Copper electroplating technique for development of HTS current leads bottom joints using MgB2 wires

Abstract

Low resistance, helium leak tight, cryo-stable superconducting (SC) joints with sufficient strength to absorb thermo-mechanical stress are commonly used in large sized magnets, their feeders and current leads (CL). Among high temperature superconductor (HTS), cuprate oxide based tapes are the popular options for CL applications. Recently composite magnesium diboride (MgB2) wires with superconducting transition temperature of 39 K are getting wide attention for low field applications. Use of Monel as a sheath material in MgB2 wires poses a major challenge while fabricating SC joints due to its low wettability and flowability using PbSn alloy based solder material. To tackle this issue, we exploited Cu electroplating technique to ease fabrication of MgB2 – NbTi: Cu as well as HTS: Cu - MgB2 joints. The details of in-house electroplating technique developed to fabricate bottom joints of BSCCO-2223 based HTS CL using MgB2 wires are reported. To study the morphology of such Cu coated MgB2 wires microstructural investigation is performed using Scanning Electron Microscope with energy dispersive X-ray spectroscopy (SEM/EDX) analysis. Similar Cu plated MgB2 wires are used to fabricate bottom joints of a prototype HTS CL. The performance of joints is preliminary attempted in the HTS CL test experiment up to 1.5 kA at 20 K.