ICRF wall conditioning at TEXTOR-94 in the presence of a 2.25 T magnetic field

To investigate alternative conditioning concepts for future fusion devices with permanent magnetic fields, plasmas produced by the coupling of ICRF power to He and gas mixtures of helium + silane, have been analyzed 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 × 10^-3 < P_He (Pa) < 1 × 10^-1 ) and ICRF power, P_ICRF (100 < P_ICRF (kW) < 800). Electron densities n_e averaged along different radial lines of sight across the vacuum vessel from the top to the bottom have been obtained in the range 5 × 10¹⁰ < n_e (cm^-3) < 3 × 10¹². To study quantitatively the efficiency of hydrogen desorption from the first wall at different ICRF plasma conditions in a reproducible way, the first wall was presaturated by RG-glow discharges in H₂. The amount and the evolution of the H₂ desorption from rf discharge to rf discharge was determined by ion gauge measurements combined with mass spectrometry. To demonstrate the capability of the new method for plasma assisted thin film deposition, different 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 disappearing from the gas phase and that measured by post mortem surface analyses of collector probes at the wall position was found.