Hidezumi Moriai

Enhanced current capacity of jelly-roll processed and transformed Nb/sub 3/Al multifilamentary conductors

In order to enhance the current carrying capacity, we have developed an improved fabrication process where the wire diameter can be increased from 0.5 to 1.25 mm and the Nb-matrix ratio decreased from 1.5 to 0.52, without degrading the critical current density, J/sub c/, of Nb/sub 3/Al phase. The critical current for a monolithic conductor at 21 T and 4.2 K has now been enhanced to 166 A which used to be 15 A. The compacted-strand-cables were fabricated to investigate feasibility for large-scale application uses. We have found that stranding and flat-rolling the as-quenched Nb/Nb(Al)/sub ss/ composite cause no degradation in J/sub c/. Attempts were made to stabilize the resulting high current conductors.

Incorporation of stabilizer to rapid-quenched and transformed Nb/sub 3/Al multifilamentary superconductors

A Nb/sub 3/Al superconducting conductor based on Jelly-Rolled (JR) Nb/Al composites is fabricated by the rapid-heating and quenching of a wire of such composites with subsequent transformation-annealing to form Nb/sub 3/Al phase. Since the conductor is heated up to 1900/spl deg/C, far above the melting point of Cu, during fabrication, Cu stabilizer cannot directly be included as a basic composite constituent. In the present study, attempts have been made to incorporate the stabilizer to the Nb/sub 3/Al conductor. In a mechanical-cladding method, the as-quenched composite is wrapped longitudinally with a Cu tape, and groove-rolled for mechanical bonding. For internal stabilization, Jelly-Rolled Nb/Al filaments are isolated from the stabilizers by a Nb-filaments barrier and the formation of unwanted Nb-Al-Cu (Ag) compounds are prevented. Ag-stabilizer, little reactive with Nb, can dramatically decrease the Nb-barrier thickness required and allows the increase in the volume fraction of the stabilizer instead.

Development of Nb3Al Superconductors

Stoichiometric Nb3Al has higher values of T c and H c2 than those of the commercial Nb3Sn superconducting wire, but stoichiometric Nb3Al is unstable except at high temperatures near 2000°C, while non-stoichiometric Nb-rich Nb3Al that has relatively low values of T c and H c2 is stable at low temperatures [1]. In addition, the Nb3Al formation rate through the solid-state diffusion reaction between Nb and Al is very slow.

Improved multifilamentary Nb3Sn conductors produced by the titanium-bronze process

The effects of a titanium addition to the bronze matrix of superconducting Nb3Sn wires have been investigated. The titanium addition to the matrix remarkably increases the Nb3Sn growth rate and the high-field, critical current density of the wire. An overall critical-current density of 3.8 × 104 A/cm2 at 15 T has been obtained for the multifilamentary Nb/Cu−7.5 at % Sn−0.4 at.% Ti wire with 4.7 μm-diameter 31 × 331 cores. The anisotropy in the critical current with respect to the field direction becomes larger with increasing aspect ratio of the rectangular-shaped multifilamentary wires. A 9.5 mm wide and 1.8 mm thick Nb/Cu-7.5Sn-0.4Ti conductor with 5 μ m-diameter 349 × 361 = 125 989 cores has been successfully fabricated on an industrial scale. This conductor carries a superconducting current of over 1300 A at 16.5 T. The newly developed Ti-bronze Nb3Sn conductor makes it feasible to generate a field of ~15 T in a large diameter bore.

Development of bronze-processed (NbTi)/sub 3/Sn superconducting wires for central solenoid model coil of ITER

Investigations to improve the critical current density of bronze-processed (NbTi)/sub 3/Sn superconducting wires for the central solenoid model coils in ITER were carried out. The effects of concurrent additions of Ti to the bronze matrix and of Ta to the filaments on the critical current density of the bronze-processed (NbTi)/sub 3/Sn wires were examined. By applying a high Sn concentration bronze matrix, a wire with a non-Cu J/sub c/ of 772 A/mm/sup 2/ at 12 T was developed.<<ETX>>

Current Developments of the Cu/Nb-Ti Superconducting Cables for SSC in Hitachi Cable, LTD.

In this few years, Cu/Nb-Ti superconducting cables for the dipole magnets of SSC projects have been developed in the in-dustrial scale in Hitachi Cable, Ltd. The features of these developed conductors are as follows. (1) The diameter of Nb-Ti filaments is very small, 4–6 µ m. (2) The critical current density (Jc) is very high, 2850–3050 A/mm2 at 5 T on wires, 2750–2950 A/mm2 at 5 T on cables in industrial scale. The champion Jc of wires is 3460 A/mm2 at 5 T in the laboratory scale.(3) The RRR (Residual Resistivity Ratio; ρ at293K/ ρ at4.2K) values of developed cables is very high, approximately 200, due to the newly developed high purity Oxygen Free Copper (0FC).(4) The conductors have been wound to the 1 m length dipole magnet in Hitachi Ltd., and it has generated 6. 7 T in the central magnetic field at 6595 A.

Fracture of filaments and its influence on critical current and residual strength of fatigued Nb-Ti/Cu superconducting composite

Abstract Fatigue behavior at room temperature and its influence on critical current at 4.2 K and residual strength at room temperature of multifilamentary Nb–Ti/Cu superconducting composite wire with a filament volume fraction of 0.49 (copper ratio of 1.04) were studied. The fatigue crack nucleated in the copper in the circumferential region and propagated stably into the inner region, causing fracture of the Nb–Ti filaments in the late stage of the fatigue life. Once the fracture of the filaments started, the number of the fractured filaments increased steeply with increasing number of stress cycles, and correspondingly, the current-transportable and stress-carrying capacity of the composite decreased steeply. In this process, both the critical current and residual strength of the fatigued composite decreased nearly linearly with decreasing fraction of surviving filaments. Thus, the critical current of the fatigued composite was proportional to residual strength as a first approximation.

Inter-Comparison Test Results for Residual Resistance Ratio of Cu/Nb3Sn Composite Conductors

The residual resistance ratio (RRR) measurement method for Cu/Nb3Sn was selected to be one of the next new work proposal items for submission to IEC-TC90 in the near future. Inter-comparison tests have been carried out for some years in Japan and show that the RRR of Cu/Nb3Sn for the external Cu type is very sensitive to heat-treatment conditions and has large scattering. On the other hand, a relatively low scattering factor of under 9% was obtained in the test in which the scattering problems at heat-treatment were removed. If pre-reacted Cu/Nb3Sn is treated, the goal of the standardization in RRR measurement method will be attained before long.

The outer cable phase IB results at Hitachi Cable, Ltd., for the SSC-VQP

Hitachi has been qualified to proceed to the phase II portion of the VQP (Vendor Qualification Program) from the standpoint of QA (quality assurance) as a result of the second QA survey practiced by the SSCL (Superconducting Super Collider Laboratory). The authors describe the fabrication procedures for Phase IB SSC outer strands and cables and summarize the strand production and the strand test results for the 0.648-mm-diameter outer strands fabricated from the first five pieces of the multibillets among the 20 pieces required to be processed in Phase IB. The first run for the 930-m-long cabling operating is discussed by considering a record of the inline cable measuring machine used for monitoring the cable dimensions. The QA program for Phase IB is also reviewed.<<ETX>>

Multifilamentary (Nb,Ti)3Sn Conductors for 15-T-Class Magnet Application

Ti-bronze multifilamentary Nb3Sn conductors have been successfully fabricated in full production scale through triple extrusion and drawing process, for a 15 T-class magnet. The heat treatment condition has been optimized using reduced size small capacity conductors. The 2.84. mm wide and 0.55 mm thick 5 ym-diam 31 × 361-core Nb/Cu-7.5at%Sn-0.4.at%Ti rectangular shaped conductor with the aspect ratio of 5.2 shows an overall critical current density Jc(overall) of 300 A/mm2 at 15 T in magnetic field parallel to the flat surface of the specimen after optimum heat treatment. A 190 mm winding inner diameter magnet has been constructed from double pancakes wound from 9.5 mm wide and 1.8 mm thick 5 ym-diam 34–9 × 361-core Ti-bronze Nb3Sn conductors heat treated at 660°C for 200 hr. The (Nb,Ti)3Sn intermediate coil is connected in series to the NbTi outer coil. The magnet has been successfully excited to a central magnetic field of 14.2 T in 28 min., and held at that field for 30 min. The magnet has been designed to have an enough margin for generating 15 T.