High density ECRH and shear related confinement with ECCD in W7-AS

An overview on physics studies with ECRH and ECCD at W7-AS is presented. Two distinct topics are discussed: In the first section the focus is on ECR heating of high density plasmas beyond the limitations given by the cut-off condition for standard heating scenarios (e.g. X2 mode cutoff for 140 GHz, B-0 = 2.5 T: 1.2 x 10(20) m(-3)). The conversion process from the ordinary to extraordinary and finally to the Bernstein mode (O-X-B) provides a possibility to overcome this density limit. Effective plasma heating with this scenario was measured in W7-AS far beyond the X2 cut-off density for resonant and nonresonant conditions. In the second section experimental results on the response of energy confinement on magnetic sheer are reported. W7-AS, with its almost shearless vacuum configuration, was operated in a hybrid mode with significant plasma currents. Stationary profiles of the rotational transform with positive or negative magnetic shear are introduced by inductive and/or EC driven currents in the co- and counter-directions with respect to the confining magnetic field. The confinement was investigated in both net current free and current carrying discharges. The electron heat transport is analysed.