Design of Divertor Scraper Elements for the W7-X Stellarator

A PPPL/ORNL/LANL team is partnering with the Max-Planck Institut f\"{u}r Plasmaphysik in the Wendelstein 7-X (W7-X) stellarator project. W7-X is a large superconducting, steady-state stellarator (R = 5.5, a = 0.5, B = 3T) with P =15-30 MW that will begin operation in 2015. The US team is focusing on control of the magnetic configuration and divertor heat flux. The W7-X divertor consists of cooled CFC plates arranged as a magnetic island divertor outside the last closed flux surface. While the W7-X configuration is optimized to minimize both Pfirsch-Schl\"{u}ter and bootstrap currents, the $\sim $30 sec evolution of the plasma to its final equilibrium drives bootstrap currents which transiently alter the distribution of divertor heat flux. This necessitates the addition of 10 actively cooled scraper elements (dimensions $\sim $0.2 m x 1 m) capable of absorbing localized heat fluxes $<$ 12 MW/m$^{2}$. ORNL/IPP are developing an engineering design for the scraper elements using ITER CFC monoblock technology.