Protection of W7-X diagnostics from radiation heat loads

Abstract

Wendelstein 7-X (W7-X), a modular advanced stellarator, is presently under construction at the Max Planck Institute for Plasma Physics in Greifswald. Part of the inner plasma vessel (PV) surface is covered by graphite tiles which are bolted onto water-cooled CuCrZr heat sink plates. This so-called heat shield is an in-vessel component and shall protect the PV wall and diagnostic components from plasma-edge-radiation and convective heat loads. Numerous diagnostics (e.g. diamagnetic loops, Rogowski coils, etc.) are located in between the heat shield and plasma vessel wall. They are loaded by thermal radiation from the hot heat shield backside, stray radiation from the electron cyclotron resonance heating (ECRH), and plasma radiation through remaining gaps between the tiles. In order to keep the temperatures of the diagnostics within their specified limits, additional shielding is required. The paper presents the design of such a shielding for the diamagnetic loops. Several options of copper shields and loosely attached Sigraflex®-layers, and combinations thereof, are compared with respect to their abilities to reduce the temperatures of the sensitive parts of the diagnostics. To increase the confidence in the results, a sensitivity study with widely varying radiative material properties has been carried out. The advantages and disadvantages of the potential solutions are discussed. As a result two copper shield variants which also reduce the load onto the plasma vessel are presented.