Neutral pressure behavior for diverted discharges in the Wendelstein 7-AS Stellarator

On the W7-AS stellarator, the subdivertor neutral pressure in an up–down divertor pair as well as at two points in the vicinity of a lower divertor module in the main chamber are measured. Results are presented for ι ̵ a = 5 / 9 island divertor discharges under conditions of normal confinement (NC) and the HDH-mode for: n ¯ e ∼ 0.1 – 4 × 10 20 m - 3 , Pecrh = 0.5−1.5 MW, Pnbi = 2 MW, and H+ and D+ plasmas, with both normal- and reversed-Bt for H+. Subdivertor pressures are in the range 1–2 × 10−3 mbar for HDH conditions. For plasma detachment at the target plates a strong up–down pressure asymmetry arises, with pup/pdown ⩽ 5. The asymmetry reverses with reversed Bt. Main vessel pressures are a factor of 5–10 lower than the average subdivertor pressure for H+, with D+ plasmas exhibiting still lower values.