Plasma-dust Structures Research Articles

Control of properties of ordered plasma-dust structures

The key factors for forming ordered plasma-dust structures (OPDSs) by which their properties can be controlled were determined in recent studies. In particular, it was found that the type and pressure of plasma-forming gas, the discharge current density, the macroparticle density and physicochemical composition can be the factors controlling characteristic properties of a specific OPDS type. Quantitative dependences of OPDS characteristics on these quantities were determined.

Calculation of characterisctics of electron drift in neon under a DC electric field

The features of the energy distribution function of electrons drifting in a monatomic gas are analyzed. The case of electron drift in neon under typical experimental conditions for dust structures in plasma is considered. The results of calculation of the energy balance of electrons and drift characteristics in an electric field at strengths of 0.1 < E/N < 1000 Td taking into account inelastic collisions are presented.

On the ion drift in gas mixtures

The features of the velocity distribution function of ions during their drift in a mixture of different gases are analyzed. Examples of drift of heavy ions in light gas, a mixture of two gases with equal concentrations, and drift of light ions in heavy gas are considered. It was shown that the transition to discharge in mixtures of different gases allows the formation of an ion flux with characteristics unattainable for discharge in single-component gas under typical conditions under which experiments with dust structures in plasma are performed.

Motion of plasma-dust structures and gas in a magnetic field

Numerous studies have revealed that the superposition of magnetic field causes plasma-dust structures to rotate in the plane normal to the field. The only explanation for this rotation found in the literature is that the plasma-dust structures are acted upon by the forces of ion entrainment (ion drag forces) from ions moving under the effect of the magnetic field in the azimuthal direction. However, this study demonstrates that the experimentally observed motion of plasma-dust structures cannot be explained by the forces of ion entrainment alone. We show that the observed motion of plasma-dust structures is further affected by their entrainment by gas rotating under the effect of the moment of force IxB , which exists in regions of discharge with nonuniform magnetic field in the vicinity of solenoid end faces, as well as at the narrowing of cross section of the discharge channel. An eddy electric current exists in a discharge with strata in a uniform magnetic field; this current causes the rotation of gas and is associated with the noncollinearity of the gradients of plasma density and temperature. Estimates are provided for the density of this current and for its impact on the rotation of gas in a magnetic field. Recent experimental data by Karasev [Phys. Rev. E 74, 066403 (2003)] are discussed.

Investigation of plasma-dust structures in He-Ar gas mixture

The paper reports on the first experiments with plasma-dust formations in dc gas discharge plasma for a He-Ar mixture. It is shown that the choice of light and heavy gases for the mixture suppresses ion heating in electric field under the conventional conditions of experiments and results in a supersonic jet with high Mach numbers. Distribution functions for drifting ions in the gas mixture are calculated for various mixture concentrations, electric field strengths, and gas pressures.

Charge and potential of a dust grain versus the intergrain distance and establishment of the latter in a low-pressure plasma

Results from experimental studies of ordered dust structures in plasma are reviewed. The experimental conditions and the data on the grain size and intergrain distance in plasma dust crystals are analyzed. It is shown that intergrain distance is a function of the grain size. The range of the ratio of the dust grain size to the Debye radius within which plasma dust crystals can form is determined. A volume cell surrounding a dust grain in plasma is considered. It is found that the potential and charge of the grain depend substantially on the intergrain distance. The charge, potential, and potential energy of a dust grain in a plasma dust crystal, as well as the electrostatic force exerted by the plasma field on the grain, are calculated by the method of molecular dynamics as functions of the intergrain distance. The corresponding analytic approximations and the criterion for the establishment of a steady-state intergrain distance are proposed.

On a supersonic flow of ions in a light gas

The possibility of attainment of large Mach numbers is analyzed for the case of heavy ions drifting in a light gas. Under conditions typical of experiments with dust structures in plasmas, the use of the mixture of light and heavy gases is shown to make it possible to suppress the ion heating in the electric field and to form supersonic flows characterized by large Mach numbers. The drift of xenon ions in helium is considered as an example.

Heat transfer in dust structures in an RF discharge plasma

Results are presented from experimental studies of heat transfer in liquid-like plasma-dust structures. The experiments were performed with aluminum oxide grains ∼3–5 μm in size in an RF discharge plasma. The heat capacity of the dust grains in plasma is measured. The thermal conductivity and thermal diffusivity of liquid-like plasma-dust structures are deduced under the assumption that the observed temperature gradients and the propagation of a thermal perturbation in a dusty plasma are related to heat conduction within the dust component. The measured temperature dependences of the thermal conductivity and thermal diffusivity are in qualitative agreement with the results of numerical simulations performed in the model of a simple single-atom liquid. It is shown that quantitative discrepancy between the experimental and numerical results is related to the energy loss of dust grains in their collisions with the neutral particles of the ambient gas.

Formation of plasma dust structures at atmospheric pressure

The formation of strongly coupled stable dust structures in the plasma produced by an electron beam at atmospheric pressure was detected experimentally. Analytical expressions were derived for the ionization rate of a gas by an electron beam in an axially symmetric geometry by comparing experimental data with Monte Carlo calculations. Self-consistent one-dimensional simulations of the beam plasma were performed in the diffusion drift approximation of charged plasma particle transport with electron diffusion to determine the dust particle levitation conditions. Since almost all of the applied voltage drops on the cathode layer in the Thomson glow regime of a non-self-sustained gas discharge, a distribution of the electric field that grows toward the cathode is produced in it; this field together with the gravity produces a potential well in which the dust particles levitate to form a stable disk-shaped structure. The nonideality parameters of the dust component in the formation region of a highly ordered quasi-crystalline structure calculated using computational data for the dust particle charging problem were found to be higher than the critical value after exceeding which an ensemble of particles with a Yukawa interaction should pass to the crystalline state.

Dusty plasma liquid: structure and transfer phenomena

Results are given of the experimental investigation of three-particle correlation for liquid plasma-dust structures formed in the electrode layer of a capacitive rf discharge. The obtained three-particle correlation functions for experimental and numerical data are analysed and compared with the superposition approximation. The forming of clusters of macroparticles in plasma-dust systems being analysed is revealed. The experiments in heat transfer were performed in plasma of a capacitive radio-frequency (rf) discharge in argon (P ≈ 20 Pa) with particles 4 µm in mean diameter. The results are given of an experimental investigation of processes of heat transfer for fluid dust structures in rf-discharge. The analysis of steady-state, and unsteady-state heat transfer are used to obtain the thermal conductivity and diffusivity constants.

The Effect of a Longitudinal Magnetic Field on the Plasma–Dust Structures in Strata in a Glow Discharge

The plasma-dust structures in strata in a glow discharge exposed to a longitudinal magnetic field are studied in detail. In a weakly ordered structure, the angular velocity has a vertical gradient. A reversal of rotation of the structure in a magnetic field corresponding to the magnetization of electrons is found. With the help of the pair distribution function of particles, changes in the degree of order of the structure in the magnetic field are revealed. These changes correlate with changes in the angular velocity of rotation. To explain this effect, it is assumed that the dust structure is subjected to the action of ions in crossed electric and magnetic fields.

Experimental Study of Dusty Plasma Kinetics

The results of investigation of transport processes such as diffusion of dust particles and formation of dust vortices in strongly coupled dusty plasma of gas discharges are considered. Pair correlation functions, velocity spectra, and diffusion coefficients of macroparticles were measured. Results are given of experimental investigation of three-particle correlation for liquid plasma-dust structures. The obtained threeparticle correlation functions for experimental and numerical data are analyzed and compared with the superposition approximation.

Vapor-air discharges between electrolytic cathode and metal anode at atmospheric pressure

Results are given of experimental investigations of a dusty plasma in the strata of a dc glow discharge. Characteristic forms of plasma-dust structures formed by dust particles of spherical and highly asymmetric shapes are described. Results are given of observation of self-excited dust-acoustic waves, as well as of high-amplitude waves generated under pulsed gasdynamic stimulation. A method of measuring charges of dust particles levitating in strata is described, as well as a method of measuring the radial field of forces acting on dust particles in a plasma trap. Mention is made of methods for the investigation of plasma-dust structures under conditions of microgravity.

Formation of vortex dust structures in inhomogeneous gas-discharge plasmas

A generalized analytical model of instabilities in a dusty plasma with a nonzero grain charge gradient in a field of nonelectrostatic forces is considered. A review is given of different experimental observations of the dust self-oscillations that occur in the plasmas of an rf capacitive discharge and a dc glow discharge and whose appearance can be explained in terms of the proposed model. It is shown that the change in the grain charge gives rise to dynamic dust structures in laboratory gas-discharge plasmas. Attention is focused on the analysis of the onset of vortex motion of the dust grains.

Three-particle correlations in nonideal dusty plasma.

Results are given of experimental investigation of three-particle correlations for liquid plasma-dust structures formed in the electrode layer of a high-frequency capacitive discharge. The obtained three-particle correlation functions for experimental and numerical data are analyzed and compared with the superposition approximation. The forming of clusters of macroparticles in plasma-dust systems being analyzed is revealed.

On the hypothesis of gas rotation in the magnetomechanical effect

This study continues a cycle of works published in Optika i Spektroskopiya (Optics and Spectroscopy) on the magnetomechanical effect in a gas discharge. It is devoted to the hypothesis of the appearance of a torque acting on an object placed in a gas discharge in a magnetic field and arising due to the momentum transfer from the rotating neutral gas. The velocities of gas rotation required for the formation of the observed moment of forces are estimated. Measurements of the velocities performed using laser Doppler anemometry are analyzed. The method of observing dust structures in a magnetic field to reveal gas rotation is justified. Corresponding experiments with the use of the method of optical visualization for the observation of plasma-dust structures are conducted. The experimental dependence ω(r) obtained for a liquidlike structure is not consistent with the hypothesis of the transfer of rotation from the gas. Additional observations of dust plasma upon tilting of the discharge tube in the gravitational field are conducted. It is found that, in this case, the structure shifts from the central region of the stratum to the periphery and the rotation ceases. Neither the analysis nor the experiments performed in this study are consistent with the hypothesis of gas rotation in the magnetomechanical effect. The results of the experiments with dust structures are of independent interest for the physics of dust plasma.

Dynamic vortex dust structures in a nuclear-track plasma

Results are presented from Monte Carlo calculations of the electric charge on dust grains ina plasma produced during the slowing down of radioactive decay products of californiumnuclei in neon. The dust grain charging is explained as being due to the drift of electronsand ions in an external electric field. It is shown that the charges of the grains depend ontheir coordinates and strongly fluctuate with time. The time-averaged grain charges agreewith the experimental data obtained on ordered liquid-like dust structures in anuclear-track plasma. The time-averaged dust grain charges are used to carry out computermodelling of the formation of dynamic vortex structures observed in experiments.Evidence is obtained for the fact that the electrostatic forces experienced by thedust grains are potential in character. The paper is supplemented by a video clipshowing the typical dynamics of the simulated vortex dust structure.

Collision-dominated dust sheaths andvoids - observations in micro-gravity experiments and numericalinvestigation of the force balance relations

Numerical solutions of stationary force balance equations areused to investigate the possible dust configurations (duststructures) in complex plasmas between two floating potentialplane electrodes. The distance between electrodes is assumed tobe larger than the ion-neutral mean free path and thehydrodynamic description is used. It includes the known forcesoperating in this limit, the ionization source and the dustcharge variations. The stationary balance equations are solvedboth in the case of the presence of one-size dust grains and for thecase of a mixture of grains with two different sizes. Recentmicro-gravity experiments with single-size dust grains and two-different-size dust grains show the formation of a system ofdust sheaths and dust voids between the two plane electrodes. The observed configurations of dust structures depend strongly onthe gas pressure and the degree of ionization used. Thenumerical investigations are able to show the necessaryconditions for the types of structure to be created and givetheir size. The size of the structures observed is larger thanthe ion-neutral mean free path and is of the order of magnitudeof that obtained numerically. The numerical investigations givedetails of the spatial distributions, the dust particles,the electron/ion densities, the ion drift velocity and dustcharges inside and outside different dust structures. Thesedetails have not yet been investigated experimentally and can indicatedirections for further experimental work to be performed.The single-dust-sheath structure with single-size dust particlessurrounded by dust free regions (dust wall-voids) and floatingpotential electrodes is computed. Such a structure was observedrecently and the computational results are in agreement withobservations. It is shown that more often a dust void in thecentre is observed. It is found that a dust void in the centreregion between two electrodes is formed if the ionization rateis larger than the critical ionization rate and that in the presence ofthe floating potential walls the central void should besurrounded by two dust sheaths. The necessary condition forthis dust structure to be formed is found to be that betweenthe sheaths and the walls there are formed two other wall-voidregions. The size of the central void and the distributions of thestructure parameters in the two sheaths and in the three voidsare computed. The qualitative features of the structure obtainedin the numerical computations correspond to those observed. Thedistributions of the structure parameters in the case of the twodust sheaths are quite different from that for the case of asingle central sheath. The possible structures between theelectrodes for the case of the presence of dust particles of twodifferent sizes are analysed numerically. It is shown that dust particles with different sizes cannot coexist inequilibrium at the same position and that the regions withdifferent size dust particles must be separated in space. Thisconclusion is in agreement with most observations performed sofar. It is illustrated numerically that for the case where thecentral void is present the dust particles of larger size form aseparate dust sheath which should be located at larger distancesfrom the centre than that for the smaller dust particles. Thisresult also coincides qualitatively with the observations.Computations for the distributions of the parameters in thelarger size dust sheath were performed both in the case wherethe central part is occupied by a dust sheath with smaller sizedust particles and for the case where in the central part there existsa dust void surrounded by dust sheaths with smaller size dustparticles. The size of the dust void between the sheaths withdifferent size dust particles is calculated and shown to besmall as compared to the sheath thickness. In the sheath withlarger size dust particles the distribution of dust and plasmaparameters differs qualitatively from that of the first dustsheath with smaller size dust particles. The stability of thestationary structures both with respect to excitation of dustconvection cells and with respect to oscillations of dust voidsize is discussed.

Liquid and crystal dust structures in low temperature plasmas

A study of the strongly coupled dusty plasma was carried out over a wide range of plasma pressures and temperatures. The plasma was investigated under conditions of low pressure dc current and inductively-coupled rf gas discharges and under nuclear induced plasma conditions. The plasma was also formed from positively charged dust grains in the presence of a flux of ultraviolet photons. The results of experimental observations of ordered dust structures are reported, and the characteristic features of the dust structures and the conditions for their appearance are discussed.

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.