Engineering design for the magnetic diagnostics of Wendelstein 7-X

Abstract

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-Schlüter 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. * Corresponding author, email: endler@ipp.mpg.de 1 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.

Magnetic configuration effects on the Wendelstein 7-X stellarator

“…3b mainly result from the scatter of Thomson data and measurement errors in the observation channels propagating to relative uncertainties of 10% in the line integral derived from Thomson scattering data. The error in the plasma current measurements with the Rogowski coils [46] is about 10%. The density profiles were measured by Thomson scattering.…”

Key results from the first plasma operation phase and outlook for future performance in Wendelstein 7-X

“…The bootstrap current remains sensitive to the magnetic configuration even in the presence of large electric fields, so one should be able to measure a larger toroidal current for this configuration. Indeed, in a scan of configurations with increasing mirror term, we see the expected clear and steady reduction of the toroidal current, as measured by Rogowski coils, 33 as the current in planar coil type A is lowered successively, thereby increasing the mirror term and decreasing the expected bootstrap current (Figure 6). Three discharges are shown, all three having the same programmed power steps in ECRH, with some deviations in the first few hundred milliseconds in the actual injected power.…”

Effects of toroidal plasma current on divertor power depositions on Wendelstein 7-X

“…The experimental assessment of these effects uses quite different diagnostic capabilities of W7-X. I tor is measured experimentally by the in-vessel Rogowski coil encircling the plasma volume in W7-X [9,10] to an accuracy in the range of several Ampères. The assessment of the interactions between the three dimensional (3D) magnetic island chains and the divertor plates, which results in toroidally asymmetric but stellarator-symmetric power loads [11], requires a complete coverage of plasma facing component by real-time video diagnostics, which is also necessary for safe operation.…”