Abstract
For the first time, direct experimental evidence is presented for equilibrium pressure surfaces formed on the magnetic flux surfaces of the island divertor at Wendelstein 7-X (W7-X). These isobars are observed up to moderate densities (e> = 3 x 1019 m-3) and are supported by strong ExB drift transport in the island. Observed hollow temperature profiles, mapped to the target sheath, induce an electric field pointing towards the center of the island of several kV/m. These measurements were obtained with the thermal helium beam diagnostic in the magnetic islands of the “standard” ι = 2π (5/5) configuration. At low density, the presence of isobars conforming to the magnetic flux surfaces in the island is confirmed. In a parallel-dominated, sheath-limited regime, such isobars would be expected. However, these isobars are experimentally observed at higher densities than predicted by EMC3-EIRENE, sustained by poloidal ExB fluid drifts not included in the code. We present evidence of a significant transition in transport regimes beyond the critical density mentioned above, consistent with a reduction in poloidal drifts at increasing density. These poloidal ExB drifts also transport particles into the magnetic shadow of the divertor, shortcutting transport through the island. Evidence for these drift effects in the form of transport asymmetries corresponding to the expected directions of poloidal ExB drifts between lower and upper divertor modules are shown, as well as estimations of drift velocities. Such investigations into the fundamental local plasma properties in the island divertor and associated particle and energy transport are required to understand the functionality of the island divertor as an exhaust concept. Despite not reflecting the drift physics, comparisons to EMC3-EIRENE modeling are presented to show that profile differences at different positions around the island can be attributed to differences not just in the magnetic structure, but the competing parallel/radial transport channels as well. These results contribute to extrapolation from W7-X to future divertor scenarios in a fusion reactor.
