Performance test of REBCO CICC sub-cables with 10 kA current under 20 T background field

While commercially manufactured rare earth barium copper oxide (REBCO) tapes show significant promise in facilitating the operation of fusion magnets with magnetic fields above 15 T, the design and development of highly stable cable in conduit conductor (CICC) technology is very important to achieve their practical application. To find a good solution for this demand, the Institute of Plasma Physics, Chinese Academy of Sciences, proposed two kinds of CICC design concepts, which are both manufactured from a sub-cable formed by winding REBCO tape around a stainless steel spiral tube. As part of the ongoing activities to develop an REBCO CICC, two sections of sub-cable specimens were manufactured and bent into a U-shape for testing under magnetic fields up to 20 T. A sub-cable specimen with 30 commercial 4 mm wide REBCO tapes displayed around 10 kA at 4.2 K and a background magnetic field of up to 20 T. It also showed stable operation under an electromagnetic (EM) load of around 200 kN m-1, which is above the 150 kN m-1 required by the designed CICC sub-cable. However, the calculated I c of the other specimen degraded from 8.8 kA to 8.5 kA when cycling with an EM load of around 160 kN m-1. The lower calculated n-value at 77 K and self-field as well as the observed imprints on the disassembled tape edges suggested that defects were generated in the cable during cabling, bending to the sample holder or operation with high EM and thermal loads. These results exhibit the potential and feasibility of using high flexible REBCO cable (HFRC) sub-cables for high-field fusion magnets. However, the winding parameters need to be optimized to ensure safe operation in more complex conditions, such as in tokamaks, especially if using tapes similar to those used in sample-B in this study. Moreover, it is imperative to establish much more rigorous requirements for coil manufacturing processes in order to avoid the occurrence of defects in the tapes.