Electrical Characteristics of Soldered Metal Insulation REBCO Coil

Abstract

NI (no-insulation) superconducting coils using REBCO (rare-earth barium-copper-oxide) wires have been widely applied in various superconducting magnets due to superior electric, mechanical and thermal performance. Yet, NI coils present the intrinsic drawbacks of charging delays and heat losses by leakage current. MI (metal insulation) windings have also been proposed to increase the turn-to-turn contact resistance and improve the coil's mechanical strength. Both NI and MI REBCO conduction-cooled coils require additional thermal paths to a cryocooler, leading to the use of epoxies between interfaces. However, the use of epoxy requires precaution to prevent delamination problems or the loss of the NI characteristic when the epoxy penetrates through the winding turns. In this study, we propose a new winding technique, SMI (soldered metal insulation), to make a consistent contact resistance while providing enhanced thermal and structural characteristics compared to MI coils having dry contacts. As a preliminary step of our research, a small MI and a SMI coil are fabricated, and their electrical characteristics are compared. For the SMI coil, In-Bi solder is tinned on a STS (stainless steel) tape that is co-wound with the REBCO wire; the SMI coil is then heated to make soldered contacts between the winding turns. The time constants and contact resistances are compared using sudden discharging experiments for the MI coil, the unheated SMI coil and the heated SMI coil.