Molecular dynamics and dynamic Monte Carlo studies of mixed materials and their impact on plasma wall interactions

A molecular dynamics technique was used to study the sputtering characteristics of co-deposited carbon (C) and to investigate subsequent effects of introducing beryllium (Be) impurities. Amorphous C layers containing hydrogen (H) with atomic ratios of H/(C+H)<0.3 together with a small amount of Be with ratios of Be/(C+H+Be)<0.06 are produced on top ofa tungsten (W). Noncumulative bombardment of the amorphous C layer without H (i.e., H/(C+H)=0) by H atoms, produces no atomic C sputtering at energies less than the threshold energy for physical sputtering, as calculated by a dynamic Monte Carlo code, EDDY. By examining the H uptake in the layer, it was observed that hydrocarbon sputtering occurs when the dominant emitted species are small radicals e.g. CH and at low H/(C+H) ratios (<0.1). When the ratio was increased, larger radicals (CH₂ and CH₃) were emitted. In the eV energy range, the larger molecules were the dominant emitted species, whilst C atoms were emitted even at very low energies. A small percentage of Be was implanted in the C layer, and was found to reduce sputtering of the large molecules, whereas the emission of the small molecules and the C atoms was slightly reduced or remained unchanged.