24 research outputs found
First-principles particle simulation and Boltzmann equation analysis of Negative Differential Conductivity and Transient Negative Mobility effects in xenon
The Negative Differential Conductivity and Transient Negative Mobility
effects in xenon gas are analyzed by a first-principles particle simulation
technique and via an approximate solution of the Boltzmann transport equation
(BE). The particle simulation method is devoid of the approximations that are
traditionally adopted in the BE solutions in which (i) the distribution
function is searched for in a two- term form, (ii) the Coulomb part of the
collision integral for the anisotropic part of the distribution function is
neglected, (iii) Coulomb collisions are treated as binary events, and (iv) the
range of the electron-electron interaction is limited to a cutoff distance. The
results obtained from the two methods are, for both effects, in good
qualitative agreement, small differences are attributed to the approximations
listed above
Fano-like anti-resonances in strongly coupled binary Coulomb systems
Molecular Dynamics (MD) simulations of a strongly coupled binary ionic
mixture have revealed the appearance of sharp minima in the species resolved
dynamical density fluctuation spectra. This phenomenon is reminiscent of the
well-known Fano anti-resonance, occurring in various physical processes. We
give a theoretical analysis using the Quasi Localized Charge Approximation, and
demonstrate that the observed phenomenon in the equilibrium spectrum is a novel
manifestation of the Fano mechanism, that occurs at characteristic frequencies
of the system different from the conventional classical Fano frequencies
Factorization of 3-point static structure functions in 3D Yukawa liquids
In many-body systems the convolution approximation states that the 3-point
static structure function, , can
approximately be "factorized" in terms of the 2-point counterpart,
. We investigate the validity of this approximation in
3-dimensional strongly-coupled Yukawa liquids: the factorization is tested for
specific arrangements of the wave vectors and
, with molecular dynamics simulations. With the increase of the
coupling parameter we find a breakdown of factorization, of which a notable
example is the appearance of negative values of
, whereas the approximate factorized
form is restricted to positive values. These negative values -- based on the
quadratic Fluctuation-Dissipation Theorem -- imply that the quadratic part of
the density response of the system changes sign with wave number. Our
simulations that incorporate an external potential energy perturbation clearly
confirm this behavior
On the self-excitation mechanisms of Plasma Series Resonance oscillations in single- and multi-frequency capacitive discharges
The self-excitation of plasma series resonance (PSR) oscillations is a
prominent feature in the current of low pressure capacitive radio frequency
(RF) discharges. This resonance leads to high frequency oscillations of the
charge in the sheaths and enhances electron heating. Up to now, the phenomenon
has only been observed in asymmetric discharges. There, the nonlinearity in the
voltage balance, which is necessary for the self-excitation of resonance
oscillations with frequencies above the applied frequencies, is caused
predominantly by the quadratic contribution to the charge-voltage relation of
the plasma sheaths. Using PIC/MCC simulations of single- and multi- frequency
capacitive discharges and an equivalent circuit model, we demonstrate that
other mechanisms such as a cubic contribution to the charge-voltage relation of
the plasma sheaths and the time dependent bulk electron plasma frequency can
cause the self-excitation of PSR oscillations, as well. These mechanisms have
been neglected in previous models, but are important for the theoretical
description of the current in symmetric or weakly asymmetric discharges
Experimental observation and computational analysis of striations in electronegative capacitively coupled radio-frequency plasmas
Self-organized spatial structures in the light emission from the ion-ion
capacitive RF plasma of a strongly electronegative gas (CF4) are observed
experimentally for the first time. Their formation is analyzed and understood
based on particle-based kinetic simulations. These "striations" are found to be
generated by the resonance between the driving radio-frequency and the
eigenfrequency of the ion-ion plasma (derived from an analytical model) that
establishes a modulation of the electric field, the ion densities, as well as
the energy gain and loss processes of electrons in the plasma. The growth of
the instability is followed by the numerical simulations