Performance Assessment of PIT VIPER Cables Following Long-duration Solder Exposure During Manufacturing

PIT VIPER cables are rare-earth barium copper oxide (REBCO)-based high-temperature superconducting (HTS) cables developed at the Massachusetts Institute of Technology and Commonwealth Fusion Systems. The cable design consists of a twisted copper former with grooves filled with stacks of REBCO tape, enclosed in a structural jacket. The assembly is then filled with solder in a vacuum pressure impregnation (VPI) process. One side effect of the VPI process, however, is potential damage to the superconductor that reduces its critical current and critical exponent. Damage may result from two mechanisms: prolonged exposure to elevated temperatures, and physical erosion of the copper stabilizer layer of the HTS tape, which leaves parts of the REBCO layer unprotected. In this experiment, the effect of extended time scales (> 2 hours) of flowing tin-lead-based solder exposure on HTS in PIT VIPER cables was tested, exploring for the first time exposure to flowing solder at a time scale that is particularly relevant to largescale magnet manufacturing. During this study, two experimental samples were manufactured for electrical testing: one 2.5-meterlong straight cable, and one 20-meter-long coiled cable. Each cable was exposed to molten tin-lead solder for 2.5 hours during the VPI process and then electrically tested in a liquid nitrogen bath. Critical current, n-value, and resistance were measured. Critical current is compared to modeled values determined from characterization of the tape used, and degradation is assessed from this comparison. The measured critical current, when tested in a liquid nitrogen bath and under self-field, was uniform within reasonable experimental error. This result de-risks solder degradation for the manufacturing of SPARC cable magnets.