Investigation for contact interface of mechanical lap joint fabricated with high-temperature superconducting conductor using X-ray microtomography

Joint winding of high-temperature superconducting (HTS) helical coil with conductor segments that are connected using bridge-type mechanical lap joints is considered as a promising method of fabricating magnet for a FFHR heliotron-type fusion reactor. Although methods for joining large-scale HTS conductor has been developed using the "simple-stack" and "joint-piece" procedures, the difference between these procedures is unclear. In this study, the two-row-four-layer joint samples were fabricated via the two joining procedures and compared in terms of contact resistivity. Joint thickness and joint resistance were measured; the contact area at the contact interface was evaluated using an X-ray computer tomography scan, to obtain the precise contact resistivity of the joint. The contact resistivity of the sample fabricated via the "simple-stack procedure" ranged from 2.41 - 5.15 pΩm², whereas that of the sample fabricated using the "joint-piece procedure" ranged from 1.98 - 6.07 pΩm². There was no significant difference between the procedures in terms of contact resistivity range. Considering the characteristics of joint thickness and location distribution of each lap joint, the inhomogeneous joining pressure was the primary factor affecting contact resistivity. As the similarity of electrical performances of the two procedures was clarified, future studies should focus on the manufacturability of large-scale joints. (author)