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. The approximately 3 m tall CLs demonstrate stable operation 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. Each CL can be divided into three main parts: an upper copper heat exchanger (HEX) cooled with gas nitrogen (GN2) which connects to room temperature water cooled power supply (PS) bus; 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. First, the BCs provide high surface area, maintaining <1 K temperature difference between their surfaces and the nucleate boiling LN2 they contain. Second, each REBCO section is composed of six “petals” that could be individually qualified to verify performance prior to installation. Third, indium seals were used throughout the assembly to simultaneously provide electrical continuity for current transfer and hermetic sealing, while enabling fabrication and assembly without need of the lengthy braze or electron beam welding qualification processes typically required for alternative designs. The modular approach was adopted to meet the aggressive schedule of the TFMC project. As a further hedge against design uncertainties, the leads were designed to permit sub-cooled operation at lower LN2 boiling pressure, and thus lower temperature which significantly increases 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 that time they have been thermally cycled over 10 times and operated successfully at both 1 atm and 0.65 atm while transmitting steady state 50 kA current following transient current ramp rates of up to 50 kA/s.