Wendelstein 7-X

Collaborative grant with the Auburn University  for work on island divertor physics at Wendelstein 7-X, Germany

Title:“Impact of three-dimensional equilibrium stability on island divertor performance at Wendelstein 7-X”

Public Abstract:

Three-dimensional (3D) plasma edge transport and plasma material interaction (PMI) critically influence plasma stability and performance in stellarators. There is a strong coupling between the plasma equilibrium and boundary plasma characteristics since the overall 3D equilibrium determines the edge magnetic topology and hence plasma interaction with material surfaces. Neutral and impurity sources released from this 3D PMI can be transported back into the main plasma. This inward impurity transport not only defines possible divertor operating conditions (e.g. a radiative divertor, detachment) but also can determine core plasma performance limits due to impurity accumulation. From a reactor perspective, impurities and in particular helium fusion ash has to be efficiently exhausted. This collaborative research enterprise between Auburn University and the University of Wisconsin-Madison investigates key aspects of this self-consistent feedback loop between plasma wall interaction, edge and core plasma transport and stability of 3D stellarator equilibria in an attempt to optimize the core-edge coupling for high performance, steady state operation of Wendelstein 7-X. This research will enable understanding of the link between finite beta and trim coil effects on the 3D equilibrium of W7-X and the resultant specific edge transport characteristics during the startup and initial operations of Wendelstein 7-X. The research effort includes development and application of a sophisticated fractional impurity neutral pressure measurements system to study and optimize impurity exhaust in general and helium exhaust in particular with the island divertor at Wendelstein 7-X for the first time. This research will enable understanding of the link between nite beta and trim coil effects on the 3D equilibrium of W7-X and the resultant specific edge transport characteristics including a US designed and built scraper element for handling high heat ux loads during the island divertor campaign.

A major goal of the proposal is to not only expand our physics knowledge base concerning long pulse core-edge plasma behavior, but to also educate the next generation of US fusion physicists. With that goal in mind we have successfully engaged a total of seven PhD students to date, one of them which already graduated based on the results obtained at W7-X. Four students are presently headquartered at W7-X. The funding in this proposal is used to bring these students to graduation and start a follow-on generation of three new PhD students on topics in the proposed research framework. This collaborative University effort is embedded into the larger US team effort at W7-X which couples the students and other early career researchers directly to the eorts of the US national laboratories involved as well as our international partners. This arrangement has been proven to be very effective and has been and will continue to be a strong foundation to provide state of the art research opportunities on W7-X to educate the next generation of US fusion physicists.

Personell: Prof. Oliver Schmitz, Associate Scientist Dr. Heinke Frerichs, Dr. Thierry Kremeyer (post-doc), BSc Aysia Demby, BSc Kelly Garcia

Funding Information: funded by the Department of Energy, Office of Fusion Energy Science, DE-SC00014210, started in 07/2015, renewed 08/2018