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The magnetic diagnostics foreseen for the Wendelstein 7-X (W7-X) stellarator are diamagnetic loops to measure the plasma energy, Rogowski coils to measure the toroidal plasma current, saddle coils to measure the Pfirsch–Schluter currents, segmented Rogowski coils (poloidal magnetic field probes) to add information on the distribution of the plasma current density, and Mirnov coils to observe magnetohydrodynamic modes. All these magnetic field sensors were designed as classical pick-up coils, after the time integration of induced signals for 1/2 hour had been successfully demonstrated. The long-pulse operation planned for W7-X causes nevertheless significant challenges to the design of these diagnostics, in particular for the components located inside the plasma vessel, which may be exposed to high levels of microwave (electron cyclotron resonance) stray radiation and thermal radiation. This article focuses on the tests and modelling performed during the development of the magnetic diagnostics and on the design solutions adopted to meet the conflicting requirements. All pick-up coils foreseen for the initial operation phase of W7-X and their signal cable sections inside the plasma vessel and the cryostat are now installed, and their electronics and data acquisition are under preparation.

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.