680 research outputs found
Attractive Potential around a Thermionically Emitting Microparticle
We present a simulation study of the charging of a dust grain immersed in a
plasma, considering the effect of electron emission from the grain (thermionic
effect). It is shown that the OML theory is no longer reliable when electron
emission becomes large: screening can no longer be treated within the
Debye-Huckel approach and an attractive potential well forms, leading to the
possibility of attractive forces on other grains with the same polarity. We
suggest to perform laboratory experiments where emitting dust grains could be
used to create non-conventional dust crystals or macro-molecules.Comment: 3 figures. To appear on Physical Review Letter
The resource curse – What have we learned from two decades of intensive research:Introduction to the Special Issue
There has been increasing interest in the so-called ‘resource curse’, i.e. the tendency of resource-rich countries to underperform in several development outcomes. This has generated a mountain of (often contradictory) evidence leaving many floundering in the flood of information. The special issue compiles eight papers from some of the most prominent contributors to this literature, combining original research with critical reflection on the current stock of knowledge. The studies collectively emphasize the complexities and conditionalities of the ‘curse’ – its presence/intensity is largely context-specific, depending on the type of resources, socio-political institutions and linkages with the rest of the economy
Quasi-stationary States of Two-Dimensional Electron Plasma Trapped in Magnetic Field
We have performed numerical simulations on a pure electron plasma system
under a strong magnetic field, in order to examine quasi-stationary states that
the system eventually evolves into. We use ring states as the initial states,
changing the width, and find that the system evolves into a vortex crystal
state from a thinner-ring state while a state with a single-peaked density
distribution is obtained from a thicker-ring initial state. For those
quasi-stationary states, density distribution and macroscopic observables are
defined on the basis of a coarse-grained density field. We compare our results
with experiments and some statistical theories, which include the
Gibbs-Boltzmann statistics, Tsallis statistics, the fluid entropy theory, and
the minimum enstrophy state. From some of those initial states, we obtain the
quasi-stationary states which are close to the minimum enstrophy state, but we
also find that the quasi-stationary states depend upon initial states, even if
the initial states have the same energy and angular momentum, which means the
ergodicity does not hold.Comment: 9 pages, 7 figure
Spinodal Decomposition in Binary Gases
We carried out three-dimensional simulations, with about 1.4 million
particles, of phase segregation in a low density binary fluid mixture,
described mesoscopically by energy and momentum conserving Boltzmann-Vlasov
equations. Using a combination of Direct Simulation Monte Carlo(DSMC) for the
short range collisions and a version of Particle-In-Cell(PIC) evolution for the
smooth long range interaction, we found dynamical scaling after the ratio of
the interface thickness(whose shape is described approximately by a hyperbolic
tangent profile) to the domain size is less than ~0.1. The scaling length R(t)
grows at late times like t^alpha, with alpha=1 for critical quenches and
alpha=1/3 for off-critical ones. We also measured the variation of temperature,
total particle density and hydrodynamic velocity during the segregation
process.Comment: 11 pages, Revtex, 4 Postscript figures, submitted to PR
Experimental and Theoretical Investigation into the Effect of the Electron Velocity Distribution on Chaotic Oscillations in an Electron Beam under Virtual Cathode Formation Conditions
The effect of the electron transverse and longitudinal velocity spread at the
entrance to the interaction space on wide-band chaotic oscillations in intense
multiple-velocity beams is studied theoretically and numerically under the
conditions of formation of a virtual cathode. It is found that an increase in
the electron velocity spread causes chaotization of virtual cathode
oscillations. An insight into physical processes taking place in a virtual
cathode multiple velocity beam is gained by numerical simulation. The
chaotization of the oscillations is shown to be associated with additional
electron structures, which were separated out by constructing charged particle
distribution functions.Comment: 9 pages, 8 figure
Radio-frequency discharges in Oxygen. Part 1: Modeling
In this series of three papers we present results from a combined
experimental and theoretical effort to quantitatively describe capacitively
coupled radio-frequency discharges in oxygen. The particle-in-cell Monte-Carlo
model on which the theoretical description is based will be described in the
present paper. It treats space charge fields and transport processes on an
equal footing with the most important plasma-chemical reactions. For given
external voltage and pressure, the model determines the electric potential
within the discharge and the distribution functions for electrons, negatively
charged atomic oxygen, and positively charged molecular oxygen. Previously used
scattering and reaction cross section data are critically assessed and in some
cases modified. To validate our model, we compare the densities in the bulk of
the discharge with experimental data and find good agreement, indicating that
essential aspects of an oxygen discharge are captured.Comment: 11 pages, 10 figure
Investigation of the Chaotic Dynamics of an Electron Beam with a Virtual Cathode in an External Magnetic Field
The effect of the strength of the focusing magnetic field on chaotic dynamic
processes occurring inan electron beam with a virtual cathode, as well as on
the processes whereby the structures form in the beamand interact with each
other, is studied by means of two-dimensional numerical simulations based on
solving a self-consistent set of Vlasov-Maxwell equations. It is shown that, as
the focusing magnetic field is decreased,the dynamics of an electron beam with
a virtual cathode becomes more complicated due to the formation andinteraction
of spatio-temporal longitudinal and transverse structures in the interaction
region of a vircator. The optimum efficiency of the interaction of an electron
beam with the electromagnetic field of the vircator isachieved at a
comparatively weak external magnetic field and is determined by the
fundamentally two-dimensional nature of the motion of the beam electrons near
the virtual cathode.Comment: 12 pages, 8 figure
A Monte Carlo simulation of ion transport at finite temperatures
We have developed a Monte Carlo simulation for ion transport in hot
background gases, which is an alternative way of solving the corresponding
Boltzmann equation that determines the distribution function of ions. We
consider the limit of low ion densities when the distribution function of the
background gas remains unchanged due to collision with ions. A special
attention has been paid to properly treat the thermal motion of the host gas
particles and their influence on ions, which is very important at low electric
fields, when the mean ion energy is comparable to the thermal energy of the
host gas. We found the conditional probability distribution of gas velocities
that correspond to an ion of specific velocity which collides with a gas
particle. Also, we have derived exact analytical formulas for piecewise
calculation of the collision frequency integrals. We address the cases when the
background gas is monocomponent and when it is a mixture of different gases.
The developed techniques described here are required for Monte Carlo
simulations of ion transport and for hybrid models of non-equilibrium plasmas.
The range of energies where it is necessary to apply the technique has been
defined. The results we obtained are in excellent agreement with the existing
ones obtained by complementary methods. Having verified our algorithm, we were
able to produce calculations for Ar ions in Ar and propose them as a new
benchmark for thermal effects. The developed method is widely applicable for
solving the Boltzmann equation that appears in many different contexts in
physics.Comment: 14 page
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