Performance of demountable solder joints for no-insulation superconducting coils produced by vacuum pressure impregnation

Abstract

No-insulation high temperature superconductor (HTS) stack coils show both increased thermal and electrical stability, and present a simplified geometry for the integration of demountable joints. Demountability is a desirable feature for many superconducting applications including fusion magnets, which motivate this research. In this work, a novel vacuum pressure impregnation (VPI) solder process was developed to couple superconducting paths via a low resistance, mechanically simply, demountable joint for a non-insulated coil design. The three low temperature solders considered were (mp = 118 C), (mp = 156.6 C), and (mp = 29.8 C), all with a lower melting point than the (mp = 183 C) used to solder the HTS tapes into the coil, thus allowing for the advantages of solder in the joint, yet facilitating demountability without disturbing the primary solder in the non-insulated coil stack. A multidisciplinary campaign was undertaken to design, build, test and identify the major challenges with these small-scale demountable solder joints. A multi probe voltage tap system was used to infer the effective resistances to exit a superconducting stack, cross the solder layer, and enter the second superconducting stack, at 77 K. The experimental resistivities show good agreement with a newly developed finite element model that breaks down domains to the level of individual layers of an HTS tape. When taking into account the thermal degradation that can occur to the HTS stacks during the VPI processes, the normalized joint resistances are found to be 528, 668, and 671 for the , and solders, respectively. The benchmarked finite element model is used to predict normalized joint resistivities for fusion-relevant temperatures and magnetic fields using a 100 µm solder layer thickness, finding 38, 125, and 103 for the respective solders; these results are competitive with the lowest resistance cable joints presented in the literature.