Mechanical and Electrical Characteristics of a Bridge-Type Mechanical Lap Joint of HTS STARS Conductors

Segmented fabrication of high-temperature superconducting (HTS) coils has been proposed for the helical fusion reactor FFHR-d1; the joint-winding and 'remountable' (demountable) helical coils concept are the primary and advanced options, respectively. This paper introduces recent research activities on a bridge-type mechanical lap joint of HTS Stacked Tapes Assembled in Rigid Structure (STARS) conductors, particularly to be used for joint winding of the helical coils, and also discusses some technical issues to develop a reliable joint. First, the required joint performance is discussed from the viewpoints of operation cost and mechanical behavior, taking the present design of joint winding of the helical coils for the FFHR-d1 into consideration. Then, mechanical and electrical characteristics of a prototype STARS conductor joint, which achieved a joint resistance of 1.8 nΩ at a current of 100 kA, are analyzed based on small- and large-scale joint tests. It is suggested that the joint performance of the prototype joint can be improved more, according to tests of small-scale conductor joints. Finally, the study introduces a fabrication procedure for a reliable joint. We need to develop nondestructive testing for a joint at room temperature. For this purpose, we also perform an X-ray computed tomography inspection as a first step. The inspection can roughly judge whether a fabricated joint is acceptable.