Spectroscopic measurement of target plate erosion in the ASDEX Upgrade divertor

The erosion of the graphite divertor plates in the ASDEX Upgrade tokamak is measured spectroscopically. Spatial profiles of the D 0 and C + influxes across the outer target plate are determined from measured absolute line intensities. Plasma parameters (n e , T e ) at the target, which are required to determine the appropriate photon emission efficiencies for these lines, are obtained from an in-vessel reciprocating Langmuir probe above the target plate. Yields for the erosion of the graphite by the incident D + flux are determined from the ratio of the measured C + to D 0 fluxes. Over a range of moderate densities the measured yields of ⩽4% are explicable in terms of physical sputtering alone. Chemical sputtering by low energy Franck-Condon neutrals probably contributes, however, to the total erosion. At higher densities detachment of the plasma from the targets occurs owing to formation of a MARFE near the X point. Under these conditions localized physical sputtering of the targets ceases. The impurity level (Z eff ) is, however, maintained following detachment, indicating a corresponding maintenance of carbon influx, perhaps due to chemical erosion of the total graphite surface and/or an improvement in particle confinement in the detached state