Overview over the neutral gas pressures in Wendelstein 7-X during divertor operation under boronized wall conditions

Abstract During the first test divertor campaign of Wendelstein 7-X, 13 neutral gas pressure gauges collected data in different locations in the plasma vessel, enabling a detailed analysis of the neutral gas pressures, the compression ratios and the particle exhaust rates in different magnetic field configurations. In Wendelstein 7-X, the edge magnetic islands are intersected by the divertor target plates and used to create a plasma-wall interface. As the number and position of the islands varies in different magnetic configurations, the position of the strike line on the target plates and the neutral gas pressure in the subdivertor differs between the configurations. Neutral gas pressures on the order of few 10e-04 mbar were measured in the subdivertor region. The highest neutral gas pressure of 1.75*10e-03 mbar was obtained in high iota configuration featuring 4 edge magnetic islands per cross section. The neutral particle flux through the pumping gaps into the subdivertor volume was provided by EMC3-EIRENE simulations and allowed to analyze the relation between the particle flux and the pressure distribution in the subdivertor. Finite element simulations in ANSYS provide a detailed picture of the pressure distribution in the subdivertor and agree with the neutral gas pressure measurements in the standard configuration featuring an island chain of 5 edge magnetic islands. High neutral gas pressures that were not predicted by the simulation were measured in the subdivertor region away from the main strike line for discharges in standard configuration. While the pressure ratio between the two sections of the subdivertor volume, the low and high iota section, is 0.06 in high iota configuration, a ratio of 2-5 was obtained in other configurations, indicating significant particle loads and exhaust rates on the high iota section of the subdivertor in magnetic configurations with the main strike line on the low iota divertor targets.