Metal impurity release in diverted tokamak discharges

Plasma-materials interaction at the wall of the main plasma chamber of the divertor tokamak ASDEX was investigated by a combined probe, allowing simultaneous measurements of the erosion rate by neutral particles, and the flux and average energy of neutrals. The erosion was measured by collecting part of the released material which was analysed subsequently by electron induced X-ray analysis. The flux and energy of the impinging neutral particles were measured after each single discharge with an energy resolving carbon resistance probe. The carbon probe is an excellent detector not only for deuterium but also for energetic neutral helium atoms. Wall sputtering by CX neutrals is the dominant iron impurity source in ohmically heated deuterium discharges. However, during discharges in helium not only CX neutral sputtering but also sputtering by helium ions is an important impurity release mechanism. But during auxiliary heating by neutral beams and radio frequency in deuterium, impurity generation by CX neutrals was found to increase with heating power. (orig.)