Initial Study on Press Welding With Indium Applied to High-Temperature Superconducting DC Feeder Cables

High-temperature superconducting (HTS) DC feeder cable has been proposed for reducing energy loss in railway systems. The feeder cables are required to be installed during the night using a quickly-fabricable low-resistive joint. A soldering technique has been widely used for connecting HTS round-shaped cables in transmission or feeder cable systems. In this study, we proposed applying press welding with indium at room temperature (cold pressure welding) or with heat treatment at 90–120 °C as an alternative candidate for the cable-to-cable joint. First, we examined the necessity of the pre-joint process, polishing the HTS tape, and pickling indium foil with 10% HCl as well as the joining temperature. The examination was performed using tape-to-tape joints with copper-alloy-laminated Bi-2223 tapes. The joint resistance and mechanical strength for railway application were satisfied even by a joint fabricated at room temperature skipping the above two pre-joint processes. Then, we fabricated straight and spiral joints of the model cable made of copper-alloy-laminated Bi-2223 tapes with various joining processes and spiral pitches. The joint resistivity for the model cable joint was less influenced by the joining process and spiral pitch and it was comparable to that of a single tape-to-tape joint. As a conclusion, press welding with indium with a simplified process, joining at room temperature, and skipping some of pre-joint processes can be applied to quick and simple joint fabrication of HTS DC feeder cable.