The behaviour of RG-cleaning discharges in large vessels

The pressures at which RG-discharges can be ignited (Psub(I)) and sustained (Psub(Q)) have been measured in an unbaked SS vessel of 33 m2 surface area and 11 m3 volume. Different rf-electrode arrangements for the application in fusion devices have been tested. Psub(Q) = 5 x 10-4 mbar can be achieved with an rf-anode in the center of the vessel. Higher pressures are necessary to sustain the discharge from anodes in portholes. Ignition is easily achieved by gaspuffing in the very vicinity of the rf antenna. No bipolar arcs of considerable lifetime have been observed, whereas numerous unipolar arcs were triggered by surface contaminations. Their rate increases strongly with increasing current but decreases exponentially with time for fixed plasma conditions. A careful discharge conditioning by RG discharges in nonbakable devices or a prehandling at elevated wall temperature seem to be necessary. If they can be used, special means against unipolar arcs and their transition into a stable bipolar arc become unnecessary. (orig.)