A new concept for developing a compact joint structure for reducing joint resistance between high-temperature superconductors (HTS) and low-temperature superconductors (LTS)

Abstract

It is significantly difficult to develop a superconducting joint between REBCO and low-temperature superconductors (LTS) using a solder matrix replacement technique. This is because the REBCO superconducting layer is highly corrosive to Sn. In this work, TEM observations were first conducted on the reaction interface between the REBCO layer and Sn to reveal the reaction. Then, a new idea to create a compact low-resistance joint to reduce the joint resistance between REBCO and LTS was proposed. In this method, the REBCO tape is rolled in a metal boat, with the Cu stabilizer remaining around the tape. Then, an LTS wire, whose superconducting filament ends are coated with a superconducting solder, is set straight into a boat. Then, the boat is filled with a superconducting solder, so that the joint state between the LTS and superconducting solder matrix remains superconductive. However, the electrical joint between the REBCO tape and the solder matrix is resistive, even if the solder matrix is superconductive. Consequently, the overall joint resistance is determined by the boundary resistance between the REBCO tape and the superconducting solder matrix. However, to achieve a joint resistance below 10–10 Ω, a long joint length of more than 5 m, preferably more than 10 m, will be required. Considering the strain state of the REBCO layer when it is rolled in to a boat, the boat-type joint structure proposed in this work enables the joint size to be significantly compact, even if a length of more than 10 m is required. At present, a joint resistance of 0.7 nΩ was obtained in a field range of less than 0.4 T by using a boat with an inner size dimensions as follows: 50 mm length, 16 mm width, 7 mm height, radius of curvature of 8 mm, and tape length of 2 m.