A. Ishihara

Permanent magnet with MgB2 bulk superconductor

Superconductors with persistent zero-resistance currents serve as permanent magnets for high-field applications requiring a strong and stable magnetic field, such as magnetic resonance imaging. The recent global helium shortage has quickened research into high-temperature superconductors (HTSs)—materials that can be used without conventional liquid-helium cooling to 4.2u2009K. Herein, we demonstrate that 40-K-class metallic HTS magnesium diboride (MgB2) makes an excellent permanent bulk magnet, maintaining 3u2009T at 20u2009K for 1 week with an extremely high stability (<0.1 ppm/h). The magnetic field trapped in this magnet is uniformly distributed, as for single-crystalline neodymium-iron-boron. Magnetic hysteresis loop of the MgB2 permanent bulk magnet was detrmined. Because MgB2 is a simple-binary-line compound that does not contain rare-earth metals, polycrystalline bulk material can be industrially fabricated at low cost and with high yield to serve as strong magnets that are compatible with conventional compact cryocoolers, making MgB2 bulks promising for the next generation of Tesla-class permanent-magnet applications.

A low-cost batch process for high-performance melt-textured GdBaCuO pellets

High-Tc superconducting magnets promise a variety of industrial, medical, public, and research applications. However, the potential large-scale applications of these materials need excellent and uniform properties and a cheap production method. The batch process developed for the fabrication of GdBa2Cu3Oy pellets in air fulfils all of these requirements. The samples were melt-processed using a cold seeding method with thin film Nd-123 seeds grown on MgO crystals. We used self-made Gd-123 and Gd-211 powders mixed with 0.1xa0wt% of Pt. Up to 1–1.5xa0kg of melt-grown Gd-123 bulks could be prepared in one run. XRD analysis confirmed that all of the bulks were c-axis oriented. The superconducting and magnetic performance of the pellets was checked on several small test samples cut out at various standard positions within the bulk. The values were reasonably uniform and the performance was similar to the oxygen-controlled melt-grown Gd-123 samples. The average trapped field at 77xa0K in the 24xa0mm diameter batch samples was between 0.8 and 0.9xa0T, close to the maximum value of 1xa0T reported so far for Gd-123 single grains processed in air. The present results prove that a high-performance good-quality LREBa2Cu3Oy material can be scaled up from laboratory to industrial production.

Development of a compact, lightweight, mobile permanent magnet system based on high Tc Gd-123 superconductors

We have designed and fabricated a compact, lightweight, mobile permanent high-Tc superconducting magnet system using melt-processed GdBa2Cu3Oy rings. Magnetic field distribution inside the annular magnet reached 0.75 T to 2.65 T, for one to ten rings, respectively. The increasing number of rings enhanced magnetic field in the center position and caused the field to be flatter over the entire surface. Using this new permanent magnet, we successfully magnetized several melt-processed bulk samples at 77 K. Use of simple superconducting permanent magnet for magnetizing bulk melt-processed materials can open new technological windows in various industrial areas.

Novel seeds applicable for mass processing of LRE-123 single-grain bulks

A batch process with cold seeding was developed, which considerably reduces the production cost of various melt-processed LREBa2Cu3Oy (where LRE represents a light rare earth element) compounds. The key prerequisite for successful batch production is novel thin film Nd-123 seeds grown on MgO crystals, compatible with all LRE-123 materials and verified on large Gd-123 and NEG-123 single grains. The samples exhibit a sharp superconducting transition (around 1 K wide) with the onset Tc around 93.5 K. The trapped field measurements showed that all the samples in the batch were single domain and of good pinning performance. The trapped field of nearly 1 and 1.2 T was observed in the best 24 mm single-grain pucks of Gd-123 and NEG-123, respectively. The trapped field in the best 45 mm Gd-123 single-grain pucks reached 1.35 T and 0.35 T at 77 K and 87 K, respectively. Using these batch-processed Gd-123 bulks, we constructed a middle-sized levitation disk. This proved that the present approach leads to production of melt-processed LRE-123 materials with a reasonable cost and high performance suitable for superconducting super-magnets.

Next generation of prototype direct current superconducting cable for railway system

The authors design and develop a prototype DC superconducting cable for railways. For the preliminary evaluation 2 m Bi-2223 DC cable is used. The maximum current flowing in the cable cooled by liquid nitrogen was 1720 A in the inner sheet wire and 2430 A in the outer sheet. The experimental results and simulations led to the conclusion important for practical applications that the leakage of magnetic field was negligible. This result is of crucial importance, in particular for use in railway systems.

Development of 10 kA high temperature superconducting power cable for railway systems

The superconducting Bi2Sr2Ca2Cu3Oy “Bi-2223” tapes made with a powder-in-tube process laminated with a copper alloy have been used to design and develop a 5 m long multilayer concentric cable to be applied in railway systems. The electrical performance of the conductor has been evaluated with DC transport current at an economical 77 K (liquid nitrogen temperature) in the self-field. The inner and outer sheet wires were able to sustain electric currents of 10u2009130 A and 10u2009910 A, respectively. These values are sufficient for several types of commercial applications. The current values are significantly higher than the earlier reported ones in Bi-2223 or YBa2Cu3Oy “Y-123” superconducting cables, indicating that the tape quality has considerably improved.

Design and construction of a high temperature superconducting power cable cryostat for use in railway system applications

The primary objective of the current effort was to design and test a cryostat using a prototype five-meter long high temperature Bi2Sr2Ca2Cu3Oy (Bi-2223) superconducting dc power cable for railway systems. To satisfy the safety regulations of the Govt of Japan a mill sheet covered by super-insulation was used inside the walls of the cryostat. The thicknesses of various walls in the cryostat were obtained from a numerical analysis. A non-destructive inspection was utilized to find leaks under vacuum or pressure. The cryostat target temperature range was around 50 K, which is well below liquid nitrogen temperature, the operating temperature of the superconducting cable. The qualification testing was carried out from 77 down to 66 K. When using only the inner sheet wire, the maximum current at 77.3 K was 10 kA. The critical current (Ic) value increased with decreasing temperature and reached 11.79 kA at 73.7 K. This is the largest dc current reported in a Bi2Sr2Ca2Cu3Oy or YBa2Cu3Oy (Y-123) superconducting prototype cable so far. These results verify that the developed DC superconducting cable is reliable and fulfils all the requirements necessary for successful use in various power applications including railway systems. The key issues for the design of a reliable cryogenic system for superconducting power cables for railway systems are discussed.

Mass Production of Low-Cost GdBaCuO Bulk Superconductors Using a Novel Seed in the Batch Process

We developed a batch process with cold seeding, which considerably reduces the production cost. This batch process is appropriate for fabrication of various LREBa2Cu3Oy (where LRE represents a light rare earth element) compounds as it employs novel thin film Nd-123 seeds grown on MgO crystals, compatible with all LRE-123 materials. In the first stage we are able to fabricate GdBa2Cu3Oy compound with large single grain, enhanced pinning, and good quality at a dramatically reduced cost. Magnetization results conformed that the samples have a sharp superconducting transition (around 1 K wide) with the onset Tc around 93.5 K. The self-field Jc of 72 kA/cm2 was achieved at 77 K, H//c-axis in various positions of the pellet. The trapped field measurements showed that the samples were single-domain and of good pinning performance. The trapped field observed in the best 45 mm single-grain puck of Gd-123 was in the range of 1.35T and 0.35 T at 77.3 K and 87.3 K, respectively. The batch process technology for production of melt-processed LRE-123 provides a cost-effective method to fabricate high-performance super- conducting super-magnets for various commercial applications.

Design and Development of Superconducting DC Cable for Railway Applications

We report the recent development in the national project dealing with a prototype of the next generation dc superconducting cable for railways. The main goal of the project is to upgrade the feeder of the overhead contact line system connecting the electric train with the substation. The superconducting cables are usually considered to reduce the resistive losses generated in a conventional feeding system mainly in the catenary wires. In this paper, various cable structures existing in the railway network were studied with respect of their use in the next generation of railways. The superconducting cables need to be cooled below 77 K. They consist of coaxially configured conductors and sheaths. Liquid nitrogen is forced to flow through these coaxial cables. Various designs and configurations of the cooling systems possess always some advantages and some drawbacks. Our systematic analysis showed that a `go and return system, where the liquid nitrogen flows through the hollow former and returns through the space between the outer layer of the HTS tapes and the cryostat wall is most effective for the railway applications; it saves both space and costs.

Evaluation of Temperature Dependence of Magnetic Field Distributions of Bulk Superconductor Annuli

Industrial applications of melt-processed bulk superconductors with Tesla-class magnetic field are still tremendously limited by not only field strength but also field uniformity. Here, the authors report the method of generating a homogeneous field using bulk superconductor annuli. Two annuli with their c-axes along the vertical direction, whose dimensions are 80-mm outer and 50-mm inner diameter and 20-mm thickness, without a nonmagnetic metal ring and epoxy resin for additional mechanical strength, respectively, were vertically constructed with various gaps. Their field uniformity was simulated with changing parameters, difference of critical current density, gaps, and all. Field distributions in the composition with optimized gap (~13 mm), in terms of field uniformity, were measured. As a result, the field strength of 0.92 T and about 2% homogeneous volume in 20-mm-diameter spherical volume were achieved. Furthermore, by a decrease in operating temperature by a cryocooler, field strength was over four times stronger at 50 K than at 77 K. It is clarified that the strong and homogeneous magnetic field can be generated by optimizing the composition and operating temperature.