HYDROCARBON FORMATION DUE TO COMBINED H-+ ION AND H-0 ATOM IMPACT ON PYROLYTIC-GRAPHITE

Graphite, which is a favored material for first wall use in fusion devices, is subject to chemical erosion under hydrogenic impact. Until recently, most published results on hydrogen-induced chemical erosion of graphite have concentrated only on the production of methane. For incident H+ energies 300 ev/H+, methane is in fact the dominant reaction product. For low energy (+ ion impact on pyrolytic graphite, however, the relative formation rates of heavier hydrocarbons (C2Hx, C3Hx) become more important. For thermal H0 atom (sub-ev) impact, the formation of heavier hydrocarbons dominates the total carbon erosion process. For combined H+ ion and H0 atom impact, as would occur in a fusion reactor with a ratio [H+][H0]approx. 0.1 to 1, the total C-erosion yield is 2-5 times larger than the methane yield, depending on the ion energy and the relative fluxes. The absolute level of the maximum erosion yield under all conditions observed remains 0.1 C/H.