Liquid Hydrogen Recirculation System for Forced Flow Cooling Test of Superconducting Conductors

The knowledge of forced flow heat transfer characteristics of liquid hydrogen (LH₂) is important and necessary for design and cooling analysis of high critical temperature superconducting devices. However, there are few test facilities available for LH₂ forced flow cooling for superconductors. A test system to provide a LH₂ forced flow (∼10 m/s) of a short period (less than 100 s) has been developed. The test system was composed of two LH₂ tanks connected by a transfer line with a controllable valve, in which the forced flow rate and its period were limited by the storage capacity of tanks. In this paper, a liquid hydrogen recirculation system, which was designed and fabricated in order to study characteristics of superconducting cables in a stable forced flow of liquid hydrogen for longer period, was described. This LH₂ loop system consists of a centrifugal pump with dynamic gas bearings, a heat exchanger which is immersed in a liquid hydrogen tank, and a buffer tank where a test section (superconducting wires or cables) is set. The buffer tank has LHe cooled superconducting magnet which can produce an external magnetic field (up to 7T) at the test section. A performance test was conducted. The maximum flow rate was 43.7 g/s. The lowest temperature was 22.5 K. It was confirmed that the liquid hydrogen can stably circulate for 7 hours.