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

The surface loss probabilities of hydrocarbon radicals on the surface of amorphous hydrogenated carbon (C:H) films are investigated by depositing films inside a cavity with walls made from silicon substrates. This cavity is exposed to a discharge using different hydrocarbon source gases, and particles from the plasma can enter the cavity through a slit. The surface loss probability b is determined by analysis of the deposition profile inside the cavity. This surface loss probability corresponds to the sum of the probabilities of effective sticking on the surface and of formation of a non-reactive volatile product via surface reactions. By comparing the deposition profiles measured in CH4, C2H2 and C2H4 discharges one obtains for C2H radicals b = 0.90 ±0.05 and for C2Hx > 2 radicals b = 0.35 ±0.1, whereas 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 hydrocarbon discharge. The very same growth precursors can be formed in a divertor plasma and should therefore dominate the formation of re-deposited layers. A scenario for the occurrence of these re-deposited films in fusion experiments on the basis of typical divertor plasma and surface parameters is being discussed. Strategies are proposed for prevention of these re-deposited layers and for their removal.