Bridge-type mechanical lap joint of HTS STARS conductors using an integrated joint piece

Joint-winding of high-temperature superconducting (HTS) helical coils has been proposed as a challenging option for the large helical device (LHD)-type fusion reactors, FFHR series. The coils are wound by connecting helical conductor segments using bridge-type mechanical lap joints with indium foils inserted into the joints. A previous study achieved 1.8 nΩ using a 100-kA-class STARS (Stacked Tapes Assembled in Rigid Structure) conductor joint at 100 kA, 4.2 K and 0.45 T. However, it took eighteen hours to join the conductors because each REBCO (Rare-earth Barium Copper Oxide) tape was joined individually and in series. This study introduced an integrated joint piece to join 10-kA-class STARS conductors, which possibly makes the fabrication time to be less than three hours per one conductor joint. We also applied heat treatment at 120 °C during the joining process to obtain uniform joint pressure distribution and a large true area of contact. The joint resistivity and critical current for each REBCO tape of the joint were comparable to those for a single lap joint at 77 K. The joint resistivity was also evaluated at 4.2 K, which was one-fourth to one-third of the previous result obtained during testing of the 100-kA-class STARS conductor joint.