The surface loss probabilities of hydrocarbon radicals on the surface of amorphoushydrogenated carbon (C:H) films are investigated by depositing films inside a cavity with wallsmade 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.
非晶氢化碳（C:H）薄膜表面上的碳氢自由基的表面损失概率通过在内壁为硅衬底的腔体内沉积薄膜来研究。该腔体暴露于使用不同碳氢源气体的放电中，等离子体中的粒子可通过狭缝进入腔体。表面损失概率β通过分析腔体内的沉积剖面来确定。该表面损失概率对应于表面有效粘附概率与通过表面反应生成非反应性挥发性产物的概率之和。通过比较CH₄、C₂H₂和C₂H₄放电中测得的沉积剖面，得到C₂H自由基的β = 0.90 ± 0.05，C₂Hₓ（x>2）自由基的β = 0.35 ± 0.1，而CH₃的表面反应概率低于10⁻²，如文献所述。因此，C:H薄膜的生长速率对入射通量中任何C₂Hₓ组分的贡献非常敏感。这些生长前驱体同样可在偏滤器等离子体中形成，因此应主导再沉积层的形成。基于典型的偏滤器等离子体和表面参数，讨论了这些再沉积薄膜在聚变装置中出现的场景。提出了防止这些再沉积层形成以及去除它们的策略。