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In order to improve superconducting property of Bi-2223 superconductor bulk, 5 mm in width, 22 mm in length and ca. 2 mm in thickness, silver (Ag) wires of 0.4 mm in diameter were composed in the bulk. The composite was made by stacking alternately Ag wires and the calcined powder. The pressed sample was sintered at 840/spl deg/C for 50 hours in air. After treatment with a cold isostatic press (CIP) as an intermediate pressing, the sample was sintered again. With 6 times of CIP processing, the maximum critical current density (J/sub c/) at 77.3 K of the sample composed with twenty-four Ag wires was obtained to be 950 A/cm/sup 2/. SEM and EPMA observation results showed that highly c-axis oriented and densely structured Bi-2223 plate-like grains could be formed around the interfacial region between the superconducting oxide and the metal Ag.

We fabricated V-based Laves phase compound such as V/sub 2/(Hf,Zr) multifilamentary wire by applying a rapidly-heating/quenching (RHQ) process. Pure Hf and Zr metal powders were mixed and it put in high pure V tube composed with tantalum (Ta), and it was drawn in order to make wire (powder-in-tube (PIT) method). The wires were heat-treated by RHQ process, and some were annealed additionally in vacuum. Since Ta has a higher melting point than V, it is expected that the alloying prevents from breakage of the precursor wire during RHQ treatment because the maximum temperature exceeds 2500 K. SEM observation confirmed that the thick reaction layer within about 10 microns was formed on the boundary of the powder mixture filament and the V matrix after the RHQ. T/sub c/ values of the as-quenched samples increased with increasing maximum temperature during RHQ process, and the maximum mid point T/sub c/ value was about 8.0 K. The transport J/sub c/ properties under magnetic fields were also investigated.

A superconducting solenoid coil was constructed using a large-scale superconductor to investigate the stability of helical coil in the Large Helical Device. The coil was cooled down to 3.2-4.4 K and current of 13 kA was put in. Superconductor motion was detected directly with a high sensitivity displacement gage. At the center of the magnet, the conductor expanded to outside stably, but the conductor at the corner showed contrary and incomprehensible behavior expected the conductor rotation due to mismatching to the magnetic field.

Superconducting magnets of helical coils for the Large Helical Device (LHD) have been operated by saturated helium at 4.4 K and plasma experiments have been carried out at less than 3 T successfully. Now, it is considered that the subcooled He I are used to improve the operating condition of the superconducting magnets of the LHD helical coils. To use subcooled He I as a coolant for the superconducting magnets of the LHD helical coils, we need to investigate the cooling characteristics and the stability of a superconducting coil in the subcooled He I. So, an R & D coil, wound with superconductors for the LHD helical coils, was cooled with the subcooled and saturated helium, and then stability tests of the coil were carried out. Subcooled and saturated He I was supplied from the bottom of the coil and flowed out to a bath of current leads at the pressure of 120 kPa. The temperature of the supplied He I was 3.1, 3.5 (subcooled He I) and 4.4 K (saturated He I) at the outlet of the heat exchanger. In the present experiment, the helium temperature distribution in the cooling channels of the R & D coil was measured under steady state conditions and the transient helium temperature change in the channels was observed in stability tests. This paper discusses the thermal hydraulic characteristics of the R & D coil from these results.

To improve superconductivity and mechanical properties of Bi-2223 sintered bulk, silver (Ag) wires of 0.4 mm in diameter were added to the bulk. The sample was sintered at 840/spl deg/C for 50 hours in air. After treatment with cold isostatic pressing (CIP), the sample was re-sintered. When twenty-four Ag wires were composed, the critical current density (J/sub c/) at 4.2 K and self-field increased from 1,200 A/cm/sup 2/ to 2,000 A/cm/sup 2/ with the CIP process. SEM observation results showed that high alignment and dense structures of Bi-2223 plate-like grains were formed around the interfacial region between the superconducting oxide and the metal Ag. The mechanical properties were measured by a three point bending test. The maximum bending stress of 60 MPa was obtained in the bulk without Ag wire, which was fractured separately. With the CIP process the maximum bending stress increased to 90 MPa for the composite. The composite did not fracture but only fine cracks were induced after the maximum bending stress.

We have measured contact resistance between superconducting strands that was a parameter of coupling losses in cable-in-conduit conductors. Assuming some kinds of twist pitches of sub-cables in the multi-stranded cables, we measured the contact resistance under the condition of the intersecting strands. Since not only the resistance but also mechanical properties of the strands (applied force to the strands and deformation of the strands due to the force) were measured, the contact area could be precisely estimated, and then the surface resistance was also evaluated. The experimental results showed that the surface resistance hardly depended on the twist pitches of the sub-cables at both liquid nitrogen and liquid helium temperatures. The results become fundamental data to estimate the coupling losses caused by current loops including more than one sub-cable with long-time constants of current decay.

In the National Institute for Fusion Science (NIFS), the following three research items are being advanced as application of high temperature superconductor (HTS) to an experimental fusion plasma system, through collaboration with universities and laboratories. The first is an application of HTS in the large current feedthrough to a superconducting coil cooled with superfluid helium in order to reconcile low heat load and stable large current transport capability. A 20 kA current feedthrough from 4.4 K to 1.8 K has been being developed using YBCO bulk superconductors manufactured by the QMG process. The second is the development of a 30 kA HTS current lead. The Bi2212 bulk material fabricated by the diffusion process is considered as a candidate material for HTS current leads. The third is the development of the superconducting magnetic-levitation coil for Mini-RT wound with Bi2223 tape conductor. A magnetic-levitation coil operated in persistent-current mode has to levitate for 8 hours without a feed of refrigerant. Present status of these HTS application researches in NIFS is reported.

To improve the superconducting properties of the high-T/sub c/ superconductors, we prepared the (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub y/ (BPSCCO-2223) bulk composite with embedded Ag wires. The composites were hand-made by stacking alternately Ag wires into the oxide powder in the thickness direction, and then the prepared samples sintered at 840/spl deg/C for 50 hours. SEM and EPMA observation showed that good c-axis oriented and densely structured BPSCCO-2223 plate-like grains could be formed around the interfacial region between BPSCCO-2223 oxide and metal Ag wire. We measured critical current density (J/sub c/) performance at 77.3 K and self-field and/or 4.2 K under several magnetic fields. As the J/sub c/ values of the BPSCCO-2223/Ag wires composite bulks were higher than that of BPSCCO-2223 bulk without Ag wires, J/sub c/ values were improved with increasing the number of Ag wires at several measurement conditions. The maximum J/sub c/ was obtained about 230 A/cm/sup 2/ at 77.3 K and self-field, 800 A/cm/sup 2/ at 4.2 K and self-field when forty-two Ag wires were composed. These J/sub c/ values corresponded to about twice that of BPSCCO-2223 oxide bulk without Ag wire. The improvement of J/sub c/ value was brought about by forming good c-axis oriented and densely structured BPSCCO-2223 plate-like grains.

The National Institute for Fusion Science (NIFS), in collaboration with universities and laboratories in Japan, the Forschungszentrum Karlsruhe (FZK) and the Max-Planck Institut fur Plasma Physik (IPP) in Germany, is planning to develop high temperature superconducting (HTS) current feeders for large-scale superconducting coils. Two programs are being progressed: one is a current feedthrough for superfluid helium (He II) cooled superconducting coils; the other is current leads for experimental fusion system. The paper describes the present status of the two programs, including joint experiments of a 20 kA Bi-2223 current lead developed by FZK and development of a prototype YBCO bulk current feedthrough for He II cooled superconducting coils. In addition, the test results of Bi-2212 tubes fabricated by diffusion process, applicable to HTS current leads, are described Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8/.

The Large Helical Device (LHD) is a plasma physics experimental device having two superconducting helical coils and three pairs of poloidal coils. All assembly and attachment works of LHD were completed in January 1998. The trial operation and the first plasma ignition of the eight-year Phase I project for LHD were finished in March just as planned. After that time the first cycle plasma experiment was carried on at a plasma central field of 1.5 T. The second cycle experiment is now conducted. The LHD magnets will be excited to a central field of 3.0 T in December 1998.