Studies of divertor target plate erosion in the ASDEX-Upgrade tokamak

Passive spectroscopy has been used for in situ studies of the erosion of various divertor target materials in ASDEX-Upgrade. Yields measured for the erosion of graphite over a range of moderate densities can be explained by physical sputtering alone, although there may also be small contributions from chemical processes. At high density, detachment of the divertor plasma occurs and sputtering of the target ceases. Under such high-density conditions high-Z materials, e.g. tungsten, offer a promising alternative to graphite. First studies of the erosion of these materials confirm the expected low sputtering rates and the short ionisation lengths of sputtered neutrals. The latter implies efficient prompt redeposition which should suppress production of higher ionisation stages and associated problems with self-sputtering.