- Conference date: 5-8 October 2004
- Location: Toki (Japan)
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.
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