Microwave stray radiation: Measures for steady state diagnostics at Wendelstein 7-X

Highlights: ► We present examples of ECRH stray radiation protection by metallic housing. ► Opposed requirements: shielding, vacuum compatibility and time resolution. ► Details are given on the protection as well as on thermal connection concepts. ► Acceptance tests are presented on full-scale mockups. -- Abstract: The main heating system of W7-X will be a 10 MW, 140 GHz ECRH. It is expected that the non-absorbed ECRH power will be up to 1 MW. Multiple reflections of the ECRH beam result in a nearly isotropic radiation background. The non-absorbed ECRH power has to be considered as an additional thermal load of microwave stray radiation for all W7-X in-vessel components. Several in-vessel diagnostics, especially magnetic diagnostics, partly consist of microwave absorbing materials. The microwave stray radiation could damage these components or, by evaporation due to overheating, perturb the operation of W7-X. Usually these diagnostics are protected against direct plasma radiation and are thermally connected to actively cooled structures. Nevertheless, ECRH stray radiation can cause a critical temperature increase. We present examples of ECRH stray radiation protection by metallic housing. The use of such housing may be in conflict with vacuum compatibility and in particular for magnetic diagnostics, with the requirement to avoid eddy currents limiting the time resolution of magnetic measurements. Details are given on the protection against ECRH stray radiation as well as on thermal connection concepts. Acceptance tests are presented on full-scale mockups