Island divertor investigations on the W7-AS stellarator

Abstract

The effectiveness of 10 newly installed control coils, which had already been implemented in preparation for the upcoming divertor experiments, has been demonstrated in several important areas. Most importantly we have been able to show that the control coils can be used to influence the ratio of parallel to cross-field transport within an island by nearly a factor 2. High recycling conditions have been attained at the inboard limiters for ι( a)=5/9 but there was no indication of detachment. Excellent data consistency has been achieved during the high density phase of the high recycling discharges between the saturation current I sat measured by the target Langmuir probes and the equivalent values derived from the new absolutely calibrated signal of the 2D H α CCD camera. Poloidal as well as toroidal asymmetries caused considerable problems when we tried to relate measurements of different edge/target diagnostic signals, taken at equivalent positions around the machine. These asymmetries could with high probability be attributed to toroidally non-symmetrically distributed components like the diamagnetic loop (module 5) or the MINISOX camera (module 3).