Long-Term Evaluation of Joint Resistivity in Lap Joints of REBCO Tapes With Indium Depending on Pre-Joint Process and Storage Temperature

Bridge type mechanical lap joints with indium foil insertion have been developed for joint winding of high-temperature superconducting (HTS) helical coils in heliotron‑type stellarator fusion reactors. While joints between copper-stabilized Rare-earth Barium Copper Oxide (REBCO) tapes have demonstrated consistently low resistance, their long-term stability remains insufficiently understood. In this study, we first quantified volumetric resistivity of CuIn2 formed at the Cu/In interface using a mechanical lap joint of copper-stabilized REBCO tapes with indium insertion based on four-terminal resistance measurement and cross-sectional Scanning Electron Microscope and Energy Dispersive X-ray Spectroscopy (SEM-EDX) analysis. Subsequently, we evaluated the time-dependent evolution of joint resistivity under different pre-treatment conditions (with or without ethanol cleaning) and storage temperatures (300 K or 77 K). Based on these results, a model was developed to separate diffusion-driven and non-diffusional contributions to joint resistivity. The results revealed that storage at 300 K leads to a monotonic increase in joint resistivity, primarily due to CuIn2 growth, while storage at 77 K suppresses such changes. Additionally, in samples not treated with ethanol cleaning, a significant increase in resistivity occurring with a delay at 300 K was observed, consistent with corrosion-related mechanisms.