Divertor for W7-X: II. Comparison of island and axisymmetric divertor

The need for sufficient power and particle exhaust is common to all magnetic confinement concepts. For the new stellarator W7-X, making use of the intrinsic island structure at the edge, an island divertor is planned. The concept of the W7-X island divertor (as a prototype of general island divertors) and its specific design criteria will be presented. A comparison of the basic physics guidelines for divertor design with axisymmetric divertors (specifically with the ASDEX-Upgrade Div-II configuration) will be made. The common elements (general scrape-off layer physics due to the dominant parallel transport and atomic physics effects through neutrals) and possible differences (three-dimensional effects, differences in field-line geometries, additional flexibility through divertor coils, possible ergodicity in some configurations, core baffling of neutrals) will be discussed. (author)