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

Abstract

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 β 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 β = 0.90 ± 0.05 and for C2Hx>2 radicals β = 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.

Examination of Temperature Dependence of Chemical Sputtering on Graphite by Comparing the Langevin and Berendsen Thermostats

“…The CH 4 yield in this case had a peak value at 600 K, which was 10 −2 . This profile of temperature dependence agreed with experimental results [9][10][11][12][13][14][15][16][17]. In particular, the experimental results in the case of a low incident energy of less than 100 eV (for example, CH 4 yield due to H + injection at 50 eV [15]), agreed well.…”

Time resolved deposition measurements in NSTX

“…Water is present in the RGA spectrum in the 1e-9 torr range, but only 10% oxygen is found in the deposit so any effect of water absorption must be small. The continuous slow rise in deposition may be related to the evaporation of polymer-like films after each discharge and deposition in the interpulse period as postulated by [24].…”

“…The second crystal faced the back of the tee, 2.5 cm away and is sensitive to radicals with low sticking coefficients (e.g. C 2 H x>2 [22]) that can be reflected from the walls. With the gate valve closed, the unit could be withdrawn from the machine to change out the quartz crystals when desired.…”

Redeposition of amorphous hydrogenated carbon films during thermal decomposition

“…A number of investigations conducted in fusion plasma devices as well as in laboratory experiments have been devoted to study the deposition of such redeposited layers [3][4][5][6][7][8][9][10][11]. Redeposited layers growing in remote areas without direct plasma contact are soft hydrocarbon films with high hydrogen content and it seems that hydrocarbon species with a relatively high sticking coefficient contribute dominantly to deposition [12].…”