First experimental confirmation of island SOL geometry effects in a high radiation regime on W7-X

This work characterizes the detachment behavior and radiation characteristics of the low iota configuration in the Wendelstein 7-X (W7-X) stellarator. The island scrape-off layer (SOL) of the low iota has a poloidal mode number of six islands surrounding the last closed flux surface (LCFS). The island geometry of the low iota configuration is significantly different from that of the standard magnetic field configuration, whose detachment characteristics have already been described in previous work (Schmitz et al 2021 Nucl. Fusion 61 016026, Jakubowski et al 2021 Nucl. Fusion 61 106003, Feng et al 2021 61 086012). Experimental results show that the radiation pattern in the low iota configuration is starkly different from that of the standard magnetic field configuration, with radiation concentrated at the island SOL O-points, rather than the X-points. Additionally, this O-point localized radiation pattern is associated with unstable detachment, with both radiation oscillations in experiments and the lack of a self-consistent plasma solution at high radiated power fraction in EMC3-Eirene simulations. EMC3-Eirene simulations are used to understand the radiation distribution. It was found that the O-point localized radiation arises first from local impurity accumulation near the parallel flow stagnation, which is located close to the geometrical center of the island (the ”O-point”). Local cooling in this region leads to plasma condensation in the islands in the closest magnetic connection to the divertor target plates. The heat source to this region of the island, which is thermally isolated from the upstream heat source in terms of parallel transport, must arise via perpendicular heat transport. This heat source is expected to be large for the low iota configuration due to its very small internal island field line pitch. This work highlights the importance (complementary to previous work, e.g. Feng et al (2005 Nucl. Fusion 45 89), Feng et al (2011 Plasma Phys. Control. Fusion 53 024009)) of the internal island field line pitch not only on the radiation pattern but also the detachment performance of the island divertor.