Grain in a Plasma in the Presence of External Electric Field: Kinetic Calculation of Effective Potential and Ionic Drag Force

Kinetic calculations of the effective grain potential are presented for the case of weakly-ionized plasma in the external electric field. The drag force associated with the ionic drift in the external field is found. It is shown that the absorption of electrons and ions by the grain can cause the change of the direction of the drag force.Представлено кiнетичнi розрахунки ефективного потенцiалу порошинки для випадку слабоiонiзованої плазми у зовнiшньому електричному полi. Знайдено силу опору, яка пов’язана з дрейфом iонiв у зовнiшньому полi. Показано, що поглинання електронiв та iонiв порошинкою може привести до змiни напрямку сили опору

Generic properties of a quasi-one-dimensional classical Wigner crystal

We studied the structural, dynamical properties and melting of a quasi-one-dimensional system of charged particles, interacting through a screened Coulomb potential. The ground-state energy was calculated and, depending on the density and the screening length, the system crystallizes in a number of chains. As a function of the density (or the confining potential), the ground state configurations and the structural transitions between them were analyzed both by analytical and Monte Carlo calculations. The system exhibits a rich phase diagram at zero temperature with continuous and discontinuous structural transitions. We calculated the normal modes of the Wigner crystal and the magnetophonons when an external constant magnetic field B is applied. At finite temperature the melting of the system was studied via Monte Carlo simulations using the modified Lindemann criterion (MLC). The melting temperature as a function of the density was obtained for different screening parameters. Reentrant melting as a function of the density was found as well as evidence of directional dependent melting. The single-chain regime exhibits anomalous melting temperatures according to the MLC and as a check we study the pair-correlation function at different densities and different temperatures, which allowed us to formulate a different melting criterion. Possible connection with recent theoretical and experimental results are discussed and experiments are proposed

Vortex states in superconducting rings

The superconducting state of a thin superconducting disk with a hole is studied within the non-linear Ginzburg-Landau theory in which the demagnetization effect is accurately taken into account. We find that the flux through the hole is not quantized, the superconducting state is stabilized with increasing size of the hole for fixed radius of the disk, and a transition to a multi-vortex state is found if the disk is sufficiently large. Breaking the circular summetry through a non central location of the hole in the disk enhances the multi-vortex state.Comment: 11 pages, 23 figures (postscript). To appear in Physical Review B, Vol. 61 (2000

Topological Defects and Non-homogeneous Melting of Large 2D Coulomb Clusters

The configurational and melting properties of large two-dimensional clusters of charged classical particles interacting with each other via the Coulomb potential are investigated through the Monte Carlo simulation technique. The particles are confined by a harmonic potential. For a large number of particles in the cluster (N>150) the configuration is determined by two competing effects, namely in the center a hexagonal lattice is formed, which is the groundstate for an infinite 2D system, and the confinement which imposes its circular symmetry on the outer edge. As a result a hexagonal Wigner lattice is formed in the central area while at the border of the cluster the particles are arranged in rings. In the transition region defects appear as dislocations and disclinations at the six corners of the hexagonal-shaped inner domain. Many different arrangements and type of defects are possible as metastable configurations with a slightly higher energy. The particles motion is found to be strongly related to the topological structure. Our results clearly show that the melting of the clusters starts near the geometry induced defects, and that three different melting temperatures can be defined corresponding to the melting of different regions in the cluster.Comment: 7 pages, 11 figures, submitted to Phys. Rev.

Structure and Melting of Two-Species Charged Clusters in a Parabolic Trap

We consider a system of charged particles interacting with an unscreened Coulomb repulsion in a two-dimensional parabolic confining trap. The static charge on a portion of the particles is twice as large as the charge on the remaining particles. The particles separate into a shell structure with those of greater charge situated farther from the center of the trap. As we vary the ratio of the number of particles of the two species, we find that for certain configurations, the symmetry of the arrangement of the inner cluster of singly-charged particles matches the symmetry of the outer ring of doubly-charged particles. These matching configurations have a higher melting temperature and a higher thermal threshold for intershell rotation between the species than the nonmatching configurations.Comment: 4 pages, 6 postscript figure

Generic properties of a quasi-one dimensional classical Wigner crystal

We studied the structural, dynamical properties and melting of a quasi-one-dimensional system of charged particles, interacting through a screened Coulomb potential. The ground state energy was calculated and, depending on the density and the screening length, the system crystallizes in a number of chains. As a function of the density (or the confining potential), the ground state configurations and the structural transitions between them were analyzed both by analytical and Monte Carlo calculations. The system exhibits a rich phase diagram at zero temperature with continuous and discontinuous structural transitions. We calculated the normal modes of the Wigner crystal and the magneto-phonons when an external constant magnetic field $B$ is applied. At finite temperature the melting of the system was studied via Monte Carlo simulations using the $modified$ $Lindemann$ $criterion$ (MLC). The melting temperature as a function of the density was obtained for different screening parameters. Reentrant melting as a function of the density was found as well as evidence of directional dependent melting. The single chain regime exhibits anomalous melting temperatures according to the MLC and as a check we study the pair correlation function at different densities and different temperatures, formulating a different criterion. Possible connection with recent theoretical and experimental results are discussed and experiments are proposed.Comment: 13 pages text, 21 picture

EFFECT OF SECONDARY ELECTRON EMISSION ON SUBNANOSECOND BREAKDOWN IN HIGH-VOLTAGE PULSE DISCHARGE

A subnanosecond breakdown in high-voltage pulse discharge is studied in experiment and in kinetic simulations for mid-high pressure in helium. It is shown that the characteristic time of the current growth can be controlled by the secondary electron emission. We test the influence of secondary electron yield on plasma parameters for three types of cathodes made from titanium, silicon carbide and Cu Al Mg-alloy. By changing the pulse voltage amplitude and gas pressure, the area of existence of subnanosecond breakdown is identified.39-3

Enhanced stability of the square lattice of a classical bilayer Wigner crystal

The stability and melting transition of a single layer and a bilayer crystal consisting of charged particles interacting through a Coulomb or a screened Coulomb potential is studied using the Monte-Carlo technique. A new melting criterion is formulated which we show to be universal for bilayer as well as for single layer crystals in the case of (screened) Coulomb, Lennard--Jones and 1/r^{12} repulsive inter-particle interactions. The melting temperature for the five different lattice structures of the bilayer Wigner crystal is obtained, and a phase diagram is constructed as a function of the interlayer distance. We found the surprising result that the square lattice has a substantial larger melting temperature as compared to the other lattice structures. This is a consequence of the specific topology of the defects which are created with increasing temperature and which have a larger energy as compared to the defects in e.g. a hexagonal lattice.Comment: Accepted for publication in Physical Review