Analysis of the wendelstein 7-X test divertor unit scraper element with radiation shields

Early implementation of divertor components for the Wendelstein 7-X stellarator will include an inertially cooled system of divertor elements called the Test Divertor Unit (TDU). One part of this system is a scraper element that is intended to explore methods of mitigating heat flux on the ends of the TDU elements. This system will be in place in 2017, after a run period that will involve no divertor, and will precede steady state operation with actively cooled divertors scheduled for 2019. The TDU scraper element is an experimental device with uncertain requirements and with loading conditions which will developed as a part of the experiment. The pattern of heat flux may vary from currently predicted distributions and intensities. The design of the scraper element must accommodate this uncertainty. Originally the mechanical design was to be based on extensive studies for the monoblock-based design of an actively cooled system. An obvious simplification is the elimination of the manifolding needed for the water cooling. The wall panels on which the panels are mounted are to be maintained at 200C or less. Thermal ratcheting of the tiles, supporting structures, and backing structures is managed with adequate cooldown times, thermal anchors, where allowed, and radiative shields. Water cooling of the shields was proposed and rejected. Better radiation modeling is showing less need for multiple shields, but during initial run periods, the scraper element will have to be restricted to an acceptable operating envelope. Thermal instrumentation is recommended.