Chemical erosion of carbon in the divertor of ASDEX upgrade

Chemical erosion of carbon via hydrocarbon formation is investigated by spectroscopic particle flux measurements and with Langmuir probes in the outer Divertor II of the ASDEX Upgrade tokamak. The measurements are made about 10 cm above the strike point of the separatrix at the vertical target plate for hydrogen ion flux densities up to 10(23) m(-2) s(-1) and up to reactor-relevant values of the power per major plasma radius, P/R less than or equal to 12 MW/m. The erosion yields obtained exhibit a pronounced hydrogen/deuterium flux dependence and an isotope effect, resulting in lower yields for higher fluxes and a factor of 2 higher yields for deuterium in comparison to hydrogen. Surface temperatures are relatively low and do not exceed 100 degrees C at the position of the measurements documenting the advantageous power spreading features of carbon in combination with the Divertor II vertical target geometry. The interpretation of the measured CH molecular line emission in terms of underlying hydrocarbon molecular fluxes from the wall is hampered by the uncertainties in the atomic data and by the formation of heavier hydrocarbons. The simple model used for the interpretation of the CH band intensity in terms of flux is supported by the behaviour of carbon ion (C+, C2+) radiation. Owing to the high electron densities, carbon is found to be a very effective coolant for the divertor plasma.