140 GHZ ELECTRON-CYCLOTRON-RESONANCE HEATING EXPERIMENTS AT THE W7-ASSTELLARATOR

Electron cyclotron resonance heating (ECRH) at the W7-AS stellarator i s performed with two systems of comparable heating power at 70 GHz (0. 8 MW, 3 s) and 140 GHz (0.9 MW, 0.4 s). Experiments with 140 GHz open a new parameter window with high density operation up to 1.1 x 10(20) m(-3). H mode transitions were observed in high density discharges at 2.5 T and at lower density at 1.25 T. The operational window for the s tellarator H mode with emphasis on the density and power threshold and the influence of gas puffing on the H transitions are discussed. Pert urbation experiments with modulated ECRH power were performed to deter mine the heat transport and the power deposition profile. Density cont rol is achieved in combined heating with neutral beam injection (NBI) despite the beam particle fuelling, whereas with NBI alone a steady de nsity rise is observed. The impact of combined heating on the impurity confinement was investigated. Electron cyclotron current drive (ECCD) was studied in different magnetic field configurations and the influe nce of trapped particles on the ECCD efficiency was examined experimen tally and compared with theory.