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- ItemThe dynamics of macroparticles in a direct current glow discharge plasma under micro-gravity conditions([London] : IOP, 2003) Nefedov, A. P.; Vaulina, O. S.; Petrov, O. F.; Molotkov, V. I.; Torchinskii, V. M.; Fortov, V. E.; Chernyshev, A. V.; Lipaev, A. M.; Ivanov, A. I.; Kaleri, A. Yu.; Semenov, Yu. P.; Zaletin, S. V.The dynamics of large-sized (70-180 μm) spherical bronze particles in a direct current glow discharge plasma was studied experimentally under microgravitation conditions. The temperatures, velocities, pair correlation functions and self-diffusion coefficients of macroparticles were measured at various discharge currents. The charges of dust particles (on the order of 106 e) corresponded to high surface potentials of about 30-40 V.
- ItemCollision-dominated dust sheaths and voids - Observations in micro-gravity experiments and numerical investigation of the force balance relations([London] : IOP, 2003) Tsytovich, V. N.; Morfill, G.; Konopka, U.; Thomas, H.Numerical solutions of stationary force balance equations are used to investigate the possible dust configurations (dust structures) in complex plasmas between two floating potential plane electrodes. The distance between electrodes is assumed to be larger than the ion-neutral mean free path and the hydrodynamic description is used. It includes the known forces operating in this limit, the ionization source and the dust charge variations. The stationary balance equations are solved both in the case of the presence of one-size dust grains and for the case of a mixture of grains with two different sizes. Recent micro-gravity experiments with single-size dust grains and two-different-size dust grains show the formation of a system of dust sheaths and dust voids between the two plane electrodes. The observed configurations of dust structures depend strongly on the gas pressure and the degree of ionization used. The numerical investigations are able to show the necessary conditions for the types of structure to be created and give their size. The size of the structures observed is larger than the ion-neutral mean free path and is of the order of magnitude of that obtained numerically. The numerical investigations give details of the spatial distributions, the dust particles, the electron/ion densities, the ion drift velocity and dust charges inside and outside different dust structures. These details have not yet been investigated experimentally and can indicate directions for further experimental work to be performed. The single-dust-sheath structure with single-size dust particles surrounded by dust free regions (dustwall-voids) and floating potential electrodesis computed. Such a structure was observed recently and the computational results are in agreement with observations. It is shown that more often a dust void in the centre is observed. It is found that a dust void in the centre region between two electrodes is formed if the ionization rate is larger than the critical ionization rate and that in the presence of the floating potential walls the central void should be surrounded by two dust sheaths. The necessary condition for this dust structure to be formed is found to be that between the sheaths and the walls there are formed two other wall-void regions. The size of the central void and the distributions of the structure parameters in the two sheaths and in the three voids are computed. The qualitative features of the structure obtained in the numerical computations correspond to those observed. The distributions of the structure parameters in the case of the two dust sheaths are quite different from that for the case of a single central sheath. The possible structures between the electrodes for the case of the presence of dust particles of two different sizes are analysed numerically. It is shown that dust particles with different sizes cannot coexist in equilibrium at the same position and that the regions with different size dust particles must be separated in space. This conclusion is in agreement with most observations performed so far. It is illustrated numerically that for the case where the central void is present the dust particles of larger size form a separate dust sheath which should be located at larger distances from the centre than that for the smaller dust particles. This result also coincides qualitatively with the observations. Computations for the distributions of the parameters in the larger size dust sheath were performed both in the case where the central part is occupied by a dust sheath with smaller size dust particles and for the case where in the central part there exists a dust void surrounded by dust sheaths with smaller size dust particles. The size of the dust void between the sheaths with different size dust particles is calculated and shown to be small as compared to the sheath thickness. In the sheath with larger size dust particles the distribution of dust and plasma parameters differs qualitatively from that of the first dust sheath with smaller size dust particles. The stability of the stationary structures both with respect to excitation of dust convection cells and with respect to oscillations of dust void size is discussed.
- ItemModelling of two differently sized dust species in plasmas under micro-gravity([London] : IOP, 2003) Akdim, M. R.; Goedheer, W. J.; Dahiya, R. P.A self-consistent two-dimensional hydrodynamic model for a dusty argon plasma has been developed to model more than one dust species. Results are presented for situations where dust particles with two different diameters have been included. The final steady state solution is achieved after three injection steps of the dust particles. At every injection phase dust particles of only one size are let in and the simulation is continued until the steady state solution is achieved. Results show that the differently sized dust particles form crystals at different positions. These dust clouds have an influence on each other by means of positive space charge layers created due to the argon ions, which cannot match the steep dust crystal boundaries. The screened Coulomb interaction between the two differently sized dust species is neglected. The electric potential, ion density, electron density and electron energy show significant changes after each injection phase, even at an amount of dust that is small compared to that studied during the micro-gravity experiments.