V. V. Yaroshenko

Compressional waves in complex (dusty) plasmas under microgravity conditions

Complex plasmas consist of electrons, ions and charged microparticles, with typical charge-to-mass ratios 1:10−5:10−13. The interest in these systems has grown explosively, because they can be investigated at the kinetic level (the microparticles). However, on Earth the supporting forces (against gravity) are of the same order as the electrostatic interparticle forces—and hence only strongly compressed systems can be investigated. Under microgravity conditions these “body forces” are a factor 102 smaller which allows the experimental investigation of weakly compressed three-dimensional complex plasmas. One way to study these systems is by the controlled excitation of low-frequency compressional waves. The first such experiments, conducted with the PKE-Nefedov laboratory on the International Space Station is reported. The waves were excited by modulating the voltage on the rf electrodes. By varying the modulation frequency the dispersion relation was measured. The results are compared with existing theoret...

Potential structure around the Cassini spacecraft near the orbit of Enceladus

We present the results of numerical simulations of the potential structure around an object in a streaming plasma with parameters relevant for the Cassini spacecraft passing through Saturns plasma disk near the orbit of Enceladus. Two- and three-dimensional particle-in-cell codes have been used allowing the potential of the simulated spacecraft body to develop self-consistently through the collection of charge by its surface. The dependence of the density and potential profiles on ambient plasma density, electron temperature, and ion drift speed is discussed. The spacecraft floating potential values, found in the simulations, are compared to those deduced from the analysis of Cassini Langmuir probe characteristics.

Determination of the ion-drag force in a complex plasma

The ion-drag force acting on dust particles in the positive column of a dc discharge is measured in the pressure range of 20-120 Pa. The force is obtained by a method which does not require a priori knowledge of the particle charge, but uses the charge gradient determined from the same experiment. The method depends only on two experimentally determined quantities: the particle drift velocity and the electric field. The comparison of experimental results with theoretical models is presented and discussed.

Relationship between the ion drag and electric forces in dense dust clouds

It is shown that the relation between the ion drag and electric forces is strongly dependent on the dust number density in complex plasmas. The effect of the particle size and discharge parameters on the force balance is investigated. Examples are given for realistic complex plasma parameters and comparison with microgravity experiments is presented.

Low-frequency waves in collisional complex plasmas with an ion drift

A self-consistent model of low-frequency linear waves in collisional complex (dusty) plasmas with an ion drift is presented. Plasma conditions relevant to recent wave experiments under microgravity conditions are considered. Ion-neutral, ion-dust, and neutral-dust collisions, as well as external forces acting on the grains and grain charge variations in the presence of the wave are taken into account. A linear dispersion relation is obtained and some limiting cases are analyzed. Comparison of the obtained theoretical results with the experiments under microgravity conditions is presented.

Effect of strong electrostatic interactions of microparticles on the dust acoustic waves

It is shown that strong electrostatic interaction of highly charged microparticles (which is common for many laboratory experiments) can significantly modify the behavior of dust acoustic waves in a complex plasma giving rise to their transition, at large wave numbers, into a new regime similar to the dust thermal mode. Examples of the dispersion curves are calculated for realistic complex plasma parameters and a comparison with a recent experiment is presented. Excellent agreement is found between the theory and the experiment.

Dust-acoustic waves in collisional dusty plasmas of planetary rings

Aims. The effects of ion and neutral drag forces on stability of low-frequency wave modes is investigated from the point of view of parameters relevant to the plasma environment in the vicinity of the main Saturn rings. The electrons and ions are considered to be magnetized and corotate with the planet, while the dust grains are not magnetized and move around the planet on Keplerian orbits. Methods. The wave modes of low-frequencies in such dusty plasma, propagating along the ring in the azimuthal direction are analyzed based on the susceptibilities derived from the standard fluid approach and model presentation of the drag forces valid in the parameter regime of the dusty plasma of planetary rings. Results. It is found that the ion drag force can crucially change the stability of dust-acoustic perturbations. Inside the co-rotation distance the ion drag force can be responsible for the excitation of dust-acoustic waves, while for the region outside the synchronous orbit this mode can hardy be excited (at radial distances corresponding to the main rings). The instability due to the ion drag force could be also of some importance for perturbations in remote rings, where the relative ion-dust velocities exceed their thermal speed.

Mutual interactions of magnetized particles in complex plasmas

Various mutual dust-dust interactions in complex plasmas, including the forces due to induced magnetic and electric moments of the grains are discussed. It is shown that the dipole short-range forces can be responsible for the formation of field-aligned chains. Such chains may incorporate a few tens of individual particles, as frequently observed in experiments.

Excitation of dust density waves in weak electric fields

Influence of the weak electric fields of the order of a few V/cm, inevitably existing in plasma discharges, on propagation of dust density waves is theoretically investigated for typical experimental conditions. The model predicts significant modifications of the dispersion of the dust acoustic modes due to equilibrium ion and dust flows and an existence of cut-off wavenumbers, corresponding to aperiodically growing dust density perturbations. The theory explains the preferential range of wavenumbers, where the dust density waves can be effective excited. Examples of dispersion curves are calculated for realistic complex plasma parameters and comparison with recent experiments is presented.

Bending modes in the hexagonal dust-plasma crystal

We study modes of particle oscillations associated with out-of-plane motions propagating in a two-dimensional hexagonal lattice of charged solid particles in a plasma. The analytical dispersion relation explicitly containing dependencies on the main plasma parameters and the direction of the mode propagation is obtained. The characteristics of the modes, stability of equilibria, and a critical dependence on the dust and plasma parameters for the realistic experimental conditions are investigated.