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2024-03-13
|
Effects of wave damping and finite perpendicular scale on three-dimensional Alfvén wave parametric decay in low-beta plasmas
|
Shear Alfven wave parametric decay instability (PDI) provides a potential
path toward significant wave dissipation and plasma heating. However,
fundamental questions regarding how PDI is excited in a realistic
three-dimensional (3D) open system and how critically the finite perpendicular
wave scale -- as found in both the laboratory and space plasmas -- affects the
excitation remain poorly understood. Here, we present the first 3D,
open-boundary, hybrid kinetic-fluid simulations of kinetic Alfven wave PDI in
low-beta plasmas. Key findings are that the PDI excitation is strongly limited
by the wave damping present, including electron-ion collisional damping
(represented by a constant resistivity) and geometrical attenuation associated
with the finite-scale Alfven wave, and ion Landau damping of the child acoustic
wave. The perpendicular wave scale alone, however, plays no discernible role,
with different wave scales exhibiting similar instability growth. These
findings are corroborated by theoretical analysis and estimates. The new
understanding of 3D kinetic Alfven wave PDI physics is essential for laboratory
study of the basic plasma process and may also help evaluate the relevance/role
of PDI in low-beta space plasmas.
|
2403.08179v1
|
2024-03-19
|
Polarization Dynamics in Paramagnet of Charged Quark-Gluon Plasma
|
It is commonly understood that the strong magnetic field produced in heavy
ion collisions is short-lived. The electric conductivity of the quark-gluon
plasma is unable to significantly extend the life time of magnetic field. We
propose an alternative scenario to achieve this: with finite baryon density and
spin polarization by the initial magnetic field, the quark-gluon plasma behaves
as a paramagnet, which may continue to polarize quark after fading of initial
magnetic field. We confirm this picture by calculations in both quantum
electrodynamics and quantum chromodynamics. In the former case, we find a
splitting in the damping rates of probe fermion with opposite spin component
along the magnetic field with the splitting parametrically small than the
average damping rate. In the latter case, we find a similar splitting in the
damping rates of probe quark with opposite spin components along the magnetic
field. The splitting is parametrically comparable to the average damping rate,
providing an efficient way of polarizing strange quarks by the quark-gluon
plasma paramagnet consisting of light quarks.
|
2403.12615v1
|
2024-03-25
|
Radiation damping of a Rayleigh scatterer illuminated by a plane wave
|
We investigate the radiation damping experienced by a dielectric spherical
particle when it is illuminated by an electromagnetic plane wave within the
Rayleigh regime. We derive the equivalent electric dipole of the moving
particle and subsequently calculate the electromagnetic force acting on it from
two different approaches. In the first approach, we calculate the force from
the integration of stress tensor and field momentum. In the second one, we
calculate the force directly from the integration of the force density. Our
derivations reveal that the damping coefficient is equal to $6P_{scat}/mc^2$
along the propagation direction, whereas it is $P_{scat}/mc^2$ along
perpendicular directions. Here, $P_{scat}$ denotes the power scattered by the
particle, and $mc^2$ represents the particle's mass energy. The radiation
damping derived in this study sets upper limits on the quality factor of
optically levitated objects and ensures the existence of a steady-state
solution of the particle's dynamics.
|
2403.16618v1
|
2006-04-14
|
The UCSD Radio-Selected Quasar Survey for Damped Lyman alpha System
|
As large optical quasar surveys for damped Lya become a reality and the study
of star forming gas in the early Universe achieves statistical robustness, it
is now vital to identify and quantify the sources of systematic error. Because
the nature of optically-selected quasar surveys makes them vulnerable to dust
obscuration, we have undertaken a radio-selected quasar survey for damped Lya
systems to address this bias. We present the definition and results of this
survey. We then combine our sample with the CORALS dataset to investigate the
HI column density distribution function f(N) of damped Lya systems toward
radio-selected quasars. We find that f(N) is well fit by a power-law f(N) = k_1
N^alpha_1, with log k_1 = 22.90 and alpha_1 = -2.18. This power-law is in
excellent agreement with that of optically-selected samples at low N(HI), an
important yet expected result given that obscuration should have negligible
effect at these gas columns. However, because of the relatively small size of
the radio-selected sample, 26 damped Lya systems in 119 quasars, f(N) is not
well constrained at large N(HI) and the first moment of the HI distribution
function, Omega_g, is, strictly speaking, a lower limit. The power-law is steep
enough, however, that extrapolating it to higher column densities implies only
a modest, logarithmic increase in Omega_g. The radio-selected value of Omega_g
= 1.15 x 10^-3, agrees well with the results of optically-selected surveys.
While our results indicate that dust obscuration is likely not a major issue
for surveys of damped Lya systems, we estimate that a radio-selected sample of
approximately 100 damped Lya systems will be required to obtain the precision
necessary to absolutely confirm an absence of dust bias.
|
0604334v1
|
2012-04-12
|
Evidence of Gunn-Peterson damping wings in high-z quasar spectra: strengthening the case for incomplete reionization
|
The spectra of several high-redshift (z>6) quasars have shown evidence for a
Gunn-Peterson (GP) damping wing, indicating a substantial mean neutral hydrogen
fraction (x_HI > 0.03) in the z ~ 6 intergalactic medium (IGM). However,
previous analyses assumed that the IGM was uniformly ionized outside of the
quasar's HII region. Here we relax this assumption and model patchy
reionization scenarios for a range of IGM and quasar parameters. We quantify
the impact of these differences on the inferred x_HI, by fitting the spectra of
three quasars: SDSS J1148+5251 (z=6.419), J1030+0524 (z=6.308), and J1623+3112
(z=6.247). We find that the best-fit values of x_HI in the patchy models agree
well with the uniform case. More importantly, we confirm that the observed
spectra favor the presence of a GP damping wing, with peak likelihoods
decreasing by factors of > few - 10 when the spectra are modeled without a
damping wing. We also find that the Ly alpha absorption spectra, by themselves,
cannot distinguish the damping wing in a relatively neutral IGM from a damping
wing in a highly ionized IGM, caused either by an isolated neutral patch, or by
a damped Ly alpha absorber (DLA). However, neutral patches in a highly ionized
universe (x_HI < 0.01), and DLAs with the large required column densities (N_HI
> few x 10^{20} cm^{-2}) are both rare. As a result, when we include reasonable
prior probabilities for the line of sight (LOS) to intercept either a neutral
patch or a DLA at the required distance of ~ 40-60 comoving Mpc away from the
quasar, we find strong lower limits on the neutral fraction in the IGM, x_HI >
0.1 (at 95% confidence). This strengthens earlier claims that a substantial
global fraction of hydrogen in the z~6 IGM is in neutral form.
|
1204.2838v2
|
2013-05-31
|
Highly inclined and eccentric massive planets I: Planet-disc interactions
|
In the Solar System, planets have a small inclination with respect to the
equatorial plane of the Sun, but there is evidence that in extrasolar systems
the inclination can be very high. This spin-orbit misalignment is unexpected,
as planets form in a protoplanetary disc supposedly aligned with the stellar
spin. Planet-planet interactions are supposed to lead to a mutual inclination,
but the effects of the protoplanetary disc are still unknown. We investigate
therefore planet-disc interactions for planets above 1M_Jup. We check the
influence of the inclination i, eccentricity e, and mass M_p of the planet. We
perform 3D numerical simulations of protoplanetary discs with embedded
high-mass planets. We provide damping formulae for i and e as a function of i,
e, and M_p that fit the numerical data. For highly inclined massive planets,
the gap opening is reduced, and the damping of i occurs on time-scales of the
order of 10^-4 deg/yr M_disc/(0.01 M_star) with the damping of e on a smaller
time-scale. While the inclination of low planetary masses (<5M_Jup) is always
damped, large planetary masses with large i can undergo a Kozai-cycle with the
disc. These Kozai-cycles are damped in time. Eccentricity is generally damped,
except for very massive planets (M_p = 5M_Jup) where eccentricity can increase
for low inclinations. The dynamics tends to a final state: planets end up in
midplane and can then, over time, increase their eccentricity as a result of
interactions with the disc. The interactions with the disc lead to damping of i
and e after a scattering event of high-mass planets. If i is sufficiently
reduced, the eccentricity can be pumped up because of interactions with the
disc. If the planet is scattered to high inclination, it can undergo a
Kozai-cycle with the disc that makes it hard to predict the exact movement of
the planet and its orbital parameters at the dispersal of the disc.
|
1305.7330v1
|
2014-10-20
|
Frequency-dependent attenuation and elasticity in unconsolidated earth materials: effect of damping
|
We use the Discrete Element Method (DEM) to understand the underlying
attenuation mechanism in granular media, with special applicability to the
measurements of the so-called effective mass developed earlier. We consider
that the particles interact via Hertz-Mindlin elastic contact forces and that
the damping is describable as a force proportional to the velocity difference
of contacting grains. We determine the behavior of the complex-valued normal
mode frequencies using 1) DEM, 2) direct diagonalization of the relevant
matrix, and 3) a numerical search for the zeros of the relevant determinant.
All three methods are in strong agreement with each other. The real and the
imaginary parts of each normal mode frequency characterize the elastic and the
dissipative properties, respectively, of the granular medium. We demonstrate
that, as the interparticle damping, $\xi$, increases, the normal modes exhibit
nearly circular trajectories in the complex frequency plane and that for a
given value of $\xi$ they all lie on or near a circle of radius $R$ centered on
the point $-iR$ in the complex plane, where $R\propto 1/\xi$. We show that each
normal mode becomes critically damped at a value of the damping parameter $\xi
\approx 1/\omega_n^0$, where $\omega_n^0$ is the (real-valued) frequency when
there is no damping. The strong indication is that these conclusions carry over
to the properties of real granular media whose dissipation is dominated by the
relative motion of contacting grains. For example, compressional or shear waves
in unconsolidated dry sediments can be expected to become overdamped beyond a
critical frequency, depending upon the strength of the intergranular damping
constant.
|
1410.5484v2
|
2020-08-05
|
Fast optimization via inertial dynamics with closed-loop damping
|
In a Hilbert space $H$, in order to develop fast optimization methods, we
analyze the asymptotic behavior, as time $t$ tends to infinity, of inertial
continuous dynamics where the damping acts as a closed-loop control. The
function $f: H \to R$ to be minimized (not necessarily convex) enters the
dynamic through it gradient, which is assumed to be Lipschitz continuous on the
bounded subsets of $H$. This gives autonomous dynamical systems with nonlinear
damping and nonlinear driving force. We first consider the case where the
damping term $\partial \phi (\dot{x}(t))$ acts as a closed-loop control of the
velocity. The damping potential $\phi : H \to [0,+\infty)$ is a convex
continuous function which achieves its minimum at the origin. We show the
existence and uniqueness of a global solution to the associated Cauchy problem.
Then, we analyze the asymptotic convergence properties of the generated
trajectories generated. We use techniques from optimization, control theory,
and PDE's: Lyapunov analysis based on the decreasing property of an energy-like
function, quasi-gradient and Kurdyka-Lojasiewicz theory, monotone operator
theory for wave-like equations. Convergence rates are obtained based on the
geometric properties of the data $f$ and $\phi$. When $f$ is strongly convex,
we give general conditions which provide exponential convergence rates. Then,
we extend the results to the case where an additional Hessian-driven damping
enters the dynamic, which reduces the oscillations. Finally, we consider an
inertial system involving jointly the velocity $\dot{x}(t)$ and the gradient
$\nabla f(x(t))$. In addition to its original results, this work surveys the
numerous works devoted in recent years to the interaction between continuous
damped inertial dynamics and numerical algorithms for optimization, with the
emphasis on autonomous systems, closed-loop adaptive procedures, and
convergence rates.
|
2008.02261v3
|
2023-01-10
|
Cosmic Ray Drag and Damping of Compressive Turbulence
|
While it is well-known that cosmic rays (CRs) can gain energy from turbulence
via second order Fermi acceleration, how this energy transfer affects the
turbulent cascade remains largely unexplored. Here, we show that damping and
steepening of the compressive turbulent power spectrum are expected once the
damping time $t_{\rm damp} \sim \rho v^{2}/\dot{E}_{\rm CR} \propto E_{\rm
CR}^{-1}$ becomes comparable to the turbulent cascade time. Magnetohydrodynamic
(MHD) simulations of stirred compressive turbulence in a gas-CR fluid with
diffusive CR transport show clear imprints of CR-induced damping, saturating at
$\dot{E}_{\rm CR} \sim \tilde{\epsilon}$, where $\tilde{\epsilon}$ is the
turbulent energy input rate. In that case, almost all the energy in large scale
motions is absorbed by CRs and does not cascade down to grid scale. Through a
Hodge-Helmholtz decomposition, we confirm that purely compressive forcing can
generate significant solenoidal motions, and we find preferential CR damping of
the compressive component in simulations with diffusion and streaming,
rendering small-scale turbulence largely solenoidal, with implications for
thermal instability and proposed resonant scattering of $E > 300$ GeV CRs by
fast modes. When CR transport is streaming dominated, CRs also damp large scale
motions, with kinetic energy reduced by up to to an order of magnitude in
realistic $E_{\rm CR} \sim E_{\rm g}$ scenarios, but turbulence (with a reduced
amplitude) still cascades down to small scales with the same power spectrum.
Such large scale damping implies that turbulent velocities obtained from the
observed velocity dispersion may significantly underestimate turbulent forcing
rates, i.e. $\tilde{\epsilon} \gg \rho v^{3}/L$.
|
2301.04156v2
|
2024-02-12
|
Relaxation of weakly collisional plasma: continuous spectra, Landau eigenmodes, and transition from the collisionless to the fluid limit
|
The relaxation of a weakly collisional plasma is described by the
Boltzmann-Poisson equations with the Lenard-Bernstein collision operator. We
perform a perturbative analysis of these equations, and obtain, for the first
time, exact analytic solutions, enabling definitive resolutions to
long-standing controversies regarding the impact of weak collisions on
continuous spectra and Landau eigenmodes. Unlike some previous studies, we
retain both damping and diffusion terms in the collision operator. We find that
the linear response is a temporal convolution of a continuum that depends on
the continuous velocities of particles, and discrete normal modes that
encapsulate coherent oscillations. The normal modes are exponentially damped
over time due to collective effects (Landau damping) as well as collisional
dissipation. The continuum is also damped by collisions but somewhat
differently. Up to a collision time, which is the inverse of the collision
frequency $\nu_{\mathrm{c}}$, the continuum decay is driven by velocity
diffusion and occurs super-exponentially over a timescale $\sim
\nu^{-1/3}_{\mathrm{c}}$. After a collision time, however, the continuum decay
is driven by the collisional damping of particle velocities and diffusion of
their positions, and occurs exponentially over a timescale $\sim
\nu_{\mathrm{c}}$. This hitherto unknown, slow exponential decay causes
perturbations to damp the most on scales comparable to the mean free path, but
very slowly on larger scales, which establishes the local thermal equilibrium,
the essence of the fluid limit. The long-term decay of the response is driven
by the normal modes on scales smaller than the mean free path, but, on larger
scales, is governed by the slowly decaying continuum and the least damped
normal mode. Our analysis firmly establishes a long-sought connection between
the collisionless and fluid limits of weakly collisional plasmas.
|
2402.07992v1
|
1995-09-21
|
Damped Lyman-alpha and Lyman Limit Absorbers in the Cold Dark Matter Model
|
We study the formation of damped \lya and Lyman limit absorbers in a
hierarchical clustering scenario using a gas dynamical simulation of an $\Omega
= 1$, cold dark matter universe. In the simulation, these high column density
systems are associated with forming galaxies. Damped \lya absorption, $N_{HI}
\simgt 10^{20.2}\cm^{-2}$, arises along lines of sight that pass near the
centers of relatively massive, dense protogalaxies. Lyman limit absorption,
$10^{17}\cm^{-2} \simlt N_{HI} \simlt 10^{20.2}\cm^{-2}$, develops on lines of
sight that pass through the outer parts of such objects or near the centers of
smaller protogalaxies. The number of Lyman limit systems is less than observed,
while the number of damped \lya systems is quite close to the observed
abundance. Damped absorbers are typically $\sim 10$ kpc in radius, but the
population has a large total cross section because the systems are much more
numerous than present day $L_*$ galaxies. Our results demonstrate that high
column density systems like those observed arise naturally in a hierarchical
theory of galaxy formation and that it is now possible to study these absorbers
directly from numerical simulations.
|
9509106v1
|
1995-09-21
|
Nonlinear Damping of Oscillations in Tidal-Capture Binaries
|
We calculate the damping of quadrupole f and low order g modes (primary
modes) by nonlinear coupling to other modes of the star. This damping is orders
of magnitude more rapid than direct radiative damping when the primary
amplitude is large, as in tidal capture.
Primary modes destabilize high degree g-modes of half their frequency
(daughter modes) by 3-mode coupling in radiative zones. In sunlike stars, the
growth time $\equiv\eta^{-1}\approx 4 E_{0,42}^{-1/2}$ days, where $E_{0,42}$
is the initial energy of the primary mode in units of $10^{42}~$erg, and of
order $10^{10}E_{0,42}^{5/4}$ daughters are unstable. The growth rate is
approximately equal to the angular frequency of the primary mode times its
dimensionless radial amplitude, $\delta R/R_*\approx 0.002E_{0,42}^{1/2}$.
Although the daughter modes are limited by their own nonlinearities,
collectively they absorb most of the primary mode's energy after a time $\sim
10\eta^{-1}$ provided $E_{0}> 10^{40}~\mbox{erg}$. In fact nonlinear mode
interaction may be the dominant damping process if $E_0\gtrsim
10^{37}~\mbox{erg}$.
Our results have application to tidally captured main sequence globular
cluster stars of mass $\ge 0.5 M_{\sun}$; the tidal energy is dissipated in the
radiative core of the star in about a month, which is less than the initial
orbital period.
|
9509112v1
|
1997-08-12
|
Spectroscopy of PKS 0528-260: New Limits on CO Absorption and Emission
|
We have obtained a moderate resolution spectrum of the quasar PKS 0528-250
with the Red Channel Spectrograph on the Multiple Mirror Telescope (MMT) in
order to study a damped Lyman alpha absorption line system at z = 2.8115.
We obtain a new upper limit for the CO column density for the z = 2.8108
velocity component in the z = 2.8115 damped Lyman alpha system. The ionization
of different species in this component rules out a quasar spectral energy
distribution (SED) as the ionization field,and implies an ultraviolet radiation
field intensity a few times that of the Milky Way value. The estimated total
number density is n(H) about 20 cm^{-3}. The physical size for the z = 2.8108
component implied by these models is about 40 parsecs. The ionization of
different species also suggests a structure with a hot intercloud medium
associated with a H I cloud in this component, that is, most low ionized ions
are from the cold medium where photoionization and photodissociation dominates.
The highly ionized species may be from the intercloud medium where collisional
ionization dominates. We also present newly identified Ni II absorption lines
in the z = 2.1408 and z = 2.8115 damped Ly$\alpha$ systems. The derived
depletion of nickel by dust confirms previous results that the dust-to-gas
ratio in these two damped Lyman alpha systems is about 10% of the Milky Way
ratio. Millimeter wavelength observations obtained at the NRAO 12 meter
telescope provide new upper limits on CO (3-2) emission in the z = 2.8115
damped Lyman alpha system.
|
9708104v1
|
1998-11-04
|
GMRT Observations of Low z Damped Lyman-alpha Absorbers
|
We present Giant Metrewave Radio Telescope (GMRT) observations of redshifted
HI 21cm absorption in two low redshift (z=0.2212, z=0.0912) damped Lyman-alpha
systems seen towards the gigahertz peaked source OI 363 (z_em = 0.630). The
object at z=0.0912 is the lowest redshift damped Lyman-alpha system known to
date. Ground based imaging (Rao & Turnshek, 1998) shows that at neither
redshift is there a large spiral galaxy at low impact parameter to the line of
sight to OI 363, in contradiction with the suggestion that these systems are
large proto-disks.
Since OI 363 is a highly compact, core dominated source, the covering factor
of the HI gas is likely to be unity. Nonetheless, the spin temperatures derived
from the 21cm optical depth (and using the N_HI measured from HST spectra, Rao
& Turnshek, 1998) are high, viz. 1120 +/- 200 K and 825 +/- 110 K for the high
and low redshift systems respectively. These values are considerably higher
than typical values (100 - 200 K) measured in our Galaxy and Andromeda and are,
in fact, similar to those obtained in high redshift damped Lyman-alpha systems.
Our observations hence suggest that evolutionary effects may not be crucial in
understanding the difference in derived spin temperature values between local
spiral disks and high redshift damped Lyman-alpha systems.
|
9811068v1
|
2002-01-25
|
Galaxies Associated with z~4 Damped Lya Systems: I. Imaging and Photometric Selection
|
This paper describes the acquisition and analysis of imaging data for the
identification of galaxies associated with z~4 damped Lya systems. We present
deep BRI images of three fields known to contain four z~4 damped systems. We
discuss the reduction and calibration of the data, detail the color criteria
used to identify z~4 galaxies, and present a photometric redshift analysis to
complement the color selection. We have found no galaxy candidates closer to
the QSO than 7'' which could be responsible for the damped Lya systems.
Assuming that at least one of the galaxies is not directly beneath the QSO, we
set an upper limit on this damped Lya system of L < L*/4. Finally, we have
established a web site to release these imaging data to the public.
|
0201417v2
|
2002-02-25
|
Eccentricity Evolution for Planets in Gaseous Disks
|
We investigate the hypothesis that interactions between a giant planet and
the disk from which it forms promote eccentricity growth. These interactions
are concentrated at discrete Lindblad and corotation resonances. Interactions
at principal Lindblad resonances cause the planet's orbit to migrate and open a
gap in the disk if the planet is sufficiently massive. Those at first order
Lindblad and corotation resonances change the planet's orbital eccentricity.
Eccentricity is excited by interactions at external Lindblad resonances which
are located on the opposite side of corotation from the planet, and damped by
co-orbital Lindblad resonances which overlap the planet's orbit. If the planet
clears a gap in the disk, the rate of eccentricity damping by co-orbital
Lindblad resonances is reduced. Density gradients associated with the gap
activate eccentricity damping by corotation resonances at a rate which
initially marginally exceeds that of eccentricity excitation by external
Lindblad resonances. But the corotation torque drives a mass flux which reduces
the density gradient near the resonance. Sufficient partial saturation of
corotation resonances can tip the balance in favor of eccentricity excitation.
A minimal initial eccentricity of a few percent is required to overcome viscous
diffusion which acts to unsaturate corotation resonances by reestablishing the
large scale density gradient. Thus eccentricity growth is a finite amplitude
instability. Formally, interactions at the apsidal resonance, which is a
special kind of co-orbital Lindblad resonance, appears to damp eccentricity
faster than external Lindblad resonances can excite it. However, apsidal waves
have such long wavelengths that they do not propagate in protoplanetary disks.
This reduces eccentricity damping by the apsidal resonance to a modest level.
|
0202462v1
|
2003-07-23
|
Dusty Molecular Cloud Collapse in the Presence of Alfvén Waves
|
It has been shown that magnetic fields play an important role in the
stability of molecular clouds, mainly perpendicularly to the field direction.
However, in the parallel direction the stability is a serious problem still to
be explained. Interstellar turbulence may allow the generation of Alfv\'en
waves that propagate through the clouds in the magnetic field direction. These
regions also present great amounts of dust particles which can give rise to new
wave modes, or modify the pre-existing ones. The dust-cyclotron damping affects
the Alfv\'en wave propagation near the dust- cyclotron frequency. On the other
hand, the clouds present different grain sizes, which carry different charges.
In this sense, a dust particle distribution has several dust-cyclotron
frequencies and it will affect a broad band of wave frequencies. In this case,
the energy transfer to the gas is more efficient than in the case where the
ion-cyclotron damping is considered alone. This effect becomes more important
if a power law spectrum is considered for the wave energy flux, since the major
part of the energy is concentrated in low-frequency waves. In this work we
calculate the dust- cyclotron damping in a dusty and magnetized dwarf molecular
cloud, as well as determine the changes in the Alfv\'en wave flux. Then, we use
these results to study the gravitational stability of the cloud. We show that,
considering the presence of charged dust particles, the wave flux is rapidly
damped due to dust-cyclotron damping. Then the wave pressure acts in a small
length scale, and cannot explain the observable cloud sizes, but can explain
the existence of small and dense cores.
|
0307411v1
|
2005-02-28
|
Thermal Evolution of a Pulsating Neutron Star
|
We have derived a set of equations to describe the thermal evolution of a
neutron star which undergoes small-amplitude radial pulsations. We have taken
into account, in the frame of the General Theory of Relativity, the pulsation
damping due to the bulk and shear viscosity and the accompanying heating of the
star. The neutrino emission of a pulsating non-superfluid star and its heating
due to the bulk viscosity are calculated assuming that both processes are
determined by the non-equilibrium modified Urca process. Analytical and
numerical solutions to the set of equations of the stellar evolution are
obtained for linear and strongly non-linear deviations from beta-equilibrium.
It is shown that a pulsating star may be heated to very high temperatures,
while the pulsations damp very slowly with time (a power law damping for
100-1000 years), as long as the damping is determined by the bulk viscosity.
The contribution of the shear viscosity to the damping becomes important in a
rather cool star with a low pulsation energy.
|
0502583v2
|
2005-05-02
|
Collisionless Damping of Fast MHD Waves in Magneto-rotational Winds
|
We propose collisionless damping of fast MHD waves as an important mechanism
for the heating and acceleration of winds from rotating stars. Stellar rotation
causes magnetic field lines anchored at the surface to form a spiral pattern
and magneto-rotational winds can be driven. If the structure is a magnetically
dominated, fast MHD waves generated at the surface can propagate almost
radially outward and cross the field lines. The propagating waves undergo
collisionless damping owing to interactions with particles surfing on magnetic
mirrors that are formed by the waves themselves. The damping is especially
effective where the angle between the wave propagation and the field lines
becomes moderately large ($\sim 20$ to $80^{\circ}$). The angle tends naturally
to increase into this range because the field in magneto-rotational winds
develops an increasingly large azimuthal component. The dissipation of the wave
energy produces heating and acceleration of the outflow. We show using
specified wind structures that this damping process can be important in both
solar-type stars and massive stars that have moderately large rotation rates.
This mechanism can play a role in coronae of young solar-type stars which are
rapidly rotating and show X-ray luminosities much larger than the sun. The
mechanism could also be important for producing the extended X-ray emitting
regions inferred to exist in massive stars of spectral type middle B and later.
|
0505013v5
|
2006-08-05
|
The nature of damped Lyman alpha and sub-damped Lyman alpha absorbers
|
We present arguments based on the measured abundances in individual damped
Lyman alpha systems (DLAs) and sub-damped Lyman alpha systems (sub-DLAs), and
also the average abundances inferred in large samples of QSO absorption line
systems, to suggest that the amount of dust in intervening QSO absorbers is
small and is not responsible for missing many QSOs in magnitude limited QSO
surveys. While we can not totally rule out a bimodal dust distribution with a
population of very dusty, metal rich, absorbers which push the background QSOs
below the observational threshold of current optical spectroscopic studies,
based upon the current samples it appears that the metallicity in QSO absorbers
decreases with increase in H I column densities beyond 10^{19} cm^{-2}. Thus
the sub-DLA population is more metal rich than the DLAs, a trend which may
possibly extend to the non-damped Lyman limit systems (NDLLS). Based on the
recently discovered mass-metallicity relation for galaxies, we suggest that
most sub-DLAs and possibly NDLLS, are associated with massive spiral/elliptical
galaxies while most DLAs are associated with low mass galaxies. The sub-DLA
galaxies will then contribute a larger fraction of total mass (stellar and ISM)
and therefore metals, to the cosmic budget, specially at low redshifts, as
compared to the DLAs.
|
0608127v2
|
2007-02-12
|
The Ucsd/Keck Damped Lya Abundance Database: A Decade of High Resolution Spectroscopy
|
We publish the Keck/HIRES and Keck/ESI spectra that we have obtained during
the first 10 years of Keck observatory operations. Our full sample includes 42
HIRES spectra and 39 ESI spectra along 65 unique sightlines providing abundance
measurements on ~85 damped Lya systems. The normalized data can be downloaded
from the journal or from our supporting website:
http://www.ucolick.org/~xavier/DLA/. The database includes all of the
sightlines that have been included in our papers on the chemical abundances,
kinematics, and metallicities of the damped Lya systems. This data has also
been used to argue for variations in the fine-structure constant. We present
new chemical abundance measurements for 10 damped Lya systems and a summary
table of high-resolution metallicity measurements (including values from the
literature) for 153 damped Lya systems at z>1.6. We caution, however, that this
metallicity sample (and all previous ones) is biased to higher N(HI) values
than a random sample.
|
0702325v1
|
1998-06-30
|
Structure and Spin Dynamics of La$_{0.85}$Sr$_{0.15}$MnO$_3$
|
Neutron scattering has been used to study the structure and spin dynamics of
La$_{0.85}$Sr$_{0.15}$MnO$_3$. The magnetic structure of this system is
ferromagnetic below T_C = 235 K. We see anomalies in the Bragg peak intensities
and new superlattice peaks consistent with the onset of a spin-canted phase
below T_{CA} = 205 K, which appears to be associated with a gap at q = (0, 0,
0.5) in the spin-wave spectrum. Anomalies in the lattice parameters indicate a
concomitant lattice distortion. The long-wavelength magnetic excitations are
found to be conventional spin waves, with a gapless (< 0.02 meV) isotropic
dispersion relation $E = Dq^2$. The spin stiffness constant D has a $T^{5/2}$
dependence at low T, and the damping at small q follows $q^4T^{2}$. An
anomalously strong quasielastic component, however, develops at small wave
vector above 200 K and dominates the fluctuation spectrum as T -> T_C. At
larger q, on the other hand, the magnetic excitations become heavily damped at
low temperatures, indicating that spin waves in this regime are not eigenstates
of the system, while raising the temperature dramatically increases the
damping. The strength of the spin-wave damping also depends strongly on the
symmetry direction in the crystal. These anomalous damping effects are likely
due to the itinerant character of the $e_g$ electrons.
|
9806381v1
|
1999-02-01
|
Damping Rates and Mean Free Paths of Soft Fermion Collective Excitations in a Hot Fermion-Gauge-Scalar Theory
|
We study the transport coefficients, damping rates and mean free paths of
soft fermion collective excitations in a hot fermion-gauge-scalar plasma with
the goal of understanding the main physical mechanisms that determine transport
of chirality in scenarios of non-local electroweak baryogenesis. The focus is
on identifying the different transport coefficients for the different branches
of soft collective excitations of the fermion spectrum. These branches
correspond to collective excitations with opposite ratios of chirality to
helicity and different dispersion relations. By combining results from the hard
thermal loop (HTL) resummation program with a novel mechanism of fermion
damping through heavy scalar decay, we obtain a robust description of the
different damping rates and mean free paths for the soft collective excitations
to leading order in HTL and lowest order in the Yukawa coupling. The space-time
evolution of wave packets of collective excitations unambiguously reveals the
respective mean free paths. We find that whereas both the gauge and scalar
contribution to the damping rates are different for the different branches, the
difference of mean free paths for both branches is mainly determined by the
decay of the heavy scalar into a hard fermion and a soft collective excitation.
We argue that these mechanisms are robust and are therefore relevant for
non-local scenarios of baryogenesis either in the Standard Model or extensions
thereof.
|
9902218v2
|
2002-08-29
|
Some notes on ideology of waves in plasmas
|
Our last three papers provide an occasion to make some brief notes on
ideology of waves in plasmas and to rehabilitate Vlasov prescription to
calculate relevant logarithmically divergent integrals in the principal value
sense. In this approach asymptotical solutions of plasma oscillations are
selected according to self-consistent boundary physical conditions. Landau
damping is absent in this case by definition. Boundary electrical field
together with conditions of absence of unphysical backward and kinematical
waves define single-valued dependence of boundary distribution function on
electron velocity \vec{v} in the case of transversal waves and on the surface
break of the normal electrical field in the case of longitudinal oscillations.
We have proposed physically more justified modified iteration procedure of
collisional damping calculation and demonstrated some results of damping
decrements calculations in a low-collision electron-ion plasma. Dispersion
smearing of both longitudinal and transversal high-frequency waves, for which
the smearing decrement \delta_x is proportional to
\Delta\omega/(\omega\sqrt{\omega^2-\omega_L^2}), might be the main cause of
waves amplitude damping in collisionless plasmas imitating Landau damping.
|
0208098v7
|
2004-11-26
|
Open quantum systems
|
The damping of the harmonic oscillator is studied in the framework of the
Lindblad theory for open quantum systems. A generalization of the fundamental
constraints on quantum mechanical diffusion coefficients which appear in the
master equation for the damped quantum oscillator is presented; the
Schr\"odinger, Heisenberg and Weyl-Wigner-Moyal representations of the Lindblad
equation are given explicitly. On the basis of these representations it is
shown that various master equations for the damped quantum oscillator used in
the literature are particular cases of the Lindblad equation and that not all
of these equations are satisfying the constraints on quantum mechanical
diffusion coefficients. The master equation is transformed into Fokker-Planck
equations for quasiprobability distributions and a comparative study is made
for the Glauber $P$ representation, the antinormal ordering $Q$ representation
and the Wigner $W$ representation. The density matrix is represented via a
generating function, which is obtained by solving a time-dependent linear
partial differential equation derived from the master equation. The damped
harmonic oscillator is applied for the description of the charge equilibration
mode observed in deep inelastic reactions. For a system consisting of two
harmonic oscillators the time dependence of expectation values, Wigner function
and Weyl operator are obtained and discussed. In addition models for the
damping of the angular momentum are studied. Using this theory to the quantum
tunneling through the nuclear barrier, besides Gamow's transitions with energy
conservation, additional transitions with energy loss, are found. When this
theory is used to the resonant atom-field interaction, new optical equations
describing the coupling through the environment are obtained.
|
0411189v1
|
2006-02-17
|
Damped quantum harmonic oscillator
|
In the framework of the Lindblad theory for open quantum systems the damping
of the harmonic oscillator is studied. A generalization of the fundamental
constraints on quantum mechanical diffusion coefficients which appear in the
master equation for the damped quantum oscillator is presented; the
Schr\"odinger and Heisenberg representations of the Lindblad equation are given
explicitly. On the basis of these representations it is shown that various
master equations for the damped quantum oscillator used in the literature are
particular cases of the Lindblad equation and that the majority of these
equations are not satisfying the constraints on quantum mechanical diffusion
coefficients. Analytical expressions for the first two moments of coordinate
and momentum are also obtained by using the characteristic function of the
Lindblad master equation. The master equation is transformed into Fokker-Planck
equations for quasiprobability distributions. A comparative study is made for
the Glauber $P$ representation, the antinormal ordering $Q$ representation and
the Wigner $W$ representation. It is proven that the variances for the damped
harmonic oscillator found with these representations are the same. By solving
the Fokker-Planck equations in the steady state, it is shown that the
quasiprobability distributions are two-dimensional Gaussians with widths
determined by the diffusion coefficients. The density matrix is represented via
a generating function, which is obtained by solving a time-dependent linear
partial differential equation derived from the master equation. Illustrative
examples for specific initial conditions of the density matrix are provided.
|
0602149v1
|
2007-04-12
|
The effect of the solar corona on the attenuation of small-amplitude prominence oscillations. I. Longitudinal magnetic field
|
Context. One of the typical features shown by observations of solar
prominence oscillations is that they are damped in time and that the values of
the damping times are usually between one and three times the corresponding
oscillatory period. However, the mechanism responsible for the attenuation is
still not well-known. Aims. Thermal conduction, optically thin or thick
radiation and heating are taken into account in the energy equation, and their
role on the attenuation of prominence oscillations is evaluated. Methods. The
dispersion relation for linear non-adiabatic magnetoacoustic waves is derived
considering an equilibrium made of a prominence plasma slab embedded in an
unbounded corona. The magnetic field is orientated along the direction parallel
to the slab axis and has the same strength in all regions. By solving the
dispersion relation for a fixed wavenumber, a complex oscillatory frequency is
obtained, and the period and the damping time are computed. Results. The effect
of conduction and radiation losses is different for each magnetoacoustic mode
and depends on the wavenumber. In the observed range of wavelengths the
internal slow mode is attenuated by radiation from the prominence plasma, the
fast mode by the combination of prominence radiation and coronal conduction and
the external slow mode by coronal conduction. The consideration of the external
corona is of paramount importance in the case of the fast and external slow
modes, whereas it does not affect the internal slow modes at all. Conclusions.
Non-adiabatic effects are efficient damping mechanisms for magnetoacoustic
modes, and the values of the obtained damping times are compatible with those
observed.
|
0704.1566v2
|
2007-10-01
|
Lyman-alpha Damping Wing Constraints on Inhomogeneous Reionization
|
One well-known way to constrain the hydrogen neutral fraction, x_H, of the
high-redshift intergalactic medium (IGM) is through the shape of the red
damping wing of the Lya absorption line. We examine this method's effectiveness
in light of recent models showing that the IGM neutral fraction is highly
inhomogeneous on large scales during reionization. Using both analytic models
and "semi-numeric" simulations, we show that the "picket-fence" absorption
typical in reionization models introduces both scatter and a systematic bias to
the measurement of x_H. In particular, we show that simple fits to the damping
wing tend to overestimate the true neutral fraction in a partially ionized
universe, with a fractional error of ~ 30% near the middle of reionization.
This bias is generic to any inhomogeneous model. However, the bias is reduced
and can even underestimate x_H if the observational sample only probes a subset
of the entire halo population, such as quasars with large HII regions. We also
find that the damping wing absorption profile is generally steeper than one
would naively expect in a homogeneously ionized universe. The profile steepens
and the sightline-to-sightline scatter increases as reionization progresses. Of
course, the bias and scatter also depend on x_H and so can, at least in
principle, be used to constrain it. Damping wing constraints must therefore be
interpreted by comparison to theoretical models of inhomogeneous reionization.
|
0710.0371v1
|
2008-02-11
|
Eccentricity of masing disks in Active Galactic Nuclei
|
Observations of Keplerian disks of masers in NCG 4258 and other Seyfert
galaxies can be used to obtain geometric distance estimates and derive the
Hubble constant. The ultimate precision of such measurements could be limited
by uncertainties in the disk geometry. Using a time-dependent linear theory
model, we study the evolution of a thin initially eccentric disk under
conditions appropriate to sub-pc scales in Active Galactic Nuclei. The
evolution is controlled by a combination of differential precession driven by
the disk potential and propagating eccentricity waves that are damped by
viscosity. A simple estimate yields a circularization timescale of
approximately 10 Myr at 0.1 pc. Numerical solutions for the eccentricity
evolution confirm that damping commences on this timescale, but show that the
subsequent decay rate of the eccentricity depends upon the uncertain strength
of viscous damping of eccentricity. If eccentricity waves are important further
decay of the eccentricity can be slow, with full circularization requiring up
to 50 Myr for disks at radii of 0.1 pc to 0.2 pc. Observationally, this implies
that it is plausible that enough time has elapsed for the eccentricity of
masing disks to have been substantially damped, but that it may not be
justified to assume vanishing eccentricity. We predict that during the damping
phase the pericenter of the eccentric orbits describes a moderately tightly
wound spiral with radius.
|
0802.1524v1
|
2008-02-20
|
The Effect of Charon's Tidal Damping on the Orbits of Pluto's Three Moons
|
Pluto's recently discovered minor moons, Nix and Hydra, have almost circular
orbits, and are nearly coplanar with Charon, Pluto's major moon. This is
surprising because tidal interactions with Pluto are too weak to damp their
eccentricities. We consider an alternative possibility: that Nix and Hydra
circularize their orbits by exciting Charon's eccentricity via secular
interactions, and Charon in turn damps its own eccentricity by tidal
interaction with Pluto. The timescale for this process can be less than the age
of the Solar System, for plausible tidal parameters and moon masses. However,
as we show numerically and analytically, the effects of the 2:1 and 3:1
resonant forcing terms between Nix and Charon complicate this picture. In the
presence of Charon's tidal damping, the 2:1 term forces Nix to migrate outward
and the 3:1 term changes the eccentricity damping rate, sometimes leading to
eccentricity growth. We conclude that this mechanism probably does not explain
Nix and Hydra's current orbits. Instead, we suggest that they were formed
in-situ with low eccentricities.
We also show that an upper limit on Nix's migration speed sets a lower limit
on Pluto-Charon's tidal circularization timescale of >10^5 yrs. Moreover,
Hydra's observed proper eccentricity may be explained by the 3:2 forcing by
Nix.
|
0802.2939v1
|
2008-03-18
|
Non-adiabatic magnetohydrodynamic waves in a cylindrical prominence thread with mass flow
|
High-resolution observations show that oscillations and waves in prominence
threads are common and that they are attenuated in a few periods. In addition,
observers have also reported the presence of material flows in such prominence
fine-structures. Here we investigate the time damping of non-leaky oscillations
supported by a homogeneous cylindrical prominence thread embedded in an
unbounded corona and with a steady mass flow. Thermal conduction and radiative
losses are taken into account as damping mechanisms, and the effect of these
non-ideal effects and the steady flow on the attenuation of oscillations is
assessed. We solve the general dispersion relation for linear, non-adiabatic
magnetoacoustic and thermal waves supported by the model, and find that slow
and thermal modes are efficiently attenuated by non-adiabatic mechanisms. On
the contrary, fast kink modes are much less affected and their damping times
are much larger than those observed. The presence of flow has no effect on the
damping of slow and thermal waves, whereas fast kink waves are more (less)
attenuated when they propagate parallel (anti-parallel) to the flow direction.
Although the presence of steady mass flows improves the efficiency of
non-adiabatic mechanisms on the attenuation of transverse, kink oscillations
for parallel propagation to the flow, its effect is still not enough to obtain
damping times compatible with observations.
|
0803.2600v2
|
2008-07-28
|
Thermal fluctuations in moderately damped Josephson junctions: Multiple escape and retrapping, switching- and return-current distributions and hysteresis
|
A crossover at a temperature T* in the temperature dependence of the width s
of the distribution of switching currents of moderately damped Josephson
junctions has been reported in a number of recent publications, with positive
ds/dT and IV characteristics associated with underdamped behaviour for lower
temperatures T<T*, and negative ds/dT and IV characteristics resembling
overdamped behaviour for higher temperatures T>T*. We have investigated in
detail the behaviour of Josephson junctions around the temperature T* by using
Monte Carlo simulations including retrapping from the running state into the
supercurrent state as given by the model of Ben-Jacob et al. We develop
discussion of the important role of multiple escape and retrapping events in
the moderate-damping regime, in particular considering the behaviour in the
region close to T*. We show that the behaviour is more fully understood by
considering two crossover temperatures, and that the shape of the distribution
and s(T) around T*, as well as at lower T<T*, are largely determined by the
shape of the conventional thermally activated switching distribution. We show
that the characteristic temperatures T* are not unique for a particular
Josephson junction, but have some dependence on the ramp rate of the applied
bias current. We also consider hysteresis in moderately damped Josephson
junctions and discuss the less commonly measured distribution of return
currents for a decreasing current ramp. We find that some hysteresis should be
expected to persist above T* and we highlight the importance, even well below
T*, of accounting properly for thermal fluctuations when determining the
damping parameter Q.
|
0807.4502v1
|
2009-02-26
|
Viscous propagation of mass flow variability in accretion discs
|
We study mass flow rate through a disc resulting from a varying mass supply
rate. Variable mass supply rate occurs, e.g., during disc state transitions,
and in interacting eccentric binaries. It is, however, damped by the viscosity
of the disc. Here, we calculate this damping in detail. We derive an analytical
description of the propagation of the flow rate using the solution of
Lynden-Bell & Pringle, in which the disc is assumed to extend to infinity. In
particular, we derive the accretion-rate Green's function, and its Fourier
transform, which gives the fractional damping at a given variability frequency.
We then compare this model to that of a finite disc with the mass supply at its
outer edge. We find significant differences with respect to the infinite disc
solution, which we find to overestimate the viscous damping. In particular, the
asymptotic form of the Green's function is power-law for the infinite disc and
exponential for the finite one. We then find a simple fitting form for the
latter, and also calculate its Fourier transform. In general, the damping
becomes very strong when the viscous time at the outer edge of the disc becomes
longer than the modulation time scale. We apply our results to a number of
astrophysical systems. We find the effect is much stronger in low-mass X-ray
binaries, where the disc size is comparable to that of the Roche lobe, than in
high-mass binaries, where the wind-fed disc can have a much smaller size.
|
0902.4530v2
|
2010-04-09
|
Oscillations of weakly viscous conducting liquid drops in a strong magnetic field
|
We analyse small-amplitude oscillations of a weakly viscous electrically
conducting liquid drop in a strong uniform DC magnetic field. An asymptotic
solution is obtained showing that the magnetic field does not affect the shape
eigenmodes, which remain the spherical harmonics as in the non-magnetic case.
Strong magnetic field, however, constrains the liquid flow associated with the
oscillations and, thus, reduces the oscillation frequencies by increasing
effective inertia of the liquid. In such a field, liquid oscillates in a
two-dimensional (2D) way as solid columns aligned with the field. Two types of
oscillations are possible: longitudinal and transversal to the field. Such
oscillations are weakly damped by a strong magnetic field - the stronger the
field, the weaker the damping, except for the axisymmetric transversal and
inherently 2D modes. The former are overdamped because of being incompatible
with the incompressibility constraint, whereas the latter are not affected at
all because of being naturally invariant along the field. Since the magnetic
damping for all other modes decreases inversely with the square of the field
strength, viscous damping may become important in a sufficiently strong
magnetic field. The viscous damping is found analytically by a simple energy
dissipation approach which is shown for the longitudinal modes to be equivalent
to a much more complicated eigenvalue perturbation technique. This study
provides a theoretical basis for the development of new measurement methods of
surface tension, viscosity and the electrical conductivity of liquid metals
using the oscillating drop technique in a strong superimposed DC magnetic
field.
|
1004.1548v2
|
2011-02-03
|
Damping of Electron Density Structures and Implications for Interstellar Scintillation
|
The forms of electron density structures in kinetic Alfven wave turbulence
are studied in connection with scintillation. The focus is on small scales $L
\sim 10^8-10^{10}$ cm where the Kinetic Alfv\'en wave (KAW) regime is active in
the interstellar medium. MHD turbulence converts to a KAW cascade, starting at
10 times the ion gyroradius and continuing to smaller scales. These scales are
inferred to dominate scintillation in the theory of Boldyrev et al. From
numerical solutions of a decaying kinetic Alfv\'en wave turbulence model,
structure morphology reveals two types of localized structures, filaments and
sheets, and shows that they arise in different regimes of resistive and
diffusive damping. Minimal resistive damping yields localized current filaments
that form out of Gaussian-distributed initial conditions. When resistive
damping is large relative to diffusive damping, sheet-like structures form. In
the filamentary regime, each filament is associated with a non-localized
magnetic and density structure, circularly symmetric in cross section. Density
and magnetic fields have Gaussian statistics (as inferred from Gaussian-valued
kurtosis) while density gradients are strongly non-Gaussian, more so than
current. This enhancement of non-Gaussian statistics in a derivative field is
expected since gradient operations enhance small-scale fluctuations. The
enhancement of density gradient kurtosis over current kurtosis is not obvious,
yet it suggests that modest fluctuation levels in electron density may yield
large scintillation events during pulsar signal propagation in the interstellar
medium. In the sheet regime the same statistical observations hold, despite the
absence of localized filamentary structures. Probability density functions are
constructed from statistical ensembles in both regimes, showing clear formation
of long, highly non-Gaussian tails.
|
1102.0810v2
|
2011-09-28
|
Different dimensionality trends in the Landau damping of magnons in iron, cobalt and nickel: time dependent density functional study
|
We study the Landau damping of ferromagnetic magnons in Fe, Co, and Ni as the
dimensionality of the system is reduced from three to two. We resort to the
\textit{ab initio} linear response time dependent density functional theory in
the adiabatic local spin density approximation. The numerical scheme is based
on the Korringa-Kohn-Rostoker Green's function method. The key points of the
theoretical approach and the implementation are discussed. We investigate the
transition metals in three different forms: bulk phases, free-standing thin
films and thin films supported on a nonmagnetic substrate. We demonstrate that
the dimensionality trends in Fe and Ni are opposite: in Fe the transition from
bulk bcc crystal to Fe/Cu(100) film reduces the damping whereas in Ni/Cu(100)
film the attenuation increases compared to bulk fcc Ni. In Co, the strength of
the damping depends relatively weakly on the sample dimensionality. We explain
the difference in the trends on the basis of the underlying electronic
structure. The influence of the substrate on the spin-wave damping is analyzed
by employing Landau maps representing wave-vector resolved spectral density of
the Stoner excitations.
|
1109.6217v2
|
2011-10-06
|
Dissipative and conservative nonlinearity in carbon nanotube and graphene mechanical resonators
|
Graphene and carbon nanotubes represent the ultimate size limit of one and
two-dimensional nanoelectromechanical resonators. Because of their reduced
dimensionality, graphene and carbon nanotubes display unusual mechanical
behavior; in particular, their dynamics is highly nonlinear. Here, we review
several types of nonlinear behavior in resonators made from nanotubes and
graphene. We first discuss an unprecedented scenario where damping is described
by a nonlinear force. This scenario is supported by several experimental facts:
(i) the quality factor varies with the amplitude of the motion as a power law
whose exponent coincides with the value predicted by the nonlinear damping
model, (ii) hysteretic behavior (of the motional amplitude as a function of
driving frequency) is absent in some of our resonators even for large driving
forces, as expected when nonlinear damping forces are large, and (iii) when we
quantify the linear damping force (by performing parametric excitation
measurements) we find that it is significantly smaller than the nonlinear
damping force. We then review parametric excitation measurements, an
alternative actuation method which is based on nonlinear dynamics. Finally, we
discuss experiments where the mechanical motion is coupled to electron
transport through a nanotube. The coupling can be made so strong that the
associated force acting on the nanotube becomes highly nonlinear with
displacement and velocity. Overall, graphene and nanotube resonators hold
promise for future studies on classical and quantum nonlinear dynamics.
|
1110.1234v1
|
2012-06-02
|
Slow Mode Oscillations and Damping of Hot Solar Coronal Loops
|
The effect of temperature inhomogeneity on the periods, their ratios
(fundamental vs. first overtone), and the damping times of the standing slow
modes in gravitationally stratified solar coronal loops are studied. The
effects of optically thin radiation, compressive viscosity, and thermal
conduction are considered. The linearized one-dimensional magnetohydrodynamic
(MHD) equations (under low-$\beta$ condition) were reduced to a fourth--order
ordinary differential equation for the perturbed velocity. The numerical
results indicate that the periods of non-isothermal loops (i.e. temperature
increases from the loop base to apex) are smaller compared to those of
isothermal loops. In the presence of radiation, viscosity, and thermal
conduction, an increase in the temperature gradient is followed by a monotonic
decrease in the periods (compared with the isothermal case), while the period
ratio turns out to be a sensitive function of the gradient of the temperature
and the loop lengths. We verify that radiative dissipation is not a main
cooling mechanism of both isothermal and non-isothermal hot coronal loops and
has a small effect on the periods. Thermal conduction and compressive viscosity
are primary mechanisms in the damping of slow modes of the hot coronal loops.
The periods and damping times in the presence of compressive viscosity and/or
thermal conduction dissipation are consistent with the observed data in
specific cases. By tuning the dissipation parameters, the periods and the
damping times could be made consistent with the observations in more general
cases.
|
1206.0366v1
|
2012-09-15
|
Damped kink oscillations of flowing prominence threads
|
Transverse oscillations of thin threads in solar prominences are frequently
reported in high-resolution observations. Two typical features of the
observations are that the oscillations are damped in time and that simultaneous
mass flows along the threads are detected. Flows cause the dense threads to
move along the prominence magnetic structure while the threads are oscillating.
The oscillations have been interpreted in terms of standing magnetohydrodynamic
(MHD) kink waves of the magnetic flux tubes which support the threads. The
damping is most likely due to resonant absorption caused by plasma
inhomogeneity. The technique of seismology uses the observations combined with
MHD wave theory to estimate prominence physical parameters. This paper presents
a theoretical study of the joint effect of flow and resonant absorption on the
amplitude of standing kink waves in prominence threads. We find that flow and
resonant absorption can either be competing effects on the amplitude or both
can contribute to damp the oscillations depending on the instantaneous position
of the thread within the prominence magnetic structure. The amplitude profile
deviates from the classic exponential profile of resonantly damped kink waves
in static flux tubes. Flow also introduces a progressive shift of the
oscillation period compared to the static case, although this effect is in
general of minor importance. We test the robustness of seismological estimates
by using synthetic data aiming to mimic real observations. The effect of the
thread flow can significantly affect the estimation of the transverse
inhomogeneity length scale. The presence of random background noise adds
uncertainty to this estimation. Caution needs to be paid to the seismological
estimates that do not take the influence of flow into account.
|
1209.3382v1
|
2013-04-13
|
Parametric survey of longitudinal prominence oscillation simulations
|
It is found that both microflare-sized impulsive heating at one leg of the
loop and a suddenly imposed velocity perturbation can propel the prominence to
oscillate along the magnetic dip. An extensive parameter survey results in a
scaling law, showing that the period of the oscillation, which weakly depends
on the length and height of the prominence, and the amplitude of the
perturbations, scales with $\sqrt{R/g_\odot}$, where $R$ represents the
curvature radius of the dip, and $g_\odot$ is the gravitational acceleration of
the Sun. This is consistent with the linear theory of a pendulum, which implies
that the field-aligned component of gravity is the main restoring force for the
prominence longitudinal oscillations, as confirmed by the force analysis.
However, the gas pressure gradient becomes non-negligible for short
prominences. The oscillation damps with time in the presence of non-adiabatic
processes. Compared to heat conduction, the radiative cooling is the dominant
factor leading to the damping. A scaling law for the damping timescale is
derived, i.e., $\tau\sim l^{1.63} D^{0.66}w^{-1.21}v_{0}^{-0.30}$, showing
strong dependence on the prominence length $l$, the geometry of the magnetic
dip (characterized by the depth $D$ and the width $w$), and the velocity
perturbation amplitude $v_0$. The larger the amplitude, the faster the
oscillation damps. It is also found that mass drainage significantly reduces
the damping timescale when the perturbation is too strong.
|
1304.3798v1
|
2013-06-08
|
Observation of a Berry phase anti-damping spin-orbit torque
|
Recent observations of current-induced magnetization switching at
ferromagnet/normal-conductor interfaces have important consequences for future
magnetic memory technology. In one interpretation, the switching originates
from carriers with spin-dependent scattering giving rise to a relativistic
anti-damping spin-orbit torque (SOT) in structures with broken space-inversion
symmetry. The alternative interpretation combines the relativistic spin Hall
effect (SHE), making the normal-conductor an injector of a spin-current, with
the non-relativistic spin-transfer torque (STT) in the ferromagnet. Remarkably,
the SHE in these experiments originates from the Berry phase effect in the band
structure of a clean crystal and the anti-damping STT is also based on a
disorder-independent transfer of spin from carriers to magnetization. Here we
report the observation of an anti-damping SOT stemming from an analogous Berry
phase effect to the SHE. The SOT alone can therefore induce magnetization
dynamics based on a scattering-independent principle. The ferromagnetic
semiconductor (Ga,Mn)As we use has a broken space-inversion symmetry in the
crystal. This allows us to consider a bare ferromagnetic element which
eliminates by design any SHE related contribution to the spin torque. We
provide an intuitive picture of the Berry phase origin of the anti-damping SOT
and a microscopic modeling of measured data.
|
1306.1893v1
|
2013-08-20
|
Stringent constraints on the H I spin temperature in two z > 3 Damped Lyman-alpha systems from redshifted 21 cm absorption studies
|
Physical properties of Damped Lyman-alpha absorbers and their evolution are
closely related to galaxy formation and evolution theories, and have important
cosmological implications. H I 21 cm absorption study is one useful way of
measuring the temperature of these systems. In this work, very strong
constraints on the temperature of two Damped Lyman-alpha absorbers at z > 3 are
derived from low radio frequency observations. The H I spin temperature is
found to be greater than 2000 K for both the absorbers. The high spin
temperature of these high-redshift systems is in agreement with the trend found
in a compilation of temperatures for other Damped Lyman-alpha absorbers. We
also argue that the temperature - metallicity relation, reported earlier in the
literature, is unlikely to be a spurious line of sight effect, and that the
redshift evolution of the spin temperature does not arises due to a selection
effect. All of these are consistent with a redshift evolution of the warm gas
fraction in Damped Lyman-alpha systems.
|
1308.4410v1
|
2013-09-26
|
Non-Landau damping of magnetic excitations in systems with localized and itinerant electrons
|
We discuss the form of the damping of magnetic excitations in a metal near a
ferromagnetic instability. The paramagnon theory predicts that the damping term
should have the form $\Omega/\Gamma (q)$ with $\Gamma (q) \propto q$ (the
Landau damping). However, the experiments on uranium metallic compounds UGe$_2$
and UCoGe showed that $\Gamma (q)$ tends to a constant value at vanishing $q$.
A non-zero $\Gamma (0)$ is impossible in systems with one type of carriers
(either localized or itinerant) because it would violate the spin conservation.
It has been conjectured recently that a non-zero $\Gamma (q)$ in UGe$_2$ and
UCoGe may be due to the presence of both localized and itinerant electrons in
these materials, with ferromagnetism involving predominantly localized spins.
We present microscopic analysis of the damping of near-critical localized
excitations due to interaction with itinerant carriers. We show explicitly how
the presence of two types of electrons breaks the cancellation between the
contributions to $\Gamma (0)$ from self-energy and vertex correction insertions
into the spin polarization bubble and discuss the special role of the
Aslamazov-Larkin processes. We show that $\Gamma (0)$ increases with $T$ both
in the paramagnetic and ferromagnetic regions, but in-between it has a peak at
$T_c$. We compare our theory with the available experimental data.
|
1309.7065v3
|
2014-06-16
|
Design of the Readout Electronics for the Qualification Model of DAMPE BGO Calorimeter
|
The DAMPE (DArk Matter Particle Explorer) is a scientific satellite being
developed in China, aimed at cosmic ray study, gamma ray astronomy, and
searching for the clue of dark matter particles, with a planned mission period
of more than 3 years and an orbit altitude of about 500 km. The BGO
Calorimeter, which consists of 308 BGO (Bismuth Germanate Oxid) crystal bars,
616 PMTs (photomultiplier tubes) and 1848 dynode signals, has approximately 32
radiation lengths. It is a crucial sub-detector of the DAMPE payload, with the
functions of precisely measuring the energy of cosmic particles from 5 GeV to
10TeV, distinguishing positrons/electrons and gamma rays from hadron
background, and providing trigger information for the whole DAMPE payload. The
dynamic range for a single BGO crystal is about 2?105 and there are 1848
detector signals in total. To build such an instrument in space, the major
design challenges for the readout electronics come from the large dynamic
range, the high integrity inside the very compact structure, the strict power
supply budget and the long term reliability to survive the hush environment
during launch and in orbit. Currently the DAMPE mission is in the end of QM
(Qualification Model) stage. This paper presents a detailed description of the
readout electronics for the BGO calorimeter.
|
1406.3886v1
|
2014-11-24
|
Damping of liquid sloshing by foams
|
When a container is set in motion, the free surface of the liquid starts to
oscillate or slosh. Such effects can be observed when a glass of water is
handled carelessly and the fluid sloshes or even spills over the rims of the
container. However, beer does not slosh as readily as water, which suggests
that foam could be used to damp sloshing. In this work, we study experimentally
the effect on sloshing of a liquid foam placed on top of a liquid bath. We
generate a monodisperse two-dimensional liquid foam in a rectangular container
and track the motion of the foam. The influence of the foam on the sloshing
dynamics is experimentally characterized: only a few layers of bubbles are
sufficient to significantly damp the oscillations. We rationalize our
experimental findings with a model that describes the foam contribution to the
damping coefficient through viscous dissipation on the walls of the container.
Then we extend our study to confined three-dimensional liquid foam and observe
that the behavior of 2D and confined 3D systems are very similar. Thus we
conclude that only the bubbles close to the walls have a significant impact on
the dissipation of energy. The possibility to damp liquid sloshing using foam
is promising in numerous industrial applications such as the transport of
liquefied gas in tankers or for propellants in rocket engines.
|
1411.6542v2
|
2015-04-16
|
Attenuation of short strongly nonlinear stress pulses in dissipative granular chains
|
Attenuation of short, strongly nonlinear stress pulses in chains of spheres
and cylinders was investigated experimentally and numerically for two ratios of
their masses keeping their contacts identical. The chain with mass ratio 0.98
supports solitary waves and another one (with mass ratio 0.55) supports
nonstationary pulses which preserve their identity only on relatively short
distances, but attenuate on longer distances because of radiation of small
amplitude tails generated by oscillating small mass particles. Pulse
attenuation in experiments in the chain with mass ratio 0.55 was faster at the
same number of the particles from the entrance than in the chain with mass
ratio 0.98. It is in quantitative agreement with results of numerical
calculations with effective damping coefficient 6 kg/s. This level of damping
was critical for eliminating the gap openings between particles in the system
with mass ratio 0.55 present at lower or no damping. However with increase of
dissipation numerical results show that the chain with mass ratio 0.98 provides
faster attenuation than chain with mass ratio 0.55 due to the fact that the
former system supports the narrower pulse with the larger difference between
velocities of neighboring particles. The investigated chains demonstrated
different wave structure at zero dissipation and at intermediate damping
coefficients and the similar behavior at large damping.
|
1504.04344v1
|
2015-04-17
|
Chiral damping of magnetic domain walls
|
Structural symmetry breaking in magnetic materials is responsible for a
variety of outstanding physical phenomena. Examples range from the existence of
multiferroics, to current induced spin orbit torques (SOT) and the formation of
topological magnetic structures. In this letter we bring into light a novel
effect of the structural inversion asymmetry (SIA): a chiral damping mechanism.
This phenomenon is evidenced by measuring the field driven domain wall (DW)
motion in perpendicularly magnetized asymmetric Pt/Co/Pt trilayers. The
difficulty in evidencing the chiral damping is that the ensuing DW dynamics
exhibit identical spatial symmetry to those expected from the
Dzyaloshinskii-Moriya interaction (DMI). Despite this fundamental resemblance,
the two scenarios are differentiated by their time reversal properties: while
DMI is a conservative effect that can be modeled by an effective field, the
chiral damping is purely dissipative and has no influence on the equilibrium
magnetic texture. When the DW motion is modulated by an in-plane magnetic
field, it reveals the structure of the internal fields experienced by the DWs,
allowing to distinguish the physical mechanism. The observation of the chiral
damping, not only enriches the spectrum of physical phenomena engendered by the
SIA, but since it can coexists with DMI it is essential for conceiving DW and
skyrmion devices.
|
1504.04411v1
|
2015-07-28
|
Spatial damping of propagating sausage waves in coronal cylinders
|
Sausage modes are important in coronal seismology. Spatially damped
propagating sausage waves were recently observed in the solar atmosphere. We
examine how wave leakage influences the spatial damping of sausage waves
propagating along coronal structures modeled by a cylindrical density
enhancement embedded in a uniform magnetic field. Working in the framework of
cold magnetohydrodynamics, we solve the dispersion relation (DR) governing
sausage waves for complex-valued longitudinal wavenumber $k$ at given real
angular frequencies $\omega$. For validation purposes, we also provide
analytical approximations to the DR in the low-frequency limit and in the
vicinity of $\omega_{\rm c}$, the critical angular frequency separating trapped
from leaky waves. In contrast to the standing case, propagating sausage waves
are allowed for $\omega$ much lower than $\omega_{\rm c}$. However, while able
to direct their energy upwards, these low-frequency waves are subject to
substantial spatial attenuation. The spatial damping length shows little
dependence on the density contrast between the cylinder and its surroundings,
and depends only weakly on frequency. This spatial damping length is of the
order of the cylinder radius for $\omega \lesssim 1.5 v_{\rm Ai}/a$, where $a$
and $v_{\rm Ai}$ are the cylinder radius and the Alfv\'en speed in the
cylinder, respectively. We conclude that if a coronal cylinder is perturbed by
symmetric boundary drivers (e.g., granular motions) with a broadband spectrum,
wave leakage efficiently filters out the low-frequency components.
|
1507.07724v1
|
2015-10-19
|
On the branching of the quasinormal resonances of near-extremal Kerr black holes
|
It has recently been shown by Yang. et. al. [Phys. Rev. D {\bf 87}, 041502(R)
(2013)] that rotating Kerr black holes are characterized by two distinct sets
of quasinormal resonances. These two families of quasinormal resonances display
qualitatively different asymptotic behaviors in the extremal ($a/M\to 1$)
black-hole limit: The zero-damping modes (ZDMs) are characterized by relaxation
times which tend to infinity in the extremal black-hole limit ($\Im\omega\to 0$
as $a/M\to 1$), whereas the damped modes (DMs) are characterized by non-zero
damping rates ($\Im\omega\to$ finite-values as $a/M\to 1$). In this paper we
refute the claim made by Yang et. al. that co-rotating DMs of near-extremal
black holes are restricted to the limited range $0\leq
\mu\lesssim\mu_{\text{c}}\approx 0.74$, where $\mu\equiv m/l$ is the
dimensionless ratio between the azimuthal harmonic index $m$ and the spheroidal
harmonic index $l$ of the perturbation mode. In particular, we use an
analytical formula originally derived by Detweiler in order to prove the
existence of DMs (damped quasinormal resonances which are characterized by
finite $\Im\omega$ values in the $a/M\to 1$ limit) of near-extremal black holes
in the $\mu>\mu_{\text{c}}$ regime, the regime which was claimed by Yang et.
al. not to contain damped modes. We show that these co-rotating DMs (in the
regime $\mu>\mu_{\text{c}}$) are expected to characterize the resonance spectra
of rapidly-rotating (near-extremal) black holes with $a/M\gtrsim 1-10^{-9}$.
|
1510.05604v1
|
2016-02-16
|
Damping and power spectra of quasi-periodic intensity disturbances above a solar polar coronal hole
|
We study intensity disturbances above a solar polar coronal hole seen in the
AIA 171 \AA\ and 193 \AA\ passbands, aiming to provide more insights into their
physical nature. The damping and power spectra of the intensity disturbances
with frequencies from 0.07 mHz to 10.5 mHz are investigated. The damping of the
intensity disturbances tends to be stronger at lower frequencies, and their
damping behavior below 980" (for comparison, the limb is at 945") is different
from what happens above. No significant difference is found between the damping
of the intensity disturbances in the AIA 171 \AA\ and that in the AIA 193 \AA.
The indices of the power spectra of the intensity disturbances are found to be
slightly smaller in the AIA 171 \AA\ than in the AIA 193 \AA, but the
difference is within one sigma deviation. An additional enhanced component is
present in the power spectra in a period range of 8--40 minutes at lower
heights. While the power spectra of spicule is highly correlated with its
associated intensity disturbance, it suggests that the power spectra of the
intensity disturbances might be a mixture of spicules and wave activities. We
suggest that each intensity disturbance in the polar coronal hole is possibly a
series of independent slow magnetoacoustic waves triggered by spicular
activities.
|
1602.04883v1
|
2016-04-20
|
Nonlinear wave damping due to multi-plasmon resonances
|
For short wavelengths, it is well known that the linearized Wigner-Moyal
equation predicts wave damping due to wave-particle interaction, where the
resonant velocity shifted from the phase velocity by a velocity $v_q = \hbar
k/2m$. Here $\hbar$ is the reduced Planck constant, $k$ is the wavenumber and
$m$ is the electron mass. Going beyond linear theory, we find additional
resonances with velocity shifts $n v_q$, $n = 2, 3, \ldots$, giving rise to a
new wave-damping mechanism that we term \emph{multi-plasmon damping}, as it can
be seen as the simultaneous absorption (or emission) of multiple plasmon
quanta. Naturally this wave damping is not present in classical plasmas. For a
temperature well below the Fermi temperature, if the linear ($n = 1$) resonant
velocity is outside the Fermi sphere, the number of linearly resonant particles
is exponentially small, while the multi-plasmon resonances can be located in
the bulk of the distribution. We derive sets of evolution equations for the
case of two-plasmon and three-plasmon resonances for Langmuir waves in the
simplest case of a fully degenerate plasma. By solving these equations
numerically for a range of wave-numbers we find the corresponding damping
rates, and we compare them to results from linear theory to estimate the
applicability. Finally, we discuss the effects due to a finite temperature.
|
1604.05983v2
|
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
|
2019-05-22
|
Ultra-low magnetic damping in Co 2 Mn-based Heusler compounds: promising materials for spintronic
|
The prediction of ultra-low magnetic damping in Co 2 MnZ Heusler half-metal
thin-film magnets is explored in this study and the damping response is shown
to be linked to the underlying electronic properties. By substituting the Z
elements in high crystalline quality films (Co 2 MnZ with Z=Si, Ge, Sn, Al, Ga,
Sb), electronic properties such as the minority spin band gap, Fermi energy
position in the gap and spin polarization can be tuned and the consequence on
magnetization dynamics analyzed. The experimental results allow us to directly
explore the interplay of spin polarization, spin gap, Fermi energy position and
the magnetic damping obtained in these films, together with ab initio
calculation predictions. The ultra-low magnetic damping coefficients measured
in the range 4.1 10-4-9 10-4 for Co 2 MnSi, Ge, Sn, Sb are the lowest values
obtained on a conductive layer and offers a clear experimental demonstration of
theoretical predictions on Half-Metal Magnetic Heusler compounds and a pathway
for future materials design.
|
1905.08987v1
|
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
|
2019-07-29
|
Breather arrest in a chain of damped oscillators with Hertzian contact
|
We explore breather propagation in the damped oscillatory chain with
essentially nonlinear (non-linearizable) nearest-neighbour coupling.
Combination of the damping and the substantially nonlinear coupling leads to
rather unusual two-stage pattern of the breather propagation. The first stage
occurs at finite fragment of the chain and is characterized by power-law decay
of the breather amplitude. The second stage is characterized by extremely small
breather amplitudes that decay hyper-exponentially with the site number. Thus,
practically, one can speak about finite penetration depth of the breather. This
phenomenon is referred to as breather arrest (BA). As particular example, we
explore the chain with Hertzian contacts. Dependencies of the breather
penetration depth on the initial excitation and on the damping coefficient on
the breather penetration depth obey power laws. The results are rationalized by
considering beating responses in a system of two damped linear oscillators with
strongly nonlinear (non-linearizable) coupling. Initial excitation of one of
these oscillators leads to strictly finite number of beating cycles. Then, the
beating cycle in this simplified system is associated with the passage of the
discrete breather between the neighbouring sites in the chain. Somewhat
surprisingly, this simplified model reliably predicts main quantitative
features of the breather arrest in the chain, including the exponents in
numerically observed power laws.
|
1907.12462v1
|
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
|
2019-12-09
|
Analytical solution of linearized equations of the Morris-Lecar neuron model at large constant stimulation
|
The classical biophysical Morris-Lecar model of neuronal excitability
predicts that upon stimulation of the neuron with a sufficiently large constant
depolarizing current there exists a finite interval of the current values where
periodic spike generation occurs. Above the upper boundary of this interval,
there is four-stage damping of the spike amplitude: 1) minor primary damping,
which reflects a typical transient to stationary dynamic state, 2) plateau of
nearly undamped periodic oscillations, 3) strong damping, and 4) reaching a
constant asymptotic value of the neuron potential. We have shown that in the
vicinity of the asymptote the Morris-Lecar equations can be reduced to the
standard equation for exponentially damped harmonic oscillations. Importantly,
all coefficients of this equation can be explicitly expressed through
parameters of the original Morris-Lecar model, enabling direct comparison of
the numerical and analytical solutions for the neuron potential dynamics at
later stages of the spike amplitude damping.
|
1912.04083v4
|
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
|
2022-05-10
|
Nonlinear damping quantification from phase-resonant tests under base excitation
|
The present work addresses the experimental identification of
amplitude-dependent modal parameters (modal frequency, damping ratio, Fourier
coefficients of periodic modal oscillation). Phase-resonant testing has emerged
as an important method for this task, as it substantially reduces the amount of
data required for the identification compared to conventional
frequency-response testing at different excitation/response levels. In the case
of shaker-stinger excitation, the applied excitation force is commonly measured
in order to quantify the amplitude-dependent modal damping ratio from the
phase-resonant test data. In the case of base excitation, however, the applied
excitation force is challenging or impossible to measure. In this work we
develop an original method for damping quantification from phase-resonant
tests. It relies solely on response measurement; it avoids the need to resort
to force measurement. The key idea is to estimate the power provided by the
distributed inertia force imposed by the base motion. We develop both a
model-free and a model-based variant of the method. We validate the developed
method first in virtual experiments of a friction-damped and a geometrically
nonlinear system, and then in a physical experiment involving a thin beam
clamped at both ends via bolted joints. We conclude that the method is highly
robust and provides high accuracy already for a reasonable number of sensors.
|
2205.04735v1
|
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
|
2023-03-15
|
Blow-up and decay for a class of variable coefficient wave equation with nonlinear damping and logarithmic source
|
In this paper, we consider the long time behavior for the solution of a class
of variable coefficient wave equation with nonlinear damping and logarithmic
source. The existence and uniqueness of local weak solution can be obtained by
using the Galerkin method and contraction mapping principle. However, the long
time behavior of the solution is usually complicated and it depends on the
balance mechanism between the damping and source terms. When the damping
exponent $(p+1)$ (see assumption (H3)) is greater than the source term exponent
$(q-1)$ (see equation (1.1)), namely, $p+2>q$, we obtain the global existence
and accurate decay rates of the energy for the weak solutions with any initial
data. Moreover, whether the weak solution exists globally or blows up in finite
time, it is closely related to the initial data. In the framework of modified
potential well theory, we construct the stable and unstable sets (see (2.8))
for the initial data. For the initial data belonging to the stable set, we
prove that the weak solution exists globally and has similar decay rates as the
previous results. For $p+2<q$ and the initial data belonging to the unstable
set, we prove that the weak solution blows up in finite time for a little
special damping $g(u_{t})=|u_{t}|^{p}u_{t}$.
|
2303.08629v1
|
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
|
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