Bending and Twisting Characteristics of REBCO Lap Joint With Indium

Joint-winding of high-temperature superconducting (HTS) magnets has been proposed for an advanced helical reactor, the FFHR-d1 reactor. In the fabrication method, stacked tapes assembled in rigid structure (STARS) conductors—in which a simple stack of rare-earth barium copper oxide (REBCO) tapes within a metal casing—are connected using a bridge-type mechanical lap joint with indium foil inserted. Previous studies showed sufficiently low joint resistances in straight conductors, although curved joints would be necessary for the fabrication of helically-wound HTS coils. In this study, we devised joint, twist, and bend (JTB) and twist, bend and joint (TBJ) procedures for the curved-joint fabrication. We evaluated the joint resistance as a function of bending and torsional strain for the JTB procedure as well as the joint resistance as a function of bending radius or twist pitch for the TBJ procedure. Based on the experimental results and discussion, the JTB procedure could be applied to large-scale helical coils—such as those used in the FFHR-d1 reactor—whereas the TBJ procedure could be adopted for smaller-scale helical coils such as those used in the FFHR-a1 reactor.