Influence of radial electric fields on the heating efficiency of neutral beam injection in the W7-AS stellarator

A version of the FAFNER neutral beam injection code that takes a radial electric field into account is applied to the W7-AS stellarator. The influence of the strength and radial profile of an assumed electric field is studied with respect to heating profiles and global efficiency. The resulting profiles decisively depend on the electric field profile in contrast to the global efficiency. With increasing electric potential (from -5 kV to +5 kV) the heating efficiency decreases for co-injection and increases for counter-injection. This is different to the corresponding ODIN results obtained for the W7-A stellarator, where the confinement of fast ions was strongly enhanced by negative as well as positive electric potentials. These differences are mainly caused by the different beam injection geometries in these devices: perpendicular in W7-A and tangential in W7-AS. (author)