Surface loss probabilities of hydrocarbon radicals on amorphous hydrogenated carbon film surfaces: Consequences for the formation of re-deposited layers in fusion experiments

The surface loss probabilities of hydrocarbon radicals on the surface of am orphous hydrogenated carbon (C:H) films are investigated by depositing film s inside a cavity with walls made from silicon substrates. This cavity is e xposed to a discharge using different hydrocarbon source gases, and particl es from the plasma can enter the cavity through a slit. The surface loss pr obability beta is determined by analysis of the deposition profile inside t he cavity. This surface loss probability corresponds to the sum of the prob abilities of effective sticking on the surface and of formation of a non-re active volatile product via surface reactions. By comparing the deposition profiles measured in CH4, C2H2 and C2H4 discharges one obtains for C2H radi cals beta = 0.90 +/- 0.05 and for C2Hx>2 radicals beta = 0.35 +/- 0.1, wher eas the surface reaction probability for CH3 is below 10(-2), as known from the literature. The growth rate of C:H films is, therefore, very sensitive to any contribution of C2Hx species in the impinging flux from a hydrocarb on discharge. The very same growth precursors can be formed in a divertor p lasma and should therefore dominate the formation of re-deposited layers. A scenario for the occurrence of these re-deposited films in fusion experime nts on the basis of typical divertor plasma and surface parameters is being discussed. Strategies are proposed for prevention of these re-deposited la yers and for their removal.