79 research outputs found
Modification of classical electron transport due to collisions between electrons and fast ions
A Fokker-Planck model for the interaction of fast ions with the thermal
electrons in a quasi-neutral plasma is developed. When the fast ion population
has a net flux (i.e. the distribution of the fast ions is anisotropic in
velocity space) the electron distribution function is significantly perturbed
from Maxwellian by collisions with the fast ions, even if the fast ion density
is orders of magnitude smaller than the electron density. The Fokker-Planck
model is used to derive classical electron transport equations (a generalized
Ohm's law and a heat flow equation) that include the effects of the
electron-fast ion collisions. It is found that these collisions result in a
current term in the transport equations which can be significant even when
total current is zero. The new transport equations are analyzed in the context
of a number of scenarios including particle heating in ICF and MIF
plasmas and ion beam heating of dense plasmas
Deviations from the local field approximation in negative streamer heads
Negative streamer ionization fronts in nitrogen under normal conditions are
investigated both in a particle model and in a fluid model in local field
approximation. The parameter functions for the fluid model are derived from
swarm experiments in the particle model. The front structure on the inner scale
is investigated in a 1D setting, allowing reasonable run-time and memory
consumption and high numerical accuracy without introducing super-particles. If
the reduced electric field immediately before the front is >= 50kV/(cm bar),
solutions of fluid and particle model agree very well. If the field increases
up to 200kV/(cm bar), the solutions of particle and fluid model deviate, in
particular, the ionization level behind the front becomes up to 60% higher in
the particle model while the velocity is rather insensitive. Particle and fluid
model deviate because electrons with high energies do not yet fully run away
from the front, but are somewhat ahead. This leads to increasing ionization
rates in the particle model at the very tip of the front. The energy overshoot
of electrons in the leading edge of the front actually agrees quantitatively
with the energy overshoot in the leading edge of an electron swarm or avalanche
in the same electric field.Comment: The paper has 17 pages, including 15 figures and 3 table
Formation of plasma around a small meteoroid: 1. Kinetic theory
This article is a companion to Dimant and Oppenheim [2017] https://doi.org/10.1002/2017JA023963.This paper calculates the spatial distribution of the plasma responsible for radar head echoes by applying the kinetic theory developed in the companion paper. This results in a set of analytic expressions for the plasma density as a function of distance from the meteoroid. It shows that at distances less than a collisional mean free path from the meteoroid surface, the plasma density drops in proportion to 1/R where R is the distance from the meteoroid center; and, at distances much longer than the meanâfreeâpath behind the meteoroid, the density diminishes at a rate proportional to 1/R2. The results of this paper should be used for modeling and analysis of radar head echoes.This work was supported by NSF grant AGS-1244842. (AGS-1244842 - NSF
Index
The interest in relativistic beam-plasma instabilities has been greatly rejuvenated over the past two decades by novel concepts in laboratory and space plasmas. Recent advances in this long-standing field are here reviewed from both theoretical and numerical points of view. The primary focus is on the two-dimensional spectrum of unstable electromagnetic waves growing within relativistic, unmagnetized, and uniform electron beam-plasma systems. Although the goal is to provide a unified picture of all instability classes at play, emphasis is put on the potentially dominant waves propagating obliquely to the beam direction, which have received little attention over the years. First, the basic derivation of the general dielectric function of a kinetic relativistic plasma is recalled. Next, an overview of two-dimensional unstable spectra associated with various beam-plasma distribution functions is given. Both cold-fluid and kinetic linear theory results are reported, the latter being based on waterbag and MaxwellâJĂŒttner model distributions. The main properties of the competing modes (developing parallel, transverse, and oblique to the beam) are given, and their respective region of dominance in the system parameter space is explained. Later sections address particle-in-cell numerical simulations and the nonlinear evolution of multidimensional beam-plasma systems. The elementary structures generated by the various instability classes are first discussed in the case of reduced-geometry systems. Validation of linear theory is then illustrated in detail for large-scale systems, as is the multistaged character of the nonlinear phase. Finally, a collection of closely related beam-plasma problems involving additional physical effects is presented, and worthwhile directions of future research are outlined.Original Publication: Antoine Bret, Laurent Gremillet and Mark Eric Dieckmann, Multidimensional electron beam-plasma instabilities in the relativistic regime, 2010, Physics of Plasmas, (17), 12, 120501-1-120501-36. http://dx.doi.org/10.1063/1.3514586 Copyright: American Institute of Physics http://www.aip.org/</p
Collisionless heating in radio-frequency discharges: a review
Radio-frequency discharges are practically and scientifically interesting. A practical understanding of such discharges requires, among other things, a quantitative appreciation of the mechanisms involved in heating electrons, since this heating is the proximate
cause of the ionization that sustains the plasma. When these discharges are operated at sufficiently low pressure, collisionless electron heating can be an important and even the dominant mechanism. Since the low pressure regime is important for many applications, understanding collisionless heating is both theoretically and
practically important. This review is concerned with the state of theoretical knowledge of collisionless heating in both inductive and capacitive discharges
Generalized Whittle-Matrn random field as a model of correlated fluctuations
This paper considers a generalization of Gaussian random field with
covariance function of Whittle-Matrn family. Such a random
field can be obtained as the solution to the fractional stochastic differential
equation with two fractional orders. Asymptotic properties of the covariance
functions belonging to this generalized Whittle-Matrn family
are studied, which are used to deduce the sample path properties of the random
field. The Whittle-Matrn field has been widely used in
modeling geostatistical data such as sea beam data, wind speed, field
temperature and soil data. In this article we show that generalized
Whittle-Matrn field provides a more flexible model for wind
speed data.Comment: 22 pages, 10 figures, accepted by Journal of Physics
Ionospheric gas dynamics of satellites and diagnostic probes
The gas dynamics of interactions of a tenuous ionosphere with moving satellites and probes that have bearings on the diagnostics of the ionosphere are discussed. Emphasis is on the cases where the body is moving at mesothermal speeds, namely intermediate between the thermal speeds of ions and electrons of the ambient ionosphere. Methods of collision-free plasma kinetics with self-consistent field are used. The development of the topics for discussion starts with stationary Langmuir probe which entails the basic mechanism of body-plasma interaction that becomes further intricated as the body moves at a higher and higher speed. Applications of the theory of plasma interaction to meteors which move in the ionosphere are also presented.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43801/1/11214_2004_Article_BF00212707.pd
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