Recently, high speed trains over 1000 km/h have been proposed in an evacuated tunnel and a maglev train using REBCO magnets can be considered as a feasible option due to their excel-lent mechanical and electromagnetic characteristics. A cryogenic cooling system for REBCO magnet occupies an installation space and burdens system weight. For a maglev train with a limited space, it is important to minimize the installation space and reduce the system weight. The electric power is also limited while the magnet is running. This paper describes thermal characteristics of a prototype REBCO magnet with a solid nitrogen vessel as a thermal battery, a detachable cryocooler and a quasi-persistent current switch to reduce the installation space and system weight. Solid nitrogen vessel, attached on the magnet bobbin, is designed to maintain the magnet temperature below an operation limit without an on-board cryocooler, which simulates an external cryogenic refrigeration system that periodically cools the magnet at stations. Heat capacity of the solid nitrogen, which includes a solid phase change at 35.6 K, is used to maintain the magnet temperature. The experimental results with solid nitrogen are compared with a design and simulation results. The result shows the possible application of external cooling system and the quasi-persistent current mode of the REBCO magnet for a high-speed maglev train.

Researches on a magnetic levitation (maglev) train traveling in a vacuum tube have been actively conducted as a next generation ultra-high speed vehicle. For strong propulsion and levitation, a REBCO magnet is considered a feasible option. Due to the limited facility space of the maglev train, it has been proposed to eliminate the cumbersome cryogenic cooling system and power supply while the train is running. Applying a thermal battery instead of the on-board cooling system can suppress the temperature rise of the REBCO magnet while the maglev train is running. In this study, a solid nitrogen (SN ₂ ) is proposed as the thermal battery since it has a large sensible heat and an additional latent heat by α-β solid phase transition at 35.6 K. This can delay the temperature increase during the operating time. In addition, a semi-persistent current switch (semi-PCS) is applied with detachable current leads to operate the magnet without the power supply while the train is running. The detachable current leads can eliminate heat loss through the current leads. This paper describes the experimental results on the thermal and electromagnetic performance of REBCO magnet with the thermal battery after disconnecting the cooling system and the power supply.

We developed a prototype of a high-Tc superconducting (HTS) linear synchronous motor (LSM) and evaluated its operational performances to obtain the preliminary design data for a test vehicle having a speed of 518.4 km/h. The HTS electromagnet was fabricated using a non-insulated (NI) GdBCO HTS coil, which was designed to produce a magnetomotive force of 300 kA-turns at a cooling temperature of 20 K. A two-stage Gifford-McMahon cryocooler was used to cool the electromagnet. The prototype was tested up to 2.7 kN sufficient to the required thrust of the test vehicle. The thrust characteristics shown by changing the operating conditions were also consistent with the simulation results. The HTS electromagnet was successfully operated during the test, and there was no fault in the NI HTS coils. Finally, we confirmed the design feasibility of HTS LSM for a test vehicle.

NI (no-insulation) superconducting coils using rare-earth barium-copper-oxide (REBCO) wires have been widely applied in various superconducting magnets due to superior electric, mechanical and thermal performance. Yet, NI coils present the intrinsic drawbacks of charging delays and heat losses by leakage current. MI (metal insulation) windings have also been proposed to increase the turn-to-turn contact resistance and improve the coil's mechanical strength. Both NI and MI REBCO conduction-cooled coils require additional thermal paths to a cryocooler, leading to the use of epoxies between interfaces. However, the use of epoxy requires precaution to prevent delamination problems or the loss of the NI characteristic when the epoxy penetrates through the winding turns. In this study, we propose a new winding technique, soldered metal insulation (SMI), to decrease the contact resistance of MI coils while providing enhanced thermal characteristics. To verify the effectiveness of SMI, both a small MI coil and a small SMI coil are fabricated, and their electrical performances are compared. For the SMI coil, In-Bi solder is tinned on a STS (stainless steel) tape that is co-wound with the REBCO wire; the SMI coil is then heated to make soldered contacts between the winding turns. The resulting time constant and contact resistance are investigated by performing sudden discharging experiments.

Infrared detectors on satellites and spacecraft require cooling to increase their measurement sensitivity. To efficiently cool infrared detectors in a zero gravity environment and in limited spaces, a cryogenic loop heat pipe (CLHP) can be used to transfer heat over a certain distance by the capillary forces generated from porous wicks without a mechanical power source. The CLHP presented in this study transfers the heat load to a condenser 0.5 m away from an evaporator at temperatures below −150 °C. The CLHP with two evaporators includes a subloop for initial start-up, and uses a pressure reduction reservoir (PRR) for the supercritical start-up from room to cryogenic temperature. Nitrogen is used as the working fluid to verify the thermal behavior of the CLHP, and the heat-transfer capacity according to the nitrogen charging pressure of the PRR is investigated. To simulate a cryogenic environment, the CLHP is installed inside a space environment simulator, including a single-stage GM (Gifford McMahon) cryocooler to cool the condenser. The CLHP is horizontally installed to simulate zero gravity. The heat-transfer characteristics are experimentally evaluated through the loop circulation of the CLHP.

We estimated the critical current of the second generation (2G) high-temperature superconducting (HTS) conductor using neural network fitting methods. The critical current of 2G HTS conductors depends on magnetic field strength and angle as well as on temperature, I_c(T, B, θ). Moreover, the critical current values vary for 2G HTS conductors from different manufacturers. In this study, we addressed three challenging issues in critical current assessment by neural network fitting methods, namely 90° asymmetry, a wide range of temperature-field-angle dependence, and different manufacturer conductor differences. Prediction models for three commercial HTS conductors were trained and evaluated by convergence, accuracy, and robustness. The linear regression correlation coefficient R was approximately equal to 1 for the three models. The critical current estimation obtained from the proposed method was compared with the critical current estimation from the interpolation method at different fixed temperatures using a multi-width no-insulation magnet. The model computation speed was also discussed. The proposed model needed only 2.7 s to compute 10 million data sets. Therefore, the convergence, accuracy, reliability, and speed of the proposed method prove that it can be used in a wide range of industrial applications and academic fields.