Detachment Instability in the W7-X Stellarator

The Wendelstein 7-X stellarator has shown that the island divertor concept can achieve steady-state detached scenarios. However, it was found in the latest experimental campaign that plasma operation in the low iota (ι=5/6) magnetic configuration was unstable starting at f_rad ≈ 40% for intrinsic carbon and nitrogen seeding. Above this radiation level, the plasma stored energy dropped, a rapid rise of f_rad to near unity and strong oscillations of P_rad were observed. We use EMC3-Eirene simulations to characterize how detachment in the island divertor becomes unstable. The instability seems to arise from the small internal field line pitch (Θ) in the low iota island SOL, which increases the ratio of perpendicular to parallel transport significantly. The simulations show strongly localized radiation and density build-up near the island center, at the parallel flow stagnation point. With increasing f_rad, the radiation moves towards the separatrix. As a result, the island neutral screening is lost locally. The strong parallel T_e gradients that allow for the localized radiation is sustained due to weak parallel heat conduction. Both of these phenomena can trigger thermal instabilities, the former of which is reminiscent of W7-AS observations. Preliminary results indicate that other, stable configurations can be destabilized using the island control coils to reduce Θ, which supports the above hypothesis regarding the effect of parallel-to-perpendicular transport ratio on detachment stability.