Study of impacts of island resonance and location on detachment transition in W7-X using EMC3-EIRENE

Abstract A numerical survey was conducted using EMC3-EIRENE to investigate the impact of island resonance and location on detachment characteristics during density ramp-up in Wendelstein 7-X (W7-X), prior to corresponding experiments. The radial position of the islands in the scrape-off layer (SOL) was modulated by varying the planar-coil current I PC within the range −1.5 kA ≤ I PC ≤ 1.5 kA, while keeping the non-planar-coil current fixed in the standard configuration (SDC). Detachment characteristics — including detachment onset density, impurity radiation patterns, total target heat load, and divertor neutral pressure — depend strongly on the values of I PC : an inward-shifted island with I PC = −1.5 kA and an outward-shifted island with I PC = +1.5 kA produce markedly different behavior. The detachment onset occurs at lower density for negative I PC (inward-shifted island) and at higher density for positive I PC (outward-shifted island) relative to the reference SDC. In all configurations, the divertor recycling rollover coincides with a radiation fraction of ≈ 0.5–0.6, indicating that power starvation plays a dominant role in the detachment transition. Impurity radiation peaks near the island O‑point for negative I PC , whereas it peaks near the X‑point in the reference SDC and for positive I PC . Regarding the power and particle control, a clear trade-off is observed: negative I PC configurations are most effective for reducing the total target heat load, whereas the reference SDC shows the highest divertor neutral compression for efficient pumping. These results demonstrate that edge magnetic geometry — specifically the positional relationship between the divertor plate and the island O-point, the strike‑line location relative to the island X‑ or O‑point, and the connection length L c in the island SOL, all of which change with I PC — strongly impact detachment operation in W7‑X and can be potential levers for its control.