ICRF wall conditioning experiments in the W7-AS stellarator

Wall conditioning by means of ICRF-produced helium discharges is shown to be applicable in the presence of a confining stellarator magnetic field. Gas breakdown and stable discharge operation are easily achieved by poloidal double strap antennas over wide ranges of ICRF power, pressure and magnetic field. Mean plasma densities up to 3×1018 m−3 have been obtained with electron temperatures of about 20 eV at the edge. In spite of the large ionization probability at such temperatures pulsed discharges in particular are suited to remove hydrogen from the vessel leading to improved density control in the following stellarator discharges. The efficiency is comparable to that of a conventional He glow discharge. Differences with the recent tokamak results are assumed to arise from the confinement properties of the stellarator field. For example, the hydrogen removal efficiency seems to increase with power density, which is reciprocal to the tokamak results.