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 + a = 5/9 island divertor discharges under conditions of normal confinement (NC) and the HDH-mode for: n e ∼ 0.1-4 x 10 20 m -3 , P ecrh = 0.5-1.5MW, P nbi = 2MW, and H + and D + plasmas, with both normal- and reversed-B t for H + . Subdivertor pressures are in the range 1-2 x 10 -3 mbar for HDH conditions. For plasma detachment at the target plates a strong up-down pressure asymmetry arises, with p up /p down ≤ 5. The asymmetry reverses with reversed B t . Main vessel pressures are a factor of 5-10 lower than the average subdivertor pressure for H + , with D + plasmas exhibiting still lower values.