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When dust particles are introduced in a radio-frequency discharge under micro-gravity conditions, usually a dust free void is formed due to the ion drag force pushing the particles away from the center. Experiments have shown that it is possible to close the void by reducing the power supplied to the discharge. This reduces the ion density and with that the ratio between the ion drag force and the opposing electric force. We have studied the behavior of a discharge with a large amount of dust particles (radius 3.4 micron) with our hydrodynamic model, and simulated the closure of the void for conditions similar to the experiment. We also approached the formation of a Yukawa ball from the other side, starting with a discharge at low power and injecting batches of dust, while increasing the power to prevent extinction of the discharge. Eventually the same situation could be reached. [ABSTRACT FROM AUTHOR]

Dust particles in a radio-frequency discharge generally obtain a negative charge due to the high mobility of the electrons compared to that of the positive ions. When a sufficiently intense flux of photons in the UV range is present, however, the dust charge will be less negative due to photodetachment. The resulting charge distribution will depend on the competition between collection of electrons and ions from the plasma and the detachment by photons. In this paper we study the influence of photodetachment on the charge of dustparticles suspended in a radio-frequency discharge, as well as its influence on the electron energy distribution function and the electron and ion density profiles. An existing Particle-In-Cell code has been modified to include the photodetachment process. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Processing plasmas often produce clusters, ranging in size from a few nanometer up to micrometers. Due to their negative charge, clusters are confined by the sheath electric fields until their mass enables gravity to pull them out of the discharge. Examples are discharges in SiH4 or C2H2. Although there is agreement on the global aspects of the chemistry, details on many processes are lacking. This concerns attachment of electrons to large molecules, restructuring leading to a reduction of the hydrogen content, and the interaction between large negative ions and (excited) molecules, radicals, and positive ions of the parent gas. Results from a one-dimensional model for a radio-frequency discharge in SiH4/H2 will be used to illustrate the consequences of various assumptions regarding these basic steps in the chemistry. For discharges in mixtures containing hydrocarbons the incorporation of C2 groups in polycyclic aromatic hydrocarbons has been proposed as an additional mechanism for dust formation. This is the main process adopted in astrophysics. Also in Tokamaks the formation of carbonaceous dust is observed, caused mostly by the erosion of carbon containing divertor tiles and redeposited layers on plasma facing components. In case of detached operation the plasma in the divertor will be similar to that of a processing discharge, favoring homogeneous processes. In ITER this will be accompanied by hydrogen ion fluxes up to 1024 m-2s-1 and power fluxes up to 10 MWm-2, leading to evaporation of wall material. Here we will discuss the chemistry in these situations (processing discharges and divertors), indicating open questions regarding cluster formation. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]