Thermo-hydraulic and 3D mechanical analysis of the stresses in the electrical insulation of the W7-X non-planar coil during quench

Temperature variations during quench are a known cause of thermal strains and associated stresses due to variation of thermal expansion coefficients between materials comprising the superconducting cable. In the case of the W7-X CICC, a NbTi superconducting cable is placed inside an aluminum jacket wrapped with glass-epoxy electrical insulation. The strength of this composite material is limited to ~55 MPa in shear. The potential risk of the mechanical failure of the insulation during quench was studied in this work. The thermo-hydraulic-electric analysis was conducted using the Cryosoft THEA software, which simulated two out of the six double layers of the entire winding pack (WP) of the non-planar (NPC) W7-X coil. Heat transfer between the adjacent turns and layers was accounted for, along with the inductance matrix computed using a dedicated FEM model. The impact of strand–jacket electrical contact conductance on temperature evolution and hot-spot temperature was studied The mechanical analysis involved developing a detailed parametric mechanical model of the entire WP and combining it with the existing FEM model of the NPC coil via a bonded interface. The geometry and mesh were generated parametrically using a bottom-up method in ANSYS Mechanical APDL, allowing control of the mesh both in-plane and along the coil centerline. A mapping algorithm was created to read temperatures from THEA and apply them to the 3D mechanical model. The orthotropy of the electrical insulation was included by defining coordinate systems aligned with material orientation. The model was solved for initial deformations and stresses after cool-down to 4.1 K and for temperature fields at selected times from 0.1 to 25 s, representing the most severe thermal cases. The resulting stresses were compared with the allowable stresses obtained based on experimental data and the Large Helical Device (LHD) criterion. Mesh dependence study was performed for the mechanical model to assess the sensitivity of the results.