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 sm
all contributions from chemical processes. At high density, detachment
of the divertor plasma occurs and sputtering of the target ceases. Un
der such high-density conditions high-Z materials, e.g. tungsten, offe
r a promising alternative to graphite. First studies of the erosion of
these materials confirm the expected low sputtering rates and the sho
rt ionisation lengths of sputtered neutrals. The latter implies effici
ent prompt redeposition which should suppress production of higher ion
isation stages and associated problems with self-sputtering.