Island Divertors for Helical Devices

Abstract

Basic features of the island divertor concepts for low-shear advanced stellarators like W7-AS and W7-X (intrinsic island divertors) and for high-shear heliotrons like CHS and LHD (local island divertors, LID) and first results for both concepts are shortly reviewed. The diverting fields of island divertors are either intrinsic (low-shear case) or externally imposed (high- shear case). The associated field perturbations are very small compared to tokamak poloidal field divertors, which explains the high flexibility of island divertor configurations, but sufficiently large to generate divertor-viable islands. Although the physics of island divertors is expected to be similar to that of tokamak poloidal field divertors, leading geometrical parameters significantly differ from those of comparable-size tokamaks. Furthermore, strong three-dimensional effects arise from toroidally discontinuous target plates. For both the intrinsic and local island divertor configurations, the island structure and the plasma divers on have been verified experimentally. For W7-AS, stable high recycling conditions could be demonstrated in a stellarator for a simple island divertor geometry. In CHS with the LID field switched on, a strong increase of the pumping efficiency was measured, resulting in a 50% reduction of the core density for the same gas-puff rate as in the non-LID case. 3D numerical transport studies predict, for low densities, significant momentum losses associated with the island divertor geometry. For high densities, low island temperatures lead to detachment of the ionisation front from the targets and to a strong toroidal modulation of the plasma parameter due to the discontinuous targets.