Hideaki Miura

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.

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.

This paper reports the optimum design results of REBCO high-temperature superconducting magnets for 3 T-whole-body magnetic resonance imaging (MRI). The REBCO magnet consists of a main coil, compensation coil, and shield coil, and optimum design is carried out using an immunogenetic algorithm. A magnetic field uniformity of 20 ppm was observed in the uniform sphere inside the coil, and a 5-gauss region of 2.67 m in radial length and 4.50 m in axial length was realized. The local magnetic field vector distribution of the designed coil was also calculated for the designed magnet using electromagnetic field analysis, and the local voltage was obtained using the magnetic field/temperature dependence of the electric field vs. current density properties evaluated with the REBCO short sample. Then, the voltage vs. current characteristics of the magnet were evaluated by adding it over the entire coil length. Defining the current load factor (operating current/critical current) of the designed magnet using the above analysis results makes it possible to clarify the relationship between the operating current, optimized magnet shape, and required wire length for the operating temperature. In particular, we found that the required HTS tape length and transport current have a simple linear relationship in the operating temperature range below approximately 50 K.

This study investigated the axial compressive stress dependence of the critical current of Rare Earth Barium-Copper-Oxide (REBCO) double-pancake coils. We measured the critical currents of REBCO coils cooled by liquid nitrogen while various axial compressive stresses from 0 MPa to 130 MPa were applied. After each stress was applied, the critical currents of the coils were also measured without the stress application to examine if the REBCO coils were irreversibly deteriorated. The experimental results showed that the REBCO coils were deteriorated at more than 100 MPa and the critical currents of the coils decreased by about 30% by applying 120 MPa to the coils. Moreover, reversible reduction of the critical currents of the coils were observed while the stresses of less than 100 MPa were applied. The reversible critical current decrease occurred even at a small stress, and the critical currents of the coils were reversibly reduced by about 15% while the stress of 100 MPa was applied. This study quantitatively clarified the dependence of the critical current of the REBCO coil on the axial compressive stress and the limitation of the axial compressive stress to prevent the deterioration of the REBCO coil.

This paper investigated a suitable excitation method for a conduction-cooled REBCO coil to reduce the temporal attenuation of the screening current induced field (screening field) for MRI. We investigated the effect of the pre-excitation of the coil and high temperature magnetization method on the screening field attenuation by measuring the variation rate of the magnetic field at the side surface of a conduction-cooled REBCO double pancake coil. Experimental results showed that the pre-excitation of the coil and high temperature magnetization increased the temporal stability of the coil magnetic field when the pre-excitation current is higher than the main operational current, or the temperature change of the high temperature magnetization is over 5 K, respectively. We also clarified that the combination of the pre-excitation, overshooting, and high temperature magnetization method greatly suppressed the attenuation of the screening field and also the load factor of the coil than in the case when only the high temperature magnetization is applied.

Research and development for the practical application of a med-ical-use magnetic resonance imaging system (MRI) superconduct-ing magnet that does not require liquid helium started as the New Energy and Industrial Technology Development Organization's (NEDO) supported project in fiscal 2016. Development of a liquid helium-free medical MRI superconducting magnet has been de-sired. Another purpose is to reduce the size and weight of high magnetic field magnets. By using the high temperature supercon-ducting coil, it is possible to make the 3T magnet of the same shape, weight, leakage magnetic field as 1.5T magnet. In this pro-ject, we are developing a half size active shield-type 3T REBCO coil for MRI. This magnet has active shield coils with a maximum diameter of 1200 mm, and its room bore diameter is 480 mm. This magnet is one of the world's largest magnets using a REB-CO wire with an accumulated energy of 1.6 MJ at the rated mag-netic field. It generates a magnetic field of high homogeneity and stability necessary for imaging. In this paper, we report the half-size active shield-type 3T coil and its cooling system that can re-duce the initial cooling time.

We have been developing a liquid-helium-free magnet using High-Temperature Superconducting (HTS) wire for MRI. The aim of this research and development is to commercialize a 3-T HTS magnet for whole body MRI which has the characteristics of being liquid-helium-free, small, and light-weight. HTS coil requires tolerance of a high magnetic field, 7 T, and it must have high current density, 200 A/mm ² , for developing a small and light weight magnet used for 3-T MRI. A half-size 5-T HTS magnet will be manufactured in order to demonstrate the tolerance of high magnetic field and high current density of the coil, which are equivalent to a whole-body 3-T MRI magnet in a half-size HTS magnet. In this paper, we have investigated the arrangement of coils of a half-size 5-T coil. The designed coil consists of 124 double-pancake coils, whose inner diameter is 560 mm, outer diameter is 1200 mm, and axial length is 1100 mm. The uniformity of designed magnetic field is under 2.0 ppm (250 mmDSV).