Investigation on the Electrical Contact Resistance of Soldered Metal Insulation REBCO Coil

Abstract

No-insulation (NI) winding techniques are widely used in fabricating thermally, mechanically, and electrically robust REBCO magnets. However, NI coils have disadvantages during charging operation, such as charging delay and heat loss due to leakage current. Leakage current can be reduced by increasing contact resistance. Metal-insulation (MI) winding methods are also used by co-winding high-resistive metal insulators between the winding turns of REBCO wires. Although the contact resistance of MI coils can be substantially increased, quantitative estimation of the magnet contact resistance is difficult owing to the dry-contact between the winding turns. Moreover, special care is needed during epoxy impregnation of MI or NI magnets owing to delamination problems induced by the epoxy layer between the winding turns. In this study, a soldered metal insulation (SMI) method is proposed to remove the drawbacks of the MI coil, such as the hardly predictable contact resistance and the delamination issue during conduction cooling applications. The SMI coils were wound using a tin-plated stainless-steel (SS) tape between the REBCO wires as in the MI coil and they were heated in a furnace to produce soldered contacts between winding turns. To investigate the electric characteristics, sudden discharge and charge/discharge experiments were conducted at 77 K. In this paper, the test coils were fabricated using the NI, MI, and SMI winding methods and the contact resistance of each coil was investigated. The results show that the SMI winding method can be used to create expected or controllable contact resistance.