Thermodynamic optimization of 10-30 kA gas-cooled current leads with REBCO tapes for superconducting magnets at 20 K

Thermodynamic optimization is carried out to minimize the refrigeration work of gas-cooled current leads at a current level of 10-30 kA for superconducting magnets at 20 K. The binary HTS lead is a serial combination of REBCO (rare-earth barium copper oxide) tapes as cold part and copper conductor as warm part. In gas-cooled leads, liquid nitrogen is not used, but cold helium gas is supplied for forced-flow cooling through the channel between spiral fins of copper conductor. A special attention is paid to the conditions of gas-cooling, which can be integrated with a closed refrigeration cycle without any heat intercept or boil-off loss of liquid. The input power for refrigeration is rigorously calculated with the temperature-dependent properties of conductors. When a safety margin is selected on the critical current of REBCO, it is proven that there exists a unique optimum in the cooling -gas temperature and the dimensional size of copper conductor to minimize the required work for refrigeration. The results are compared with the optimized cases of conduction-cooled and vapor-cooled binary leads for 20 K magnets. The details of optimization procedure and design data are presented for practical application.