Reducing Joint Resistance by Heat Treatment During Fabrication of a Mechanical Joint of High-Temperature Superconducting Conductors

A mechanical joint of high-temperature superconducting (HTS) conductors has been investigated for segmented fabrication of HTS magnets proposed for a future fusion reactor. In a previous study, joint resistance of 1.8 $\text{n}\Omega$ at 100 kA was achieved for a large-scale simple-stacked gadolinium barium copper oxide (GdBCO) conductor having a bridge-type mechanical lap joint with indium foils inserted between joint surfaces. The obtained joint resistivity (the product of joint resistance and contact area) was, however, ten times larger than that of a single mechanical lap joint achieved in previous studies, which may be caused by nonuniform distribution of contact pressure. In order to reduce this joint resistance, therefore, we propose a new method by introducing heat treatment in fabricating process of the joint. By applying the method, the joint resistance became the lowest at heating temperatures of 75 °C-90 °C, which was reduced to 60% of the value obtained without heat treatment. The reduction of joint resistance was due to a decrease in thickness of the inserted indium and an increase in true area of the contact surface. On the other hand, relatively large voids were observed in the indium by X-ray computed tomography when the heating temperature exceeds 120 °C. The voids were caused by emission of the gases absorbed in the tape and foil, and the voids decreased by baking the conductor tapes and indium foil before the heat treatment. Applying the heat treatment to a large conductor joint, the voids can be formed due to the temperature exceeding 120 °C since the temperature control in the large joint becomes more difficult. Therefore, the baking process before the joint fabrication with heat treatment is one of the options to avoid the void formation.