Design of the Wendelstein 7-X inertially cooled Test Divertor Unit Scraper Element

Abstract

The Wendelstein 7-X stellarator is scheduled to begin operation in 2015, and to achieve full power steady-state operation in 2019. Computational simulations have indicated that for certain plasma configurations in the steadystate operation, the ends of the divertor targets may receive heat fluxes beyond their qualified technological limit. To address this issue, a high heat-flux "scraper element" (HHF-SE) has been designed that can protect the sensitive divertor target region. The surface profile of the HHF-SE has been carefully designed to meet challenging engineering requirements and severe spatial limitations through an iterative process involving physics simulations, engineering analysis, and computer aided design rendering. The desire to examine how the scraper element interacts with the plasma, both in terms of how it protects the divertor, and how it affects the neutral pumping efficiency, has led to the consideration of installing an inertially cooled version during the short pulse operation phase. This Test Divertor Unit scraper element (TDU-SE) would replicate the surface profile of the HHF-SE. The design and instrumentation of this component must be completed carefully in order to satisfy the requirements of the machine operation, as well as to support the possible installation of the HHF-SE for steady-state operation.

Electron-cyclotron-resonance heating in Wendelstein 7-X: A versatile heating and current-drive method and a tool for in-depth physics studies

“…One example is the scraper element. For OP1.2b a test version has been installed [48] which will be used to validate the passive protection of the edges of the divertor tiles. An alternative could be the application of ECCD to compensate the effect of the bootstrap current.…”

Operation of W7-X With an Inertially Cooled Divertor—On the Way to Steady State Operation

“…The mission for this phase was the development of divertor scenarios for the later steady-state phases with the HHF divertor [7]. In the middle of this second operation phase, a break was put in to install two so-called scraper elements [ 8] to protect the divertor during the build-up of a toroidal bootstrap current [9], if significant current may be present. This additional installation divided the second operation phase into OP 1.2a and OP 1.2b.…”

Access to edge scenarios for testing a scraper element in early operation phases of Wendelstein 7-X

“…The overload phase lasts tens of seconds. To lower the heat load of the divertor target plate edge during the transitional phase, an additional divertor target plate was designed, called the SE [14,15]. A separate SE must be placed in front of each divertor unit (DU) to provide the mentioned protection, i.e.…”