Backside Protection for Wendelstein 7-X In-Vessel Components

Successful physical experiment campaigns have been performed on the most advanced modular stellarator Wendelstein 7-X (W7-X) during the second operation phase (OP1.2). The completion phase (CP2) lasting until 2021 is devoted to the installation of cryo-vacuum pumps (CVPs), new diagnostics, and actively cooled in-vessel components instead of the inertially cooled ones used in OP1.2. This update allows us to move forward to achieve the steady state operation during the next operation phase (OP2). Several first wall components which are exposed to heat flux in the range from 250 to 500 kW/m
²
are covered by graphite tiles facing the plasma. Due to construction constraints, the backside area of the graphite tiles is not fully covered by the actively cooled CuCrZr heat sinks. This results in the presence of high-temperature graphite rims, which become an additional heat radiation source for the components behind the first wall. This article presents stepwise development of backside protections (BSPs) to mitigate the problem mentioned above. The process is supported by: 1) the study of BSP shielding performances through thermal analysis taking into account plasma radiation and electron-cyclotron resonance heating (ECRH) loads; 2) BSP sizing restricted by electromagnetic forces during main coil/plasma current decay; and 3) mechanical analysis to confirm the structural fixation of BSP. In addition, several analysis iterations for CVP have been repeated to minimize BSP cost by identification of first wall components with minor heat radiation to the CVP and marginal influence on its cooling system capacity.