Oxygen glow discharge cleaning in ASDEX Upgrade

Oxygen-containing glow discharges have been proposed for the removal of redeposited hydrocarbon films in fusion experiments. After the 2004/2005 experimental campaign a 49 h He/O2 glow discharge experiment was performed in ASDEX Upgrade. A total of 25 g of carbon was removed as CO and CO2. In order to probe erosion in various locations in the vessel 48 well defined laboratory-deposited a-C:H film samples were distributed poloidally and toroidally. In principle, the films were successfully removed on all types of surfaces directly facing the discharge. Additionally, two sets of tungsten-coated tiles were removed from the vessel, one prior to and one after the cleaning discharge. Analysis of films on the tiles revealed that they consisted mainly of boron with only little carbon and deuterium. Neither erosion of these films nor any further reduction of their carbon and deuterium content due to the glow discharge could be detected.