Electron Cyclotron Resonance Heating Transmission Line and Launching System for the Wendelstein VII-AS Stellarator

A 70-GHz electron cyclotron resonance heating (ECRH) system (4 × 0.2 MW for 3 s and 0.2 MW for 0.1 s) has been installed on the Wendelstein VII-AS stellarator for plasma buildup, heating, and current drive. Five commercially available gyrotrons, each delivering 0.2-MW radio-frequency (rf) output power, serve as the ECRH sources. The microwaves are transmitted to the stellarator by conventional oversized waveguides, and they convert the gyrotron output mode to a linearly polarized Gaussian free-space mode, which is launched to the plasma at arbitrary poloidal and toroidal angles by a quasi-optical launcher. The transmission line and launching system as well as the related rf diagnostics are discussed in detail. Experimental results on heating and current drive obtained with various kinds of microwave launching are presented.