Refined electromagnetic analysis of W7-X thermal insulation during fast plasma current decay event

During the second operation phase of the most advanced stellarator Wendelstein 7-X (W7-X) in 2018, a fast plasma current decay event was observed with the time constant of similar to 1 ms. The event is much more rapid than the design phase assumptions: 100 kA and 2.7 MA plasma current decay with the time constants of 140 and 50 ms for toroidal and diamagnetic currents, respectively. As a result, significantly higher eddy currents are expected to be induced in some of the W7-X components. The comprehensive campaign of electromagnetic (EM) analyses has been launched in order to identify the necessity of proper reinforcement of critical in-vessel diagnostics or lowering the plasma currents for future operation. One of the critical W7-X system found is the complex thermal insulation (TI). The TI is located in the cryostat to separate cryogenically cooled magnet system and warm vessels. The system consists of actively cooled thermal radiation panels and port tubes, multi-layer insulations, supports and clamps. The TI panels and tubes form the main frame of TI covering the plasma vessel (PV) and ports, respectively. Preliminary EM analysis with few panels and tubes indicates that the EM forces are increased by about 7 times due to the newly postulated faster plasma current decay event in comparison with original design loads. To obtain more accurate results, an elaborated EM model has been created, which includes the excitation coils for toroidal and diamagnetic plasma currents, superconducting coils, TI panels and tubes, the PV and ports. The paper introduces at first the TI structure, continues with the field and eddy current accuracy studies and the lessons learned for the modelling accuracy improvement in ANSYS (R). Then the EM analysis results of different plasma current decay scenarios are presented and discussed. Finally, the EM forces for static and transient mechanical analyses are chosen and extracted for further mechanical analyses.