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

Abstract

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 (RJ) 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 RJ of the In52Sn48 soldered joints of better than 10% at temperatures below Tc 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.

An Electric-Circuit Model on the Inter-Tape Contact Resistance and Current Sharing for REBCO Cable and Magnet Applications

“…We set R c = 1000 Ω for the insulated case. For the soldered case, R c was 46 nΩ based on the conductivity of the solder layer of 0.1 mm thickness (27 MSm −1 at 77 K [29]). The calculated voltages at the tape sections for high and low R c are shown in Fig.…”

Dipole Magnets above 20 Tesla: Research Needs for a Path via High-Temperature Superconducting REBCO Conductors

“…Joints between conductors need low electrical resistance to minimize the heat generation during magnet operation. Although HTS conductors and magnets can tolerate higher heat load and hence possibly higher joint resistances, a low joint resistance on the order of 1 nΩ at self field is still desirable to minimize the refrigeration load [126] for accelerator magnets operating at 10-20 kA current range. This low resistance has been routinely achieved in joints for Nb-Ti and Nb 3 Sn Rutherford cables [65] ( Table 1) but the joint technology for multi-tape REBCO conductors remains to be demonstrated to achieve the same low joint resistance.…”

“…Joints for CORC R conductors demonstrated 2-6 nΩ resistance at 4.5 K [68]. With the well defined tape-tape contact condition, several studies show that the interfacial resistance inside a REBCO tape (Cu/Ag, Ag/REBCO) contribute strongly to the joint resistance [64,126,127]. Collaboration with tape manufacturers is needed to achieve a low and consistent interfacial resistance with the industrial tape manufacturing process.…”

Could High H98-Factor Commercial Tokamak Power Plants Use Nb–Ti Toroidal Field Coils?

“…Such joints would offer significant improvements in the availability of a reactor, from the 75% assumed in this work, by improving the ease of replacing coils. They would also enable modular construction, likely decreasing capital costs [43]. Novel designs of the TF coils are also expected to drive costs down.…”