New scenarios of ICRF wall conditioning in TEXTOR and ASDEX Upgrade

Inter-machine studies of wall conditioning with the ICRF discharges have been performed in the limiter (TEXTOR) and divertor (ASDEX Upgrade (AUG)) tokamaks in the presence of a toroidal magnetic field (⩾2 T) using the conventional ICRF antennas without modifications in hardware. The vessel oxidation treatment by pulsed ICRF discharges in (He + O2)-mixture (TEXTOR) is analyzed in terms of ratios of the RF pulse length to the O2-puff duration. A successive set of deuterium and helium ICRF discharges was developed for post-oxidation wall cleaning and analyzed in the light of TEXTOR recovery to the normal plasma operation. A new scenario of ICRF wall conditioning in (He + H2)-mixture at two frequencies was applied in AUG and compared with the standard glow discharge in terms of outgassing efficiency. Modeling of the absorbed RF power was done to clear up a role of the H2 concentration in the homogeneity of ICRF plasmas and the generation of high-energy ions.