Speaker: Kenneth Hammond, Princeton Plasma Physics Lab
Title: Drift effects on W7-X edge heat and particle fluxes
Abstract: Classical particle drifts have substantial impacts on the fluxes of heat and particles to the plasma-facing components of magnetic fusion devices. Drift flows are known to cause asymmetric loading of divertor targets [1,2] and have been theorized to affect the scrape-off layer width in tokamaks [3]. Here we present the first dedicated study of drift effects in the Wendelstein 7-X (W7-X) stellarator [4]. By comparing target deposition patterns in similar plasmas with the magnetic field direction reversed, we isolate the asymmetries arising from drift fluxes. In low-density plasmas, the asymmetries included radial offsets in the strike lines of up to 3 cm between upper and lower targets, as well as uneven deposition in areas that are nominally shadowed from parallel flow. These effects appear to arise primarily from poloidal ExB drift flows driven by radial electric fields in the edge plasma. This interpretation is supported by a comparison of the locations of asymmetric features in the flux distributions with the footprints on the divertors of key topological regions of the edge magnetic field. In higher-density plasmas, the target fluxes were quite different and the underlying drift mechanisms are not yet as well understood. Also, unlike in the low-density case, the targets collected asymmetric non-ambipolar currents that reversed in response to magnetic field reversal.
This work was carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom Research and Training Programme 2014-2018 and 2019-2020 under Grant Agreement No. 633053.
[1] P. C. Stangeby and A. V. Chankin, Nucl. Fusion 36, 839 (1996)
[2] Y. Feng et al., Plasma Phys. Control. Fusion 40, 371 (1998)
[3] R. J. Goldston, Nucl. Fusion 52, 013009 (2012)
[4] K. C. Hammond et al., Plasma Phys. Control. Fusion (in press)