Delamination and current-carrying degradation behavior of epoxy-impregnated superconducting coil winding with 2G HTS tape caused by thermal stress

A thermo-mechanical-electromagnetic model is developed for estimating the delamination and current-carrying degradation of epoxy-impregnated pancake coils. The mixed-mode traction-separation law and the Weibull distribution of delamination strength are considered in simulating the onset and extension of the delamination caused by thermal stress. Because of the considerable differences in thermal shrinkage between the epoxy resin, insulation tape, and second-generation high temperature superconducting (2G HTS) tape, the accumulated radial thermal stress locally exceeds the electro-mechanical delamination strength and even the mechanical delamination strength. The electro-mechanical delamination strength of the 2G HTS tape is the transverse tensile or shear stress level corresponding to an abrupt irreversible critical current degradation. The mechanical delamination strength is the transverse tensile or shear stress limit corresponding to a structural failure. After injecting current into the coil, we obtained the E-I curve of the coil. The critical current and n-value of the superconducting coil indicate a strong degradation after epoxy-impregnation. The current-carrying degradation precedes delamination because the electro-mechanical delamination strength is smaller than the mechanical delamination strength. The degradation is more obvious in large than in small superconducting coils because the radial thermal stress is larger. The onset of degradation depends on the minimum delamination strength, suggesting that caution is required in screening the 2G HTS tape before winding the coil. The simulation results indicate that reducing the thickness of the insulation tape and the amount of epoxy resin effectively reduces the degradation of epoxy-impregnated pancake coils.