Transport properties of quasi-two-dimensional dissipative systems with a screened Coulomb potential

Results are presented from numerical investigations of the dynamics of quasi-two-dimensional systems in the form of a monolayer of dust grains interacting by means of a screened Coulomb potential and suspended in the Earth’s gravitational field by an external electric field. The structural and transport characteristics of such a system, including the pair correlation functions and the viscosity and diffusion coefficients, were investigated over a broad parameter range corresponding to the conditions under which dust structures are observed in RF capacitive discharge plasmas. An analysis of the results obtained revealed that there is a specific topological transition typical of systems having a low spatial dimension.     

Numerical simulation of the dynamics of cylindrical dust grains in an external electric field

Numerical simulation of the dynamics of charged cylindrical macroparticles confined in an external electric field is carried out. The orientation of two identical cylindrical grains in the field of the trap is considered. The conditions for the onset of a new layer in an extended quasi-two-dimensional system of such grains are analyzed.     

Positive column of a glow discharge in neon with charged dust grains (a review)

The effect of charged micron-size dust grains (microparticles) on the electric parameters of the positive column of a low-pressure dc glow discharge in neon has been studied experimentally and numerically. Numerical analysis is carried out in the diffusion-drift approximation with allowance for the interaction of dust grains with metastable neon atoms. In a discharge with a dust grain cloud, the longitudinal electric field increases. As the number density of dust grains in an axisymmetric cylindrical dust cloud rises, the growth of the electric field saturates. It is shown that the contribution of metastable atoms to ionization is higher in a discharge with dust grains, in spite of the quenching of metastable atoms on dust grains. The processes of charging of dust grains and the dust cloud are considered. As the number density of dust grains rises, their charge decreases, while the space charge of the dust cloud increases. The results obtained can be used in plasma technologies involving microparticles.     

Charging of dust grains in a nonequilibrium plasma of a stratified glow discharge

A theoretical model is presented that describes the charging of dust grains in the positive plasma column of a stratified glow dc discharge in argon. A one-dimensional self-consistent model is used to obtain axial profiles of the electric field, as well as the electron energy distribution function along the axis of the discharge tube. Radial profiles of the electric field are determined in the ambipolar diffusion approximation. It is assumed that, in the radial direction, the electron distribution function depends only on the total electron energy. Two-dimensional distributions of the discharge plasma parameters are calculated and used to determine the potential and charge of a test dust grain at a certain point within the discharge and the electrostatic forces acting on it. It is shown that the grain charge distribution depends strongly on the nonequilibrium electron distribution function and on the nonuniform distribution of the electric field in a stratified glow discharge. A discussion is presented on the suspension of dust grains, the separation of grains by size in the discharge striations, and a possible mechanism for the onset of vortex dust motion at the edge of a dust cloud.     

Numerical simulations of ion and electron transport in a low-temperature dusty plasma

Charged particle transport and kinetic processes in a low-temperature dusty plasma are numerically simulated. Dust grains are represented as spheres with a given radius. The self-consistent electric field in the plasma surrounding a charged dust grain is calculated taking into account the perturbations of plasma quasineutrality near the grains. It is shown that applying an external electric field leads to a rearrangement of the plasma space charge and a break of the spherical symmetry of the electron and ion density distributions around the grain. The mutual influence of two identical charged dust grains is considered, and the energy of the electrostatic interaction between the grains is calculated. It is shown that this energy has a minimum at a certain finite distance between the grains.     

Dust grain charges in a nuclear-track plasma and the formation of dynamic vortex dust structures

Results are presented from Monte Carlo calculations of the electric charge of dust grains in a plasma produced during the slowing down of the radioactive decay products of californium nuclei in neon. The dust grain charging is explained for the first time as being due to the drift of electrons and ions in an external electric field. It is shown that the charges of the grains depend on their coordinates and strongly fluctuate with time. The time-averaged grain charges agree with the experimental data obtained on ordered liquidlike dust structures in a nuclear-track plasma. The time-averaged dust grain charges are used to carry out computer modeling of the formation of dynamic vortex structures observed in experiments. Evidence is obtained of the fact that the electrostatic forces experienced by the dust grains are potential in character.     

Plasma Effects in Dust Devils near the Martian Surface

Plasma?dust effects in the martian atmosphere are discussed. A specific feature of the martian atmosphere is the presence of dust grains in a wide range of altitudes. Taking into account the presence of the martian ionosphere and the high conductivity of the medium at lower altitudes, the appearance of plasma systems in the martian atmosphere can be considered quite a common phenomenon. Special attention is paid to dust devils that frequently form in the martian atmosphere and can efficiently lift dust grains. The processes of dust grain charging as a result of triboelectric effect and generation of electric fields in a dust devil are discussed. The dynamics of dust grains in such a vortex is simulated with allowance for their charging and the generated electric field.     

Nonlinear Wave Structures and Plasma−Dust Effects in the Earth’s Atmosphere

The interaction of charged dust grains with nonlinear vortical structures in the Earth’s atmosphere is analyzed. Certain aspects of the atmosphere?ionosphere interaction, in particular, mechanisms for the appearance of dust grains at ionospheric altitudes, are discussed. It is shown that, at certain altitudes, there are regions in the wavenumber space in which conditions leading to the excitation of acoustic?gravity waves are satisfied. The interaction of nonlinear acoustic?gravity waves with dust grains of meteoric origin at ionospheric altitudes, which leads to the mixing and redistribution of dust grains over the region where vortices exist, is investigated. The possibility of formation of vertical and horizontal dust flows in dusty ionospheric plasma as a result of modulational instability is analyzed. The dynamics of dust grains in dust devils frequently arising in the atmosphere above well-heated surfaces is modeled. The vortical structure of such a dust devil is characterized by a reduced pressure in the center, which facilitates the lifting of small dust grains from the surface. The formulated model is used to calculate the trajectories of dust grains in dust devils with allowance for the influence of the electric field generated in the vortex by colliding dust grains. The calculations show that dust devils play an important role in the transport of dust grains.     

Dusty Plasma near the Martian Satellite Deimos

The formation of dusty plasma in the near-surface layer above the illuminated part of the Deimos, the satellite of Mars, due to photoelectric and electrostatic processes is analyzed. Using a physicomathematical model self-consistently describing the densities of photoelectrons and dust grains above the illuminated part of Deimos, the distribution function of photoelectrons near its surface is calculated and the altitude dependences of the electric field, as well as of the number density, charge, and size of dust grains, are determined. It is noted that, due to the lower gravity, substantially larger grains are lifted above the surface of Deimos compared to those lifted above the Moon’s surface. In this case, adhesion, which is believed to significantly hamper the detachment of dust grains from the lunar surface, plays a substantially smaller role on Deimos.     

Dynamics of plasma−dust structures formed in a trap created in the narrowing of a current channel in a magnetic field

The geometry and dynamics of plasma?dust structures in a longitudinal magnetic field is studied experimentally. The structures are formed in a glow-discharge trap created in the double electric layer produced as a result of discharge narrowing by means of a dielectric insert introduced in the discharge tube. Studies of structures formed in the new type of glow-discharge trap are of interest from the standpoint of future experiments with complex plasmas in superstrong magnetic fields in which the dust component is magnetized. Different types of dielectric inserts were used: conical and plane ones with symmetric and asymmetric apertures. Conditions for the existence of stable dust structures are determined for dust grains of different density and different dispersity. According to the experimental results, the angular velocity of dust rotation is ≥10 s^–1, which is the fastest type of dust motion for all types of discharges in a magnetic field. The rotation is interpreted by analyzing the dynamics of individual dust grains.     

Effect of Ion Streaming on Diffusion of Dust Grains in Dissipative System

The presence of strong electric fields in the sheath region of laboratory complex plasma induces an ion drift and perturbs the field around dust grains. The downstream focusing of ions leads to the formation of oscillatory kind of attractive wake potential which superimpose with the normal Debye-Hückel (DH) potential. The structural properties of complex plasma and diffusion coefficient of dust grains in the presence of such a wake potential have been investigated using Langevin dynamics simulation in the subsonic regime of ion flow. The study reveals that the diffusion of dust grains is strongly affected by the ion flow, so that the diffusion changes its character in the wake potential to the DH potential dominant regimes. The dependence of the diffusion coefficient on the parameters, such as the neutral pressure, dust grain size, ion flow velocity, and Coulomb coupling parameter, have been calculated for the subsonic regime by using the Green-Kubo expression, which is based on the integrated velocity autocorrelation function. It is found that the diffusion and the structural property of the system is intimately connected with the interaction potential and significantly get affected in the presence of ion flow in the subsonic regime.     

Longitudinal low-frequency waves in the flows of self-gravitating charged dust grains in an electron-ion plasma

A study is made of the features of wave processes in the individual flows of self-gravitating dust grains in a plasma and the electric and gravitational interactions in a system of several dusty plasma flows. It is shown that, in a dusty plasma, Debye screening can substantially weaken the electric coupling between the beams of self-gravitating grains, without affecting the gravitational forces between them, and that the electrostatic perturbations are exchanged between the grain flows via gravitational fields, as happens in vacuum.     

Skin Effects in a Dusty Plasma

A multifluid MHD model is applied to study the magnetic field dynamics in a dusty plasma. The motion of plasma electrons and ions is treated against the background of arbitrarily charged, immobile dust grains. When the dust density gradient is nonzero and when the inertia of the ions and electrons and the dissipation from their collisions with dust grains are neglected, we are dealing with a nonlinear convective penetration of the magnetic field into the plasma. When the dust density is uniform, the magnetic field dynamics is described by the nonlinear diffusion equations. The limiting cases of diffusion equations are analyzed for different parameter values of the problem (i.e., different rates of the collisions of ions and electrons with the dust grains and different ratios between the concentrations of the plasma components), and some of their solutions (including self-similar ones) are found. The results obtained can also be useful for research in solid-state physics, in which case the electrons and holes in a semiconductor may be analogues of plasma electrons and ions and the role of dust grains may be played by the crystal lattice and impurity atoms.     

Magnetorotational instability of a weakly ionized accretion disk with vertical and azimuthal magnetic fields

Magnetorotational instability of a weakly ionized accretion disk with an admixture of charged dust grains in a magnetic field with the axial and toroidal components is analyzed. The dispersion relation for perturbations perpendicular to the disk plane is derived with allowance for both the Hall current and the finite transverse plasma conductivity. It is shown that dust grains play an important role in the disk magnetic dynamics. Due to the effect of dust grains, the Hall current can reverse its direction as compared to the case of electron-ion plasma. As a result, the instability threshold shifts toward the short-wavelength range. Under certain conditions, electromagnetic fluctuations of any length can become unstable. It is established that the instability criterion for waves of any scale length is satisfied within a finite interval of the density ratio between the dust and electron plasma components. The width of this interval and the instability growth rate as functions of the plasma parameters and the configuration of the magnetic field in the disk are analyzed.     

Formation of chain structures in systems of charged grains interacting via isotropic pair potentials

Conditions for the formation of chain structures of charged grains confined in the gravitational field by external electric fields are studied analytically and numerically. The relationships between the parameters of the pair interaction potential, the number of grains, and the electric field gradient in the trap are found. A criterion for the violation of stable equilibrium in a quasi-one-dimensional chain of grains and the formation of a new configuration in the system is proposed.     

Nonlinear equilibrium spherical dust structures II: Dependence of equilibrium states on volume ionization,number of trapped dust grains,and neutral gas pressure

A study is made of how equilibrium spherical dust structures depend on the volume ionization power, the neutral gas pressure, the number of trapped dust grains, and the ion-to-electron temperature ratio. It is shown that the structures are charged negatively and their charge is determined by the floating potential, which depends on the radius of the structure and on the ion temperature. The structures are charged mainly by absorbing a plasma flux. Conditions are determined under which the polarization fields and charges out-side the structures change sign, indicating the presence of overscreening effects, previously known only for individual dust grains. It is shown that overscreening outside the structures results exclusively from the plasma fluxes that are generated by the structure itself and are required to maintain it in equilibrium.     

Behavior of a dust grain in the double layer of an electric probe in a gas-discharge plasma

Equations for the motion of an individual dust grain in the double layer of a negatively charged cylindrical probe in a glow discharge plasma are derived and solved numerically. The distribution of the electric potential near the probe is determined, and the grain charge is calculated as a function of the distance from the probe for different probe potentials. The trajectories of grains with different initial energies are traced. An analysis of the grain trajectories shows that, at a certain distance from the probe, high-energy grains may be recharged; i.e., the grain charge may change sign. The grains are found to have no direct effect on the probe current in a dusty plasma of a glow discharge.     

Effect of microwave radiation on a dusty plasma

The effect of microwave radiation on a complex plasma produced by an external ionizer is studied using numerical simulations. It is shown that, as the radiation intensity increases, the scattering of the incident radiation by charged metal grains is enhanced and radiation at the second harmonic of the incident radiation appears in the scattered spectrum. This effect is associated with the grain charge oscillations caused by the nonlinear action of the microwave field. It is found that, under the action of strong microwave radiation, the grain charge can increase by one order of magnitude. It is shown that, when the microwave intensity is high enough, the distribution of the electric field near a dust grain is shown to change so radically that the field component normal to the grain surface can even change its sign.     

Helical structures in complex plasma II: Collective interaction

The conditions for the formation and stability of quasi-crystal helical structures formed of charged dust grains of equal size in a complex plasma are investigated. A study is made of both the transverse confinement of such structures by means of an external parabolic potential well and the possibility of their self-confinement or confinement by the field of the smaller background grains. It is shown that, in the presence of dissipation, any initial random distribution of dust grains in a cylindrical external potential evolves into an equilibrium helical structure with a constant pitch angle. It is found that, when the external control parameter is smoothly varied, the pitch angle of the helix changes abruptly and the structure of the quasi-crystal undergoes bifurcations, during which the number of interwoven helices changes. The smaller the confinement parameter and the weaker the attractive forces between the grains, the larger the number of bifurcations. In addition, it is found that, because of the attraction (both noncollective and collective) between the dust grains, stable helical structures may exist not only at zero value of the external confinement parameter, but also at a negative one, and that the collective interaction between the grains not only increases the number of bifurcations but also changes their character.     

Development of a self-consistent model of dust grain charging at elevated pressures using the method of moments

A model of dust grain charging is constructed using the method of moments. The dust grain charging process in a weakly ionized helium plasma produced by a 100-keV electron beam at atmospheric pressure is studied theoretically. In simulations, the beam current density was varied from 1 to 10⁶ μA/cm². It is shown that, in a He plasma, dust grains of radius 5 μm and larger perturb the electron temperature only slightly, although the reduced electric field near the grain reaches 8 Td, the beam current density being 10⁶ μA/cm². It is found that, at distances from the grain that are up to several tens or hundreds of times larger than its radius, the electron and ion densities are lower than their equilibrium values. Conditions are determined under which the charging process may be described by a model with constant electron transport coefficients. The dust grain charge is shown to be weakly affected by secondary electron emission. In a beam-produced helium plasma, the dust grain potential calculated in the drift-diffusion model is shown to be close to that calculated in the orbit motion limited model. It is found that, in the vicinity of a body perturbing the plasma, there may be no quasineutral plasma presheath with an ambipolar diffusion of charged particles. The conditions for the onset of this presheath in a beam-produced plasma are determined. __________ Translated from Fizika Plazmy, Vol. 29, No. 3, 2003, pp. 214–226. Original Russian Text Copyright ? 2003 by Filippov, Dyatko, Pal’, Starostin.