ICRF WALL CONDITIONING AT TEXTOR-94 IN THE PRESENCE OF A 2.25 T MAGNETIC-FIELD

To investigate alternative conditioning concepts for future fusion dev ices with permanent magnetic fields, plasmas produced by the coupling of ICRF power to He and gas mixtures of helium + silane, have been ana lyzed in the presence of a 2.25 T toroidal magnetic field at TEXTOR-94 . Their qualification for wall conditioning has been investigated for different He-pressures, P-He (1.X 10(-3) < P-He (Pal < 1 X 10(-1)) and ICRF power, P-ICRF (100 <P-ICRF (kW)< 800). Electron densities n(e) a veraged along different radial lines of sight across the vacuum vessel from the top to the bottom have been obtained in the range 5 X 10(10) < n(e) (cm(-3))< 3 X 10(12). To study quantitatively the efficiency o f hydrogen desorption from the first wall at different ICRF plasma con ditions in a reproducible way, the first wall was presaturated by RG-g low discharges in H-2. The amount and the evolution of the H-2 desorpt ion from rf discharge to rf discharge was determined by ion gauge meas urements combined with mass spectrometry. To demonstrate the capabilit y of the new method for plasma assisted thin film deposition, differen t amounts of silane (< 50%) were added to the He gas. During the ICRF pulses, the silane molecules were dissociated in the plasma and the Si atoms stick to the wall. A good balance of the amount of Si disappear ing from the gas phase and that measured by post mortem surface analys es of collector probes at the wail position was found.