Wall conditioning for Wendelstein 7-X by glow discharge

The basic design of a reliable and robust glow discharge cleaning (GDC) system for Wendelstein 7-X is presented. Preliminary experiments at the DEMO cryostat, a full size prototype sector of W7-X, result in conclusions about the operating range, power consumption and discharge stability of a GDC in W7-X dimensions. Ten electrodes are planned to be integrated into the first wall of W7-X, thus avoiding complex manipulators. A spherical calotte anode made of graphite mounted in a shielding trough was found as the optimum electrode design assuring stable discharge operation without active cooling, even when exposed to steady state stellarator discharges.