Divertor operation in stellarators: results from W7-AS and implications for future devices

The research on divertors for stellarators is at the beginning. Extensive studies are being prepared on large helical device (LHD) and W7-X. W7-AS is now being operated with an open island divertor (ID) which serves as a test bed for the W7-X divertor. The divertor enables access to a new NBI-heated, high-density operating regime with improved confinement properties. This regime—the high-density H-mode (HDH)—displays no evident mode activity, is extant above a threshold density and characterized by flat density profiles, high-energy- and low-impurity-confinement times and edge-localized radiation. Impurity accumulation, normally associated with ELM-free H-modes, is avoided. Quasi-steady-state discharges with n̄e up to 4×1020 m−3, edge radiation levels up to 90% and plasma partial detachment at the divertor targets can be simultaneously realized. The accessibility to other improved confinement modes in W7-AS (conventional H-mode and OC-mode) is not restricted by the divertor. The results provide a promising basis for future experiments, in particular on W7-X, and recommend the ID as a serious candidate for solving the plasma exhaust problem in stellarators.