignificant optically, but coincident with the
highest surface brightness peak in the mid-IR (12-18 micron) ISO image
presented by Mirabel et al. (1998). We obtained high signal-to-noise 2.03 -
2.45 micron spectra of the nucleus and the obscured star cluster at R ~ 1900.
The cluster is very young (4 Myr old), massive (16e6 M_sun), and compact
(density ~ 115 M_sun pc^(-3) within a 32 pc half-light radius), assuming a
Salpeter IMF (0.1 - 100 M_sun). Its hot stars have a radiation field
characterized by T_eff ~ 39,000 K, and they ionize a compact H II region with
n_e ~ 1e4 cm^(-3). The stars are deeply embedded in gas and dust (A_V ~ 9-10
mag), and their strong FUV field powers a clumpy photodissociation region with
densities n_H >= 1e5 cm^(-3) on scales of up to 200 pc, radiating L[H_2 1-0
S(1)] = 9600 L_sun. | 9912369v1 |
2000-09-07 | Practical Quantum Cryptography: A Comprehensive Analysis (Part One) | We perform a comprehensive analysis of practical quantum cryptography (QC)
systems implemented in actual physical environments via either free-space or
fiber-optic cable quantum channels for ground-ground, ground-satellite,
air-satellite and satellite-satellite links. (1) We obtain universal
expressions for the effective secrecy capacity and rate for QC systems taking
into account three important attacks on individual quantum bits, including
explicit closed-form expressions for the requisite amount of privacy
amplification. Our analysis also includes the explicit calculation in detail of
the total cost in bits of continuous authentication, thereby obtaining new
results for actual ciphers of finite length. (2) We perform for the first time
a detailed, explicit analysis of all systems losses due to propagation, errors,
noise, etc. as appropriate to both optical fiber cable- and satellite
communications-based implementations of QC. (3) We calculate for the first time
all system load costs associated to classical communication and computational
constraints that are ancillary to, but essential for carrying out, the pure QC
protocol itself. (4) We introduce an extended family of generalizations of the
Bennett-Brassard (BB84) QC protocol that equally provide unconditional secrecy
but allow for the possibility of optimizing throughput rates against specific
cryptanalytic attacks. (5) We obtain universal predictions for maximal rates
that can be achieved with practical system designs under realistic
environmental conditions. (6) We propose a specific QC system design that
includes the use of a novel method of high-speed photon detection that may be
able to achieve very high throughput rates for actual implementations in
realistic environments. | 0009027v5 |
2009-08-07 | The Dominance of Metal-Rich Streams in Stellar Halos: A Comparison Between Substructure in M31 and Lambda-CDM Models | Extensive photometric and spectroscopic surveys of the Andromeda galaxy (M31)
have discovered tidal debris features throughout M31's stellar halo. We present
stellar kinematics and metallicities in fields with identified substructure
from our on-going SPLASH survey of M31 red giant branch stars with the DEIMOS
spectrograph on the Keck II 10-m telescope. Radial velocity criteria are used
to isolate members of the kinematically-cold substructures. The substructures
are shown to be metal-rich relative to the rest of the dynamically hot stellar
population in the fields in which they are found. We calculate the mean
metallicity and average surface brightness of the various kinematical
components in each field, and show that, on average, higher surface brightness
features tend to be more metal-rich than lower surface brightness features.
Simulations of stellar halo formation via accretion in a cosmological context
are used to illustrate that the observed trend can be explained as a natural
consequence of the observed dwarf galaxy mass-metallicity relation. A
significant spread in metallicity at a given surface brightness is seen in the
data; we show that this is due to time effects, namely the variation in the
time since accretion of the tidal streams' progenitor onto the host halo. We
show that in this theoretical framework a relationship between the
alpha-enhancement and surface brightness of tidal streams is expected, which
arises from the varying times of accretion of the progenitor satellites onto
the host halo. Thus, measurements of the alpha-enrichment, metallicity, and
surface brightness of tidal debris can be used to reconstruct the luminosity
and time of accretion onto the host halo of the progenitors of tidal streams. | 0908.1111v1 |
2009-09-25 | The SPLASH Survey: A Spectroscopic Portrait of Andromeda's Giant Southern Stream | The giant southern stream (GSS) is the most prominent tidal debris feature in
M31's stellar halo. The GSS is composed of a relatively metal-rich, high
surface-brightness "core" and a lower metallicity, lower surface brightness
"envelope." We present Keck/DEIMOS spectroscopy of red giant stars in six
fields in the vicinity of M31's GSS and one field on Stream C, an arc-like
feature on M31's SE minor axis at R=60 kpc. Several GSS-related findings and
measurements are presented here. We present the innermost kinematical detection
of the GSS core to date (R=17 kpc). This field also contains the continuation
of a second kinematically cold component originally seen in a GSS core field at
R=21 kpc. The velocity gradients of the GSS and the second component in the
combined data set are parallel over a radial range of 7 kpc, suggesting a
possible bifurcation in the line-of-sight velocities of GSS stars. We also
present the first kinematical detection of substructure in the GSS envelope.
Using kinematically identified samples, we show that the envelope debris has a
~0.7 dex lower mean photometric metallicity and possibly higher intrinsic
velocity dispersion than the GSS core. The GSS is also identified in the field
of the M31 dSph satellite And I; the GSS in this field has a metallicity
distribution identical to that of the GSS core. We confirm the presence of two
kinematically cold components in Stream C, and measure intrinsic velocity
dispersions of ~10 and ~4 km/s. This compilation of the kinematical (mean
velocity, intrinsic velocity dispersion) and chemical properties of stars in
the GSS core and envelope, coupled with published surface brightness
measurements and wide-area star-count maps, will improve constraints on the
orbit and internal structure of the dwarf satellite progenitor. | 0909.4540v1 |
2011-05-28 | Transmission Control of Two-User Slotted ALOHA Over Gilbert-Elliott Channel: Stability and Delay Analysis | In this paper, we consider the problem of calculating the stability region
and average delay of two user slotted ALOHA over a Gilbert-Elliott channel,
where users have channel state information and adapt their transmission
probabilities according to the channel state. Each channel has two states,
namely, the 'good' and 'bad' states. In the 'bad' state, the channel is assumed
to be in deep fade and the transmission fails with probability one, while in
the 'good' state, there is some positive success probability. We calculate the
Stability region with and without Multipacket Reception capability as well as
the average delay without MPR. Our results show that the stability region of
the controlled S-ALOHA is always a superset of the stability region of
uncontrolled S-ALOHA. Moreover, if the channel tends to be in the 'bad' state
for long proportion of time, then the stability region is a convex Polyhedron
strictly containing the TDMA stability region and the optimal transmission
strategy is to transmit with probability one whenever the nodes have packets
and it is shown that this strategy is delay optimal. On the other hand, if the
channel tends to be in the 'good' state more often, then the stability region
is bounded by a convex curve and is strict subset of the TDMA stability region.
We also show that enhancing the physical layer by allowing MPR capability can
significantly enhance the performance while simplifying the MAC Layer design by
the lack of the need of scheduling under some conditions. Furthermore, it is
shown that transmission control not only allows handling higher stable arrival
rates but also leads to lower delay for the same arrival rate compared with
ordinary S-ALOHA. | 1105.5676v2 |
2012-10-11 | Global Properties of M31's Stellar Halo from the SPLASH Survey. I. Surface Brightness Profile | We present the surface brightness profile of M31's stellar halo out to a
projected radius of 175 kpc. The surface brightness estimates are based on
confirmed samples of M31 red giant branch stars derived from Keck/DEIMOS
spectroscopic observations. A set of empirical spectroscopic and photometric
M31 membership diagnostics is used to identify and reject foreground and
background contaminants. This enables us to trace the stellar halo of M31 to
larger projected distances and fainter surface brightnesses than previous
photometric studies. The surface brightness profile of M31's halo follows a
power-law with index -2.2 +/- 0.2 and extends to a projected distance of at
least ~175 kpc (~ 2/3 of M31's virial radius), with no evidence of a downward
break at large radii. The best-fit elliptical isophotes have b/a=0.94 with the
major axis of the halo aligned along the minor axis of M31's disk, consistent
with a prolate halo, although the data are also consistent with M31's halo
having spherical symmetry. The fact that tidal debris features are
kinematically cold is used to identify substructure in the spectroscopic fields
out to projected radii of 90 kpc, and investigate the effect of this
substructure on the surface brightness profile. The scatter in the surface
brightness profile is reduced when kinematically identified tidal debris
features in M31 are statistically subtracted; the remaining profile indicates a
comparatively diffuse stellar component to M31's stellar halo exists to large
distances. Beyond 90 kpc, kinematically cold tidal debris features can not be
identified due to small number statistics; nevertheless, the significant
field-to-field variation in surface brightness beyond 90 kpc suggests that the
outermost region of M31's halo is also comprised to a significant degree of
stars stripped from accreted objects. | 1210.3362v2 |
2014-09-12 | Global Properties of M31's Stellar Halo from the SPLASH Survey: II. Metallicity Profile | We present the metallicity distribution of red giant branch (RGB) stars in
M31's stellar halo, derived from photometric metallicity estimates for over
1500 spectroscopically confirmed RGB halo stars. The stellar sample comes from
38 halo fields observed with the Keck/DEIMOS spectrograph, ranging from 9 to
175 kpc in projected distance from M31's center, and includes 52 confirmed M31
halo stars beyond 100 kpc. While a wide range of metallicities is seen
throughout the halo, the metal-rich peak of the metallicity distribution
function becomes significantly less prominent with increasing radius. The
metallicity profile of M31's stellar halo shows a continuous gradient from 9 to
~100 kpc, with a magnitude of -0.01 dex/kpc. The stellar velocity distributions
in each field are used to identify stars that are likely associated with tidal
debris features. The removal of tidal debris features does not significantly
alter the metallicity gradient in M31's halo: a gradient is maintained in
fields spanning 10 to 90 kpc. We analyze the halo metallicity profile, as well
as the relative metallicities of stars associated with tidal debris features
and the underlying halo population, in the context of current simulations of
stellar halo formation. We argue that the large scale gradient in M31's halo
implies M31 accreted at least one relatively massive progenitor in the past,
while the field to field variation seen in the metallicity profile indicates
that multiple smaller progenitors are likely to have contributed substantially
to M31's outer halo. | 1409.3843v1 |
2016-07-15 | Solving the stochastic Landau-Lifshitz-Gilbert-Slonczewski equation for monodomain nanomagnets : A survey and analysis of numerical techniques | The stochastic Landau-Lifshitz-Gilbert-Slonczewski (s-LLGS) equation is
widely used to study the temporal evolution of the macrospin subject to spin
torque and thermal noise. The numerical simulation of the s-LLGS equation
requires an appropriate choice of stochastic calculus and numerical integration
scheme. In this paper, we comprehensively evaluate the accuracy and complexity
of various numerical techniques to solve the s-LLGS equation. We focus on
implicit midpoint, Heun, and Euler-Heun methods that converge to the
Stratonovich solution of the s-LLGS equation. By performing numerical tests for
both strong (path-wise) and weak (statistical) convergence, we quantify the
accuracy of various numerical schemes used to solve the s-LLGS equation. We
demonstrate a new method intended to solve Stochastic Differential Equations
(SDEs) with small noise (RK4-Heun), and test its capability to handle the
s-LLGS equation. We also discuss the circuit implementation of nanomagnets for
large-scale SPICE-based simulations. We evaluate the efficacy of SPICE in
handling the stochastic dynamics of the multiplicative noise in the s-LLGS
equation. Numerical schemes such as Euler and Gear, typically used by
SPICE-based circuit simulators do not yield the expected outcome when solving
the Stratonovich s-LLGS equation. While the trapezoidal method in SPICE does
solve for the Stratonovich solution, its accuracy is limited by the minimum
time step of integration in SPICE. We implement the s-LLGS equation in both its
cartesian and spherical coordinates form in SPICE and compare the stability and
accuracy of the two implementations. The results in this paper will serve as
guidelines for researchers to understand the tradeoffs between accuracy and
complexity of various numerical methods and the choice of appropriate calculus
to solve the s-LLGS equation. | 1607.04596v4 |
2016-11-30 | Low Energy Supergravity Revisited (I) | General forms of the K\"ahler and superpotenials that lead to consistent low
energy broken Supersymmetry originating from $N=1$ Supergravity have been
classified and used for model building since more than three decades. We point
out the incompleteness of this classification when hidden sector vacuum
expectation values are of the order of the Planck mass. Focusing in this paper
mainly on the case of minimal K\"ahler potential, we adopt a rigorous approach
that retrieves on the one hand the known forms, and demonstrate on the other
hand the existence of a whole set of new forms for the superpotential of which
we give a complete classification. The latter forms involve a new type of
chiral superfields having the unusual property of belonging neither to the
hidden sector nor to the conventional observable sector. Comparing the obtained
forms with the conventional ones, we argue how new possibilities for model
building can arise, and discuss the gravity mediation of soft as well as
additional hard (but parametrically small) Supersymmetry breaking, in the
presence of the new type of chiral superfields. In the simplest case, we study
the vacuum structure, characterize the masses and couplings of the scalar
components to the hidden and observable sectors and discuss briefly the
physical role they could play. In the generic case, we estimate the magnitude
and possible consequences of the hard breaking of Supersymmetry in terms of the
interplay between hidden and visible sectors mass scales. | 1611.10327v2 |
2018-09-24 | Ionic Tuning of Cobaltites at the Nanoscale | Control of materials through custom design of ionic distributions represents
a powerful new approach to develop future technologies ranging from spintronic
logic and memory devices to energy storage. Perovskites have shown particular
promise for ionic devices due to their high ion mobility and sensitivity to
chemical stoichiometry. In this work, we demonstrate a solid-state approach to
control of ionic distributions in (La,Sr)CoO$_{3}$ thin films. Depositing a Gd
capping layer on the perovskite film, oxygen is controllably extracted from the
structure, up-to 0.5 O/u.c. throughout the entire 36 nm thickness. Commensurate
with the oxygen extraction, the Co valence state and saturation magnetization
show a smooth continuous variation. In contrast, magnetoresistance measurements
show no-change in the magnetic anisotropy and a rapid increase in the
resistivity over the same range of oxygen stoichiometry. These results suggest
significant phase separation, with metallic ferromagnetic regions and
oxygen-deficient, insulating, non-ferromagnetic regions, forming percolated
networks. Indeed, X-ray diffraction identifies oxygen-vacancy ordering,
including transformation to a brownmillerite crystal structure. The unexpected
transformation to the brownmillerite phase at ambient temperature is further
confirmed by high-resolution scanning transmission electron microscopy which
shows significant structural - and correspondingly chemical - phase separation.
This work demonstrates room-temperature ionic control of magnetism, electrical
resistivity, and crystalline structure in a 36 nm thick film, presenting new
opportunities for ionic devices that leverage multiple material
functionalities. | 1809.08728v1 |
2019-04-10 | The Convergence of Iterative Delegations in Liquid Democracy in a Social Network | Liquid democracy is a collective decision making paradigm which lies between
direct and representative democracy. One of its main features is that voters
can delegate their votes in a transitive manner such that: A delegates to B and
B delegates to C leads to A indirectly delegates to C. These delegations can be
effectively empowered by implementing liquid democracy in a social network, so
that voters can delegate their votes to any of their neighbors in the network.
However, it is uncertain that such a delegation process will lead to a stable
state where all voters are satisfied with the people representing them. We
study the stability (w.r.t. voters preferences) of the delegation process in
liquid democracy and model it as a game in which the players are the voters and
the strategies are their possible delegations. We answer several questions on
the equilibria of this process in any social network or in social networks that
correspond to restricted types of graphs.
We show that a Nash-equilibrium may not exist, and that it is even
NP-complete to decide whether one exists or not. This holds even if the social
network is a complete graph or a bounded degree graph. We further show that
this existence problem is W[1]-hard w.r.t. the treewidth of the social network.
Besides these hardness results, we demonstrate that an equilibrium always
exists whatever the preferences of the voters iff the social network is a tree.
We design a dynamic programming procedure to determine some desirable
equilibria (e.g., minimizing the dissatisfaction of the voters) in polynomial
time for tree social networks. Lastly, we study the convergence of delegation
dynamics. Unfortunately, when an equilibrium exists, we show that a best
response dynamics may not converge, even if the social network is a path or a
complete graph. | 1904.05775v2 |
2019-06-20 | Ongoing Vaccine and Monoclonal Antibody HIV Prevention Efficacy Trials and Considerations for Sequel Efficacy Trial Designs | Four randomized placebo-controlled efficacy trials of a candidate vaccine or
passively infused monoclonal antibody for prevention of HIV-1 infection are
underway (HVTN 702 in South African men and women; HVTN 705 in sub-Saharan
African women; HVTN 703/HPTN 081 in sub-Saharan African women; HVTN 704/HPTN
085 in U.S., Peruvian, Brazilian, and Swiss men or transgender persons who have
sex with men). Several challenges are posed to the optimal design of the sequel
efficacy trials, including: (1) how to account for the evolving mosaic of
effective prevention interventions that may be part of the trial design or
standard of prevention; (2) how to define viable and optimal sequel trial
designs depending on the primary efficacy results and secondary 'correlates of
protection' results of each of the ongoing trials; and (3) how to define the
primary objective of sequel efficacy trials if HIV-1 incidence is expected to
be very low in all study arms such that a standard trial design has a steep
opportunity cost. After summarizing the ongoing trials, I discuss statistical
science considerations for sequel efficacy trial designs, both generally and
specifically to each trial listed above. One conclusion is that the results of
'correlates of protection' analyses, which ascertain how different host
immunological markers and HIV-1 viral features impact HIV-1 risk and prevention
efficacy, have an important influence on sequel trial design. This influence is
especially relevant for the monoclonal antibody trials because of the focused
pre-trial hypothesis that potency and coverage of serum neutralization
constitutes a surrogate endpoint for HIV-1 infection... (see manuscript for the
full abstract) | 1906.08409v1 |
2019-08-12 | Elemental Abundances in M31: First Alpha and Iron Abundance Measurements in M31's Giant Stellar Stream | We present the first measurements of [Fe/H] and [$\alpha$/Fe] abundances,
obtained using spectral synthesis modeling, for red giant branch stars in M31's
giant stellar stream. The spectroscopic observations, obtained at a projected
distance of 17 kpc from M31's center, yielded 61 stars with [Fe/H]
measurements, including 21 stars with [$\alpha$/Fe] measurements, from 112
targets identified as M31 stars. The [Fe/H] measurements confirm the
expectation from photometric metallicity estimates that stars in this region of
M31's halo are relatively metal-rich compared to stars in the MW's inner halo:
more than half the stars in the field, including those not associated with
kinematically identified substructure, have [Fe/H] abundances $> -1.0$. The
stars in this field are $\alpha$-enhanced at lower metallicities, while
[$\alpha$/Fe] decreases with increasing [Fe/H] above metallicities of [Fe/H]
$\gtrsim -0.9$. Three kinematical components have been previously identified in
this field: the giant stellar stream, a second kinematically cold feature of
unknown origin, and M31's kinematically hot halo. We compare probabilistic
[Fe/H] and [$\alpha$/Fe] distribution functions for each of the components. The
giant stellar stream and the second kinematically cold feature have very
similar abundance distributions, while the halo component is more metal-poor.
Although the current sample sizes are small, a comparison of the abundances of
stars in the giant stellar stream field with abundances of M31 halo and dSph
stars from the literature indicate that the progenitor of the stream was likely
more massive, and experienced a higher efficiency of star formation, than M31's
existing dSphs or the dEs NGC147 and NGC185. | 1908.04429v1 |
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