Critical Current Degradation and Joint Resistance Rise of REBCO Tape and Coil During Heat Treatment

High temperature superconducting (HTS) REBCO tape has great advantages on high field magnets due to its high critical current density and superior mechanical strength. It is inevitably exposed to high temperatures during certain stages of coil manufacturing, such as joint welding and coil impregnation, which can potentially result in a decrease in the critical current of the coils and an increase in the joint resistance. In this paper, this issue is studied by measurement using three post-processing types of REBCO tapes: copper-plated tapes, tin-coated tapes, and copper-laminated tapes. To simulate possible heat treatment conditions, the samples are heated to between 105 C-degrees and 265 C-degrees in different atmospheres for durations ranging from 1 min to 4 h, and then the critical current of the tapes and coils are investigated at 77 K. Its influences on the REBCO-to-Skeleton joint resistance are also investigated. The results show that heat treatments below 185 C-degrees with a duration of less than 60 min are suitable for common heat treatment processes due to a critical current degradation of less than 5%. For short heat treatment (duration < 60 min), the tin-coated tape may have a smaller critical current decay. The REBCO-to-Skeleton joint resistance of coils increases with increasing heat treatment temperature and duration, and an internal joint welding method is proposed that has little effect on the critical current of the coil (degradation < 3%).