Abstract
The flow of metal impurities from the wall and limiter to the plasma, and back towards the wall, is investigated using surface collection probes and subsequent surface analysis in order to understand impuritiy generation and impurity transport. Impurity fluxes and their scrapeoff lengths have been investigated for several years in a large number of tokamaks. The results are summarized and discussed. Erosion exceeding deposition was first observed to occur at limiterlike structures closest to the plasma edge. Recently, a new probe has been developed to measure quantitatively the erosion in ASDEX. Subsequent quantitative surface analysis is performed in situ by electron induced x-ray analysis. Erosion caused either by ions (limiter) or charge exchange neutrals (wall) can be investigated separately. The erosion at the wall is about two orders of magnitude smaller than the erosion at limiterlike structures, which is of the order of one monolayer per discharge. Simultaneous measurements of deposition and erosion have been performed to elucidate net values of deposition and erosion.
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