Impact of magnetic islands in the plasma edge on particle fueling and exhaust in the HSX and W7-X stellarators

L. Stephey, A. Bader, F. Effenberg, O. Schmitz, G. A. Wurden, D. T. Anderson, F. S.B. Anderson, C. Biedermann, A. Dinklage, Y. Feng, H. Frerichs, G. Fuchert, J. Geiger, J. H. Harris, R. König, P. Kornejew, M. Krychowiak, J. D. Lore, E. A. Unterberg, I. Waters

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Abstract

The edge magnetic structure in the Helically Symmetric eXperiment (HSX) and Wendelstein 7X (W7-X) stellarators has been shown to have a significant impact on the particle fueling and exhaust of the plasma main species (hydrogen) as well as impurity helium. For HSX, the plasma sourcing to exhaust ratio, quantified by the effective and global particle confinement times τ p∗ and τ p, H, respectively, increases when a magnetic island chain is located in the plasma edge. The fueling efficiency is reduced by 25% when the plasma boundary is deformed by the magnetic islands. The X-point geometry also yields higher plasma temperatures in front of the main recycling region. When the island is moved radially inward, both τ p∗ and τp decrease by 10 % - 25 % depending on plasma density. The τ p, H results rely heavily on EMC3-EIRENE modeling which confirms reduced fueling efficiency due to more rapid ionization in the outward shifted island position. These findings suggest that for a helically optimized system like HSX, the plasma fueling from the recycling source, as well as from active gas injection, can be controlled by the magnetic island chain in the plasma edge - which is a basic requirement for a divertor system. This process is also effective for the control of effective helium exhaust times, as τ p, H e∗ measured by perturbative gas puff experiments is reduced by up to 40% when the islands are shifted inwards. For Wendelstein 7-X, a similar reduction of τ p, H e∗ was inferred when magnetic islands were moved from the far plasma edge into the confined plasma region. However, the effective confinement features of H as the main plasma species were not affected due to the non-optimal position of the magnetic islands with respect to the highly localized ionization domain during the limiter startup campaign.

Original languageEnglish
Article number062501
JournalPhysics of Plasmas
Volume25
Issue number6
DOIs
StatePublished - Jun 1 2018

Funding

This work was supported in part by the U.S. Department of Energy (DOE) under Grant Nos. DE-SC0014210, DE-FG02-93ER54222, and DE-AC05-00OR22725, and DOE LANS Contract No. DE-AC52–06NA25396. The publisher, by accepting the article for publication acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014–2018 under Grant Agreement No. 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.

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