Hiroki Kamijo

Preliminary study of a superconducting bulk magnet for the Maglev train

One of the prospective applications of high critical temperature superconductors is a superconducting magnet for the magnetically levitated (Maglev) train. Development shows that RE (rare earth) BaCuO and LRE (light rare-earth) BaCuO superconductors prepared by melt processes have a high critical current density at 77 K and high magnetic fields. LRE-Ba-Cu-O bulk superconductors melt-processed in a reduced oxygen atmosphere, named oxygen-controlled-melt-growth (OCMG) process, are very promising for high field application as a superconducting permanent magnet with liquid nitrogen refrigeration. Compared to good quality melt-grown REBaCuO bulks, LREBaCuO bulks exhibit larger critical current densities in high magnetic fields and much improved irreversibility field at 77 K, implying that more effective flux pinning can be realized in a commercially feasible way. In this study, we discuss the possibility of a superconducting bulk magnet for a Maglev train. A preliminary design of the bulk magnet and also melt processing for REBaCuO and LREBaCuO bulk superconductors and their characteristic superconducting properties are presented.

Tests of Superconducting Traction Transformer for Railway Rolling Stock

We have been developing a lightweight and high-efficiency superconducting traction transformer for railway rolling stock. We designed and fabricated a prototype superconducting traction transformer of a floor-mount type for Shinkansen rolling stock. In this study, we present test results such as type test, system test and vibration test. We performed the type test in accordance with JIS, E5007. In this test, we measured the basic electrical characteristics. In the system test, we tested the transformer with a dynamic simulator for rolling stock, as a converter connected at secondary winding, and verified the operating characteristics. We performed vibration tests in accordance with JIS, E4031, vibrating the transformer on a shaking table with three-dimensional movement. As a result, we could verify that the transformer satisfied the requirement almost exactly as initially planned.

Experimental and numerical analysis of lift force in magnetic levitation system

A new type of magnetic levitation system using the shielding effect of the HTS bulk is studied. The system includes a permanent magnet and three HTS bulks as a vehicle, and a ferromagnetic bar as a guide way. The simple structure of the guide way reduces large amount of construction costs. The system also provides stable lift force without any active control system. The characteristics of lift force are numerically analyzed using IEM(Integral Element Method). Experiment was performed using a small model. The two results are compared.

Fabrication of Superconducting Traction Transformer for Railway Rolling Stock

We designed a floor type single-phase 4 MVA superconducting traction transformer for Shinkansen rolling stock. In this study, we fabricated a prototype superconducting traction transformer based on this design. This transformer of the core-type design has a primary winding, four secondary windings and a tertiary winding. The windings are wound by Bi2223 superconducting tapes and cooled by subcooled liquid nitrogen. The core is kept at room temperature. The cryostat is made of GFRP with two holes to pass core legs through. The outer dimensions are about 1.2m × 0.7m × 1.9m excluding the compressor. Its weight is 1.71t excluding that of refrigerator and compressor. The transformer was tested according to Japanese Industrial Standards (JIS)-E5007. We confirmed that the performance of transformer has been achieved almost exactly as planned. The rated capacity is equivalent to 3.5MVA in the superconducting state.

Increase of Levitation Properties on Magnetic Levitation System Using Magnetic Shielding Effect of GdBCO Bulk Superconductor

We have studied a magnetic levitation system using magnetic shielding effect of a superconducting bulk made of gadolinium. We extended the experimental apparatus from the previously used basic model to parallely placed 2-basic model. The peak levitation force in the 2-basic model was more than double the force in the basic model due to the bending of magnetic force lines. We could successfully obtain the large levitation force.

Repeated pulsed-field magnetization with temperature control in a high-T/sub c/ bulk superconductor

We investigated the applicability of high-Tc superconducting bulk magnets to the superconducting Maglev system. A bulk superconductor needs to generate as high a magnetic field as possible to obtain a powerful bulk magnet. The bulk magnet requires a larger external coil and larger power supply, and has a problem that large electromagnetic force is applied to the bulk in the process of magnetization. Therefore, a relatively small magnetic field is preferable to magnetize a high-Tc bulk superconductor. In this study, we magnetized a melt-processed YBCO bulk superconductor by repeated pulsed-magnetic field while controlling its temperature. In this method, a comparatively small pulsed-magnetic field was repeatedly applied to the bulk while lowering the temperature of the bulk from the critical level at intervals of magnetization. For example, the TBCO bulk was magnetized by a series of seven pulsed-magnetic fields, while temperature was lowered from 94 K to 82 K at intervals of 2 K. As a result, it is found that the bulk superconductor can generate a higher magnetic field by using a comparatively small magnetic field.

Development of Low-AC-Loss Bi-2223 Superconducting Multifilamentary Wires

We have been designing and developing advanced Ag-sheathed Bi-2223 multifilamentary wires for the future applications to superconducting transformers of electric rolling stocks. Trial wires were twisted without remarkable degradation in the transport property for suppressing AC losses in a perpendicular magnetic field configuration. The AC losses were measured by a saddle-shaped pickup coil in a wide range of frequency up to a commercial frequency. The experimental results show that the AC loss per cycle has linear frequency dependence in a region of the practical amplitude of AC magnetic field for the transformers, where the excess loss may come from an electromagnetic coupling between the filaments. The hysteresis loss evaluated as a low frequency limit of the AC loss was also reduced with the decrease in the twist pitch, which may be considered as a similar effect to the so-called ldquotwist effectrdquo observed in in-situ Nb3Sn wires.

Dependence of size and position of ferromagnetic bars on lift force in magnetic levitation system

Basic experiment for examining characteristics of lift force using magnetic shielding effect of a superconducting bulk is performed. The new levitation system provides stable lift force without any active control system. Simple structure of guide way on the ground with only a ferromagnetic bar makes it possible to reduce great amount of cost. The characteristics of lift force for various size of ferromagnetic bar are measured and examined. We have confirmed that there is a limit in the size of ferromagnetic bar to obtain stable lift force. The ferromagnetic bar with a width of 18 mm was the most applicable size in obtaining stability and larger lift force. The results also showed that magnitude of lift force depends on the width of ferromagnetic bar (when the height is within the parameter of 2 to 10 mm), not the volume.

Trapped magnetic field of a mini-bulk magnet using YBaCuO at 77 K

Abstract Melt-processed rare earth (RE)123 superconductors have a high Jc at 77 K and high magnetic field. Solidification processes for producing (L)RE123 superconductors and pinning centers in the (L)RE123 matrix are effective for obtaining high Jc, leading to high-field application as a superconducting quasi-permanent bulk magnet with the liquid nitrogen refrigeration. One of the promising applications is a superconducting magnet for the magnetically levitated train. We fabricated a mini-superconducting bulk magnet of 200×100 mm 2 , consisting of 18 bulks, which are a square 33 mm on a side and 10 mm in thickness, and magnetized the mini-magnet by field cooling. The mini-magnet showed the trapped magnetic field of larger than 0.1 T on the surface of the outer vessel of the magnet. The present preliminary study discusses trapped magnetic field properties of the mini-bulk magnet using YBaCuO superconductors at 77 K.

Flux Trapping Characteristics of YBCO Bulks using Pulse Magnetization

We have been studying magnetization characteristics of plural HTS bulks arranged in array. Previously, we presented experimental results using field-cool method in copper coils [1],a superconducting coil [2]. This time, we have carried out a pulse magnetization experiment using a capacitor bank and a pulse coil wound by copper wire. Plural HTS bulks are arranged in array in the magnetization coil. The trapped magnetic flux densities of the bulks are measured 2-dimensionally using Hall probe.