Hirofumi Kasahara

An optimal configuration design method for HTS-SMES coils

Recently, the properties of high temperature superconducting tapes have been in advance and high temperature superconducting magnets have been constructed and demonstrated. However, the high temperature superconducting tapes have different thermal characteristics compared with low temperature superconducting wires. Therefore, it is necessary to consider these characteristics of high temperature superconducting tapes at the magnet design stage. We proposed an optimal design method for superconducting coils wound with Bi2223/Ag tapes. In this paper, the configuration of 72 MJ SMES coils wound with Bi2223/Ag tapes are optimized.

Material and electro-magnetic aspects of newly developed Nb-Ti wires for AC use with Cu-Si alloy matrix

The material characteristics of Cu-Si alloy as a new matrix material for AC superconducting wires are presented. The successful trial fabrication of submicron-diameter Nb-Ti multifilamentary wires with Cu-2.5 wt.% Si alloy matrix is described. The wires, which had filament diameters down to 0.1 mu m, showed high enough critical current density and low AC loss compared to Nb-Ti AC superconductors with a Cu-Ni alloy matrix.<<ETX>>

Ultrafine multifilamentary Nb Ti wires with Cu Si alloy matrix

Abstract The Cu  Si alloy has been proposed as a new matrix material for filamentary Nb  Ti wires in a.c. use. The Cu  Si alloy shows appropriate mechanical and electrical properties, and is economically more favourable than the Cu  Ni alloy matrix used currently. Moreover, the addition of Si to Cu prevents the formation of intermetallic compounds around the filaments. After extensive investigations on CuSi alloy as a matrix material, an ultrafine multifilamentary Nb  Ti/Cu − 2.5 wt% Si composite wire has been successfully fabricated. The d.c. superconducting properties of the wire were adequate for use in electrical apparatus. A preliminary study has revealed that the a.c. loss of the new wire is equivalent to that of a typical ultrafine multifilamentary Nb  Ti/Cu  Ni composite wire.

Research and development of high-T/sub c/ SMES

High-T/sub c/ superconducting technology is thought to provide many merits for SMES systems. For example, a cryocooled system can be used as a cooling system for High-T/sub c/ superconducting coils, indicating that an operation temperature can be selected from a wide-temperature range below critical temperatures. Refrigerator cooling operation temperature for High-T/sub c/ SMES can be elevated more than 20 K from conventional 4.2 K. As a result, the heat capacity of coil system becomes much larger than that at 4.2 K, indicating that thermal diffusion time constant becomes much longer. If we could absorb transient heat generation with heat capacity of the coil, SMES systems can be designed under the over current state of critical current for a short duration. As a cooling capacity for an average heat load will be enough to cool High-T/sub c/ superconducting coil system for SMES, the refrigerator system cost can be much lower than that for a SMES system using Low-T/sub c/ superconductors. Moreover, we are developing high critical current superconducting wire for SMES system. The Bi2212 Rutherford conductors can carry 4 kA at 26 K under cryocooling. We also estimate the superconducting wire cost of the whole coil system, which is designed to minimize the superconductor volume. The conclusion is that the cost of High-T/sub c/ SMES system will be reduced by using the low-cost YBCO superconducting wires in the future.

Applications of Low Temperature, AC Superconducting Magnets to Material Processing

Superconducting technology has conventionally been used in magnetic separation using high DC field as its industrial application. In the field of electromagnetic processing of materials (EPM), on the other hand, it has a lot of advantages in the production of high quality, high value-added materials, such as uniformity in crystals, faster refining speed, facilitation of separation of inclusions, and attracts considerable attention. In the present study, we intended to lay a new path to industrial application of superconducting technology, and aimed at applying AC superconducting technology to high AC field as is expected in EPM. We examined thermal insulation technology regarding high temperature metals and low temperature superconducting magnets, and showed a path to solutions. We also designed magnets for applied stirring field, and made a prototype of magnets in which effective countermeasures are adopted against the shape of magnets and electromagnetic oscillation

Critical currents of Nb/sub 3/Sn superconductors for generators under cyclic mechanical load

Critical currents of Nb/sub 3/Sn cabled superconductors were measured under cyclic mechanical compressive force of 30 MPa up to 10000 times. The conductors were developed for a superconducting generator under the Super-GM project. The conductors were designed to apply the field winding of the 70 MW-class superconducting generator. The 10000 repetitions of force application simulate 30 years of centrifugal and electromagnetic force produced by the daily start-and-stop (DSS) operation of the superconducting generator. In the test facility constructed at the Akagi Testing Center of the Central Research Institute of Electric Power Industry (CRIEPI), mechanical compressive force is generated by the cyclic fatigue test machine, which can generate 100 kN of force and is located on top of a cryostat at room temperature. Measured critical currents of two types of Nb/sub 3/Sn conductor did not change throughout the test. >

AC loss properties of Bi-2212 Rutherford-type cables

We investigated the AC loss properties of a Bi-2212 Rutherford-type cable with large current capacity for future SMES. The cable is composedof 20 strands with a diameter of 0.8 mm and reinforced by Ni based alloy, which is located in the center of the cable. The width and thickness of cable are 9.05 and 2.25 mm respectively. The strand has 61×7 filaments and the average diameter of the 61 filament bundle is about 0.2 mm. The critical current of the cable is about 2.9 kA at 4.2 K and 0 T. We measured the AC losses in the strand and the cable at 4.2 K by pick-up coil method. The AC loss in the strand was scarcely dependent on frequency since it was not twisted. On the other hand, the AC losses in the cable had remarkable frequency dependence. It may be due to the coupling current between strands. We quantitatively discuss the AC loss of the cable in comparison with theoretical evaluation.

Hysteresis loss and critical current density of multifilamentary (NbW)3Sn composite conductor for AC use

Abstract A multifilamentary Nb3Sn composite conductor for AC use was fabricated by the internal tin-core method. In this conductor, Nb–1at.%W is used for filaments and Cu–1at.%Ni for matrix. The conductor has 33,252 filaments of a diameter of 0.36 μm. Samples of conductor were heat treated at 450–700°C for 24–740 h and the critical current and magnetization were measured at 4.2 K in transverse fields up to 5 T. Overall critical current density of 4.9 GA/m2 at 1.5 T is obtained with a heat treated at 600°C for 200 h. The effective filament diameter was estimated to be 3–5 μm at 1.5 T.

Development of an Nb/sub 3/Sn AC coil with react & wind method

The Nb/sub 3/Sn superconductor possesses a higher critical temperature than the NbTi strand. Therefore, it is now possible to make highly stable superconducting strands. We examined the manufacturing process of AC Nb/sub 3/Sn strands. The internal Sn diffusion process showed a higher critical current density than the conventional process. Nb/sub 3/Sn strands for AC use have a high Jc with low temperature reaction heat treatment, because they have fine filaments to decrease AC loss. We made a 400 kVA class superconducting coil using the developed Nb/sub 3/Sn cable with the React and Wind method. The loss density of this coil was 25.7 MW/m/sup 3/ at the point just before the quench. In this case, the temperature of the cable increases about 3.39 K. This means that the coil using Nb/sub 3/Sn cables has a very high stability in AC use.

Development of 100 kVA AC superconducting coil using NbTi cables with a CuSi alloy matrix

For implementation of AC superconducting equipment, it is imperative to develop low loss cables having highly stable characteristics. Here, newly developed NbTi superconducting cables using a CuSi alloy matrix are of low loss and are very promising as cables for practical application. However, since the CuSi alloy is a new material as a matrix for NbTi superconducting cables, many unknown factors as regards to optimum conditions for the manufacture of long cables, as well as superconducting characteristics are involved. For this new superconducting cable, a long strand (km class) was manufactured as a step for practical application, and a primary twisted cable was fabricated. Using this cable, a coil of the 100 kVA class was fabricated for trial, and its performance characteristic with transport current was evaluated. This coil had no training phenomenon and had a high stabilities. Furthermore, it permitted full AC current transmission of up to DCIc. Upon analysis of the coil loss, the hysteresis loss was smaller than coupling loss, and there was little increase of loss due to the current flow to the coil. Consequently, by using CuSi alloy matrix superconducting cables, it was possible to provide an AC coil of low loss and high stability, and the present cable was found to be promising as a new AC superconducting cable in the future.