Electron cyclotron resonance heating and EC-current drive experiments at W7-AS, status at W7-X

Abstract

Electron Cyclotron Resonance Heating (ECRH) is the main heating method for both the W7-AS stellarator, which is operational, and W7-X, which is under construction. An overview on physics studies on ECRH and EC-current drive (ECCD) in an extended parameter range at W7-AS is presented. Current drive was investigated with an ECRH power of up to 1.3 MW at 140 GHz in a wide range of plasma density and launch angle. The experimental results are compared to predictions from quasilinear theory. Electron temperatures of up to 5.7 keV were measured at low to intermediate densities, which can only be explained by the beneficial effect of positive radial electric fields (‘electron root feature’). The results from W7-AS establish the experimental and technological basis for W7-X, which aims at demonstrating the inherent steady state capabality of stellarators at reactor relevant plasma parameters. The status of the design and construction of the ECRH system for W7-X is presented.