736 research outputs found
Attracted Diffusion-Limited Aggregation
In this paper, we present results of extensive Monte Carlo simulations of
diffusion-limited aggregation (DLA) with a seed placed on an attractive plane
as a simple model in connection with the electrical double layers. We compute
the fractal dimension of the aggregated patterns as a function of the
attraction strength \alpha. For the patterns grown in both two and three
dimensions, the fractal dimension shows a significant dependence on the
attraction strength for small values of \alpha, and approaches to that of the
ordinary two-dimensional (2D) DLA in the limit of large \alpha. For
non-attracting case with \alpha=1, our results in three dimensions reproduce
the patterns of 3D ordinary DLA, while in two dimensions our model leads to
formation of a compact cluster with dimension two. For intermediate \alpha, the
3D clusters have quasi-2D structure with a fractal dimension very close to that
of the ordinary 2D-DLA. This allows one to control morphology of a growing
cluster by tuning a single external parameter \alpha.Comment: 6 pages, 6 figures, to appear in Phys. Rev. E (2012
A number-conserving linear response study of low-velocity ion stopping in a collisional magnetized classical plasma
The results of a theoretical investigation on the low-velocity stopping power
of the ions moving in a magnetized collisional plasma are presented. The
stopping power for an ion is calculated employing linear response theory using
the dielectric function approach. The collisions, which leads to a damping of
the excitations in the plasma, is taken into account through a
number-conserving relaxation time approximation in the linear response
function. In order to highlight the effects of collisions and magnetic field we
present a comparison of our analytical and numerical results obtained for a
nonzero damping or magnetic field with those for a vanishing damping or
magnetic field. It is shown that the collisions remove the anomalous friction
obtained previously [Nersisyan et al., Phys. Rev. E 61, 7022 (2000)] for the
collisionless magnetized plasmas at low ion velocities. One of major objectives
of this study is to compare and contrast our theoretical results with those
obtained through a novel diffusion formulation based on Dufty-Berkovsky
relation evaluated in magnetized one-component plasma models framed on target
ions and electrons.Comment: Submitted to Phys. Rev. E, 17 pages, 4 figure
Electron Correlations in an Electron Bilayer at Finite Temperature: Landau Damping of the Acoustic Plasmon
We report angle-resolved Raman scattering observations of the temperature
dependent Landau damping of the acoustic plasmon in an electron bilayer system
realised in a GaAs double quantum well structure. Corresponding calculations of
the charge-density excitation spectrum of the electron bilayer using forms of
the random phase approximation (RPA), and the static local field formalism of
Singwi, Tosi, Land and Sj\"{o}lander (STLS) extended to incorporate non-zero
electron temperature and phenomenological damping, are also
presented. The STLS calculations include details of the temperature dependence
of the intra- and inter-layer local field factors and pair-correlation
functions. Good agreement between experiment and the various theories is
obtained for the acoustic plasmon energy and damping for , the Fermi temperature. However, contrary to current expectations,
all of the calculations show significant departures from our experimental data
for . From this, we go on to demonstrate
unambiguously that real local field factors fail to provide a physically
accurate description of exchange correlation behaviour in low dimensional
electron gases. Our results suggest instead that one must resort to a
{\em{dynamical}} local field theory, characterised by a {\em{complex}} field
factor to provide a more accurate description.Comment: 53 pages, 16 figure
Stopping of Charged Particles in a Magnetized Classical Plasma
The analytical and numerical investigations of the energy loss rate of the
test particle in a magnetized electron plasma are developed on the basis of the
Vlasov-Poisson equations, and the main results are presented. The Larmor
rotation of a test particle in a magnetic field is taken into account. The
analysis is based on the assumption that the energy variation of the test
particle is much less than its kinetic energy. The obtained general expression
for stopping power is analyzed for three cases: (i) the particle moves through
a collisionless plasma in a strong homogeneous magnetic field; (ii) the fast
particle moves through a magnetized collisionless plasma along the magnetic
field; and (iii) the particle moves through a magnetized collisional plasma
across a magnetic field. Calculations are carried out for the arbitrary test
particle velocities in the first case, and for fast particles in the second and
third cases. It is shown that the rate at which a fast test particle loses
energy while moving across a magnetic field may be much higher than the loss in
the case of motion through plasma without magnetic field.Comment: 14 pages, 3 figures, LaTe
A long-lived coronal X-ray arcade
A large, long-lived, soft X-ray emitting arch system observed during a Skylab mission is analyzed. The supposition is that these arches owe their stability to the stable coronal magnetic-field configuration. A global constant alpha force-free magnetic field analysis, is used to describe the arches which stayed in the same approximate position for several solar rotations. A marked resemblance is noted between the theoretical magnetic field configuration and the observed X-ray emmitting feature
Axiomatic geometrical optics, Abraham-Minkowski controversy, and photon properties derived classically
By restating geometrical optics within the field-theoretical approach, the
classical concept of a photon (and, more generally, any elementary excitation)
in arbitrary dispersive medium is introduced, and photon properties are
calculated unambiguously. In particular, the canonical and kinetic momenta
carried by a photon, as well as the two corresponding energy-momentum tensors
of a wave, are derived from first principles of Lagrangian mechanics. As an
example application of this formalism, the Abraham-Minkowski controversy
pertaining to the definitions of these quantities is resolved for linear waves
of arbitrary nature, and corrections to the traditional formulas for the photon
kinetic energy-momentum are found. Several other applications of axiomatic
geometrical optics to electromagnetic waves are also presented
Cosmic polarimetry in magnetoactive plasmas
Polarimetry of the Cosmic Microwave Background (CMB) represents one of the
possible diagnostics aimed at testing large-scale magnetism at the epoch of the
photon decoupling. The propagation of electromagnetic disturbances in a
magnetized plasma leads naturally to a B-mode polarization whose angular power
spectrum is hereby computed both analytically and numerically. Combined
analyses of all the publicly available data on the B-mode polarization are
presented, for the first time, in the light of the magnetized CDM
scenario. Novel constraints on pre-equality magnetism are also derived in view
of the current and expected sensitivities to the B-mode polarization.Comment: 34 pages, 13 figure
Compressible hydromagnetic nonlinearities in the predecoupling plasma
The adiabatic inhomogeneities of the scalar curvature lead to a compressible
flow affecting the dynamics of the hydromagnetic nonlinearities. The influence
of the plasma on the evolution of a putative magnetic field is explored with
the aim of obtaining an effective description valid for sufficiently large
scales. The bulk velocity of the plasma, computed in the framework of the
LambdaCDM scenario, feeds back into the evolution of the magnetic power spectra
leading to a (nonlocal) master equation valid in Fourier space and similar to
the ones discussed in the context of wave turbulence. Conversely, in physical
space, the magnetic power spectra obey a Schroedinger-like equation whose
effective potential depends on the large-scale curvature perturbations.
Explicit solutions are presented both in physical space and in Fourier space.
It is argued that curvature inhomogeneities, compatible with the WMAP 7yr data,
shift to lower wavenumbers the magnetic diffusivity scale.Comment: 29 page
Heating of gas inside radio sources to mildly relativistic temperatures via induced Compton scattering
Measured values of the brightness temperature of low-frequency synchrotron
radiation emitted by powerful extragalactic sources reach 10^11--10^12 K. If
some amount of nonrelativistic ionized gas is present within such sources, it
should be heated as a result of induced Compton scattering of the radiation. If
this heating is counteracted by cooling due to inverse Compton scattering of
the same radio radiation, then the plasma can be heated up to mildly
relativistic temperatures kT~10--100 keV. The stationary electron velocity
distribution can be either relativistic Maxwellian or quasi-Maxwellian (with
the high-velocity tail suppressed), depending on the efficiency of Coulomb
collisions and other relaxation processes. We derive several easy-to-use
approximate expressions for the induced Compton heating rate of mildly
relativistic electrons in an isotropic radiation field, as well as for the
stationary distribution function and temperature of electrons. We also give
analytic expressions for the kernel of the integral kinetic equation (one as a
function of the scattering angle and another for the case of an isotropic
radiation field), which describes the redistribution of photons in frequency
caused by induced Compton scattering in thermal plasma. These expressions can
be used in the parameter range hnu<< kT<~ 0.1mc^2 (the formulae earlier
published in Sazonov, Sunyaev, 2000 are less accurate).Comment: 22 pages, 7 figures, submitted to Astronomy Letter
Nyquist method for Wigner-Poisson quantum plasmas
By means of the Nyquist method, we investigate the linear stability of
electrostatic waves in homogeneous equilibria of quantum plasmas described by
the Wigner-Poisson system. We show that, unlike the classical Vlasov-Poisson
system, the Wigner-Poisson case does not necessarily possess a Penrose
functional determining its linear stability properties. The Nyquist method is
then applied to a two-stream distribution, for which we obtain an exact,
necessary and sufficient condition for linear stability, as well as to a
bump-in-tail equilibrium.Comment: 6 figure
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