Effect of thermophoretic forces on the formation of ordered structures of macroparticles in a thermal plasma

The thermophoretic interaction of macroparticles and its effect on the formation of ordered structures of macroparticles in plasma was studied. It was shown that coexistence of regions with a chaotic arrangement of particles and regions of ordered structures is typical of a thermal plasma with strong interaction of the macroparticles. Computer simulation of a system of strongly interacting macroparticles, taking account of the thermophoretic interaction of the particles, was performed. The results showed that the thermophoretic attractive forces explain the form of the spatial nonuniformity associated with the grouping of particles in small domains. The experimentally obtained correlation function was very close to the correlation function obtained in the computer simulation. Zh. éksp. Teor. Fiz. 116, 1601–1615 (November 1999)     

Diffraction of optical radiation on spatially ordered structures of macroparticles in a strongly nonideal thermal plasma

The first observation of the diffraction of optical radiation on ordered structures of macroparticles in a low-temperature thermal plasma was reported. The experiments were conducted in an air thermal plasma at atmospheric pressure with CeO₂ particles at temperatures 1800–2200 K. The parameters of the plasma medium were determined using probe and optical diagnostics methods. The binary correlation function of the system of macroparticles was reconstructed from measurements of the structure factor. Simulation of nonideal plasma with the parameters corresponding to experiment was performed by the molecular dynamic method. The computed and experimental correlation functions were compared, and it was noted that they are in good agreement with one another.     

Creation of ordered structures in a classical thermal plasma containing macroparticles: Experiment and computer simulation

We have compared experimental measurements of ordered structures in a thermal plasma containing macroparticles of CeO₂ at atmospheric pressure and a temperature around 1700 K with the results of numerical Monte Carlo calculations for the Yukawa model. We describe several distinctive features of the way the experiments were done, including how the ordered macroparticle structures were detected. We discuss a theoretical model of the behavior of an equilibrium system of charged macroparticles in a plasma and the effective interaction potential between them. Good agreement between the experimental and numerical results is noted, and possible reasons for the observed discrepancies are discussed. Zh. éksp. Teor. Fiz. 111, 889–902 (March 1997)     

Formation of ordered structures of charged macroparticles in a photoemission trap

The possibility of confining positively charged macroscopic dust particles in a unique photoemission trap was studied. The spatial distributions of the potentials for a cylindrical geometry of a phototrap were obtained (by the particles in a cell method) and the dynamics of the formation of ordered structures of dust particles in the potential field of the trap was studied (by the molecular-dynamics method). The dependence of the number of dust particles confined by a phototrap on the particle energies and sizes and the buffer-gas pressure were obtained.     

Dynamics of the formation of two-dimensional ordered structures

The growth of ordered domains in lattice gas models, which occurs after the system is quenched from infinite temperature to a state below the critical temperatureT _c, is studied by Monte Carlo simulation. For a square lattice with repulsion between nearest and next-nearest neighbors, which in equilibrium exhibits fourfold degenerate (2×1) superstructures, the time-dependent energy E(t), domain size L(t), and structure functionS(q, t) are obtained, both for Glauber dynamics (no conservation law) and the case with conserved density (Kawasaki dynamics). At late times the energy excess and halfwidth of the structure factor decrease proportional tot ^–x, whileL(t) t ^x, where the exponent x=1/2 for Glauber dynamics and x1/3 for Kawasaki dynamics. In addition, the structure factor satisfies a scaling lawS(k,t)=t ^2xS(kt^x). The smaller exponent for the conserved density case is traced back to the excess density contained in the walls between ordered domains which must be redistributed during growth. Quenches toT>T _c, T=T_c (where we estimate dynamic critical exponents) andT=0 are also considered. In the latter case, the system becomes frozen in a glasslike domain pattern far from equilibrium when using Kawasaki dynamics. The generalization of our results to other lattices and structures also is briefly discussed.     

Interaction of two macroparticles in a nonequilibrium plasma

The interaction of two macroparticles in a nonequilibrium plasma at elevated pressures has been investigated. An asymptotic theory of screening, which leads to a two-exponential dependence of the macroparticle potential on distance with different screening constants, is used to determine the electrostatic energy of the system of charges associated with the two macroparticles. The dependence of the electrostatic energy on interparticle distance has been found to have a minimum, as in an equilibrium plasma. The interaction force between the macroparticles has been determined; it turned out to be asymmetric—for different charges, the forces acting on the first and second macroparticles are not equal. This is the result of an asymmetric charge separation near macroparticles with differing charges and indicates that the interaction force in a nonequilibrium plasma is nonpotential. The forces are equal for identical macroparticles or in an equilibrium plasma and the potential energy of the interaction between the macroparticles has been determined for these cases. Attraction between likely charged particles with different (in magnitude) charges has been found to be possible when they come very close together. Relations to determine the modified coupling parameter for an interaction potential that consists of two exponential terms with different screening constants have been derived.     

Dynamics of macroparticles in a dusty plasma under microgravity conditions (First experiments on board the ISS)

The results of experimental investigation of macroparticle transport in the dusty plasma of a capacitive high-frequency discharge under microgravity conditions are considered. Experimental data were obtained for monodisperse polymer particles of radius a _p =1.7 mm in a wide range of plasma parameters on the International Space Station. Analysis of macroparticle dynamics for a strongly nonideal dusty plasma, including diffusion and dust vortex formation processes, is carried out.     

Anomalously high kinetic energy of charged macroparticles in a plasma

A number of recent experimental investigations of nonideal plasmas containing macroparticles have revealed an anomalous increase in that part of the kinetic energy of these macroparticles that corresponds to their random motion. In this paper a model is proposed for the dynamic motion of charged macroparticles that explains this phenomenon.Calculations based on this model are compared with experimental results. Zh. éksp. Teor. Fiz. 112, 499–506 (August 1997)     

The diffusion of macroparticles and criteria of phase transitions for dust structures in weakly ionized plasma

The dynamics of charged particles is investigated under conditions close to those of experiments in a weakly ionized laboratory gas-discharge dust plasma. The existing phenomenological criteria of phase transitions for dust structures in such a plasma are treated, and new criteria are suggested. The parameters responsible for the order and scaling of dynamic processes in Yukawa dissipative systems are determined. The relation for the diffusion coefficient D of macroparticles in strongly correlated liquid structures is derived.     

Effective interaction potential and ordered structures of dust particles in a gas-discharge plasma

An effective potential is proposed for the interaction between dust particles in a gas-discharge plasma which takes account of the following physical factors: the spatial dependence of the particle charges on the floating potential of the plasma, anisotropy of the interaction, resulting from focusing of the negatively charged particles of the drift ion current, and aspects of screening of the dust particles by plasma electrons and ions which interact strongly with them and recombine faster in their vicinity and on their surface. Monte Carlo calculations explain the formation of threadlike structures of dispersed particles, and also “transverse crystallization” of these “threads” in a stratified gas-discharge plasma. Zh. éksp. Teor. Fiz. 115, 819–836 (March 1999)     

Electrostatic interaction of macroparticles in a plasma in the strong screening regime

We have studied the electrostatic interaction of spherical particles in an equilibrium plasma or an electrolyte in the moderate and strong screening regimes when the macroparticle size is comparable with or much larger than the Debye screening radius. We have developed an approximate theory of the electrostatic interaction of macroparticles in the case of constant potentials of their surfaces in the weak or moderate screening regimes. In this theory, the charges of macroparticles with a fixed spacing between them are determined using vacuum capacitive coefficients, which are corrected taking into account the plasma screening effects. The force of interaction with the resultant charges is calculated based on the solution of the problem of interaction in a homogeneous dielectric (vacuum) and is multiplied by the plasma factor. We have also obtained an approximate solution to the problem in the strong screening regime. Comparison with the exact solution has demonstrated high accuracy of the proposed methods of calculation.     

Experimental study of heat transfer processes for macroparticles in a dusty plasma

The results of the experimental study of heat transfer processes in liquid dusty structures are reported. The experiments have been carried out for aluminum oxide particles in an rf discharge plasma. The thermal conductivity and thermal diffusivity of the dusty plasma component have been determined by analyzing the steady and unsteady heat transfer processes. The temperature dependence has been obtained for these quantities, which is in qualitative agreement with numerical simulation results for a simple one-atomic fluid.     

Experimental study of transport of macroparticles in plasma RF-discharge

Experimental study of the kinematic viscosity and the diffusion is performed for macroparticles of different sizes in plasma of a capacitive radio-frequency (RF-) discharge. Experimental examination of the Einstein-Stokes relation between the viscosity and diffusion constants is carried out. A comparison of the measured transport constants with the existing numerical data is presented.     

Simulation of the dynamics of strongly interacting macroparticles in a weakly ionized plasma

The method of molecular dynamics is used to study the dynamic behavior of a nonideal system of particles interacting through screened Coulomb potential. The behavior of the self-diffusion coefficient of particles as a function of the nonideality parameter is investigated. The conditions of the crystallization of such a system are discussed, as well as the possibility of using the crystallization criterion, based on the dynamic characteristics of the system.     

Effect of the size of macroparticles on their electrostatic interaction in a plasma

The electrostatic interaction of two spherical macroparticles in a plasma has been considered. Primary attention has been focused on investigating the electrostatic interaction at short distances where polarization effects of the surface charge of finite-size macroparticles begin to play a dominant role. The first part of this study is devoted to the interaction of a point charge with a charged conducting sphere in an equilibrium plasma. It has been shown that the presence of a plasma in the system leads to a decrease in the potential barrier when two like-charged macroparticles approach each other and that this decrease proves to be the most significant in the case where the macroparticle radius is comparable to the Debye screening length. The second part of this study is concerned with the interaction of two conducting spheres in the bispherical system of the coordinates under the assumption that the charges of the conducting spheres are constant and under the assumption that the surface potentials of the spheres are constant. The latter case is closer to the physics of electrostatic interaction of two macroparticles in a plasma medium where the electrostatic potential of their surface is determined by the floating potential of the plasma. It has been demonstrated that the interaction potentials in these two cases are substantially different from each other and that, at constant macroparticle charges, the energy of the electrostatic field is an interaction potential, but, in the case of macroparticles with constant surface potentials, which are independent of the interparticle distance, the energy of the electrostatic field is not an interaction potential. In the latter case, account must be taken of the work done by external sources on the macroparticle potentials to maintain them constant. The form of the interaction potential has been established in this case from the analysis of the interaction force in terms of the Maxwell tension tensor. In the third part of this study, the interaction of two macroparticles has been considered in the spherical system of coordinates and analytical expressions for the interaction potentials have been derived for both the case of constant macroparticle charges and the case of constant surface potentials of the macroparticles.     

Cooperative formation of dust structures in plasma

The formation and destruction of ordered dust structures in glow discharges are investigated experimentally. The initial construction phase of an ordered structure is related to the construction of its cooperative field and is determined by the number of particles and by the existence of crystallization centers. After the structure has been constructed, it influences the local plasma properties and the discharge current-voltage characteristics. The recovery of the structure after weak exposure takes place at local equilibrium, while, after intense exposure to high-voltage nanosecond pulses, it is determined by the fluctuation level and the degree of chaotization in the system.     

Dynamics of antiproton cooling in a positron plasma during antihydrogen formation

We demonstrate cooling of 10⁴ antiprotons in a dense, cold plasma of 10⁸ positrons, confined in a nested cylindrical Penning trap at about 15 K. The time evolution of the cooling process has been studied in detail, and several distinct types of behavior identified. We propose explanations for these observations and discuss the consequences for antihydrogen production. We contrast these results with observations of interactions between antiprotons and “hot” positrons at about 3000 K, where antihydrogen production is strongly suppressed.     

Self-oscillations of macroparticles in the dust plasma of glow discharge

Two types of instabilities emerging in dust-plasma systems with a spatial gradient of the macroparticle charge are considered. It is shown that the change in the macroparticle charge is an effective mechanism for exciting dust self-oscillations in a laboratory plasma. The results of experimental observations and an analysis of the conditions for the development of various self-oscillations of macroparticles in the strata of a dc glow discharge are presented.