Film Deposition and their Removal in Gaps and Regions Shaded from the Plasma in the Presence of RF Fields

Post-mortem analyses of gap facing surfaces of tungsten tiles of T-10 ring limiter

“…One must mention that the gap between the SS holder and the tile was very narrow (below 100 µm); nevertheless, large amount of carbon was deposited in this gap. This observation is in line with observations described in [24,25], where carbon films were found in very narrow gaps between two surfaces tightly attached to each other. This was explained by decomposition of volatile hydrocarbons in the high frequency electric fields produced by auto-oscillating high frequency discharges in the magnetic field [24].…”

“…One can hardly expect that particles from plasma may penetrate so deep into so narrow gaps. For explanation of this observation, we suggest a mechanism associated with auto-oscillating discharges, which was proposed in [24][25][26]. These discharges arise between two surfaces (for example the external surface of a W tile and the wall of the tokamak) connected by the magnetic field.…”

“…Deep penetration of carbon in gaps was observed also in [24,25] in simulation experiments with gaps subjected to the beam-plasma discharge. Deposition of carbon films was detected through the entire depth of the gaps including those that were as narrow as 0.1 mm.…”

“…This current provokes high frequency electric fields in the gap, and these fields are supposed to be the reason of dissociation of volatile hydrocarbons and C deposition on surfaces in the gap. The effect was demonstrated in [24,25], where preferential deposition of carbon films in very narrow gaps (below 0.1 mm) was associated with high frequency electric fields produced by high frequency currents through the gap facing surfaces. Frequencies were estimated to be of about 50 MHz, and the electric field strength was estimated to be 10 7 V/m in [24] and 1.5×10 4 V/m in [25].…”

“…The effect was demonstrated in [24,25], where preferential deposition of carbon films in very narrow gaps (below 0.1 mm) was associated with high frequency electric fields produced by high frequency currents through the gap facing surfaces. Frequencies were estimated to be of about 50 MHz, and the electric field strength was estimated to be 10 7 V/m in [24] and 1.5×10 4 V/m in [25].…”