WALL CONDITIONING IN FUSION DEVICES AND ITS INFLUENCE ON PLASMA PERFORMANCE

Proper wall conditioning has turned out to be an essential element for achieving the highest possible plasma performance in present day fusi on devices. The main issues art controlling the generation of plasma i mpurities, liberated by plasma-surface interactions, and controlling t he recycling hydrogenic fluxes. The underlying mechanisms are discusse d in this paper. The paper presents a review of the different wall con ditioning methods. It focuses on low-Z wall coatings (beryllium evapor ation, boronization, siliconization, lithium pellet injection) and on helium glow discharge cleaning and assesses their effects on fusion pl asmas. New wall conditioning concepts, compatible with steady-state ma gnetic fields, are discussed in view of future large devices with supe rconducting coils.