ECRH at W7-AS

The capability and versatility of ECRH is reviewed using W7-AS as an example experiment. A two frequency system with 70 GHz (0.4 MW) and 140 GHz (2.0 MW) is routinely operated at W7-AS. Basic heating scenarios such as 1st harmonic O-mode (70 GHz at 2.5 T), 2nd harmonic X-Mode (70 GHz at 1.25 T and 140 GHz at 2.5 T) as well as advanced scenarios like mode-conversion heating via the OXB process were investigated at W7-AS and selected results are reported. Recently the OXB mode conversion heating was extended to densities above the O2-mode cut-off at 2.4 · 1020m-3. The experiments cover a wide plasma parameter range from ne=0.1 to 3.5 · 1020m-3 and electron temperatures of up to 7 keV. ECRH related heat transport phenomena, which are attributed to the existence of strong radial electric fields are discussed. Experiments with combined NBI and ECRH at low and high density are reported. The absence of strong ohmic currents qualifies stellarators for Electron Cyclotron Current Drive investigations, because even small driven currents can be measured with high accuracy. Experimental results are compared to linear and nonlinear theory. First experiments with current drive by Bernstein-waves are reported. (author)