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Description
Temperature variations during quench are a known cause of thermal strains and associated thermal stresses due to variation of the coefficient of thermal expansion between materials comprising the superconducting cable. In the case of the W7-X CICC, 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 this insulation was the subject of this study.
The thermo-hydraulic and electric analysis was conducted using the Cryosoft THEA software, which simulated two double layers out of six double layers of the entire winding pack (WP) of the non-planar W7-X coil (NPC). The heat transfer between the adjacent turns and layers was included, along with the inductance matrix computed by a dedicated FEM model. The impact of the electrical conductance for the contact between strands and jacket on the temperature evolution and hot spot temperature was studied.
The mechanical analysis consisted of building a detailed mesh of the entire WP and combining it with the existing FEM model of the NPC coil via bonded interface. The geometry and meshing were built via bottom-up method, parametrically, within the Ansys Mechanical APDL environment. With the possibility of controlling the mesh both in-plane and along the centerline of the coil. A mapping algorithm was created to read the temperatures from THEA and apply to the 3D mechanical model.
The orthotrophy of the electrical insulation was included by defining coordinate systems aligned with material orientation. The model was solved for the initial deformations and stresses after cool-down to 4.1 K and for temperature field at various time moments between 0.15-25 s identified as the most severe cases from the thermal point of view. The resulting stresses were compared with the allowable stresses obtained based on experimental data. Mesh dependence study was performed for the mechanical model to assess the sensitivity of the results.
