Impurity collection in the SOL plasma and at the wall of W7-AS

In order to study the impurity production, transport and deposition on W7-AS a retractable collector probe, spatially distributed long term samples (LTS) and a thermal Li beam have been applied. Surface analysis of the LTS shows strongly enhanced impurity fluxes to restricted regions of the wall. Using the retractable collector probe impurity deposition was followed through several successive wall coatings by boronization, and a change of the limiter material from TiC coated graphite to B doped graphite. With the TiC limiter the dependence of the impurity deposition on the plasma density was investigated. The thermal Li beam is used to demonstrate the penetration behaviour of impurity influxes. (orig.)