Impurity fluxes in ASDEX studied with a solid graphite collector probe

Abstract

A high-purity graphite probe for collecting impurities in ASDEX has been used for the first time. The probe head consists of a rotatable graphite cylinder housed in a fixed molybdenum shield containing an exposure slit. Measurements were made on the scrape-off layer plasma streaming to the upper outer divertor plate and, after extraction of the probe, collected impurities were identified and quantified using surface physics techniques. Single null ohmic discharges produce a threefold increase in erosion of the Cu plate of the active upper divertor compared to similiar double null discharges. The radial distributions of scrape-off layer impurities change in both shape and intensity with vertical plasma position. Boronization of the main vessel reduces the amount of Fe collected by more than a factor of two but the oxygen deposit increases. Radial distributions of Cu and Fe deposits as a function of time for a neutral beam heated discharge are also discussed.