Wall conditioning by microwave generated plasmas in a toroidal magnetic field

The suitability of microwave generated plasmas for the purpose of wall coating in pure toroidal magnetic fields is investigated in a special test bench. We report on the results of layer deposition using methane plasmas as a systematic case study for future boron or silicon deposition by other gases (B2H6,SiH4). The produced coatings can be characterized as polymer-like soft a-C:D-films with a high D/C-ratio due to the low energy of particles hitting the wall. Neutral hydrocarbon radicals could be identified to play the major role for film deposition. On the other hand, strong re-erosion induced by deuterium ions is observed in regions with plasma-wall contact. The spatial homogeneity and the characteristics of produced coatings are presented and observations are correlated with measured plasma parameters. The use of pulsed plasmas for wall conditioning is compared with steady-state discharges.