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 investigated using spectroscopic particle flux measurements in the divertor of the ASDEX Upgrade tokamak. The methane formation at the graphite surface in hydrogen and deuterium plasmas is derived from CH/CD molecular band emission, and the corresponding hydrogenic fluxes are obtained from H-beta/D-beta spectroscopy, supported by Langmuir probe measurements in many cases. Under conditions of high particle flux densities above 10(22) m(-2).s(-1), which are typical for high recycling divertor operation, a strong decrease of the apparent chemical erosion yield derived from CH release with increasing hydrogen flux density (and decreasing impact energy) is observed, Y-CH proportional to Gamma(H)(-0.8). The erosion yields depend on the hydrogen isotope; the values for deuterium are about a factor of 2 higher than those for hydrogen but exhibit the same flux dependence. These results are obtained for relatively low target temperatures between 300 and 360 K; the electron temperature 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 obtained from extrapolation of laboratory experiments performed under low flux density conditions. Three explanations are possible for the observed reduction of CH fluxes with increasing hydrogen flux densities: the flux dependence of the underlying chemical erosion yield Y-chem, the energy dependence of Y-chem entering via the experimental correlation of E and 1/Gamma, and the plasma parameter dependence of the used molecular photon efficiency arising from the increasing prompt redeposition of CH4 fragmentation products with rising Gamma(H).