Structure improvement and joint resistance estimation in demountable butt and edge joints of a stacked rebco conductor within a metal jacket

Electrical (mechanical) butt and edge joints of a stacked REBCO conductor within a metal jacket have been investigated as candidates for demountable (remountable) electrical joints within demountable high-temperature superconducting magnets. This paper summarizes recent progress in the R&D of the two electrical joints. First, we have described recent R&D for the butt joint. We have previously established fabrication methods for the butt joint samples. In this paper, we have tried to establish a predictive method of joint resistance for a large-scale stacked REBCO conductor within a metal jacket by using small-scale experiments and current distribution analyses. For this purpose, we evaluated joint resistivity, particularly in the butt joint where an indium film is inserted between joint surfaces. Based on the small-scale experiments and 3-D current distribution analyses, the joint resistivity between the contact surface and the indium film inserted into the joint section is evaluated to be $1.1 \times 10^{-12}\ \Omega\hbox{m}². According to the discussion based on the results, the joint resistance for a 100-kA-class REBCO conductor is estimated to be 3.7 n\Omega$, which is a reasonably acceptable value from the viewpoint of electric power for cooling. Second, we recapitulate present R&D for the edge joint. In the case of the edge joint, adequate methods for fabricating samples had not been established because this joint has only been recently proposed. In a previous study, the experimentally measured joint resistance of the edge joint was much higher than predicted. We expect that the reason for the discrepancy is degradation of the conductor edge during the fabrication process and strains due to forces during the joint test. In addition, poor fabrication of the joint faces resulted in limited local area contact across the joint. In this paper, we improved the methods of fabrication and the structure of the test conductors based on numerical analysis. We then carried out a testing program of the edge joint of a stacked GdBCO conductor within a copper jacket fabricated with the improved process. The experimentally evaluated joint resistance agreed with numerical predictions. Therefore, the satisfactory performance of the edge joint was demonstrated in this paper.