Fundamental investigation on tensile characteristics of a mechanical lap joint of REBCO tapes

This paper discusses tensile characteristics of a mechanical lap joint, which has been proposed and tested for joint-winding of a high-temperature superconducting (HTS) helical coil in the Large Helical Device type helical fusion reactor, FFHR-d1. Structural analysis for the helical coils in FFHR-d1 indicates that the maximum tensile strain along the HTS conductors and the maximum shear stress in HTS tapes region are 0.145% and 32 MPa, respectively under electromagnetic forces. Tensile test of the mechanical lap joint using Gadolinium Barium Copper Oxide (GdBCO) tapes with an inserted indium foil showed that the joint having 5-mm length and 5-mm width with a joint pressure of > 11 MPa had tensile strength exceeding that of the GdBCO tape itself, which also allows the tensile strain of 0.145% when used in the helical coil. To tolerate the shear stress of 32 MPa, the joint pressure of > 50 MPa was found to be needed according to the experimental results.