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Physics/engineering design of high-heat-flux divertor scraper elements for the W7-X stellarator

Author: Jeremy D Lore
Requested Type: Poster Only
Submitted: 2012-12-12 11:24:58

Co-authors: T. Andreeva, J. Boscary, S. Bozhenkov, J. Geiger, J.H. Harris, H. Hoelbe, A. Lumsdaine, D. McGinnis, A. Peacock, J. Tipton

Contact Info:
ORNL
1 Bethel Valley Rd
Oak Ridge, TN   37831
USA

Abstract Text:
Wendelstein 7-X (W7-X) is a large (R = 5.5 m) superconducting advanced stellarator under construction at the Max-Planck Institute in Greifswald, Germany, that will begin plasma operations in 2015. W7-X will ultimately be capable of quasi-steady state operation (plasma durations of up to 30 minutes) with ~20 MW of plasma heating power, and will use a helical magnetic-island divertor to exhaust heat and particle flux. A set of ten high-heat flux (~15MW/m2), actively cooled, divertor components, referred to as ‘scraper elements’ (SE), is being designed for W7-X. The SE will intercept field lines and associated plasma fluxes that would be incident on regions of the divertor that are predicted to be overloaded in certain high-power operational scenarios. In these scenarios, the bootstrap current evolves on the order of 40s, resulting in a change to the boundary rotational transform ~ 10%, which alters the configuration of the magnetic islands relative to the divertor plates To account for this change, and in order to yield an island divertor (ID) power exhaust configuration when the bootstrap current is fully evolved, the vacuum transform is reduced such that the initial configuration is limited by the divertor plates. As the configuration evolves to ID the heat flux patterns sweep across lower-rated regions of the divertor and into the pumping region, resulting in overload. The SE are designed considering constraints due to plasma physics (heat fluxes, pumping efficiency, magnetic field models), fluid dynamics (cooling water pressure drop and temperature rise), and engineering (manufacturing and alignment tolerances, CFC properties). In each of these areas it is important to consider sensitivity to input parameters and tolerances, and attempt to make uncertainty estimates for the output quantities. The current SE design and design procedure, as well as an uncertainty analysis will be presented.
Work supported by D.O.E. contract DE-AC05-00OR22725.

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