W7-X Magnet System Thermal Insulation Analysis Under Fast Plasma Decay Conditions

The Wendelstein 7-X (W7-X) is the world largest stellarator with a sophisticated superconducting (SC) magnet system (MS) in operation since 2015. The W7-X components were designed in a framework of aspects: a plasma current decay of 100 kA and 2.7 MA with time constants of 140 and 50 ms for toroidal and diamagnetic currents, respectively, and a SC coil current decay of a 3T field on axis within 3s. However, during the second phase of operation in 2018, a fast plasma current decay event with a time constant of approximately 3 ms for toroidal current was observed. This observation has forced to reconsider the fast decay expectations and to initiate a massive W7-X torus components reanalysis campaign. One of the most critical components under higher induced eddy currents is the complex system of the MS thermal insulation (TI). The TI system separates the MS operated at ∼4 K from the plasma vessel and outer vessel (PV TI and OV TI, respectively) and is located in the cryostat. The two main elements of the PV TI are actively cooled thermal panels and port shielding tubes. The multilayer thermal panels of the PV are connected to each other with clamps and attached to the PV by three different types of supports. Two complex finite element (FE) models have been developed to analyze the PV TI during postulated fast plasma current decay events with 1 ms time constant: an electromagnetic (EM) model to calculate eddy currents and EM forces on TI components [1], and a mechanical model to calculate the structural response of the PV TI. The paper presents main features of structural FE model and most important results from static and dynamic analyses of postulated fast plasma current decay events. Finally, required limitations for plasma currents for future W7-X operation are discussed.