EVIDENCE FOR HYDROGEN FLUX DEPENDENCE OF THE APPARENT CHEMICAL EROSION YIELD OF GRAPHITE UNDER HIGH-FLUX CONDITIONS

The erosion yield of carbon by chemical hydrocarbon generation is inve stigated using spectroscopic particle flux measurements in the diverto r of the ASDEX Upgrade tokamak. The methane formation at the graphite surface in hydrogen and deuterium plasmas is derived from CH/CD molecu lar band emission, and the corresponding hydrogenic fluxes are obtaine d from H-beta/D-beta spectroscopy, supported by Langmuir probe measure ments in many cases. Under conditions of high particle flux densities above 10(22) m(-2).s(-1), which are typical for high recycling diverto r operation, a strong decrease of the apparent chemical erosion yield derived from CH release with increasing hydrogen flux density (and dec reasing impact energy) is observed, Y-CH proportional to Gamma(H)(-0.8 ). The erosion yields depend on the hydrogen isotope; the values for d euterium are about a factor of 2 higher than those for hydrogen but ex hibit the same flux dependence. These results are obtained for relativ ely low target temperatures between 300 and 360 K; the electron temper ature at the target for the lowest yields/highest fluxes is about 5 eV . The yields at the highest fluxes are much lower than the results obt ained from extrapolation of laboratory experiments performed under low flux density conditions. Three explanations are possible for the obse rved reduction of CH fluxes with increasing hydrogen flux densities: t he flux dependence of the underlying chemical erosion yield Y-chem, th e energy dependence of Y-chem entering via the experimental correlatio n of E and 1/Gamma, and the plasma parameter dependence of the used mo lecular photon efficiency arising from the increasing prompt redeposit ion of CH4 fragmentation products with rising Gamma(H).