Influence of beryllium plasma impurities on the erosion of graphite

This paper is concerned with the importance of mixed-material effects that could mitigate the problems associated with chemical erosion of carbon in the next step fusion device ITER. It presents experiments on the influence of beryllium (Be) impurities in deuterium (D) plasmas on the erosion properties of graphite. Graphite (C) targets were exposed to a Be seeded steady state D plasma in the PISCES-B divertor simulator. The Be plasma concentrations and the changes in chemical and physical erosion were measured using a visible light spectroscopy system. During all experiments a bias voltage of −50 V was applied to the C target, the target temperatures were in the range from 500 to 1280 K and the Be plasma concentration was in the range from 0.02% to 0.2%. It was found that even at these low Be concentrations, the C targets are covered with a shielding Be layer that eliminates both chemical and physical erosion of the underlying C target. The formation of this shielding Be layer also results in diminishing C and large Be concentrations in the material co-deposited on a collector probe that was shielded from direct plasma impact.