Focusing on the circulating cooling of superconducting coils, the author introduces an example of research and development that cools the coil by circulating liquid helium, gas helium, and liquid hydrogen. In order to realize more efficient cooling, the author explains a method that combines a circulation method with magnetic refrigeration and oscillating heat pipes, and a method that involves circulation with a cryofan in the state of low temperature fluid without going through room temperature. In addition, the benefits of circulating cooling are also mentioned.

The Turbo-Brayton refrigerator is needed to keep the sub-cooled state of liquid nitrogen in a cooling system for the High Temperature Superconducting (HTS) Cable. Our developed Turbo-Brayton refrigerator were applied in two HTS Cable applications of electric power and railway feeder. In this paper, the specifications and performance test results of Turbo-Brayton refrigerators aimed for electric power and railway feeder applications are described. Further, this Turbo-Brayton refrigerator had been proven reliable in the HTS Cable demonstration Project at Yokohama. These test results of the cooling system, including the Turbo-Brayton refrigerator, are also introduced.

Introducing a superconducting cable is expected to reduce power loss. However, it needs cooling to maintain the superconducting state. In this paper, we outline the superconducting system which consists of superconducting cables and a cooling system. Subsequently, we report the examination of the efficient refrigerant circulation methods, the construction of the cooling system, and the results of the cooling tests on the railway site.

A model of superconducting magnet energy storage device has been fabricated to demonstrate its cooling scheme by using liquid hydrogen flow through thermosiphon loops. A stack of three double pancake coils wound with MgB₂ Rutherford cables are in thermal contact with thermosiphon pipes through pure copper plates which cover the each pancake coil surface. To prevent eddy current from being induced in the copper plates, the latter are slit into strips. After completion of the model device, performance test was carried out by actually supplying liquid hydrogen. The coil has been kept at a temperature lower than 21 K and has been charged up to a nominal current of 600 A. On the other hand, a large amount of evaporation of liquid hydrogen due to oscillation of some sort and gradual temperature rise at the coil terminals resulting in quench were observed, and these issues should be resolved in the future.

The response characteristics of a 54-pixel transition edge sensor (TES) to an applied magnetic field B were evaluated with a superconducting solenoid coil and a microwave superconducting quantum interference device (SQUID) multiplexer (MW-Mux). Three figures of merits, i.e., height of output pulse current I_P, its fall time 𝜏, and noise referred to SQUID 𝑆_Φ are systematically measured as a function of B and bias point of TES 𝑅/𝑅_n where 𝑅 is the TES resistance at the operation point in the transition edge and 𝑅_n is that in its normal state. It has been found that 54 pixels can be categorized into three groups due to their behavior toward B. The first group, to which the majority of the pixels belongs, exhibits the clear B dependence of I_P and 𝜏, but not of 𝑆_Φ. The second group consisting of about 10 % of the pixels does not show obvious B dependence of I_P, 𝜏, and 𝑆_Φ. The third group provides 𝑆_Φ which reaches the local maxima at the specific value of B at the lower bias current region (𝑅/𝑅_n ≈ 9 % and 14 %). This indicates the importance of systematic characterization of multi-pixel TES with varying (B, 𝑅/𝑅_n ) and understanding of the optimal conditions prior to applications such as astronomical observations in space that are demanding in terms of measurement time. Combination of coil and MW-Mux is useful for this purpose and scalable for the future large format TES array.

The computational fluid dynamics (CFD) analysis was carried out on the experimental results of measuring heat transfer from the inside of a vertical pipe of 6 mm inner diameter and 100 mm length to forced flow of hydrogen at the pressure of 1.5 MPa. In the CFD analysis, heat flux was applied to the pipe surface as in the experiment, and the effect of flow velocity was clarified. The slope of the heat transfer curve obtained by the CFD analysis decreased as the wall heat flux increased, showing the same tendency as the experimental results. The friction loss coefficient was calculated based on the analytical results, and a new evaluation formula was proposed to express the friction loss coefficient obtained by numerical analysis.