Satoshi Fukui’s research while affiliated with Niigata University and other places

High temperature superconducting REBCO tape has the characteristic of maintaining high critical current density under strong external magnetic field, which makes it an ideal material for the construction of air-core electromagnets of accelerator and electron cyclotron resonance (ECR) ion source. In Research Center for Nuclear Physics, Osaka University, a non-insulated air-cored REBCO coil assembly has been constructed. This coil assebmly consists of three circular REBCO solenoid and six racetrack REBCO coil. This coil assembly will be used as an electromagnet of a multi-frequency ECR ion source, and is also developed as a key technology development of an air-core cyclotron. The magnetic field of this ion source are designed, and 77K performance tests of the assembly are carried out in order to examine the capability of REBCO coils of inducing magnetic field under external field. In this work, the test results and the magnetic field designed for the ECR ion source will be presented and discussed.

In a previous study, the AC loss characteristics of each layer of a high-temperature superconducting (HTS) AC cable were experimentally investigated [1]–[3]. Based on these data, we modeled the AC loss in a superconducting cable by considering the maxi-mum and minimum values of the loss. A comparison between this model and the experimental data showed good agreement. Hence, it is possible to estimate AC loss under electromagnetic conditions in a high-temperature superconducting cable. This model makes it possible to determine the losses of each layer in the superconducting cable and to design the minimum AC loss load of the superconducting cable. This model estimates the AC losses of a three-layer twisted HTS cable.

In this paper, design, fabrication, and test results of the demonstration device for 400 kW-class aluminum billet heating using the HTS magnet were reported. Based on our user's request, the target of the demonstration device was to heat the aluminum billet of 155 mm in diameter and 500 mm in length from room temperature to 500 ℃ within 60 seconds. In the heating test using the demonstration device, it was demonstrated that the input heating power to the aluminum billet of 155 mm in diameter and 500 mm long was more than 400 kW. Based on the measured data of the heating tests, the heating efficiency was estimated to be 74.5%. It was shown that at least 30% reduction of energy consumption could be possible compared to conventional induction heating devices.

Herein, a compact cryogen-free nuclear magnetic resonance (NMR) system is developed using a stacked high-temperature superconducting (HTS) bulk system. To compensate for magnetic field uniformity, an HTS cylinder is installed inside the measurement space. The cylinder is wound with REBCO tapes placed on the surface of a copper cylinder. The uniformity of its critical current density is important for NMR signal detection. Such an operation requires an examination of the quality of the critical-current-density distribution of the cylinder. A noncontact measurement method is required for such an examination. When a magnetic field is applied, the superconductor exhibits a demagnetization effect. Moreover, if any defect exists in any part of the superconducting tape, that particular part would contain no demagnetization, thus disturbing the relevant magnetic field distribution. In this study, this phenomenon is investigated using DC and AC magnetic fields. The magnets and sensor are placed inside the HTS cylinder, and magnetic fields are applied to the HTS cylinder surface. The results confirm that the proposed method is effective and applicable for evaluating the critical-current distribution.

We have been conducting research and development of the DC induction heating device of aluminum billet using HTS magnets. In this R&D, following the development of main components such as the HTS magnet system, the heating control system and the mechanism for grasping and rotating the aluminum billet, the demonstration device for 400 kW-class billet heater was developed and the heating power of over 400 kW was demonstrated. In this study, as the final step of the R&D, we conducted a continuous repetitive heating test and evaluation of the electromagnetic and mechanical influence due to the heating operation on the HTS coil. It was demonstrated that the continuous repetitive heating of 90 billets（more than two hours）with the rated power was possible. The temperature increase and the behavior of mechanical stress of the HTS coil during the continuous heating operation were evaluated. It was confirmed that the soundness of the HTS coil through the continuous repetitive heating was not compromised.

This paper investigates the AC loss characteristics in the resonator of the superconducting wireless power transmission system proposed by Prof. Sekiya. This superconducting resonator is characterized by the fact that the current distribution is not constant in the tape length, the REBCO tapes are double layered, and the operating frequency is 10 MHz. Since the influence of eddy current loss cannot be ignored due to the high operating frequency, we investigated the effect of the REBCO tape configuration using the finite element method.

A new High-Temperature Superconducting ECR (HTS-ECR) ion source is under development in Research Center for Nuclear Physics (RCNP), Osaka University. This ion source will be used for production of high intensity proton, deuteron and He ion beams. The HTS-ECR magnets are composed of three solenoid coils and a set of sextupole coils made of REBCO tapes, a high-temperature superconductor. The HTS-ECR ion source is designed to operate at frequency of 2.45 GHz and 10 GHz. Performance test of the HTS coils had been carried out at 31 K and 77 K. This HTS coil technology will be applied to development of a meter-size HTS coil system of a high intensity compact AVF cyclotron. This paper introduces the basic design of the HTS-ECR ion source. The performance test results showed that REBCO solenoids remain superconducting state with a current up to 400 A. Simulation results of the magnetic field and electromagnetic field distributions in a plasma chamber fulfilled the requirements of electron cyclotron resonance conditions at 2.45 GHz and 10 GHz. Simulation result of mirror ratios and electromagnetic field amplitudes are also presented in this paper.

This paper reported the fabrication and the test results of the high temperature superconducting (HTS) magnet developed for the aluminum billet heater for aluminum extrusion processing. In this magnet, the HTS coils were combined with iron cores, because the required magnetic field in the working region was about 1T. Two HTS coils wound directly on the convex shape iron cores were fabricated. The HTS coils were made by the no-insulation winding of REBCO tape with the YOROI-coil technique. Each of them was separately installed in an individual cryostat. Two HTS coils with iron cores were disposed oppositely to each other and were magnetically connected by a C-shape yoke placed in room temperature. The HTS coil was cooled down to about 15 K by the GM cryocooler in the initial cooling. The magnetic field of 1.06 T at the center of the heating region was stably generated with the rated current of 200 A. It was also demonstrated that the HTS magnet could be charged up to 300 A without quench. According to the observation of the electromagnetic force direction from the outside of the cryostat, it was confirmed the validity of the force balance design of the magnet.

When the MgB2 bulk body is magnetized by the pulse magnetizing method, the MgB2 bulk can take the role of a pseudo permanent magnet. However, it was confirmed that the temperature at which MgB2 is expected to be used is close to the critical temperature (TC) of MgB2 and the specific heat is small, so that the heat generated by the pulse magnetizing method causes flux jump and the trapped magnetic field performance deteriorates. In this study, pulse magnetizing was performed on different samples to verify and consider changes in the trapped magnetic field performance. As the sample, the sample obtained by changing the amount of titanium added was synthesized by uniaxial hot pressing and the sample synthesized by spark plasma sintering (SPS). Figure (a) shows a comparison of the trapped magnetic fields of the samples synthesized by the uniaxial hot press. From Fig. (A), it was shown that the capture magnetic field performance may be improved or deteriorated by the composition ratio of the material, the addition of impurities, and the method of synthesizing the bulk material.

The magnetic flux capturing of MgB 2 bulk magnets made by spark plasma sintering process has been precisely investigated to clarify the mechanism of flux motions during the pulse-field magnetization processes. The field trapping ratio B T / B P was evaluated as a key parameter of field trapping ability which strongly relates to the heat generation due to the rapid flux motion in the samples. The time dependence of magnetic flux density revealed the actual flux motion which penetrated the samples. The trapped fields B T and field trapping ratios B T / B P of various samples were classified into three regions of ‘no flux flow’, ‘fast flux flow’ and ‘flux jump’ according to the generation of heat and its propagation. A flux jump was observed late at 280 ms from the beginning of PFM process, while the field penetration B P showed its peak at 10 ms. Considering the heat propagation speed, the long-delayed flux jump should be attributed to the macroscopic barriers against the heat propagation to the surface centre of bulk magnet.