A. G. Khrapak

Mechanism of dust-acoustic instability in a direct current glow discharge plasma

An observation of low frequency waves spontaneously excited in a dc glow discharge dusty plasma is reported. To analyze possible reasons for the instability observed, a linear dispersion relation which takes into account collisions with neutrals, dust grain charge variations, ion drift, and forces acting on dust particles is derived. Numerical analysis of the dispersion relation shows that the observed instability is the result of dust charge variations in the presence of external charge-dependent forces together with the ion drift effect.

Dust acoustic waves in a dc glow-discharge plasma

The spontaneous excitation of low-frequency oscillations of the macroparticle density in ordered dust structures levitating in standing striations of a dc glow discharge is discovered. It is concluded on the basis of a simplified linear model of an ideal collisionless plasma that the observed instability is caused by the drift motion of ions relative to the dust, which leads to the excitation of dust acoustic oscillations of the plasma.

Charging properties of a dust grain in collisional plasmas

Charging related properties of a small spherical grain immersed in a collisional plasma are investigated. Asymptotic expressions for charging fluxes, grain surface potential, long range electrostatic potential, and the properties of grain charge fluctuations due to the discrete nature of the charging process are obtained. These analytical results are in reasonable agreement with the available results of numerical modeling.

Crystallization of a dusty plasma in the positive column of a glow discharge

The formation of macroscopic ordered structures in the standing striations of a stationary glow discharge in Ne is observed. A Coulomb quasicrystal is formed by spherical glass particles with diameters of 50–63 μm and charge Zp~7·105e. The interparticle distance is approximately 300 μm. This corresponds to a nonideality parameter Γ~5·104, which leads to crystallization in the Yukawa model. The factors leading to the formation of a quasicrystal in the striations are discussed.

Ordered structures in a nonideal dusty glow-discharge plasma

The formation of ordered structures of charged macroparticles in a constant-current neon glow-discharge plasma is investigated. Experiments were performed with two types of particles: thin-walled glass spheres 50–63 μm in diameter and particles of Al2O3, 3–5 μm in diameter. Formation of quasicrystalline structures is observed in the standing strata and in an artificially created double electric layer. The formation of extended filamentary structures of macroparticles in the absence of visible stratification of the positive column has been observed for the first time. The influence of the discharge parameters on the formation of the ordered structures and their melting is examined. The form of the interaction potential between the charged macroparticles is considered, as well as changes in the conditions for maintaining the discharge in the presence of high concentrations of dust particles.

Anomalous heating of a system of dust particles in a gas-discharge plasma

The coexistence of regions of negatively charged macroparticles with substantially different kinetic temperatures in a highly nonideal dusty plasma in a dc glow discharge has been observed experimentally. An explanation of the observed anomalous heating of the system of dust particles in a gas-discharge plasma is proposed on the basis of a molecular-dynamics model.

Model of grain charging in collisional plasmas accounting for collisionless layer

Grain charging in collision dominated plasmas is investigated. The transition from a thin collisionless region around the grain, i e a, to a thick one, i e a, is studied under the assumptions i e D and a D, where i e is the ion electron mean free path, a is the grain radius, and D is the plasma screening length. It is also assumed that no ionization and recombination occur in the vicinity of the grain. With these assumptions, the analytical model of grain charging is constructed, the expressions for the ion and electron fluxes to the grain surface are derived, and the grain charge is obtained from their balance. The analytical results are then compared with the available experimental results. The behavior of ion and electron number densities in the vicinity of the grain is briefly discussed.

Liquid plasma crystal: Coulomb crystallization of cylindrical macroscopic grains in a gas-discharge plasma

Ordered structures formed of 300-μm-long cylindrical nylon grains with diameters of 15 and 7.5 μm are obtained. In contrast to conventional spherical monodisperse grains, which, under certain conditions, form the plasma-dust Coulomb crystal, the cylindrical grains in the plasma acquire a charge of ∼7×105 electrons and form a structure similar to a liquid crystal. The parameter characterizing the nonideality of the dusty component of the plasma attains 106. Grains are suspended in the striation in the horizontal plane and line up in parallel with each other.

Simple estimation of thermodynamic properties of Yukawa systems

A simple analytical approach to estimate thermodynamic properties of model Yukawa systems is presented. The approach extends the traditional Debye-Hückel theory into the regime of moderate coupling and is able to qualitatively reproduce thermodynamics of Yukawa systems up to the fluid-solid phase transition. The simplistic equation of state (pressure equation) is derived and applied to the hydrodynamic description of the longitudinal waves in Yukawa fluids. The relevance of this study to the topic of complex (dusty) plasmas is discussed.

Ionic mobilities in liquid xenon

The drift mobility of positive ions of tetramethylsilane (TMSi) and of negative oxygen ions in liquid xenon was measured as a function of temperature from 161 K (triple point) to 205 K. The mobility of the TMSi/sup +/ ion is more than one order of magnitude lower than the intrinsic mobility of positive charge carriers (holes) in very pure liquid xenon. The positive ionic mobility is influenced by electrostrictive effects, which lead to the formation of solid layers of Xe atoms around the ion, and to a locally enhanced viscosity. A simple model for the cluster formation is discussed. Qualitative agreement is obtained over most of the temperature range, but serious deviations occur at the triple point. Another shortcoming of the model is the fact that it cannot explain the higher mobility of the negative oxygen ions. >