The erosion yield of carbon by chemical hydrocarbon generation isinvestigated using spectroscopic particle flux measurements in the divertorof 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 Hβ/Dβ spectroscopy, supported by Langmuir probe measurements in many cases.Under conditions of high particle flux densities above 1022 m-2·s-1,which are typical for high recycling divertor operation, astrong decrease of the apparent chemical erosion yield derived from CH release with increasing hydrogen flux density (and decreasing impact energy)is observed, YCH ∝ ΓH-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 lowerthan the results obtained from extrapolationof laboratory experimentsperformed under low flux density conditions.Three explanations are possible for the observed reduction of CH fluxeswith increasing hydrogen flux densities: the flux dependence of the underlying chemical erosion yield Ychem, the energy dependence of Ychem entering via the experimental correlation of E and 1/Γ, and the plasma parameter dependence of the used molecular photon efficiencyarising from the increasing prompt redeposition of CH4 fragmentation products with rising ΓH.
High flux dependence of erosion and retention in beam experiments and its significance to fusion systems