記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：IEEE Transactions on Applied Superconductivity  

An electric propulsion system is one of the solutions to achieve low carbon dioxide emissions from aircraft. In this regard, a superconducting motor is being developed to achieve higher power density than conventional motors. Because a REBa2Cu3O7-σ (REBCO) wire is difficult to bend freely owing to its tape shape, racetrack windings are often used for the armature coils of the superconducting motors. Although the output power can be increased for the same armature current by widening the armature winding pitch angle, the racetrack armature has only a small pitch of 30°. A full-pitched armature with the maximum pitch angle has not been realized in past studies. Therefore, in this study, we propose an unprecedented configuration method of a four-pole full-pitched armature coil composed of trapezoidal windings. To investigate the advantages of the armature, we made a 2D cross-sectional model and determined torque using the finite element method of electromagnetic field analysis. Consequently, we verified that the trapezoidal four-pole armature had lower torque ripple and higher power density than the conventional two-pole racetrack armature. This study confirmed that the proposed novel armature had better characteristics than conventional ones. The armature can make the motor lightweight and low-noise.

DOI： 10.1109/TASC.2024.3358250

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掲載種別：研究論文（学術雑誌）   出版者・発行元：IEEE Transactions on Applied Superconductivity  

We are conducting research and development of an all-superconducting induction motor with lightweight. A lightweight induction motor can be realized by making the rotor without an iron core. To evaluate the characteristics of the induction motor without an iron core and to design an optimal induction motor, it is necessary to quantitatively clarify the characteristics of the current flowing in the superconducting rotor with an air core. In this study, an experimental apparatus was developed to measure the induced current flowing in a rotor exposed to a rotating magnetic field using a pickup coil. In the experiment, a squirrel-cage rotor sample made of YBCO tape 81 mm in diameter and 170 mm in length was placed in a space 120 mm in diameter, and a rotating magnetic field was applied. The temperature of the rotor sample is liquid nitrogen temperature. The rotor sample consisted of six rotor bars and end rings. The rotor bars consisted of one wire with a critical current of 110 A. The magnetic moment of the rotor was measured with three pickup coils, and then the induced current in the rotor was evaluated.

DOI： 10.1109/TASC.2024.3385737

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掲載種別：研究論文（学術雑誌）   出版者・発行元：IEEE Transactions on Applied Superconductivity  

A reciprocating type active magnetic regenerative refrigeration (AMR) utilizes the magnetocaloric effect of magnetic materials which occurs during magnetization and demagnetization processes by moving magnetic materials in and out of a magnet. Estimating the induced voltage and electromagnetic force in the superconducting magnet during the reciprocation is crucial to optimize the design of the AMR system. This paper represents the behavior of the superconducting magnet during the AMR operation that the magnetic material HoAl2 weighing 250 g enters and exits the 120 mm bore NbTi superconducting coil. The calculation and measurement results of the induced voltage and electromagnetic force in the magnetic field of 0-5 T at 25 K were reported.

DOI： 10.1109/TASC.2024.3384342

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記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：IEEE Transactions on Applied Superconductivity  

Fully superconducting synchronous motors employing superconducting armature and field coils, have a great possibility for electric propulsion systems in aircraft applications, owing to their high output power density (kW/kg). To realize a large current capacity in REBa2Cu3Oy superconducting armature coils, we have proposed a cabling design for the transposed multi-strand parallel conductors. In this study, branch currents among eight- and 16-strand parallel conductors having only one transposition into a coil, were investigated. The analytical calculations showed approximately uniform branch current for the eight- and 16-strand conductors. According to the calculations, the deviations to the uniform branch current of eight-strand parallel conductors were ±7%. The approximately uniform current for the eight-strand conductors were also demonstrated experimentally by small-sized sample coils.

DOI： 10.1109/TASC.2024.3356436

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出版者・発行元：Journal of Physics: Conference Series  

The global demand for aircraft is currently increasing, paralleled by a growing emphasis on decarbonization through the replacement of fossil fuel engines with electric propulsion. In response to this trend, we have developed a superconducting motor distinguished by its high efficiency, output, and low weight. Our focus centers on two critical aspects. Firstly, we addressed the issue of the field coil's shape. The widely used REBCO wire in high-temperature superconductivity is tape-shaped, making a racetrack coil the most straightforward method for winding the wire. However, this configuration necessitates dividing the motor shaft as it comes in contact with the field coil, raising concerns about the mechanical durability of motors. To overcome this challenge, we introduce a saddle-type coil and design a coil shape that achieves a single continuous through-axis. Utilizing the Frenet-Serre formula, we created a 3D model of the coil's end without applying load to the wire. This method, previously employed in accelerators, enhances durability, potentially leading to increased output due to higher rotation speeds and reduces weight through part savings. Secondly, introducing a saddle coil presents challenges in forming an ideal magnetic field distribution. In a rotating-field-type motor using an iron core, the winding can be directly wound around it, resulting in multiple coils forming similar magnetic fields that can be superimposed to create one large magnetic pole. However, with a saddle coil, coils are arranged along the circumference, and the magnetic field distribution and an anticipated decrease in output. To address this, we aimed to optimize the magnetic field distribution and increase output through a parameter survey. Specifically, we conducted an analysis under fixed armature conditions (current, number of turns, and shape) in three cases. In Case1, with a fixed total number of turns for the field coil, we compared magnetic field distributions and output by varying the number and arrangement of coils. In Case2, building on the findings of Case1, we aimed to enhance output by fixing the coil arrangement and increasing the number of turns. Finally, in Case 3, we examined the change in motor size with fixed coil parameters while altering the coil arrangement and pitch.

DOI： 10.1088/1742-6596/2776/1/012012

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出版者・発行元：Journal of Physics: Conference Series  

The armature coil of the superconducting synchronous motor for electric aircraft requires a current of 1,000 A or more. However, a single superconducting tape at liquid nitrogen temperatures is insufficient for this operating current. Therefore, the armature coil should be composed of multi-strand parallel conductors. If the coil is constructed from parallel conductors, the inductance balance between the parallel conductors will be collapsed, so the current distributions will be non-uniform. As a solution for this issue, the uniform current distribution can be achieved by the transposition technique that changes the order in which tapes are stacked. Our previous research has developed the transposition method for the 2n-strand parallel conductors (n = 1, 2, 3, and so on). However, some of the superconducting synchronous motors are actually designed using a number of parallel conductors other than 2n. This study focused on the three- and the six-strand parallel conductors. We analytically clarified the unique transposition configuration for the three-strand parallel conductors that achieves uniform current distribution, and the transposition configuration for the six-strand parallel conductors by combining the transposition configuration for the three- and two-strand parallel conductors. As a result, the current distribution ratio became less than ±5% when the number of turns of the double pancake coil was a multiple of six by applying the transposition.

DOI： 10.1088/1742-6596/2776/1/012004

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出版者・発行元：Journal of Physics: Conference Series  

The estimation of AC loss in REBa2Cu3Oy (REBCO; RE = rare earth, such as Y, Eu, and Gd) superconducting tapes within superconducting synchronous motors and generators for electric aircraft is imperative during the design stage phase to optimize efficiency and cooling considerations. Our previous studies clarified the AC loss characteristics, focusing on estimating the AC loss in armature windings. While the magnetic field applied to the armature windings is exclusively as an AC field, the field windings are subject to a composite field comprising both AC and DC components. Notably, the amplitude of the AC component is markedly smaller than the magnitude of DC component. This study experimentally investigated to consider the AC loss properties in field windings. The experimental results were as follows: When subjected to operational conditions of field windings within synchronous motor and generator, the AC loss increased with the DC field strength. This property is attributed to a reduction in the critical current induced by the external field. Hence, we modified the method for estimating AC loss. Subsequently, we validated the effectiveness of the modified method.

DOI： 10.1088/1742-6596/2776/1/012003

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記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：IEEE Transactions on Applied Superconductivity  

To realize a large current capacity in REBa2Cu3Oy superconducting coils, we have proposed a cabling design for the transposed multi-strand parallel conductors. This cabling design is also useful for a superconducting armature coil in a fully superconducting rotating machine system. Very recently, we developed an analytical method based on simple modeling, to roughly calculate the current distributions among the continuous strands in rotating machine conditions. In this manuscript, the current distributions predicted by the analytical method were compared with the measured values. Further, to understand the complex current distribution property for the three-strand parallel conductors, the turn number dependence of the current distribution with various transposition combinations was investigated. The current branches were measured under liquid nitrogen temperature of 77.3 K by Rogowski coils by applying AC transport current. The analytical method described the complex current distribution of the strands well, especially the coil with a large number of turns.

DOI： 10.1109/TASC.2023.3241824

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記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：IEEE Transactions on Applied Superconductivity  

We propose the concept of uniform current between groups (CUCG) in superconducting armature coils composed of two or more strands of transposed parallel conductors. We validate the proposed concept by measuring the current distributions in several double pancake test coils comprising two and four parallel conductors using Rogowski coils by applying AC signals of varying frequencies and amplitudes. Accordingly, uniform transport currents across four strands were confirmed. The proposed concept produced uniform current distributions across four groups of parallel conductors with a small number of transpositions. This work paves the way for developing armature coils composed of more than eight strands in parallel conductors by introducing a small number of transpositions based on CUCG.

DOI： 10.1109/TASC.2023.3242621

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記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：IEEE Transactions on Applied Superconductivity  

We are currently developing superconducting electric propulsion armature coils which utilize REBa2Cu3Oy (REBCO) strands. To implement the REBCO coils in aircraft, the voltage must be reduced for safety concerns by using high-current capacity. Parallel conductors can be used to expand the current capacity, but the current cannot flow equally through each strand owing to the slight difference in inductance. We have investigated transposed parallel conductors which can almost equalize the inductance of each conductor. Conversely, the distributed windings for the armature coils can improve the magnetic field waveform and transform it into a sine wave by reducing the distortion of the magnetic flux density about the rotating magnetic field. However, it is difficult to implement a distributed armature winding using the REBCO strands since it is tape-shaped and difficult to twist. Therefore, we propose a distributed armature winding which combines two types of racetrack double pancakes. Each double pancake comprises two-strand insulated REBCO superconducting parallel conductors transposed inside the pancake for current equalization. In this study, we perform an experimental demonstration of the current equalization based on the transposition of the distributed armature winding. We made coils with two-strand as a simplified, but the actual coils were needed more 10 strand parallel conductors. In future works, we will manufacture transposed parallel conductor coils using much more strands and measure the current distribution. Additionally, this study measured the current distribution with only the stater. Thus, we will combine the rotor and measure the magnetic field distribution of the rotating magnetic field in the armature.

DOI： 10.1109/TASC.2023.3241822

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記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Superconductor Science and Technology  

Fully superconducting rotating machines employing REBa2Cu3O y (REBCO, RE = rare earth elements or Y) superconducting armature and field coils, are particularly interesting for aircraft applications, owing to their high output power density (kW kg−1). To achieve high current capability in superconducting coils, we have proposed a cabling design for transposed multi-strand parallel conductors. In the parallel conductor design, the REBCO strands are insulated from each other, except for both terminal ends, and transposed during the winding process to achieve uniform current distribution by cancellation of interlinkage magnetic flux between the strands. In this study, a simplified analytical method considering inductances was developed based on Laplace’s equation in cylindrical coordinates to roughly calculate the current distributions of multi-strands under armature coil conditions. The validity of the analytical method was investigated through current distribution measurements of the sample coils wound with two-strand parallel conductors. Consequently, the analytical method was validated with approximately 10% deviation under the experimental coil conditions. To establish a more accurate analysis method, certain improvements are needed.

DOI： 10.1088/1361-6668/acca4f

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記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：IEEE Transactions on Applied Superconductivity  

A large AC loss is generated in armature windings composed of a REBa₂Cu₃O_y (REBCO, RE = rare earth, Eu, Gd, etc.) wire in a fully superconducting synchronous generator. In aircraft applications, liquid nitrogen is suitable for cooling windings because of its specific heat capacity and dielectric strength. The evaporation of liquid refrigerant significantly decreases the dielectric strength, which results in an increase in the risk of accidents by discharging. For safe operation, the temperature in the generator is set below the boiling point of nitrogen (~77 K). We calculated the temperature rise caused by the AC loss and others by applying a method that estimates the AC loss from the experimental results. This paper addresses the electromagnetic-thermal analysis in finite element method to calculate the temperature rise and proves the feasibility of a fully superconducting generator cooled by liquid nitrogen, that is, the temperature is maintained below 77 K. The output power is 10 MW. The temperature of liquid nitrogen was assumed to be 65 K, and its flow speed was set as a parameter. Additionally, we confirmed that designing a method that determines the number of parallel conductors of REBCO windings considering the temperature rise can enhance the output power density.

DOI： 10.1109/TASC.2022.3160660

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記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：IEEE Transactions on Applied Superconductivity  

There is an ongoing plan to upgrade the 25 T cryogen-free superconducting magnet (25T-CSM), at the High Field Laboratory for Superconducting Materials (HFLSM), IMR, Tohoku University, to a 30T-CSM by replacing the existing Bi2223 insert coils with REBCO coils. For the REBCO coils of the 30T-CSM, we will adopt a robust coil concept, which consists of two tapes co-wound in a face-to-back configuration with thin FRP plates glued on the edges. To confirm the effectiveness of this concept, we performed hoop stress tests on full-scale-size four stacked pancake coils fabricated with Fujikura EuBCO tapes at conduction-cooled 20 K under a background field of 11 T. The inner and outer diameters of the pancake coils were 68 and 268 mm, respectively. The averages of the measured strains on the outermost winding were 0.17-0.24% at a hoop stress of 400 MPa estimated by the BJR relation. The I-V properties of the coils indicated no anomalous voltage, implying that our coil structure was robust against large electromagnetic stress.

DOI： 10.1109/TASC.2022.3166469

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記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：IEEE Transactions on Applied Superconductivity  

For extensive use in electric vehicles, electric airplanes, and other such applications, superconducting motors are designed to be lightweight and have a high power density. Rare-earth barium copper oxide (REBCO) tapes are considered a potential material for use in these motors because of their high critical current density. Because REBCO can be modeled into tape shapes, motors with racetrack coils as armature windings have been studied. However, racetrack coils are inferior in terms of increasing the interlinkage flux to the rotor compared with distributed windings. In this study, we propose trapezoidal REBCO tape coils for shaping armature windings into distributed windings. Next, 2 kW-class induction motors using two types of coils - the trapezoidal and racetrack coils - were designed to compare the characteristics of the two motors. The JMAG-Designer software was used to perform electromagnetic simulations on the two models. The results revealed that the motor with trapezoidal coils required less REBCO tapes and exhibited a lower vertical component of magnetic flux density than the racetrack coils did. Therefore, the AC loss of the trapezoidal coils becomes lower than that of the racetrack coils, and the trapezoidal coils weigh less than racetrack coils.

DOI： 10.1109/TASC.2022.3160146

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記述言語：その他   掲載種別：研究論文（その他学術会議資料等）   出版者・発行元：Journal of Physics: Conference Series  

Generators used in electric aircraft require a high-power density, and AC loss is also a significant problem. We designed 10 MW REBCO superconducting synchronous generators at 64 K to achieve a power density of 20 kW/kg and low AC loss. In this study, electromagnetic analyses were performed using finite element method software. Consequently, the thickness of the back yoke is 50 mm or less so that the generator can meet the target power density. The method of decreasing the magnetic field of the armature winding and increasing that of the field winding was used to effectively reduce the AC loss. As a result, the generator achieved a high-power density of 21.0 kW/kg, reducing the AC loss from over 600 kW to 415 kW.

DOI： 10.1088/1742-6596/2323/1/012037

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記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：IEEE Transactions on Applied Superconductivity  

We investigated current distribution in three-strand transposed parallel conductors consisting of REBa₂Cu₃O_y (REBCO, RE = rare earth) superconducting tapes under applied AC magnetic fields. We recently derived the theoretical expression for the additional AC loss caused by the formation of three-strand transposed parallel conductors and proved its validity through experiments. However, the conditions of each strand and current distribution have not yet been discussed. Accordingly, we measured the current distribution and compared it with the calculated distribution derived using the theoretical expression. From the results, we found that the current branch ratio of the three strand can also be predicted via the theoretical expression.

DOI： 10.1109/TASC.2022.3154338

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記述言語：その他   掲載種別：研究論文（学術雑誌）  

The upgrade to 30 T of the existing 25 T cryogen-free magnet (25 T CSM) at the High Field Laboratory for Superconducting Materials, Tohoku Univ., will consist in replacing the existing High Temperature Superconductor insert made of Bi-2223 by one wound using REBCO tape. The new insert is designed to produce 16 T instead of 11 T. In order to have good dynamics and allow sweeping mode experiment, the insert will be conventionally isolated, as it was the case for the previous Bi-2223 version. In previous work we modelled thermal runaway phenomenon and showed that a sensitive dissipative voltage detection technique can give sufficient warning to protect an HTS coil using external dump resistor even in the case of sharp local critical current drop. For the 30 T insert, the conductor will consist of a two tape bundle so as to reduce the chance of such sharp Ic drop due to tape performances inhomogeneities or local damage. We simulate the quench dynamics in the LTS outsert coupled with the HTS insert to demonstrate that the existing detection/protection system of the 25 T CSM can be re-used and determine the current dumping dynamics. We then use our thermal runaway modelling tool to study the behavior of the future 16 T REBCO insert in case of local sharp defect and determine realistic detection threshold for its safe protection.

DOI： 10.1109/TASC.2021.3059604

記述言語：その他   掲載種別：研究論文（学術雑誌）  

The upgrade project of the 25T-CSM to 30 T is on-going at HFLSM, IMR, Tohoku University. The goal is to obtain 30 T in a 32 mm warm bore by replacing the present Bi2223 insert coil with one wound using REBCO. In order to develop a robust REBCO high field insert, we adopt a two-tape bundle conductor with a face-to-back configuration. We confirmed that this conductor configuration mitigates effectively the risk posed by local defects in an R&D coil: a stable operation was achieved with negligible degradation in coil Ic even with a severe local defect on one of the two tapes. Quench protection from possible hotspot in the REBCO insert using an early detection method is made possible by the two-tape bundle configuration and the thick Cu stabilizer. Other R&D coils using such conductor were tested with large hoop stresses up to 460 MPa with no degradation. On the basis of these R&D studies, a robust REBCO insert is designed for the upgrade. The designed REBCO insert has a capability to achieve 35 T with enough Ic margin if we accept a maximum hoop stress of 500 MPa.

DOI： 10.1109/TASC.2021.3061896

記述言語：その他   掲載種別：研究論文（学術雑誌）  

An upgrading of the 25 T cryogen-free superconducting magnet (CSM) at the HFLSM to a 30T-class CSM is considered by replacing the present Bi2223 insert coil with a REBCO ones, since REBCO tapes have better mechanical and in-field critical current properties. In our strategy for the REBCO coil of the 30T-CSM, two tapes co-winding technique will be adopted in order to increase the space current density in the coil and to avoid a hotspot generation due to local degradation. Electromagnetic and mechanical properties of two bundle co-wound (two-ply) pancake coils are investigated as an R&D test coil for the 30T-CSM. The R&D coils were fabricated with Fujikura REBCO tapes that have 4.1 mm width and 0.11 or 0.15 mm thickness depending on the coil. The coil voltages and strains were measured at 4.2 K under a background field of 11 T. The maximum hoop stress up to 460-530 MPa at the outermost winding estimated by the BJR relation was applied, and no degradation by the hoop stress was found.

DOI： 10.1109/TASC.2021.3069685

記述言語：その他   掲載種別：研究論文（学術雑誌）  

A coil without turn-to-turn insulation, called a no-insulation (NI) coil has been developed. The NI winding technique has been reported to be a promising method of quench protection. In order to apply the NI winding technique to a conduction-cooled REBCO coil, we developed a coil using an electrically conductive epoxy resin. The conductive epoxy resin, in which silver powder was mixed, was applied to the edge of the winding. If thermal runaway of the coil is observed, the excessive current could be automatically bypassed through the conductive epoxy resin. In order to confirm the effect of a larger coil with higher stored energy when using the conductive epoxy resin, we fabricated and tested a stacked coil composed of six single pancakes whose inner diameters and outer diameters were 501 mm and 567 mm, respectively. The coil was tested in liquid nitrogen and under conduction cooling conditions. When the operating current was 256 A at 40 K, the stored energy was 29 kJ, and burn-out of the coil could be avoided. These results show that thermal runaway can be avoided in larger coils with higher stored energy by using conductive epoxy resin.

DOI： 10.1109/TASC.2020.2991260

記述言語：その他   掲載種別：研究論文（その他学術会議資料等）  

High-Temperature superconducting magnets are generally driven by a power supply (driven mode), and temporal fluctuations of the power supply unit, such as ripple noise, directly affect the magnetic field stability. A method of suppressing such temporal instability by short-circuiting both ends of the coil with finite low-resistance joints, somewhat similar to a persistent current switch, has been proposed in order to form a closed loop having a large time constant, L/R. In this research, we developed two types of REBCO superconducting switches was constructed with a non-inductive winding using mainly ordinary copper-coated REBCO tapes, and type-B switch used uncoated REBCO tapes toward application to emergency shutdown. For the type-B switch, at the end of the winding, copper-coated REBCO tapes were jointed to prevent degradation by exposure to moisture. The R-T and V-I characteristics of each switch when cooled with liquid nitrogen were tested. The type-A switch was applied to a test REBCO magnet, and the temporal stability of the magnetic field was evaluated under a conduction cooling configuration in the driven mode.

DOI： 10.1088/1742-6596/1559/1/012114

記述言語：その他   掲載種別：研究論文（学術雑誌）  

When superconducting magnets are operated in a conduction-cooling system, if a malfunction of the cryocooler or an abnormality in the heat transfer path occurs, the temperature of the coil increases. In most cases, the operating temperature limit is designed so that thermal runaway of the coil can be prevented by charging-down or shutdown of the magnet. Therefore, the temperature margin should be considered carefully. In this research, a bypass circuit for a REBCO coil was developed using the flux flow resistance generated inside the coil when the load factor rises. Low resistors were connected to both ends of a test coil composed of a stack of four single pancakes wound with 4-mm-wide REBCO tape having an inner diameter of 200 mm and an outer diameter of 291 mm in the cryostat. Automatic suppression of the current flowing through the coil at high load factor was attempted. In this report, the test and simulation results of the coil operation when the cryocooler was intentionally stopped under conduction-cooling conditions are compared and discussed.

DOI： 10.1109/TASC.2019.2906978

記述言語：その他   掲載種別：研究論文（学術雑誌）  

Since REBa2Cu3O7-δ (REBCO) coils have high thermal stability, the possibility of quenching is considerably lower than low-temperature-superconducting coils. However, if thermal runaway is observed in a conduction-cooled system, a REBCO coil will almost certainly be burned out. A recently reported technique of producing a REBCO coil without turn-to-turn insulation, called a no-insulation coil, is a promising method of quench protection. In order to apply the NI winding technique to a conduction-cooled REBCO coil, a coil using electrically conductive epoxy resin was developed. If thermal runaway is observed, the excessive current can be automatically bypassed through the conductive epoxy resin. We fabricated a single pancake coil wound with REBCO-coated conductor using conductive epoxy resin and tested it at 40 K. Although the coil voltage increased rapidly at 415 A, the current was automatically bypassed through the conductive epoxy resin. Before and after an over-current test under conduction-cooled conditions, the coil was tested in liquid nitrogen. The voltage-current curve of the coil was the same as the one obtained before the over-current test, which indicates that damage due to thermal runaway can be avoided by using conductive epoxy resin.

DOI： 10.1109/TASC.2019.2909795

記述言語：その他   掲載種別：研究論文（学術雑誌）  

Magnetic field stability and hysteresis can be serious problems in a high-field high-temperature superconducting magnet, because of the large shielding current effect. Accordingly, the field stability of a 25 T cryogen-free superconducting magnet (25T-CSM) was investigated. This particular system can generate a central field of 24.6 T in a 52-mm room-temperature bore. It was found that the magnetic field relaxation rate of the 25T-CSM with a Bi2Sr2Ca2Cu3Oy (Bi2223) insert was due to the flux creep effect, and was small enough to allow nuclear magnetic resonance (NMR) experiments. Based on these experiences in the development of the 25T-CSM, primitive designs for a 30 T cryogen-free superconducting magnet (30T-CSM) were suggested based on upgrading the present Bi2223 insert to a REBa2Cu3Oy (RE123, RE: rare earth and Yttrium) insert incorporating two co-wound RE123 tapes. It was confirmed that the prototype test coil could be operated at 4.2 K under large electromagnetic stresses of up to 540 MPa without experiencing degradation.

DOI： 10.1109/TASC.2019.2898699

記述言語：その他   掲載種別：研究論文（学術雑誌）  

An R&D project funded by the Japan Science and Technology Agency under its S-Innovation Program is currently in progress. The purpose of this project is to develop fundamental technologies for accelerator magnets with REBCO coils. In the first step of this project, winding technology for fabricating REBCO coils required for HTS accelerator magnets was developed. At the last stage of this project, there are plans to fabricate cryocooler-cooled REBCO magnets and evaluate the beam guiding characteristics and stability against beam injection to the magnets at the beam line of Heavy Ion Medical Accelerator in Chiba (HIMAC). This paper describes the design of an HTS magnet fabricated for this evaluation and presents fabrication results of this magnet. The magnet had a room-temperature beam duct with dimensions 264× 40 mm for beam guiding. The magnet consisted of a pair of racetrack coil stacks each containing four coils, and a cold iron was adopted to increase the magnetic field strength. It could generate a dipole field of 2.4 T at the beam duct by 200 A excitation. Furthermore, beam windows were placed on the side of the cryostat for beam injection tests, allowing the beam to irradiate the REBCO coil directly.

DOI： 10.1109/TASC.2019.2906406

記述言語：その他   掲載種別：研究論文（学術雑誌）  

In order to use a coil wound with REBCO-coated conductor in a conduction cooling configuration, ac loss of the coil should be quantitatively predicted and designed under the operating conditions, for example during charging-up and -down, or shutdown operation. A tape-shaped REBCO-coated conductor is suited to be composed of a stack of single pancake coils for generating high magnetic field. Various researchers have measured and evaluated the ac loss of REBCO tape; however, there are few reports about the ac loss of the coil as measured at liquid nitrogen temperature. In this report, the ac loss of REBCO coil composed of a stack of 22 single pancakes with 50 mm in inner diameter, 132 mm in outer diameter, and 105 mm in height, generating 13.5T at 10 K, was measured by electrical method and evaluated with conduction cooling configuration. The experimental results were compared with the calculated results and discussed.

DOI： 10.1109/TASC.2018.2809601

記述言語：日本語  

<p>A project for developing of RE₁Ba₂Cu₃O_7-&delta; (REBCO) magnets to be utilized in ultrahigh-field magnetic resonance imaging (MRI) was started in 2013. Our final targets are 9.4 T MRI systems for whole-body and brain imaging. In this project, two different development approaches towards the final target were planned. One is a small REBCO coil that can generate 10 Tclass magnetic fields, which is the same level as the target magnetic field. The other is a conduction-cooled 1.5 T REBCO MRI magnet that has a room-temperature bore of 396 mm, which is as large as those of mid-sized model magnets. These results were reflected in the design of a conduction-cooled 9.4 T REBCO magnet for whole-body MRI systems.</p>

DOI： 10.2221/jcsj.52.224

記述言語：その他   掲載種別：研究論文（その他学術会議資料等）  

Heavy-ion radiotherapy has a high curative effect in cancer treatment and also can reduce the burden on patients. These advantages have been generally recognized. Furthermore, a rotating gantry can irradiate a tumor with ions from any direction without changing the position of the patient. This can reduce the physical dose on normal cells, and is thus commonly used in proton radiotherapy. However, because of the high magnetic rigidity of carbon ions, the weight of the rotating gantry for heavy-ion therapy is about three-times heavier than those used for proton cancer therapy, according to our estimation. To overcome this issue, we developed a small and lightweight rotating gantry in collaboration with the National Institute of Radiological Sciences (NIRS). The compact rotating gantry was composed of ten low-temperature superconducting (LTS) magnets that were designed from the viewpoint of beam optics. These LTS magnets have a surface-winding coil-structure and provide both dipole and quadrupole fields. The maximum dipole and quadrupole magnetic field of the magnets were 2.88 T and 9.3 T/m, respectively. The rotating gantry was installed at NIRS, and beam commissioning is in progress to achieve the required beam quality. In the three years since 2013, in a project supported by the Ministry of Economy, Trade and Industry (METI) and the Japan Agency for Medical Research and Development (AMED), we have been developing high-temperature superconducting (HTS) magnets with the aim of a further size reduction of the rotating gantry. To develop fundamental technologies for designing and fabricating HTS magnets, a model magnet was manufactured. The model magnet was composed of 24 saddle-shaped HTS coils and generated a magnetic field of 1.2 T. In the presentation, recent progress in this research will be reported.

DOI： 10.1088/1742-6596/871/1/012083

記述言語：その他   掲載種別：研究論文（学術雑誌）  

A conduction-cooled REBCO racetrack coil that is stored and impregnated in a stainless steel case is proposed for mechanical reinforcement of the straight parts. Two racetrack coils composed of a stack of four single pancakes wound with two kinds of the REBCO tapes were fabricated and tested using conventional and new advanced conduction cooling systems, respectively. As the latter a more simplified cooling structure for a REBCO coil without the need for a conventional thermal radiation shield is currently under development. One challenge is restricting the flow of heat to the coil when the cryocooler stops due to an unexpected electrical and/or mechanical breakdown. In this research, the stainless steel case was thermally connected to a single-stage GM cryocooler. Most of the heat load was received by the case, and thermal resistance layers were inserted between the case and the coil surface for controlling the flow of heat into the coil. The REBCO coil was cooled indirectly and operated successfully without any degradation and the temperature rise when the cryocooler stopped was evaluated by experiments and calculations.

DOI： 10.1109/TASC.2017.2674959

記述言語：その他   掲載種別：研究論文（学術雑誌）  

Superconducting magnets for magnetic resonance imaging systems require homogeneous, stable magnetic fields. The homogeneity of the magnetic field is highly dependent on the coils shape and positions. Furthermore, in REBa2Cu3O7-δ (REBCO) magnets, the screening-current-induced magnetic field, which changes the magnetic field distribution of the magnet, is one of the most critical issues. In the work described here, a small REBCO coil that can generate a magnetic field of over 10 T was fabricated and tested. The small REBCO coil was composed of a stack of 22 single pancake coils whose inner and outer diameters were 50 mm and 130 mm, respectively. The total conductor length was 1.5 km. The small REBCO coil could generate a central magnetic field of 8.27 T at 10 K. A magnetic field of over 10 T was achieved by replacing 12 of the single pancake coils with new ones wound with the latest REBCO-coated conductors. The measured value of the central magnetic field at 10 K was 13.53 T, which was lower than the calculated value of 13.94 T obtained without taking account of the screening-current-induced magnetic field. Furthermore, it was examined whether the current sweep reversal would be effective for 10 T-class magnetic field conditions.

DOI： 10.1109/TASC.2017.2657636

記述言語：その他   掲載種別：研究論文（学術雑誌）  

We started developing REBa2Cu3O7-δ (REBCO) magnets for ultrahigh-field magnetic resonance imaging (MRI) systems in 2013. Our final targets are 9.4 T MRI systems for whole-body and brain imaging. In this paper, a conduction-cooled 1.5 T REBCO MRI magnet having a room-temperature bore of 396 mm was fabricated and tested in order to evaluate the magnetic field homogeneity and stability. The magnet was composed of 60 single pancakes whose inner diameter was 500 mm. The total conductor length was 10.3 km, and the total inductance was 12.4 H. The size of the homogeneous magnetic field region was 200 mm diameter spherical volume. The central magnetic field was as high as 1.5 T at 192.7 A, and the current density of single pancakes was 301 A/mm 2. The magnet was cooled from room temperature to 4.7 K in 55 hours, and the temperature difference among the coils was 0.1 K or less during both initial cooling and excitation. The magnetic field inhomogeneity was 249.7 parts per million (ppm), and the Z2 coefficient was largest at 579.6 ppm. The main reason for the error magnetic field was dimensional errors in the positions on the z-axis. The magnetic field inhomogeneity was improved to 4.1 ppm by passive shimming using iron pieces. The magnetic field stability was about 2 ppm/h because of a reduction in screening-current induced in the REBCO-coated conductors. Current sweep reversal improved the magnetic field stability from 2 ppm/h to 0.8 ppm/h.

DOI： 10.1109/TASC.2017.2656858

記述言語：その他   掲載種別：研究論文（学術雑誌）  

In recent years, high-field magnets wound with REBCO tape and their applications to magnetic resonance imaging (MRI) have undergone considerable research and development. MRI coil systems require a highly accurate field in the imaging space. However, in REBCO coils, the irregular magnetic field generated by the screening current is induced by the radial component of the magnetic field in the REBCO tape, and may deteriorate the field quality both spatially and temporally. Therefore, a design process with consideration for the effect of the screening-current field is required for the reduction and correction of this field for a high-accuracy-field REBCO coil. In this study, the screening-current distribution in the REBCO tape and the irregular magnetic field were analyzed by using the simulation we developed for a REBCO model coil system for whole-body MRI. We additionally discuss the effect of the screening-current field in the REBCO coil based on the coil shape and arrangement to use it as feedback to the design stage.

DOI： 10.1109/TASC.2016.2645839

記述言語：その他   掲載種別：研究論文（学術雑誌）  

A cryogenic cooling system for a 25-T cryogen-free superconducting magnet (25T-CSM) was developed. The 25T-CSM consisted of 11-T high-Tc superconducting (HTS) coils and 14 T low-Tc superconducting (LTS) coils. The HTS coils, which had about 10-W AC-loss, were cooled to about 10 K by two GM cryocoolers and a helium gas circulation unit. In this system, the gas flow rate had an optimized value and was controlled by a mass flow controller. The LTS coils were cooled to about 4 K by two GM/JT cryocoolers. Each GM/JT cryocooler had two sets of precooling gas lines. These pre-cooling lines were cooled by two single-stage GM cryocoolers and the HTS coil cooling system via a heat exchanger, between the HTS gas circulation line and the LTS cool-down line. As a result of cooling test of the 25T-CSM, the LTS coils and HTS coils were cooled from room temperature to 4.3 K and 4.6 K within 164 h (about seven days), respectively. During the excitation test of the HTS coils, their maximum temperature increased to 7.6 K, which was still sufficiently low.

DOI： 10.1109/TASC.2017.2673762

記述言語：その他   掲載種別：研究論文（学術雑誌）  

A 25 T cryogen-free superconducting magnet (25T-CSM) was developed and installed at the High Field Laboratory for Superconducting Materials (HFLSM), IMR, Tohoku University. The 25T-CSM consists of a high-temperature superconducting (HTS) coil and a low-temperature superconducting (LTS) coil. A high-strength CuNb/Nb3Sn Rutherford cable with a reinforcing stabilizer CuNb composite is adopted for the middle LTS section coil. All the coils were impregnated using an epoxy resin for conduction cooling. Initially, a GdBa2Cu3Oy (Gd123) coil was designed as the HTS insert coil, and then a Bi2Sr2Ca2Cu3Oy (Bi2223) coil was also developed. The HTS insert and the LTS (CuNb/Nb3Sn and NbTi) outsert coils are cooled by two 4K GM and two GM/JT cryocoolers, respectively. The LTS coils successfully generated a central magnetic field of 14 T at an operating current of 854 A without any training quench. The Gd123 coil generated 10.15 T at an operating current of 132.6 A in the absence of a background field. Subsequently, the operating current of the Gd123 insert was increased in a step-by-step manner under a background field of 14 T. The Gd123 coil could be operated up to 124.0 A stably, which corresponds to 23.55 T, but quenched at around 124.6 A (23.61 T). The Bi2223 insert coil using a Ni-alloy reinforced Bi2223 tape successfully generated 11.48 T at an operation current of 204.7A in a stand-alone test and 24.57 T in a background field of 14 T. The differences between the calculated and the measured values of the central magnetic fields are about 0.4 T for the Gd123 insert and 0.1 T for the Bi2223 insert around 24 T.

DOI： 10.1088/1361-6668/aa6676

記述言語：その他   掲載種別：研究論文（その他学術会議資料等）  

High temperature superconducting (HTS) wires are high aspect ratio cross-section tapes. Therefore, the applications of HTS tapes lead to various problems such as AC loss, non-uniform current, and screening current. In practice, we must investigate the detail of current distribution in HTS tapes using the electromagnetic analysis technique dealing with high aspect ratio tape and nonlinearity of superconductor. We developed the numerical simulation code of electromagnetic field analysis in HTS tape using a finite element method (FEM) and fast multipole method (FMM).

DOI： 10.1109/CEFC.2016.7816041

記述言語：その他   掲載種別：研究論文（その他学術会議資料等）  

The development of ultra-high-field (∼10 T magnetic resonance imaging (MRI) device is underway. It is, therefore, required to achieve the extremely high field homogeneity at a magnet design stage, where the field homogeneity is evaluated using the Legendre polynomials. Commonly, it is supposed that the current uniformly flows on the cross section of coils. However, for REBCO magnet, the no-current-flowing region has to be taken into account, such as a copper stabilizer. In addition, the REBCO layer inside REBCO tape is very thin, 1-2 μm, so the current-flowing region is 1/100 of the REBCO tape area. Therefore, it is necessary to consider that the current flows only in the REBCO layer, for accurate evaluation of field homogeneity.

DOI： 10.1109/CEFC.2016.7815982

記述言語：その他   掲載種別：研究論文（学術雑誌）  

A project on the development of REBa2Cu3O7-δ (REBCO) magnets for ultra-high-field magnetic resonance imaging (MRI) was started in 2013. Since REBCO-coated conductors feature high mechanical strength under tensile stress and high critical current density, use of REBCO coils would allow superconducting magnets to be made smaller and lighter than conventional ones. In addition, a conduction-cooled superconducting magnet is simpler to use than one cooled by a liquid helium bath because the operation and maintenance of the cryogenic system become simpler, without the need to handle cryogenic fluid. Superconducting magnets for MRI require homogeneous, stable magnetic fields. The homogeneity of the magnetic field is highly dependent on the coil shape and position. Moreover, in REBCO magnets, the screening-current-induced magnetic field, which changes the magnetic field distribution of the magnet, is one of the critical issues. In order to evaluate the magnetic field homogeneity and the screening-current-induced magnetic field, a 1 T model magnet and some test coils were fabricated. From an evaluation of the 1 T model magnet, it was found that the main reason for magnetic field inhomogeneity was the tolerances in the z-axis positions of the coils, and therefore, it is important to control the gap between the single pancakes. In addition, we have already demonstrated the generation of an 8.27 T central magnetic field at 10 K with a small test coil. The screening-current-induced magnetic field was 0.43 T and was predictable by using an electromagnetic field simulation program. These results were reflected in the design of a conduction-cooled 9.4 T REBCO magnet for whole-body MRI systems. The magnet was composed of six main coils and two active shield coils. The total conductor length was 581 km, and the stored energy was 293 kJ. The field inhomogeneity was 24 ppm peak to peak and 3 ppm volume-root-mean-square (VRMS) for a 500 mm diameter spherical volume (DSV). The axial and radial 5 gauss line locations were less than 5 m and 4.2 m respectively.

DOI： 10.1088/0953-2048/29/10/104001

記述言語：その他   掲載種別：研究論文（学術雑誌）  

Coil systems are fabricated with various combinations of multiple coils in order to generate homogenous fields for NMR and MRI applications. In multiple coil systems using REBCO coils, the behavior of the screening current, which deteriorates field homogeneity, depends on the patterns of the transport current, the charging sequence of the coils, and the various resulting magnetic fields. It is therefore important to investigate and clarify the behavior of the irregular magnetic field and screening current in multiple coil systems under different operating environments. In a previous study, we experimentally and numerically clarified the different behaviors of irregular magnetic fields based on the charging sequence of coils in multiple-model REBCO coils [1], [2]. In this paper, we constructed the multiple-model REBCO coils in order to obtain the fundamental data for reducing or improving the resulting irregular magnetic field. Furthermore, the distribution of the irregular magnetic field in terms of the screening current was measured by using the reversal current method [3], [4]. We also calculated the spatial and temporal behavior of the irregular magnetic field by using our developed 3-D numerical simulation. Based on the experimental and numerical results, we discuss the behavior of the magnetic field in terms of the screening current when using the current sweep reversal method in multiple-model REBCO coils.

DOI： 10.1109/TASC.2016.2542980

記述言語：その他   掲載種別：研究論文（学術雑誌）  

A cryogen-free 25-T superconducting magnet with a 52-mm room temperature bore consists of an 11-T &#36; ext{REBa}-{2} ext{Cu}-{3} ext{O}-{7-delta}&#36; (REBCO) insert coil and outer 14-T lowerature superconducting (LTS) coils. The REBCO insert coil is composed of a stack of 56 single pancake coils. The inner and outer diameters of the REBCO insert coil are 102 and 263 mm, respectively, and the total conductor length is about 14 km. The REBCO insert coil is cooled by circulating helium gas using both single-stage and two-stage Gifford-McMahon cryocoolers, and the LTS coils are cooled by two Gifford-McMahon/Joule-Thomson cryocoolers. The maximum hoop stress of the REBCO insert coil was estimated to be 387 MPa when the central magnetic field was 25 T. If thermal runaway occurs in a conduction-cooled system, the REBCO coils will almost certainly be burned out. Therefore, the coil should have no damaged area in the winding because a damaged area would generate heat locally, eventually resulting in thermal runaway. In order to avoid the degradation of the coil due to thermal stress, the coil was divided into all winding parts in the radial direction. In practice, the coil was divided by inserting a polyimide tape coated with a fluorine resin, which has low adhesive strength, into the epoxy resin at every turn. Before fabricating and testing the actual REBCO insert coil, two kinds of coils were fabricated and tested in order to evaluate the validity of the winding method. Two stacked pancake coils wound with REBCO-coated conductors were fabricated and tested in liquid helium with a 4-T background magnetic field in order to evaluate whether the coil could withstand the hoop stress of over 387 MPa. Moreover, we fabricated and tested a demonstration coil which was composed of a stack of four single pancake coils with the same size as the pancake coils used for the REBCO insert coil in order to evaluate whether the coil could withstand the thermal stress and the electromagnetic force.

DOI： 10.1109/TASC.2016.2530082

記述言語：その他   掲載種別：研究論文（学術雑誌）  

In this paper, normal zone in a REBCO single-pancake coil induced by quenches in a low-temperature superconducting outsert coil is investigated in both experimental and numerical methods to discuss the quench protection of REBCO coils. The normal zone is experimentally clarified by measuring a partial voltage in the REBCO coil at temperatures from 30 K to 50 K during quenches of the outsert coil charging 5 T. Using practical fittings for the physical properties of the components of the REBCO coil in ranges of 0-5 T and 10-77 K, the numerical results have good agreement with the experimental results. The simulation reveals the heating distribution with initial temperatures from 10 K to 65 K, which indicates that the maximum temperature in a hot spot becomes higher for the lower initial temperature. Unexpectedly, the temperature distribution becomes rather homogeneous with an initial temperature of 65 K. It suggests that the hot spot at higher temperature has a lower risk of burning out. Finally, possible coil protections are discussed as the operation at the high temperature or the quench heater method to keep the REBCO coil at high temperature when a hot spot appears.

DOI： 10.1109/TASC.2016.2540918

記述言語：その他   掲載種別：研究論文（学術雑誌）  

An RandD project on higherature superconducting (HTS) magnets using (RE)Ba2Cu3O7 (REBCO; RE=rare earth) wires for ultrahigh-field (UHF) magnetic resonance imaging (MRI) systems is described. Our targets are 9.4-T MRI systems for whole-body imaging and brain imaging. REBCO wires are promising components for UHF-MRI because REBCO wires have high critical current density in high magnetic fields and high strength against hoop stresses, which allows MRI magnets to be smaller and lighter than conventional ones. The aim of the project is to establish basic magnet technologies for adapting REBCO coils for UHF-MRI. The project term is three years, and this year is the final year. We have already demonstrated the generation of an 8.27-T magnetic field with a small test coil composed of 22 REBCO pancake coils. A magnetic field spatial distribution with inhomogeneity of several hundreds of parts per million within 100-mm diameter spherical volume (DSV) was demonstrated with a 1-T model magnet. A stable magnetic field of a few parts per million per hour was also demonstrated with the 1-T model magnet. The targets of the project, to be achieved by March 2016, are to demonstrate the generation of a 9.4-T field with the small REBCO coil, and to demonstrate a homogeneous magnetic field in 200-mm DSV with a 1.5-T magnet having three pairs of split coils. Imaging will be performed with the 1.5-T magnet.

DOI： 10.1109/TASC.2016.2535863

記述言語：その他   掲載種別：研究論文（学術雑誌）  

In a REBCO superconducting magnet for magnetic resonance imaging (MRI) applications, the large screening current induced by a magnetic field perpendicular to the winding tape is a serious problem: it generates irregular magnetic field that deteriorates the spatial field homogeneity and temporal field stability in MRI applications. Therefore, we need to estimate the magnetic field generated by the screening current using electromagnetic field analysis to design and develop a REBCO magnet that generates highly accurate and stable magnetic field. Therefore, to examine the influence of the magnetic field generated by the screening current, we constructed cryocooler-cooled small bore-coil stacked pancake REBCO coils with an inner diameter of 50 mm. This coil generates a magnetic field of 10 T at the center. In this study, we report the evaluation of the current distribution and magnetic field in the REBCO coil using the developed 3-D numerical simulation code for electromagnetic field analysis in a REBCO tape using the finite-element method and fast multipole method. The numerical results agree well with the experimental results. Furthermore, we discuss the current distribution in the REBCO tape and the spatial and temporal behavior of the magnetic field on the basis of the results of the experiments and numerical simulation.

DOI： 10.1109/TASC.2016.2535965

記述言語：その他   掲載種別：研究論文（学術雑誌）  

A REBCO-coated conductor has a tape shape and is suitable for forming a stack of single pancakes for generating a high magnetic field. However, the magnetic flux going through the tape surface causes a screening current, which reduces the magnetic field in conventional coil designs. Magnetic resonance imaging systems need high uniformity of the magnetic field, both spatially and temporally, and thus, the screening field resulting from the screening current is one of the critical issues. In this paper, a 10-T class small test coil using REBCO-coated conductors was fabricated and tested in a conduction-cooled configuration from 10 to 60 K for investigating the effect of the screening field on the generated magnetic field in the inner space of the coil. The coil was composed of a stack of 22 single pancakes using 4-mm-wide REBCO tapes with an inner diameter of 50 mm, an outer diameter of 129 mm, and a height of 104 mm. For different stacking orders based on the coil critical currents, the measured values of the central magnetic field at 10 K were 7.66 and 8.27 T, which were lower than the calculated values of 8.07 and 8.70 T obtained without taking account of the screening-current fields.

DOI： 10.1109/TASC.2016.2530301

記述言語：その他   掲載種別：研究論文（学術雑誌）  

It is important to investigate the stability and behavior of an epoxy-resin-impregnated no-insulation (NI) REBCO pancake coil to implement high-field applications, such as ultra-high-field magnetic resonance imaging. We have performed sudden discharging and overcurrent tests for the impregnated NI REBCO pancake coil. From the discharging test, the contact resistivity is estimated, and it changes depending on the initial current. From the overcurrent test, the high thermal stability of the impregnated NI REBCO pancake coil is confirmed. The REBCO pancake coil is charged up to 67 A though the critical current is only 46 A, and no degradation has been found. To investigate in detail the electromagnetic behavior of an impregnated NI REBCO pancake coil, the simulation is performed by means of the partial element equivalent circuit (PEEC) model. In the sudden discharging test, the simulation results for the case of lower initial current are in good agreement with the experimental data. As can be inferred from the simulation results, the current drastically decreases from the inside of the impregnated NI REBCO pancake coil. The result of the overcurrent simulation is almost identical to the experimental one. However, since the contact resistivity is presumed to be constant in the simulation, the difference is observed in the high current region.

DOI： 10.1109/TASC.2016.2536736

記述言語：その他   掲載種別：研究論文（学術雑誌）  

In recent years, research and development of applications for a high field magnet wound with a REBCO tape into nuclear magnetic resonance and magnetic resonance imaging have been carried out. These applications require a very high accuracy field, that is, the spatial uniform and temporally stable field. However, in the REBCO coils, the irregular magnetic field is generated by the screening current, which is remarkably induced by the radial component of the magnetic field in a REBCO tape. Therefore, a nonconventional approach is required for the reduction or correction of the irregular magnetic field generated by the screening current in the stage of design for a high-accuracy-field REBCO coil. The prediction and improvement of the irregular magnetic field generated by the screening current is a serious issue for the development and design of REBCO coils and correction shims.

DOI： 10.1109/TASC.2016.2535966

記述言語：その他   掲載種別：研究論文（その他学術会議資料等）  

An R&D project on high-temperature superconducting (HTS) magnets using rare-earth Ba2Cu3O7 (REBCO) wires was started in 2013. The project objective is to investigate the feasibility of adapting REBCO magnets to ultrahigh field (UHF) magnetic resonance imaging (MRI) systems. REBCO wires are promising components for UHF-MRI magnets because of their superior superconducting and mechanical properties, which make them smaller and lighter than conventional ones. Moreover, REBCO magnets can be cooled by the conduction-cooling method, making liquid helium unnecessary. In the past two years, some test coils and model magnets have been fabricated and tested. This year is the final year of the project. The goals of the project are: (1) to generate a 9.4 T magnetic field with a small test coil, (2) to generate a homogeneous magnetic field in a 200 mm diameter spherical volume with a 1.5 T model magnet, and (3) to perform imaging with the 1.5 T model magnet. In this paper, the progress of this R&D is described. The knowledge gained through these R&D results will be reflected in the design of 9.4 T MRI magnets for brain and whole body imaging.

DOI： 10.1016/j.phpro.2016.04.034

記述言語：その他   掲載種別：研究論文（学術雑誌）  

A GdBa2Cu3Oy (Gd123) insert coil for the 25 T cryogen-free superconducting magnet was constructed, installed and tested. We succeeded in the generation of 10 T using a Gd123 insert coil without a background field. The temperature of the Gd123 coil increased from 4.5 K gradually and reached about 5.5 K, when the magnet was energized with 0.036 A/s, which corresponds to a 1 hour energizing mode. The calculated and measured central magnetic fields are 10.61 T and 10.15 T, respectively, because of the magnetization current effect in RE123 tape. The maximum heat load by the AC-losses estimated from the temperature rise is about 3 W, which is consistent with the slab model combined with tape stacking effect.

DOI： 10.1088/0953-2048/29/5/055010

記述言語：その他   掲載種別：研究論文（学術雑誌）  

Development of a higherature superconducting magnet wound with REBa2Cu3O7-δ (REBCO)-coated conductor for ultrahigh-field magnetic resonance imaging (MRI) is in progress. Our final targets are 9.4-T MRI systems for whole-body and brain imaging. Since REBCO-coated conductors feature high mechanical strength under a tensile stress and high critical current density, superconducting magnets could be made smaller by using REBCO coils. Superconducting magnets for MRI require homogeneous stable magnetic fields. The homogeneity of the magnetic field is highly dependent on the size and current density of the coils. Furthermore, in REBCO magnets, the screening-current-induced magnetic field that changes the magnetic field distribution of the magnet is one of the critical issues. In order to evaluate the magnetic field homogeneity and the screening-current-induced magnetic field of REBCO magnets, a conduction-cooled REBCO magnet with a roomerature bore of 200 mm was fabricated and tested. The REBCO coils were composed of 12 single pancakes, and the size of the homogeneous magnetic field region was 100-mm diameter spherical volume (DSV). The central magnetic field was as high as 1 T at 285 A. The magnetic field distribution on the z-axis was measured by using an NMR probe. The maximum error magnetic field was 470 parts per million (ppm) in the range from -50 to +50 mm, as well as in the coefficients of the spherical harmonic expansion for a 100-mm DSV. The error magnetic fields due to the screening-current-induced magnetic field were less than 5 ppm, because there was a sufficient distance between the coil and the homogeneous magnetic field region. The main reason for the error magnetic field was dimensional errors in the outer diameters and positions on the &#36;z&#36;-axis.

DOI： 10.1109/TASC.2016.2529841

記述言語：その他   掲載種別：研究論文（その他学術会議資料等）  

A cryogen-free 25 T superconducting magnet using a ReBCO insert coil that generates 11.5 T in a 14 T background field of outer low-temperature superconducting (LTS) coils is currently under development. The AC loss of the insert coil during field ramping is approximately 8.8 W, which is difficult to dissipate at the operating temperature of the LTS coils (4 K). However, since a ReBCO coil can operate at a temperature above 4 K, the ReBCO insert coil is cooled to about 10 K by two GM cryocoolers, and the LTS coils are independently cooled by two GM/JT cryocoolers. Two GM cryocoolers cool a circulating helium gas through heat exchangers, and the gas is transported over a long distance to the cold stage located on the ReBCO insert coil, in order to protect the cryocoolers from the leakage field of high magnetic fields. The temperature difference of the 2nd cold stage of the GM cryocoolers and the insert coil can be reduced by increasing the gas flow rate. However, at the same time, the heat loss of the heat exchangers increases, and the temperature of the second cold stage is raised. Therefore, the gas flow rate is optimized to minimize the operating temperature of the ReBCO insert coil by using a flow controller and a bypass circuit connected to a buffer tank.

DOI： 10.1016/j.phpro.2015.06.095

記述言語：その他   掲載種別：研究論文（学術雑誌）  

A 25-T cryogen-free superconducting magnet (25T-CSM) is being developed at the High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University. In the case of a cryogen-free superconducting magnet, the coil temperature rises during a sweep of an operating current due to ac losses. Hence, the ac loss estimation is very important for the cooling design. The critical current density and the magnetization of Gd123 tapes at 4.2 K were measured in order to estimate the ac loss of a Gd123 insert of the 25T-CSM. The ac loss is actually the hysteresis loss, which are calculated from the &#36;J-{c} &#36; properties taking the magnetic field distribution in the Gd123 coil into account. Regarding the effect of tape stacking (or winding) in a pancake coil, the slab approximation can be used for hysteresis loss calculation. In the case of slab model, the full penetration field at the center of the tape becomes higher than the maximum applied magnetic field in the most part of the coil. As a result, the hysteresis loss increases with increasing a magnetic field when the magnet energizes. The hysteresis losses assuming the slab model, however, show an opposite field dependence to those calculated from the strip model without the stacking effect. Hence, the ac loss of 5 W is estimated when the magnet energizes to 25.5 T within 60 min.

DOI： 10.1109/TASC.2014.2366552

記述言語：その他   掲載種別：研究論文（その他学術会議資料等）  

A project to develop an ultra-high-field magnetic resonance imaging (MRI) system based on HTS magnets using (RE)Ba2Cu3O7 (REBCO; RE=rear earth) coils is underway. The project is supported by the Japanese Ministry of Economy, Trade and Industry and aims to establish magnet technologies for a whole-body 9.4 T MRI system. REBCO wires have high critical current density in high magnetic fields and high strength against hoop stresses, and therefore, MRI magnets using REBCO coils are expected to have cryogenic systems that are smaller, lighter, and simpler than the conventional ones. A major problem in using REBCO coils for MRI magnets is the huge irregular magnetic field generated by the screening current in REBCO tapes. Thus, the main purpose of this project is to make the influence of this screening current predictable and controllable. Fundamental technologies, including treatment of the screening currents, were studied via experiments and numerical simulations using small coils. Two types of model magnets are planned to be manufactured, and the knowledge gained in the development of the model magnets will be reflected in the magnet design of a whole-body 9.4 T MRI system.

DOI： 10.1016/j.phpro.2015.05.122

記述言語：その他   掲載種別：研究論文（学術雑誌）  

In a superconducting coil using a REBCO tape, a large screening current is induced by the magnetic field perpendicular to the tape. The irregular magnetic field generated by this screening current affects the magnetic-field distribution in high-field magnet applications, which require a magnetic field with high accuracy and temporal stability. Therefore, in these applications of the REBCO coil, we have to investigate and suppress the influence of the screening current in the REBCO tapes. In this study, we carried out experiments and numerical simulations focusing on the irregular magnetic field generated by the screening currents in multiple-REBCO-coil models assuming real operations in magnetic applications such as NMR/MRI system. The results of the numerical simulation are in good agreement with the behaviors of the screening field observed in the experimental results. In addition, we discussed the differences in the screening field for various excitation conditions using the numerical results of the current distribution in the coil.

DOI： 10.1109/TASC.2014.2371133

記述言語：その他   掲載種別：研究論文（学術雑誌）  

With the advances made in accelerator technologies, particle accelerators are being employed in a wide range of fields such as physics, biology, and medical treatment. However, the diameters of the main rings in most accelerators are large; for example, even the smallest carbon cancer therapy accelerator has a main ring diameter of approximately 20 m. Since a rotating gantry for a carbon-ion beam is also very large compared with the gantry for a proton beam, widespread adoption of this therapy system has been restricted. In order to promote widespread adoption of carbon cancer therapy accelerators, it is necessary to reduce the bending radius of the carbon-ion beam. One way to achieve this is to use superconducting magnets with higher magnetic fields. High-Tc superconducting (HTS) magnets have higher thermal stability and need lower power consumption for cooling than low-Tc superconducting (LTS) magnets. Although saddle-shaped coils are suitable for accelerator magnets, an HTS conductor has anisotropic bending flexibility because of its tape shape. To evaluate winding technologies for HTS saddle-shaped coils, a number of coils were fabricated and tested. The test coils were about 400 mm long and 160 mm wide. Each test coil was wound using an approximately 45-m-long 4-mm-wide YBa2Cu3Ox (YBCO)-coated conductor. Some of the coils were excited to measure their voltage-current characteristics in liquid nitrogen. An index value over 20 was obtained throughout an electric field range down to 10-7 V/m, which indicated that the superconducting properties would not be degraded. To evaluate the quality of the magnetic field distribution and the thermal stability of a conduction-cooled HTS magnet, we fabricated and tested an HTS magnet consisting of eight saddle-shaped coils.

DOI： 10.1109/TASC.2014.2385962

記述言語：その他   掲載種別：研究論文（学術雑誌）  

A cryogen-free 20 T superconducting magnet with a 52 mm room-temperature bore was developed and installed in Tohoku University in 2013. This magnet consists of a Bi2223 insert coil, which generates 4.5 T, and outer low-temperature superconducting (LTS) coils. Both coils were cooled by a GM/JT cryocooler with 4.2 W-class cooling capacity at 4.3 K. To generate a higher magnetic field, a new cryogen-free 25 T superconducting magnet using a REBCO insert coil, which generates 11.5 T, and new outer LTS coils is now under development. The magnetic field contribution of this REBCO insert coil is considerably higher than that of the previous Bi2223 insert coil, and the ac-loss of the insert coil during field ramping rises to approximately 9.7 W. The LTS coils have to operate at about 4 K, but the REBCO coil can operate at various temperatures above 4 K. In addition, the cooling capacity of a GM cryocooler is greater than that of a GM/JT cryocooler around 10 K. Thus, the REBCO insert coil is cooled to about 10 K by using two GM cryocoolers, independently of the LTS coils, which are cooled by two GM/JT cryocoolers. To protect the cryocoolers from the leakage field of the magnet, the two GM cryocoolers cool circulating helium gas through heat exchangers, and the gas is transported over a long distance to another heat exchanger provided for the REBCO insert coil. The maximum temperature of the REBCO insert coil was calculated under the most severe condition where an insert coil ac-loss of 9.7 W was generated continuously. And it was confirmed to be less than the target maximum temperature of 12 K.

DOI： 10.1109/TASC.2014.2368713

記述言語：その他   掲載種別：研究論文（学術雑誌）  

A cryogen-free 25-T superconducting magnet with a 52-mm room-temperature bore consisting of an 11.5-T REBa2Cu3O7-δ (REBCO) insert coil and outer 14-T low-temperature superconducting (LTS) coils is currently under development. The REBCO insert coil is composed of a stack of 68 single pancake coils. The inner and outer diameters of the REBCO insert coil are 102 and 276 mm, respectively; and the total conductor length is 17.6 km. The maximum hoop stress of the REBCO insert coil was estimated to be 455 MPa when the central magnetic field was 25.5 T. The REBCO insert coil is cooled by circulating helium gas using both single- and two-stage GM cryocoolers, and the LTS coils are cooled by Gifford-McMahon/Joule-Thomson cryocoolers. If thermal runaway occurs in a conduction-cooled system, the REBCO coil will almost certainly be burned out. Therefore, it is important to avoid thermal runaway of the REBCO insert coil under all operating conditions and protected operations. As operating conditions of the REBCO insert coil, quenching of the LTS coils should be assumed in the severest situation, since the current of the REBCO insert coil will increase after the LTS coils quench. From the results of quench analysis of the LTS coils and circuit simulations, it was confirmed that thermal runaway can be avoided by setting the intermediate potential of the REBCO coil to the ground potential and by using an external dump resistance of 11 Ω.

DOI： 10.1109/TASC.2014.2380783

記述言語：その他   掲載種別：研究論文（学術雑誌）  

An impregnated pancake coil is composed of a REBa2Cu3O7-δ (REBCO)-coated conductor, a polyimide tape, and epoxy resin, so that a radial thermal stress is generated in the winding by their anisotropic thermal contraction when the coil is cooled from room temperature to 77 K or less. If this radial stress locally exceeds the delamination strength of the REBCO-coated conductor under transverse tensile stress, the coil will be degraded. Although it is important to determine the exact delamination strength of the REBCO-coated conductor to fabricate an impregnated coil that does not suffer from degradation, the delamination strengths reported until now have shown considerable variation due to the evaluation methods used. Therefore, we propose a new method for evaluating the delamination strength using small epoxy impregnated test coils, since the radial stress depends on the ratio of inner to outer diameters regardless of the coil size. The delamination strength can be obtained by investigating the ratios of inner to outer diameters that do not result in degradation. In this paper, the delamination strengths of three types of REBCO-coated conductors made with different manufacturing processes and having different configurations were evaluated. This method showed that a conductor laminated with copper plates had higher delamination strength than the other two kinds of conductors. In addition, impregnated single pancake coils whose inner diameter was 50 mm and whose outer diameter was 100 mm were fabricated using the three types of REBCO-coated conductors. Degradation was avoided in all coils by dividing the coils into a number of winding parts in the radial direction according to the delamination strength of the conductors.

DOI： 10.1109/TASC.2014.2372048

記述言語：その他   掲載種別：研究論文（その他学術会議資料等）  

The construction of a 25-T cryogen-free superconducting magnet (25T-CSM) has started in 2013 at the High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University. The 25T-CSM consists of a low-T superconducting (LTS) coil and a high-Tc superconducting (HTS) coil. A high-strength CuNb/Nb3Sn Rutherford cable with the reinforcing stabilizer CuNb composite is adopted for the middle LTS section coil. The characteristic feature of the new technology using a CuNb/Nb3Sn Rutherford cable is a react-and-wind method for the coil-winding process. The LTS coil of 300-mm winding inner diameter is fabricated, and a central magnetic field of 14 T is generated at an operation current of 851 A. The HTS insert coil wound with GdBa2Cu3Oy (Gd123) tape has a 52-mm experimental room temperature bore, and a central magnetic field of 25.5 T will be generated at an operation current of 150 A in a background field of 14 T.

DOI： 10.1088/1742-6596/568/3/032019

記述言語：その他   掲載種別：研究論文（学術雑誌）  

A YBCO insert coil has been developed for upgrading a cryogen-free 18 T superconducting magnet installed in the High Field Laboratory for Superconducting Materials (HFLSM) at Tohoku University to a 22 T superconducting magnet. The YBCO insert coil is designed to generate 6.5 T at 200 A in 15.5 T outer LTS coils. The YBCO insert coil is composed of a stack of 50 single pancake coils wound with YBCO-coated conductors (0.23 mm × 4 mm). The inner and outer diameters of the YBCO insert coil are 96 mm and 178 mm, respectively, and the total conductor length is about 3 km. The maximum hoop stress of the YBCO insert coil was estimated to be 310 MPa when the central magnetic field was 22 T. The magnet system is cooled by a GM/JT cryocooler and two single-stage GM cryocoolers. Thermal runaway may cause burnout of the YBCO insert coil, and therefore, it is important to calculate the voltage-current characteristics of the coil from the superconducting properties of the YBCO-coated conductors. The coil should have no damaged area in the windings because a damaged area would generate heat locally, eventually resulting in thermal runaway. Therefore, a demonstration coil with almost the same size as the pancake coils used for the YBCO insert coil was fabricated and tested in conduction cooling conditions in order to evaluate whether the coil could withstand the thermal stresses and electromagnetic force.

DOI： 10.1109/TASC.2013.2287059

記述言語：その他   掲載種別：研究論文（学術雑誌）  

Newly designed racetrack coil wound with REBCO-coated conductor was developed and demonstrated. The coil shape was improved to achieve consistent dimensional accuracy, and the winding structure was improved to prevent degradation in the epoxy-impregnated windings. The wire length of the fabricated large-scale coils was 574 m, and the overall size was 434 mm × 796 mm. This wire length was longer than that of any of the coils the authors fabricated previously. The outer radiuses in both the semicircular sections and the pseudo-straight sections, which had a slight curvature, were approximately the same in the two coils, to within 1 mm, indicating that good dimensional accuracy was obtained consistently. Furthermore, the n-value was sufficiently high at 24 in an electric field range from 10-9 to 10-7 V/cm at 77 K, which indicates that the coils had no damaged area in the windings.

DOI： 10.1109/TASC.2013.2284422

記述言語：その他   掲載種別：研究論文（学術雑誌）  

A radial stress is generated in the winding when an impregnated YBCO pancake coil is cooled from room temperature to 77 K or less. If this radial stress locally exceeds the delamination strength of the YBCO-coated conductor under transverse tensile stress, the coil will be degraded. Although several methods for evaluating the delamination strength of YBCO-coated conductors have been reported, the delamination strength determined by these methods has shown considerable variation. In order to fabricate an impregnated coil that does not suffer from degradation, it is important to determine the exact delamination strength. The delamination strength was evaluated using small impregnated test coils with an inner diameter of 30 mm and an outer diameter from 36 mm to 60 mm, since the radial stress depends on the ratio of inner to outer diameter regardless of the coil size. The delamination strength can be obtained by investigating the ratio of inner to outer diameter that does not result in degradation. On the other hand, for a larger ratio of inner to outer diameter, it is necessary to make the radial stress less than the delamination strength of the YBCO-coated conductor. To achieve this, the radial stress was decreased by dividing the coil into a number of parts in the radial direction. 1051-8223

DOI： 10.1109/TASC.2013.2287054

記述言語：その他   掲載種別：研究論文（学術雑誌）  

The new high magnetic field research laboratory network is recognized as one of the Japanese Master Plans of Large Research Project by the Science Council of Japan. Recently, the project of the 25 T cryogen-free superconducting magnet (25 T-CSM), which is operated under a conductive cooling condition by cryocooler, was approved under the high magnetic field research laboratory network. We adopted a high strength CuNb/Nb3Sn Rutherford cable with a prebending treatment for the middle section coils of the 25 T-CSM. The central magnetic field of 14 T is generated by the operational current of 851 A by the Nb3Sn middle section and NbTi outer section coils in a 300 mm bore. The induced maximum hoop stress in the CuNb/Nb3Sn section is about 250 MPa. In addition, the 11.5 T high temperature superconducting insert coil is also designed using Gd123 tapes. Therefore, a total central magnetic field of 25.5 T can be achieved.

DOI： 10.1109/TASC.2013.2292367

記述言語：その他   掲載種別：研究論文（その他学術会議資料等）  

Since the development of the 18T cryogen-free superconducting magnet (18T-CSM) at the High Field Laboratory for Superconducting Materials (HFLSM), IMR, Tohoku University, it has been routinely utilized for the various high field researches such as the in-field materials processing and so on. After the Great East Japan Disaster, however, the 18T-CSM is under the repairing due to the serious damages of the system. At this time, a part of the middle Nb 3 Sn section coil and the Bi2223 inner most coil were renewed. The bronze-route Nb 3 Sn coil with a low ac-loss was adopted instead of the previous internal-Sn Nb 3 Sn coil for the reduction of the heat-generation during a field ramping. The new Bi2223 coil will generate the central field of 4.5 T under the background field of 15.5 T by the LTS outer coils, although the previous Bi2223 coil generated 2.5 T. Hence the 18T-CSM was successfully upgraded to the 20T-CSM. In addition to the Bi2223 insert, the Gd123 insert coil is being constructed as well. When the Gd123 insert coil is installed instead of the Bi2223 coil, it will be upgraded to the 22T-CSM from the 20T-CSM.

DOI： 10.1063/1.4860776

記述言語：その他   掲載種別：研究論文（学術雑誌）  

We have been developing a conduction-cooled coil wound with YBCO-coated conductors for HTS applications. Previously, we have fabricated a coil composed of a stack of 12 single pancakes wound with 4 mm-wide YBCO tapes. This coil had a central magnetic field as high as 5.1 T at 10 K under conduction-cooled conditions. In the present study, we fabricated and tested a coil composed of a stack of four single pancakes wound with 12 mm-wide YBCO tapes. The total size of the coil and the Jc value of the tapes were almost the same as those of the former coil. At 77 K, the voltage-current characteristics showed a high n-value of 24, confirming that the coil had no degradation. Furthermore, in a conduction-cooled configuration at 20 K to 60 K, the coil showed a high n-value of over 20. At 20 K, the central magnetic field reached 5.9 T at 903 A, which is 1.3-times higher than that of the former coil. © 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2013.04.068

記述言語：日本語  

Development of conduction-cooled REBCO superconducting coils:– investigation of the thermal runaway currents and mechanical properties of impregnated single-pancake coils–
We are developing conduction-cooled REBCO superconducting coils. We first observed thermal runaway currents of the coils during the process of making impregnated single-pancake coils with an outer diameter of 300 mm with REBCO- coated conductors and conduction-cooling them using a GM cryocooler. In addition, we performed a numerical simulation of the electromagnetic and thermal behavior of the coils with the critical current properties of the REBCO tapes through use of a threedimensional heat conduction equation. Comparing the observed results with the theoretical prediction, we confirmed the theoretical predictability of the thermal runaway currents of the single-pancake coils. These results suggest that the winding and impregnation were successfully carried out without degradation of the properties of the prepared REBCO tapes. Further, in testing conducted on an impregnated small coil in a 4 T background magnetic field, we confirmed that the impregnated coils could withstand hoop stress as high as the mechanical strength of the REBCO tape itself.

DOI： 10.2221/jcsj.48.239, 10.1016/j.cryogenics.2019.03.009_references_DOI_Ht9nhV4FesInxDHFpnGCh3WAu1F

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

Study of the basic properties of REBCO-coated conductors for HTS coil technology:– evaluation of delamination characteristics and numerical prediction of flux flow characteristics–

記述言語：その他   掲載種別：研究論文（学術雑誌）  

We fabricated and tested a 5 T-class conduction-cooled high-temperature superconducting (HTS) coil composed of a stack of 12 single pancake coils wound with YBCO-coated conductors. The length of each single pancake coil was 25 m, and the inner diameter of the coil was 50 mm. The voltage-current characteristics were measured in liquid nitrogen and under conduction-cooled conditions at 10-60 K. At 10 K, the central magnetic field of the 12 stacked pancake coils was as high as 5.1 T at 305 A. We also tested the coil operation in a 4 T background magnetic field.

DOI： 10.1016/j.physc.2012.02.041

記述言語：その他   掲載種別：研究論文（学術雑誌）  

We have studied for upgrading an 18 T super-conducting magnet conductively cooled by a GM/JT cryocooler at Tohoku University in Japan to a 23 T superconducting magnet. In order to upgrade the cryogen-free high field superconducting magnet up to 23 T, YBa 2}Cu 3O 7(Y123)coated conductor insert is intended for replacing the insert using Bi 2Sr 2Ca 2}Cu 3}O 10}({Bi2223) tape with stainless steel reinforcement tape. We have produced double pancake coils with almost the same size as a double pancake coil for the Bi2223 insert, and have investigated the performance of the coils. And the design of the double pancake coil was reviewed from the result of the R&D coils. © 2011 IEEE.

DOI： 10.1109/TASC.2011.2179405

記述言語：その他   掲載種別：研究論文（学術雑誌）  

When a high-temperature superconducting (HTS) coil is operated using a conduction-cooling technique, the coil should be impregnated with epoxy resin because the coil is not sufficiently cooled without it. On the other hand, it is important that the coil should have no damaged area in the winding because the damaged area would generate heat locally and the coil would eventually quench. However, whenever we fabricated an impregnated HTS coil wound with YBCO tapes and evaluated its V-I characteristics, the n-value of the coil was much lower than we expected, indicating that the critical currents of some areas in the winding drastically decreased. Therefore, we have started to improve the performance of an impregnated HTS coil wound with YBCO tapes. In this paper, we investigated the cause of the degradation and found that the degradation did not occur when we decreased the radial tensile stress in the windings. Then we fabricated four single-pancake coils, stacked them, and tested them in a conduction-cooled condition. The measured V-I curves were in good agreement with the calculated ones, suggesting that we successfully developed a technique of fabricating an impregnated HTS coil wound with YBCO tapes with no degradation. We also measured thermal runaway currents of a conduction-cooled HTS coil composed of two single-pancake coils wound with YBCO tapes and numerically simulated the thermal properties by using a three-dimensional heat conduction equation in order to study the thermal stability of the YBCO coil. The measured thermal runaway currents were in good agreement with the calculated ones. © 2010 IEEE.

DOI： 10.1109/TASC.2010.209012

記述言語：その他   掲載種別：研究論文（その他学術会議資料等）  

We plan to develop a new improved cooling system for the Superconducting Kaon Spectrometer (SKS) magnet and transfer the magnet to the K1.8 beamline of the Hadron Hall of the Japan Proton Accelerator Research Complex (J-PARC) for further use in nuclear physics experiments. To replace the present 300 W cryogenic system, we will adopt a new cooling method that uses 4 K Gifford-McMahon/Joule-Thomson (GM/JT) cryocoolers. In order to decide a practical design for the new liquid helium reservoir of the magnet, which will be equipped with GM/JT cryocoolers, cooling tests on a GM/JT cryocooler were performed from February to March 2007. We constructed a new cooling test stand with a GM/JT cryocooler and measured the cooling capacities under several thermal shield temperatures with or without a baffle, which helped prevent convection. Based on the test results, we have finally decided to adopt three GM/JT cryocoolers for the new SKS along with a baffle and an additional dedicated GM cooler to cool the thermal shield of the GM/JT ports. © 2008 American Institute of Physics.

DOI： 10.1063/1.2908567

記述言語：その他   掲載種別：研究論文（学術雑誌）  

We fabricated a cryocooler-cooled Bi2223 superconducting pulsed coil and experimentally studied the thermal run-away in dc or ac operation. We carried out numerical simulation of thermal properties of the coil in order to explain the thermal runaway of the coil. Firstly, we analyzed the total heat generation of flux-flow losses and ac losses inside the winding from the experimental results of the external field losses and the E - J characteristics for the Bi2223 strands. Secondly, we numerically simulated the thermal properties by using a 2-dimensional heat conduction equation with axial symmetry. The numerical simulation shows the relation between the initiation of thermal runaway and the temperature distribution with highly concentrated heat source in the winding. We have a semi-quantitative agreement between the numerical results and the experimental ones for the condition of the thermal runaway. © 2006 IEEE.

DOI： 10.1109/TASC.2006.873266

記述言語：その他   掲載種別：研究論文（その他学術会議資料等）  

We fabricated a cryocooler-cooled Bi2223 superconducting pulsed coil and experimentally studied thermal runaway in dc or ac operation. We carried out numerical simulation of thermal properties of the coil in order to explain thermal runaway of the coil. Firstly, we analyzed the total heat generation of flux-flow loss and ac loss inside the winding from the experimental results of the external field losses and the E-J characteristics for the Bi2223 strands. Secondly, we numerically simulated the thermal properties by using 2- dimensional heat conduction equation with axial symmetry. The numerical simulation shows the relation between the initiation of thermal runaway and the temperature distribution with highly concentrated heat source in the winding. We have a semi-quantitative agreement between the numerical results and the experimental ones for the condition of the thermal runaway. © 2006 IOP Publishing Ltd.

DOI： 10.1088/1742-6596/43/1/257

記述言語：その他   掲載種別：研究論文（学術雑誌）  

We studied the thermal and electromagnetic behavior of a conduction-cooled superconducting pulse coil. It was wound with a 4-strand parallel conductor composed of silver sheathed B12223 multifilamentary tapes and impregnated with an epoxy resin. We first investigated the temperature dependence of the thermal runaway current in dc operation. It was higher than the critical current by several tens amps at any temperature from 40 to 90 K. In order to study the mechanism of the thermal runaway, we carried out the numerical simulation of the coil behavior. We found out that the thermal runaway was caused when the heat generation at the winding end close to the cryocooler head where the flux-flow loss is the largest due to perpendicular magnetic field exceeded the cooling through thermal diffusion though the thermal runaway of the coil started at the other end of the winding. If the transport current is smaller than the thermal runaway current at the coil temperature just after applying current, the temperature distribution in the longitudinal direction of the coil varied into a monotonous gradient and the steady thermal flow was established. After that the coil temperature decreased to the balanced one. © 2006 IEEE.

DOI： 10.1109/TASC.2006.873268

記述言語：その他   掲載種別：研究論文（学術雑誌）  

We experimentally studied thermal runaway in a Bi2223 pulsed coil cooled by a cryocooler. It was found the total heat generation of the coil in thermal runaway was remarkably lower than the cooling capacity of the cryocooler. We simulated the total heat generation of flux-flow loss and AC loss in the winding from the experimental results of the external field losses and the E-J characteristics for the Bi2223 strands in order to estimate the thermal runaway in AC operation. The simulation shows that the thermal runaway is induced by the flux-flow loss localized in both coil sides, which is enhanced with increasing the coil temperature. © 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2005.03.074

記述言語：その他   掲載種別：研究論文（学術雑誌）  

We designed and fabricated a l T cryocooler-cooled oxide superconducting pulsed coil with Bi2223 multifilamentary flat wires. Previously we studied temperature dependence of the thermal runaway current of the pulsed coil in dc operation. The thermal runaway current was higher than the critical current that was defined as the current where the electric field 10-4 V/m was generated on average over the whole length of the conductor. In this paper, we studied the coil temperature and the ac loss dependence of the thermal runaway current in ac operation. We operated the coil at 0.5 to 5 Hz with a sinusoidal-waveform transport current. The initial coil temperature was set between 30 and 100 K. The thermal runaway current decreased monotonically with increasing temperature in a similar way to dc operation. However, in a low temperature region, the thermal runaway current was restricted to a lower level than that in dc operation due to the ac loss. We discuss the frequency dependence of thermal runaway current in ac operation. © 2005 IEEE.

DOI： 10.1109/TASC.2005.849226

記述言語：その他   掲載種別：研究論文（学術雑誌）  

We fabricated a 1 T cryocooler-cooled oxide superconducting pulsed coil. It is a 16-layer solenoidal coil wound with an interlayer-transposed 4-strand parallel conductor composed of Bi2223 multifilamentary flat wires. The insulated strands were transposed only between layers to make all strands inductively equivalent. To suppress the temperature rise due to the ac loss, the aluminum nitride(AIN) plates were arranged between layers as heat drains. The pulsed coil was cooled down to 30 K by a single stage cryocooler. It can be operated continuously at 40 K in the 1 T triangular wave operation at 1 Hz. First we studied the temperature dependence of thermal runaway current of the coil under the application of direct transport current. The runaway current was higher than the critical current, which was defined as the current where the electric field of 10-4 V/m was generated on average over the whole length of the conductor. Next we studied the temperature variation of the coil in a sinusoidal operation of 1 Hz. The temperature increment was much more gradual than that in the dc operation. In this paper, we discuss the thermal properties of the cryocooler-cooled oxide superconducting pulsed coil, taking into account the heat generation of ac loss and flux-flow loss and the cooling capacity of a cryocooler.

DOI： 10.1109/TASC.2004.830106

記述言語：その他   掲載種別：研究論文（学術雑誌）  

We designed and fabricated a 4 kJ conduction-cooled superconducting pulse coil with a 6-strand interlayer-transposed parallel conductor composed of B12223 multifilamentary tapes. We adopted the helium gas forced-flow cooling system where the helium gas flowed inside a copper pipe soldered with the flanges of brass. We succeeded in the continuous pulse operation with an amplitude of 500 A-1.6 T at a sweep rate of 140 A/s at 30 K as designed. Even in the ac operation with an ac loss of 120 W, the difference in temperature inside the winding was only 5 K and it was possible to hold the coil temperature around 30 K. In this paper, we report the design and the test results of the coil system from the aspect of ac loss and thermal properties.

DOI： 10.1109/TASC.2003.812938

記述言語：その他   掲載種別：研究論文（学術雑誌）  

Feasibility study of an oxide superconducting transformer for Shinkansen rolling stock was made from the viewpoint of light weight. On the basis of the observed critical current and ac loss properties for the currently developed Bi2223 silver-sheathed superconducting wires, which both have large anisotropy, superconducting transformers cooled by subcooled liquid nitrogen with the same specifications as the conventional ones were designed. The total weight of the transformer system including a cryocooler was quantitatively evaluated. As compared with the weight of conventional ones, the optimum winding configuration and 1-turn voltage were searched for. As a result, the following suggestion was obtained. For the sake of light weight, the transformer should be core type with closely wound cylindrical windings so as to reduce the perpendicular magnetic field to the wide surface of superconducting thin wires and the ac loss in a superconducting wire needs to be much reduced as compared with the currently developed one. Finally, according to the obtained optimum parameters, the conceptual design of a single-phase 4 MVA-25 kV superconducting transformer for Shinkansen rolling stock was performed whereby the drastic reduction of weight was shown.

DOI： 10.1109/TASC.2002.1018529

開催年月日： 2023年11月

記述言語：英語   会議種別：口頭発表（一般）  

開催地：Wellington  

開催年月日： 2023年11月

記述言語：英語   会議種別：口頭発表（一般）  

開催地：Wellington  

開催年月日： 2023年9月

記述言語：英語   会議種別：口頭発表（一般）  

開催地：Aix-en-Provence   国名：フランス共和国  

開催年月日： 2023年9月

記述言語：英語   会議種別：口頭発表（一般）  

開催地：Aix-en-Provence   国名：フランス共和国  

開催年月日： 2023年9月

記述言語：英語   会議種別：口頭発表（一般）  

開催地：Aix-en-Provence   国名：フランス共和国  

開催年月日： 2023年9月

記述言語：英語   会議種別：口頭発表（一般）  

開催地：Aix-en-Provence   国名：フランス共和国  

開催年月日： 2023年9月

記述言語：英語   会議種別：口頭発表（一般）  

開催地：Aix-en-Provence   国名：フランス共和国  

開催年月日： 2023年9月

記述言語：英語   会議種別：口頭発表（一般）  

開催地：Bologna   国名：イタリア共和国  

開催年月日： 2023年9月

記述言語：英語   会議種別：口頭発表（一般）  

開催地：Bologna   国名：イタリア共和国  

開催年月日： 2023年5月 - 2023年6月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：東京   国名：日本国  

開催年月日： 2023年5月 - 2023年6月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：東京   国名：日本国  

開催年月日： 2023年5月 - 2023年6月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：東京   国名：日本国  

開催年月日： 2023年5月 - 2023年6月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：東京   国名：日本国  

開催年月日： 2022年12月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：岐阜   国名：日本国  

開催年月日： 2022年12月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：岐阜   国名：日本国  

開催年月日： 2022年11月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：鹿児島   国名：日本国  

開催年月日： 2022年10月

記述言語：英語   会議種別：口頭発表（一般）  

開催地：HONOLULU   国名：アメリカ合衆国  

開催年月日： 2022年10月

記述言語：英語   会議種別：口頭発表（一般）  

開催地：HONOLULU   国名：アメリカ合衆国  

開催年月日： 2022年10月

記述言語：英語   会議種別：口頭発表（一般）  

開催地：HONOLULU   国名：アメリカ合衆国  

開催年月日： 2022年10月

記述言語：英語   会議種別：口頭発表（一般）  

開催地：HONOLULU   国名：アメリカ合衆国  

開催年月日： 2022年10月

記述言語：英語   会議種別：口頭発表（一般）  

開催地：HONOLULU   国名：アメリカ合衆国  

開催年月日： 2022年9月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：オンライン   国名：日本国  

開催年月日： 2022年9月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：オンライン   国名：日本国  

開催年月日： 2022年9月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：オンライン   国名：日本国  

開催年月日： 2021年11月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：オンライン   国名：日本国  

開催年月日： 2021年5月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：オンライン   国名：日本国  

Thermal design of a cryocooler-cooled superconducting magnet using the 1D-CAE method.

開催年月日： 2021年5月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：オンライン   国名：日本国  

HIS magnet equipped with electrically conductive epoxy resin for quench protection-Design and fabrication-

開催年月日： 2021年5月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：オンライン   国名：日本国  

HTS MRI magnet equipped with electrically conductive epoxy resin for quench protection-test results-

種別：大会・シンポジウム等 

種別：大会・シンポジウム等 

種別：査読等 

外国語雑誌　査読論文数：3

日本語雑誌　査読論文数：1

種別：大会・シンポジウム等 

種別：大会・シンポジウム等 

【九州大学など　次世代電動航空機向け全超電導モーター　回転試験に世界初成功】

【空飛ぶクルマへの適用目指す　九大が次世代航空機向けモータの回転試験に世界初成功】https://www.zakzak.co.jp/article/20230611-S3CARWTCAVPFTI7GHA2QTOQT6E/

【九州大学、次世代電動航空機向け400kW級全超電導モータの回転試験に世界初成功】https://www.drone.jp/news/2023060915420467765.html

【次世代電動航空機向け 400kW 級全超電導モータの回転試験に世界で初めて成功】

【次世代電動航空機向け400kW級全超電導モータの回転試験に世界で初めて成功】https://www.kyushu-u.ac.jp/ja/notices/view/2475/