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Complex Plasma Research on the International Space Station Complex (dusty) plasma research under microgravity conditions complements the research in the laboratory. Due to reduction of the main force on microparticles in the lab, gravity, it is possible to form large, homogeneous 3D complex plasma systems in the bulk region of plasmas and to investigate phenomena other than those accessible on earth in detail. Therefore, our team designed and built different setups for microgravity research. PK-3 Plus, which ended in 2013, already gained a long series of interesting scientific results with about 40 publications. The chambers of this setup has a pair of parallel electrode plates and is therefore able to produce a cylindrical symmetric rf-plasma. Particles of different size and material can be injected into the discharge. The cloud is illuminated by a laser sheet and observed in 2D with cameras with different resolutions and fields of view. Moving this observation system in the direction of sight, tomographic reconstruction of the 3D particle configuration is possible later on. This configuration is ideal to investigate stable liquid and crystalline systems and give interesting insights into a wide range of phenomena like crystallization, phase separation of binary mixtures, instabilities like heart-beat instability or projectile interaction with a strongly coupled complex plasma cloud. This contribution will give an overview over a selection of recent results of the very successful project PK-3 Plus. This work and some of the authors were funded by DLR/BMWi FKZ 50WP0203, 50WM1203 and 50WM1441.

The experimental and theoretical investigations of the formation of dust particle clouds in the stratified positive column of a dc glow discharge in helium were performed. The size and shape of a dust cloud that levitated in the strong electric field of the striations in a vertically oriented discharge tube were measured under different helium pressures. Axial electric field strength was also experimentally estimated. A model for radial distributions of all dusty plasma parameters of the positive column of the dc glow discharge was developed to describe the obtained experimental results. The model is based on the solution of a non-local Boltzmann equation for an electron energy distribution function, drift-diffusion equations for ions and dust particles, and a Poisson equation for a self-consistent radial electric field. The experimental and calculated results show that the size of the dust cloud decreases with the increase in the gas pressure, and the axial electric field strength pressure dependencies have minima. At low gas pressures, the dust particle charge number density increases and exceeds the electron density that strongly influences the plasma parameters.The experimental and theoretical investigations of the formation of dust particle clouds in the stratified positive column of a dc glow discharge in helium were performed. The size and shape of a dust cloud that levitated in the strong electric field of the striations in a vertically oriented discharge tube were measured under different helium pressures. Axial electric field strength was also experimentally estimated. A model for radial distributions of all dusty plasma parameters of the positive column of the dc glow discharge was developed to describe the obtained experimental results. The model is based on the solution of a non-local Boltzmann equation for an electron energy distribution function, drift-diffusion equations for ions and dust particles, and a Poisson equation for a self-consistent radial electric field. The experimental and calculated results show that the size of the dust cloud decreases with the increase in the gas pressure, and the axial electric field strength pressure dependencies h...

Experiments with complex plasmas in a dc discharge using the PK-4 facility will be discussed, focusing on the formation of strings corresponding to an electrorheological fluid observed in recent parabolic flight experiments.

A new method is proposed for determining the interaction forces between particles in nonideal dissipative systems with isotropic pair potentials. The method is based on the solution of the inverse problem describing the motion of interacting particles by a system of Langevin equations and allows one to recover the parameters of the external confining potential without referring to a priori information on the friction coefficients of the particles. This procedure was tested by a numerical simulation of the problem in a wide range of parameters typical of experimental conditions in a laboratory dusty plasma. The results of the first experimental approbation of the method as applied to the analysis of the interaction of dust particles in a laboratory high-frequency capacitive discharge plasma are presented.

We conducted a series of experiments to study lane formation in complex plasma, using the PK-3 Plus Laboratory in the International Space Station. In our experiments, small (driven) microparticles penetrate into a cloud of large (background) particles, revealing a strong tendency toward lane formation: When the driving force is strong enough, particles of both sorts form interpenetrating lines. Typically, the lanes exhibit a considerable anisotropic structural order accompanied by an enhancement of their (unidirectional) mobility. Laning is quantified locally in terms of the anisotropic scaling index. Using a ?nematic? order parameter, the observed time-resolved lane-formation process is compared to a set of computer simulations of a binary Yukawa model with Langevin dynamics. This not only yields a best fit but also reveals a strong influence of the initial experimental conditions on the dynamics of lane formation.

Conventional electrorheological (ER) fluids consist of suspensions of microparticles in usually nonconducting fluids with a different dielectric constant. The interparticle interaction, and hence, the rheology of ER fluids, is determined by an external electric field, which polarizes grains and thus induces additional dipole-dipole coupling. The electric field plays a role of a new degree of freedom that allows us to ?tune? the interaction between particles. This makes the phase diagram of ER fluids remarkably diversified. Here, we report on the experimental investigations of ?ER plasmas,? where the control of the interparticle interaction by an externally applied electric field is due to distortion of the Debye spheres that surround microparticles in a plasma. Interactions in ER plasmas under weak ac fields are mathematically equivalent to those in conventional ER fluids. Microgravity experiments as well as molecular dynamics simulations show a phase transition from an isotropic to an anisotropic plasma states as the electric field is increased. The variational Gibbs-Bogoliubov approach allows us to recover the phase diagram of ER plasmas, which includes string fluids and anisotropic solid phases.

The influence of a dust cloud on the structure of the positive column of a direct current gas discharge in a cylindrical glass tube under milligravity conditions has been studied both experimentally and numerically. The discharge was produced in neon at 60 Pa in a glass tube with a diameter of 30 mm at a discharge current 1 mA. Spherical monodisperse melamine formaldehyde dust particles with a diameter of 6.86 μm were injected into the positive column and formed there a uniform dust cloud with a maximum diameter of 14.4 mm. The shape of the cloud and the dust particle number density were measured. The cloud was stationary in the radial direction and slowly drifted in the axial direction. It was found that in the presence of the dust cloud, the intensity of the neon spectral line with a wavelength by 585.25 nm emitted by the discharge plasma increased by 2.3 times and 2 striations appeared on the anode side of the cloud. A numerical simulation of the discharge was performed using the 2D (quasi-3D) nonlocal...

Two techniques for the analysis of the interaction between particles in non-ideal quasi-equilibrium extended systems are considered. The first technique is based on a solution of the inverse problem describing the movement of dust particles by a system of Langevin equations. The second technique proceeds from the basic integral approaches of statistical physics. The conditions for the correct use of these techniques are presented, together with the results of their experimental application for the analysis of inter-grain interactions in the dusty plasma of radio frequency discharge.

The paper presents experimental investigation of flow of dusty plasma medium formed by macroparticles in argon plasma. The dependences of the coefficient of shear viscosity of such liquid on the external force causing the flow of dusty plasma liquid and on the pressure of plasma-generating gas are studied. It is found that the viscosity of the dusty plasma medium decreases with increasing shear stress and increases with increasing pressure of buffer gas. An experimental investigation of the dynamics of macroparticles in an unperturbed liquid dusty plasma medium as a function of coupling parameter is performed; in so doing, formations of particles whose motion is correlated are observed in the region of high values of coupling parameter. It is assumed that the non-Newtonian pattern of dusty plasma liquid may be due to the emergence of crystal-like dusty plasma clusters in the ‘liquid’ phase. An experimental investigation of a crystalline dusty plasma structure under the effect of laser radiation is performed; in so doing, a macroscopic flow of the crystalline dusty plasma structure is observed under the effect of shear stress. The mechanism of formation and subsequent annihilation of edge misfit dislocations is observed and the threshold pattern of such flow is established; the threshold value of power of laser radiation is determined.

The possibility of levitating Coulomb clusters formed by charged diamagnetic microparticles in an inhomogeneous magnetic field has been theoretically substantiated and experimentally confirmed. An experimental setup is described, in which the Coulomb clusters can be formed from charged graphite particles 100–300 μm in size and maintained stable between the poles of an electromagnet generating a magnetic field with induction B ∼ 104 G and gradient |∂B| ∼ 105 G/cm.

A self-consistent kinetic model of a low pressure DC glow discharge with dust particles based on Boltzmann equation for electron energy distribution function is presented. The influence of dust particles concentration Nd on gas discharge and dust particles parameters were investigated. It is shown that the increase of Nd leads to the increase of averaged electric field and ion density, and to the decrease of a dust particle charge in the dusty cloud. The absorption of electrons and ions on the dust particles surface does not lead to the electron energy distribution function depletion due to a self-consistent adjustment of dust particles and discharge parameters.

Results are described of experiments in studying the effect of magnetic field on dusty plasma structures in dc discharges. The levitation of dust particles in vertically oriented dc discharges is possible either in the vicinity of the flat surface of cathode (or of special additional electrode) or in an electrostatic trap which forms at the striation head during the stratification of positive column. The most interesting one of the observed effects is that of rotation of dust structures in the case of imposition of axial magnetic field. The rotation occurs about the discharge axis of symmetry with a velocity which is a nonmonotonic function of magnetic force. An increase in the field may cause the variation of direction of rotation for the opposite. An interpretation is given of the mechanism of rotation and of the inversion of its velocity. The rotation of dust structures is caused by the tangential component of ion drag force. It is demonstrated that the inversion is, in the final analysis, associated with the competition between two mechanisms of plasma recombination, namely, those on the discharge tube wall and on the surface of dust particles. When the magnetic field increases, the latter mechanism becomes predominating as a result of magnetization of plasma, and the inversion of rotation occurs (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Results of an experimental study of mass transfer are presented for extended systems of dust particles observed in capacitively coupled RF discharge plasmas. The Green-Kubo relation and the Langevin equation are validated as applied to dust grain dynamics in laboratory plasmas. A procedure is proposed for evaluating the temperature, friction coefficient, and characteristic oscillation frequency for dust grains. Measured characteristics of the dust subsystem (diffusion coefficient, pair correlation function, and friction coefficient) are compared with available theoretical and numerical results.

A dusty plasma in a dc gas discharge is considered at low (cryogenic) temperatures of the gas. The formation of dusty plasma structures consisting of monodisperse poly(styrene) particles (d = 5.44 μm) in a dc glow discharge is experimentally investigated at cryogenic temperatures in the range from 4.2 to 77 K, and the results obtained are presented. The ion velocity distribution function and the charging of dust particles at cryogenic temperatures are calculated using the molecular dynamics method. The primary attention is focused on the correct inclusion of ion-atom collisions in the analysis. This is essential to the understanding of the main mechanisms of the experimentally observed increase in the density of dust particles with decreasing temperature of the gas in the 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.

The formation of dusty plasma structures has been experimentally investigated in a cylindrical dc discharge in axial magnetic fields up to 2500 G. The rotation of the dusty plasma structures about the discharge symmetry axis with a frequency depending on the magnetic field has been observed. When the field increases to 700 G, the displacement of dust particles from the axial region of the discharge to the periphery, along with the continuation of the rotation, has been observed. The kinetic temperatures of the dust particles, the diffusion coefficients, and the effective nonideality parameter have been determined for various magnetic fields. The explanation of the features in the behavior of the dust particles in the discharge in the magnetic field has been proposed on the basis of the analysis of ambipolar diffusion in the magnetized plasma. The maximum magnetic field at which the levitation of the dust particles in the discharge is possible has been estimated.

We present the results of our experimental study of the flow of a dusty plasma liquid produced by macroparticles in an argon plasma. The dependences of shear viscosity for such a liquid on the magnitude of the external force inducing the dusty plasma liquid flow and on the plasma-generating gas pressure are analyzed. We have established that the viscosity of a dusty plasma medium decreases with increasing shear stress in it, while the viscosity of such a liquid increases with buffer gas pressure. The flow of a dusty plasma liquid under the action of an external force has been found to resemble the plastic deformation of a Bingham body. We suggest that the formation of crystal-like dusty plasma clusters in a “liquid” phase can be responsible for the non-Newtonian behavior of the dusty plasma liquid flow.

Charging of dust particles in a plasma with the two-temperature energy distribution of electrons has been studied. It has been shown that the dust-particle potential divided by the electron temperature decreases with increasing electron temperature in the plasma with cold ions. Owing to this behavior, the potential of the dustparticle surface increases with the electron temperature more slowly than the linear function and is lower than the electron temperature (divided by the elementary charge) for T e > 5.5 eV in hydrogen and for T e > 240 eV in argon. The fraction of fast electrons at which these electrons begin to contribute to the charge of dust particles has been determined. It has been shown that the charge of micron particles can reach 106 elementary charges. The effect of the cold and thermal field emission on the charge of dust particles has been analyzed. The possibility of obtaining ultrahigh charges (to 107 elementary charges on dust particles with a radius of 50–100 μm irradiated by a 25-keV 1-mA electron beam has been demonstrated.

PK-4 is an experiment designed to investigate complex plasmas in a combined dc/RF discharge under microgravity conditions on board of the International Space Station. The dc discharge is produced in a glass tube with a length of 35 cm and a diameter of 3 cm. In addition, an RF discharge can be applied by external RF coils. The setup is especially suited for studying the liquid phase of the complex plasmas, e.g., flow phenomena such as turbulence or nozzles, and forces acting on the microparticles. Experiments in the laboratory and in parabolic flights have been used to determine the charge of the microparticles as well as the ion drag force acting on them

Results are presented from measurements of the pairwise interaction potential between dust grains in plasma. The experiments were carried out with RF capacitive discharges over a wide range of dusty plasma parameters. All the experiments under analysis revealed a repulsive interaction potential that was close in shape to the screened Coulomb potential. The grain charges and the radii at which they were screened are determined.

Ordering of dust grains suspended in glow discharge plasmas into quasi-steady liquid-or crystallike structures in an external field is considered. The self-consistent electric field generated by free electrons, ions, and dust grains is found. An estimate is obtained for the confining potential required to hold dust grains in the direction perpendicular to the discharge axis. It is shown that the potential energy of interaction between ordered dust particles has the form characteristic of ionic crystals. Critical parameters are estimated for a liquidlike dust structure. The correlation function calculated for a dusty plasma by using this approach is compared with a measured one.

PK-4 is an experiment designed to investigate complex plasmas (low-temperature plasmas containing microparticles, e.g. dust grains) in a combined dc/rf discharge under microgravity conditions on board of the International Space Station. Within the 35th and 36th ESA parabolic flight campaigns first experiments under microgravity conditions in a specially designed experiment set-up have been performed. The particle flow inside the tube, the appearance of dust waves, and lane formation in interpenetrating particle clouds have been observed.

The results from numerical simulation of the dust grain charging process in a weakly ionized gasdischarge plasma are analyzed. An empirical approximate formula for the ion current to the grain surface under conditions close to those in laboratory experiments with dusty plasmas is derived. Investigations show that the approximation proposed greatly simplifies determination of the grain charge by avoiding laborious calculations for different grain sizes and different parameters of the surrounding plasma.

Analytical relations are given for estimating the energy of the stochastic motion of an individual dust grain heated by electrostatic ion oscillations in a weakly ionized gas-discharge plasma. Dust grain charging processes are analyzed, and an empirical approximation is obtained for the ion current to the grain surface. The processes are simulated under conditions similar to those of laboratory experiments on dusty plasmas. It is found that the kinetic temperature of a dust grain heated by electrostatic ion oscillations in a gas-discharge plasma can exceed the background gas temperature.

Formation of large-scale dust vortices in the striation of dc glow discharges was experimentally investigated. Dust clouds were formed by monodisperse MF particles with diameters ≈2 µm in the regions of stable striation. Vortices obtained were about 1 cm in size and contained ~104 particles. Structural (vortex configuration, dust particle flow filaments) and dynamic (rotation frequencies and velocity spectrum) characteristics of large-scale dust vortices were measured.

The PK-4 experiment is a continuation of the successful dusty plasma experiments PK-1, PK-2 and PK-3 conducted on board of the orbital space stations Mir and International Space Station. For all these experiments it is important to avoid the strong influence of gravity, exerting an external stress on the system. Whereas PK-3 and PK-3 Plus experiments are using a planar rf capacitive discharge, PK-4 studies complex plasmas in a long cylindrical chamber with a combined dc/rf discharge. Such a configuration of the chamber will provide a particular advantage for investigation of different dynamical phenomena in complex plasmas such as sheared laminar flow of a highly nonideal dusty liquid and its transition to the turbulent regime, nozzle flow, boundary layers and instabilities, shock waves (solitons) formation and propagation, dust particle lane formation, and space dust grain separation according to their size.

In this paper results are presented from the experimental studies of the dynamics of dust grains in the plasmas of an rf capacitive discharge and a dc glow discharge. The pair correlation function, temperature, and diffusion coefficient of dust grains are measured in a wide range of the dusty plasma parameters. Experimental observations of vortices in dc-glow discharges and results of the experimental studies of the laminar dusty plasma flow are presented. Results are given of experimental investigation of three-particle correlation for liquid plasma-dust structures. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Results are presented from experimental and theoretical investigations of the behavior of dust grains in a track plasma produced by a beam of accelerated protons. The dynamic ordered dust structures in a proton-beam-produced plasma are obtained for the first time. The processes leading to the formation of such structures are simulated numerically. The experimentally obtained dynamic vortex dust structures in a track plasma of a proton beam are explained theoretically, and the theoretical model developed to describe such a plasma is verified experimentally. Numerical investigations carried out by the method of Brownian dynamics made it possible to qualitatively explain the characteristic features of the formation of vortex dust structures.

We present the results of our numerical simulations of mass-transport processes on short observation time scales for extended quasi-two-dimensional and three-dimensional nonideal dissipative systems of macroparticles interacting through a screened Coulomb potential. The simulations were performed for the parameters corresponding to the experimental conditions in laboratory dusty plasmas. The evolution of the rms macroparticle displacement on short observation time scales in nonideal liquid systems is shown to be similar to the evolution of the thermal particle oscillations at lattice sites.

It is for the first time shear viscosity of dusty plasma liquid defined by the screened Coulomb interaction of dust particles is estimated on the basis of experimental data. The results of the experimental studies of the laminar viscous dusty plasma flow are presented in the Letter.

Effective structural (pair correlation function) and transport (diffusion constant) characteristics of the system of microparticles (dusty plasma) have been measured in a set of experiments in dc- and rf gasdischarge plasmas in ground-based and microgravity conditions on the board of Russian “Mir” Space Station and International Space Station. The comparison between these measurements and numerical simulations were used for dusty plasma diagnostics. Specifically, the effective coupling parameter, which mainly governs structural and dynamical properties of the system, was determined for different experimental conditions. The application of these measurements to estimate the particle charge and the plasma screening length is discussed.

Complex plasma research is a new and rapidly developing field, with investigations under gravity and microgravity conditions. The complex plasma consists of a common plasma — with electrons, ions and neutrals — and an additional component of small solid particles typically in the range of micrometers. This heavy component in the plasma makes it necessary to perform experiments under microgravity conditions although it is possible to levitate the particles in the laboratory as well. The microparticles in the plasma are charged through the absorption of free electrons and ions to thousands of elementary charges. On Earth, the particles can be levitated in a strong electric field. But this induces stresses to the particle cloud which can only be eliminated under microgravity conditions. It also implies that under gravity conditions only a small part of the parameter phase space of complex plasmas can be investigated. Therefore, to complete the research on complex plasmas, investigations under microgravity conditions are mandatory. This paper overviews the research on complex plasmas on the International Space Station ISS achieved with the first long-term experiment, PKE-Nefedov, used for this research.

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.

The results of numerical simulations of particles structure (ordering) and dynamics (diffusion constant) in a 'liquid plasma' state of strongly coupled dusty plasmas are considered. Static and dynamical properties of microparticle systems in the liquid state have been studied in experiments under microgravity conditions. The comparison of the experimental results with the results of numerical simulations of Debye systems was used to estimate the effective coupling parameter which governs particles structural and dynamical properties. This diagnostics tool allows one to estimate particle charge and plasma screening length which are very important characteristics of dusty 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.

Results of a numerical analysis of mass transfer in systems of macroscopic particles with various isotropic interaction potentials are presented. Parameters that determine transport properties of nonideal dissipative systems are obtained for a broad class of model potentials. An approximate expression for the diffusivity of interacting particles is proposed. The relationship between diffusivity and viscosity is analyzed for strongly nonideal systems.

A new method is suggested for the generation of density waves of the dust component in a dc glow-discharge dusty plasma by means of pulsed gasdynamic stimulation. This method is applied to structures consisting of spherical and disk-shaped dust particles. High-amplitude waves are obtained (compression factor of up to two). The wave profile in the case of spherical and disk-shaped particles remains unchanged. The interaction of self-excited waves and waves initiated by the effect of the gas flow pulse is observed. A mechanism of generation and propagation of density waves under gasdynamic stimulation is suggested.

Phase state and transport of macroparticles forming fluid-like structures in a complex plasma is numerically studied for radial pair potentials of various types. Special attention is paid to non-Yukawa-type screening potentials. The parameters responsible for the phase state and microscopic dynamics of macroparticle motions are determined and investigated. Consequences for the particle condensation are discussed.

Experiments with flow of microparticles were conducted in a DC discharge. A sharp threshold in the neutral gas pressure for the onset of an unstable low-frequency dust wave mode was observed. Highly space- and time-resolved measurements of the microparticle flow combined with probe measurements of the plasma parameters have allowed detailed comparison with a theoretical model. The model demonstrates good qualitative and quantitative agreement with the experimental results providing accurate estimates of the particle charge.

Pair correlation is analyzed for systems of macroscopic particles with various isotropic interaction potentials. Under certain conditions, the behavior of the pair correlation function is determined by an effective order parameter and its decrease toward infinity follows an asymptotic power law. When the effective parameter is smaller than a certain critical value, the decay of pair correlation is much steeper. Experimental results concerning the form of the pair correlation function are presented for liquid-like dust structures localized in the near-electrode plasma sheath of a high-frequency capacitive discharge. An analysis of numerical and experimental results shows that melting dynamics in these systems are analogous to those characteristic of a topological phase transition.

The PK-4 experiment is a continuation of the successful dusty plasma experiments PK-1, PK-2 and PK-3 conducted on board of the orbital space stations Mir and ISS (International Space Station). The aim of the PK-4 experiment is an investigation of physical processes in complex (dusty) plasmas under microgravity conditions in a combined dc/rf discharge plasma.

Results of the experimental studies of the dynamics of dust grains in the plasmas of an rf capacitive discharge and a dc glow discharge are presented. The dusty plasma of a dc glow discharge was investigated in both ground-based experiments and experiments carried out under microgravity conditions (on board the Mir space station). The pair correlation function, temperature, and diffusion coefficient of dust grains are measured in a wide range of the dusty-plasma parameters. Dimensionless parameters responsible for the microscopic transport of dust-grains in a gas-discharge plasma are determined. A nonintrusive diagnostic technique for determining the dust-grain charges and screening lengths under the assumption of screened interaction between the grains is proposed. This technique is used to estimate the surface potential of dust grains of different size in a gas-discharge plasma.

It is found that the ultraviolet radiation exposed to dusty plasma can serve as an additional manipulator of dust cloud structures formed under microgravity conditions. Strong ultraviolet source of radiation, making the dust charges positive, is not favorable for the formation of compact dust structures, while a relative moderate ultraviolet source can help in changing the structures to confine larger amounts of dust grains. The possibility of using the ultraviolet irradiation for formation of compact dust structures at the center of the microgravity experimental device is investigated numerically by solving the set of self-consistent balance equations for parameters of the structure in the presence of an ultraviolet radiation source.

Spatial variation of dust particle charges are estimated numerically for typical laboratory experiment conditions in a radio-frequency (rf) capacitive discharge. The surface potentials of macroparticles levitating in the upper part of the near-electrode layer of the rf discharge are measured. A model is proposed for the formation of irregular dust oscillations due to stochastic motion of dust in the bulk of a spatially inhomogeneous plasma (in the presence of a dust charge gradient). This mechanism is used for analyzing the results of measurements of the amplitude of vertical vibrations of dust particles in the near-electrode layer of the rf discharge. It is found that the dust charge gradient may be responsible for the development of such vibrations.

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.

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...