Research on the health monitoring of ReBCO tape using fiber Bragg grating sensors

Abstract Epoxy resin is used to enhance the mechanical properties, insulation, and thermal conductivity of superconducting magnets. However, the multi-layer structure of second-generation high-temperature superconducting (ReBCO) tapes causes significant mechanical anisotropy. During curing and cooling, stress concentration arises due to the differing shrinkage rates of each layer, with epoxy resin shrinking much more than the ReBCO tape. This leads to strong stresses on the conductor, potentially causing cracks, spalling, or fractures in the superconducting layer, which degrade the critical current. In this study, fiber Bragg grating (FBG) sensors were used to monitor strain during epoxy curing and cooling processes, and the critical current was measured before and after curing. Results showed that pure epoxy resin curing and cooling to 77 K introduced about 14,334 µ strain, reducing the critical current by 52 A after five thermal cycles. By impregnating the epoxy resin with glass fiber cloth, residual strain was significantly reduced to less than 2500 µ strain, with almost no degradation in critical current. Additionally, embedded FBGs were used for quench detection, and the results demonstrated that detection sensitivity strongly depends on the FBG’s distance from to the quench point.