Masayoshi Ohya

To realize a 600 MW-class high-temperature superconducting (HTS) generator cooled by liquid hydrogen, HTS assembled conductors for the field coil have been developed. Because the current capacity of our liquid hydrogen test apparatus is limited to 500 A, an inductive energization technique that induces large current in the secondary coil using a short-circuited assembled conductor was considered. Conventional direct current (DC) sweep techniques are affected by noise and drift when measuring induced current with a Rogowski coil. Therefore, an alternating current (AC) inductive energization technique was applied. The secondary coil was fabricated by short-circuiting both ends of a single-layer assembled conductor composed of four REBCO wires. AC inductive energization tests were performed using both liquid nitrogen and liquid hydrogen for cooling. In the liquid nitrogen test, critical current ( I _c ) detection was demonstrated with an accuracy of several percent based on distortion observed in the Rogowski coil voltage waveform. In the liquid hydrogen test, an induced current of up to 5,948 A was successfully achieved. In addition, the inductive energization test was successfully conducted under an external magnetic field of approximately 0.7 T. Improvements to the test setup will be pursued for future inductive energization tests under external magnetic fields of several tesla.

This paper presents an AC inductive energization method for evaluating high-temperature superconducting (HTS) assembled conductors cooled by liquid hydrogen. A short-circuited secondary coil composed of REBCO wires was fabricated and tested under liquid-nitrogen and liquid-hydrogen environments. In liquid-nitrogen cooling, phase shifts and waveform distortions in the Rogowski-coil voltage were observed when the secondary current approached the conductor's critical current (${{{\bm{I}}}_{\bm{C}}}$), demonstrating that voltage signals can serve as reliable indicators of ${{{\bm{I}}}_{\bm{C}}}$. Under liquid-hydrogen cooling, secondary currents up to 4985 A were achieved before the power supply shut down because of overvoltage protection. These results establish AC inductive energization as a safe, non-destructive diagnostic method for HTS conductors and highlight the potential of liquid hydrogen for next-generation large-scale superconducting devices.

For high-temperature superconducting wires to be of practical use in large magnets, such as fusion magnets, they must be able to tolerate localized degradation. As a fundamental study, the thermal runaway behavior of REBCO coils was investigated with locally damaged coils immersed in liquid nitrogen and liquid hydrogen. The critical current, I_C of the damaged part was less than 10% of the original value, and only one thin side of the REBCO tape was cooled with the cryogen. In the case that the tape was wrapped with 0.025 mm thick insulating tape (Kapton), thermal runaway occurred within the nucleate boiling region before the normal zone expanded. As the temperature rose, current in the copper stabilizer increased due to a decrease in I_C, and the copper resistivity increased. The thermal runaway occurred at a temperature where the temperature dependence of the heat generation became stronger than that of the cooling power. In the case that the insulating tape was removed from the cooling surface, thermal runaway did not occur until the transition from nucleate boiling to film boiling. Under these conditions, the thermal runaway current in liquid hydrogen increased with pressure from 20 K at atmospheric pressure to 27 K at 0.54 MPa, despite the temperature rise. It is estimated that the critical heat flux increased with pressure. Since even a thin insulating layer significantly affects the temperature rise of the conductor, direct cooling of bare surfaces is preferable for good cryostability.

This study explores an alternating current (AC) inductive energization method of high-temperature superconducting (HTS) assembled conductors for power generators cooled by liquid hydrogen. To perform an inductive transport current test on 6 kA-class HTS conductors, an AC excitation technique was introduced to enhance the accuracy of induced current measurements using a Rogowski coil. The principal verification test was conducted using a short-circuited secondary coil made of a single REBCO wire. The experimentally measured induced current waveforms were consistent with the simulation results for frequencies ranging from 0.5–4 Hz. Furthermore, the phase difference between the primary and secondary currents changed rapidly beyond the critical current of the wire (${{I}_C}$), aligning with analytical predictions. We proposed a distortion rate metric to improve ${{I}_C}$ detection accuracy and demonstrated that ${{I}_C}$ can be detected with an accuracy of 10% or better. These findings support the development of a novel evaluation method for the transport current characteristics of HTS conductors immersed in liquid hydrogen.

Aiming at the establishment of design criteria for cryostable HTS magnets, we have investigated the behavior of thermal runaway of REBCO three-turn coils immersed in liquid nitrogen and liquid hydrogen. Sample 1 consisted of a REBCO tape with a copper layer 0.1 mm thick and a co-wound copper tape 0.2 mm thick, and Sample 2 consisted of the REBCO tape only. Each conductor was wound in a groove of a G10 plate, and only an upper side of the coil was cooled with cryogen. In order to simulate local degradation, the testing part, 0.1 m long, was damaged by bending, and critical currents at 77 K were reduced to less than 1/10 of the original value of 120 A. In liquid nitrogen, thermal runaway of Sample 1 occurred at 107 A after wide propagation of a normal zone, while that of Sample 2 occurred at 57 A before propagation of a normal zone. In liquid hydrogen, thermal runaway of both samples occurred before propagation of a normal zone, and the amount of heat generation of Sample 1 when starting thermal runaway was 1.5–1.8 times as high as that of Sample 2, which was lower than the ratio of the wetted areas. The co-wound copper tape was less effective for the short normal zone, and it worked effectively as a bypass-current path after wide propagation of a normal zone.

A prototype high-temperature superconducting (HTS) field coil for a liquid hydrogen-cooled HTS generator presented a current–voltage (I-V) curve that was lower than that predicted from the critical current (I_C) characteristics of the REBCO wires. It was assumed that wire degradation occurred between the shipping and winding process. Therefore, this study aimed to verify the quality of the REBCO wires (approximately 140 m × 11 rods) used to manufacture field coil by investigating the differences between shipping and acceptance tests. A section lower than the I_C specification value of 150 A was detected. Additionally, small I_C drops were also observed in this wire. Moreover, there was a two-step slope observed in the Weibull plot of the acceptance test of this wire, which was different from the shipping test result. This wire also had visual imperfections. Consequently, this wire was not used for the manufacturing of the field coil. A total of four racetrack double-pancake coils were manufactured using wires that passed the acceptance test, and its I-V characteristics were measured under liquid nitrogen cooling. The measured I-V characteristics of the coils were in good agreement with the calculated curves incorporating the I_C value, I_C-B characteristics of the wire, and the three-dimensional magnetic field analysis result. It was confirmed that the coils present the transport current characteristics as designed by conducting the acceptance test and selecting the wires.

Japan's strategy to import overseas-produced liquefied hydrogen faces an economic hurdle due to liquefaction costs. Efficiently tapping into the cold heat produced by liquid hydrogen is pivotal, with high-temperature superconducting (HTS) devices offering a promising solution. Commercializing a liquid hydrogen-cooled HTS generator that operates alongside a hydrogen gas turbine holds significant promise. This setup eliminates the need for a refrigerator by utilizing cold heat, leading to notable efficiency improvements. A national project is underway to develop liquid hydrogen-cooled HTS generators, with the ultimate objective being to commercialize a 600 MW-class generator. A 10 kW-class demonstration system should be developed and verified in the project. A notable technical challenge in applying HTS coils to large-diameter, high-speed rotating field coils is their mechanical fragility. For the 600 MW system, the anticipated compressive stress due to centrifugal force is approximately 50 MPa. A technology is being developed to realize high-strength coils five times more robust than conventional coils. Mechanical simulations revealed a degradation mechanism, in which the innermost and outermost wire edges of conventional coils are delaminated. Coil casing reinforcement proved effective in reducing the stress in the superconducting layer while maintaining the high current density of the coil. Designing a liquid hydrogen-cooled HTS field coil necessitates a comprehensive understanding of the coil's transport behavior when subjected to liquid hydrogen immersion cooling. Energizing tests for the field coils of the demonstration system were conducted; the HTS coils continued to be energized without thermal runaway, even at a generated voltage of 0.8 mV.

For aluminum-stabilized large conductors used in fusion magnets and other devices, it is essential to consider not only steady-state stability but also transient stability. In the helical coils of the Large Helical Device (LHD) of the National Institute for Fusion Science (NIFS), transient stability was not considered when designing the conductors; as a result, they quenched before reaching the rated excitation. Although the delay in current sharing with the aluminum stabilizer after the normal transition is regarded to be one of the responsible factors, it is unable to explain the transient excessive heat generation. The effect of Hall current loops generated in the conductor cross section has been noted; however, no detailed transient simulation has been performed so far. In this study, a transient simulation code that incorporates the Hall coefficient into the two-dimensional Finite Difference Time Domain (FDTD) method was developed, and the process of transport current redistribution in the stabilizer and the growth of Hall current loops in the conductor cross-section was simulated. The calculated steady-state effective resistance of the LHD conductor at 7 T agrees with the measured values, successfully explaining the anomalous magnetoresistance caused by the Hall current.

High-temperature superconducting (HTS) generators that functions in combination with a hydrogen gas turbine have the potential to reduce CO2 emissions in Japan. The system can realize zero-emission power generation, and it requires no refrigerator and has almost no cooling costs by fully using the cold energy of liquid hydrogen. The ultimate goal is to commercialize a 600 MW-class liquid hydrogen-cooled HTS generator that requires a 6 kA-class assembled conductor with an outer diameter of approximately 6 mm that can conduct 6 kA at 20 K and 5 T. A technical challenge in applying assembled conductors to the field coils of generators is realizing the interlayer current distribution during steady-state operation and quick-response excitation. A conductor with four layers of 12 wires wound on a 5 mm-diameter former was experimentally and numerically studied, and the current distribution was examined. Similar waveforms were numerically obtained when the terminal resistance of each layer was set to 5/90/60/40 nΩ/m. Such variations in the terminal resistance have a slight effect on the transport characteristics of long conductors.