Thermal analysis of Test Divertor Unit Scraper Element for Wendelstein 7-X☆

The Wendelstein-7X stellarator will go through three operational phases, with successively more energy as more complex divertors are installed. The first operational phase operated without a divertor, instead using five limiters (one in each of the five field periods), the second phase uses ten uncooled test divertor units (TDU), and the third will include an actively cooled divertor. Simulations of plasma operation have shown that the evolution of a toroidal current can lead to significant heat fluxes sweeping across sensitive regions in the divertor, possibly exceeding their qualified limit. A "Scraper Element" (SE) has been proposed to protect these sensitive divertor regions. To examine the effectiveness of this high heat-flux SE, a TDU Scraper Element (TDU-SE) is scheduled to operate with the TDU. A thermal analysis has been performed to correlate to measurements to be obtained during operation to evaluate the location and intensity of the convective plasma flux. The analysis will give an understanding of what is occurring at the plasma edge. It will also allow for a quick evaluation of the safety margin during initial, low power pulses so that the experiment can nimbly move to higher power operation.