50-kA Capacity, Nitrogen-Cooled, Demountable Current Leads for the SPARC Toroidal Field Model Coil

This paper presents design, fabrication, and operational results for a novel pair of binary Current Leads (CLs). The CLs were purpose built for the SPARC Toroidal Field Model Coil (TFMC) Test Facility at the MIT Plasma Science and Fusion Center (PSFC). The CLs operate stably at current ramp rates up to 50 kA/s and steady-state currents of 50 kA from ambient temperature power supplies to the facility's 20 K test environment. The CLs have three main sections: an upper copper heat exchanger (HEX) cooled with gas nitrogen (GN2) which connects to room temperature power supply (PS); a central liquid nitrogen (LN2) boiling chamber (BC) which vents into the HEX; and a lower Rare Earth Yttrium Barium Copper Oxide (REBCO) Section. The CLs have three distinguishing features: the BCs high surface area maintains <1 K temperature difference between their surfaces and the nucleate boiling LN2 within; each REBCO section is composed of six parallel “petals” that were individually qualified prior to installation; indium seals were used to simultaneously provide electrical continuity and hermetic sealing which enabled fabrication without need for braze or electron beam welding qualification processes. Additionally, the CLs can reduce the LN2 boiling pressure and thus temperature which significantly improves the temperature margins within the REBCO Section. The CLs were designed and built within 1.5 years and used successfully to deliver 40.5 kA of current to the SPARC TFMC. Since then they have been thermally cycled over 10 times and operated at both 1 atm and 0.65 atm.