Simulations of W7-X magnet system fault scenarios involving short circuits

The stellarator Wendelstein 7-X (W7-X) which is presently under construction at the Max-Planck-Institut für Plasmaphysik in Greifswald [(C. Beidler, G. Grieger, F. Herrnegger, E. Harmeyer, J. Kißlinger, W. Lotz, H. Maassberg, P. Merkel, J. Nührenberg, F. Rau, J. Sapper, F. Sardei, R. Scardovelli, A. Schlüter and H. Wobig., Physics and engineering design for Wendelstein 7-X, Fusion Technol., 17 (1990)148–168)], uses 70 superconducting coils, arranged in 7 groups to create the magnetic confinement for the plasma. A wide variety of tests and investigations are performed in order to ensure the later safe operation of the device. Much attention is also paid to the proper insulation of all the parts. These measures are costly and time consuming but are necessary in order to avoid the severe consequences of faults–—especially short circuits–—during operation. If a short circuit would happen during an emergency switch-off, the discharge of any shorted inductance would be delayed, and the coupled magnetic flux of the discharging system would induce additional currents into this shorted part. The currents and forces developing in such a case depend not only on the short circuit resistance and the critical current of the superconductor, but also on the shorted inductance itself, its magnetic coupling to other inductances, and its position within the system. The paper describes the influences of these factors and presents simulation results for different fault scenarios involving short circuits across a coil group, a single coil, different double layers, and a single turn. Maximum currents result from a shorted outer double layer, maximum forces from a shorted coil group, depending on its position in the magnet system.