Advanced Insulation Design for HTS Coils Under High-Vacuum Conditions

High-temperature superconducting (HTS) coils operating in cryogenic vacuum or cryostat environments must withstand both high electrical stresses and mechanical instabilities that can degrade insulation. This paper investigates the electromechanical coupling between structural defects and dielectric breakdown in fast-ramping HTS magnet coils. Although vacuum can provide excellent insulation, localized voids and partial-vacuum pockets formed by winding delamination or buckling can significantly reduce the withstand voltage through strong local electric-field enhancement. Dielectric breakdown experiments were performed on epoxy resin samples with various thicknesses in liquid nitrogen (LN₂) and in low-pressure vacuum, and finite element method (FEM) simulations of REBCO single-pancake coil were conducted to evaluate how internal voids and local buckling influence the electric-field distribution. The breakdown voltage increases with insulation thickness in both LN₂ and vacuum, whereas the effective breakdown field (kV/mm) decreases with thickness. FEM results show that mechanical defects, such as gaps or loose turns, concentrate the electric field at conductor–insulator–void triple-junction interfaces, leading to insulation failure at significantly lower voltages. An improved insulation design using epoxy impregnation at the top and bottom of the coil is proposed to fill voids and reinforce the winding structure. The simulations indicate that a 0.5 mm epoxy layer substantially reduces the peak electric field in defect regions, consistent with experimental observations. The resulting thickness-dependent breakdown characteristics provide practical design curves for insulation design. Overall, the results demonstrate that mitigating mechanical weaknesses and suppressing localized partial-vacuum pockets are essential for improving the dielectric stability of REBCO coils in vacuum-related environments.