Soldered joints—an essential component of demountable high temperature superconducting fusion magnets

Demountable superconducting magnet coils would offer significant benefits to commercial nuclear fusion power plants. Whether large pressed joints or large soldered joints provide the solution for demountable fusion magnets, a critical component or building block for both will be the many, smaller-scale joints that enable the supercurrent to leave the superconducting layer, cross the superconducting tape and pass into the solder that lies between the tape and the conductor that eventually provides one of the demountable surfaces. This paper considers the electrical and thermal properties of this essential component part of demountable high temperature superconducting (HTS) joints by considering the fabrication and properties of jointed HTSs consisting of a thin layer of solder (In₅₂Sn₄₈ or Pb₃₈Sn₆₂) sandwiched between two rare-earth-Ba₂Cu₃O₇ (REBCO) second generation HTS coated conductors (CCs). The HTS joints are analysed using numerical modelling, critical current and resistivity measurements on the joints from 300 to 4.2 K in applied magnetic fields up to 12 T, as well as scanning electron microscopy studies. Our results show that the copper/silver layers significantly reduce the heating in the joints to less than a few hundred mK. When the REBCO alone is superconducting, the joint resistivity ( R _J) predominantly has two sources, the solder layer and an interfacial resistivity at the REBCO/silver interface (∼25 nΩ cm²) in the as-supplied CCs which together have a very weak magnetoresistance in fields up to 12 T. We achieved excellent reproducibility in the R _J of the In₅₂Sn₄₈ soldered joints of better than 10% at temperatures below T _c of the REBCO layer which can be compared to variations of more than two orders of magnitude in the literature. We also show that demountable joints in fusion energy magnets are viable and need only add a few percent to the total cryogenic cost for a fusion tokamak. (paper)