Thermo-Hydraulic Calculations on W7-X Coils for Updated Quench Detection Parameters

The Wendelstein 7-X stellarator (W7-X), one of the largest stellarator fusion experiments, is in operation since 2015 at the Max Planck Institute for Plasma Physics (IPP) in Greifswald. W7-X has reached the final status in 2022 with actively cooled in-vessel components. The superconducting magnet system consist of 50 non-planar and 20 planar coils. The superconducting coils as well as the connections in between are made of the same NbTi cable-in-conduit conductor (CICC) type. Two plasma breakdowns in the past have induced voltages into the non-planar coils, which triggered the quench detection (QD) system and activated the safety system with a fast discharge of the magnet system. Plasma operation creates toroidal and poloidal currents; a plasma breakdown can change the plasma currents within 100 ms. Especially fast poloidal current changes are able to induce voltages into the superconducting coils. Using the adjustment capabilities of the QD system, new parameter sets for threshold values and shunted capacitor settings were deduced. These parameters prolongs the time until the start of the magnet discharge, which increases the hot spot temperature in the CICC in case of a quench. Thermo-hydraulic calculations with the program THEA from Cryosoft were performed to verify that the 130 K limit for the hot spot temperature is kept with the new QD parameter sets. The analysis were performed by the West Pomeranian University of Technology (ZUT) and by the Henryk Niewodniczański Institute of Nuclear Physics Polish Academy of Sciences in Poland.