Sakaru Endoh

Very high critical current density of bronze-processed (Nb,Ti)/sub 3/Sn superconducting wire

From the viewpoint of fabricability, the Sn content in the bronze matrix of bronze-processed Nb/sub 3/Sn wire is limited to below 15 wt%. This limitation results in a higher matrix-to-niobium ratio of bronze-processed Nb/sub 3/Sn wire than other high-Sn-content wires, such as the tube-processed Nb/sub 3/Sn wire. This is a reason why the non-Cu critical current density (J/sub C/) of the bronze-processed Nb/sub 3/Sn wire is lower than that of tube-processed Nb/sub 3/Sn wire. Therefore, increasing the Sn content in the bronze matrix and reducing the bronze ratio is the key to enhancing the non-Cu J/sub C/ of the bronze-processed Nb/sub 3/Sn wire. A bronze-processed Nb/sub 3/Sn superconducting wire with a Cu-16wt%Sn-0.2wt%Ti matrix was successfully fabricated on a production scale. The wire achieved a high non-Cu J/sub C/ of approximately 1000 A/mm/sup 2/ at 12 T and 4.2 K.

(Nb,Ti)/sub 3/Sn superconducting wire with CuNb reinforcing stabilizer

An in-situ CuNb composite shows a high strength and a high electrical conductivity. Therefore, it is a suitable material for the reinforcement of Nb/sub 3/Sn superconducting wire. Many studies about mechanical and electrical properties of Nb/sub 3/Sn wire with CuNb reinforcing stabilizer have been carried out, but a fabricability of the long-length wire was not confirmed. In this study, 1 mm-diameter (Nb,Ti)/sub 3/Sn superconducting wire with Cu-20 wt%Nb reinforcing stabilizer was fabricated 22 km in length. The wire showed a good fabricability throughout the production. Mechanical properties and critical currents are evaluated in comparison with un-reinforced one.

(Nb,Ti)/sub 3/Sn superconducting wire reinforced with Nb-Ti-Cu compound

Reinforcement of Nb/sub 3/Sn wires is required for high field superconducting magnets because the Nb/sub 3/Sn superconductor is very sensitive to mechanical strain and its critical current degrades drastically at a higher absolute strain than 0.3%. Recently, Nb/sub 3/Sn superconducting wires reinforced with tantalum, Cu-Nb alloy and Al/sub 2/O/sub 3/-dispersed copper, have been developed. But, these high-strength wires do not show very high strength at an absolute strain of 0.3% because these reinforcements have a similar Youngs modulus to copper. In addition, Nb/sub 3/Sn layers of these wires experience an additional compressive strain caused by the differences in thermal contraction between reinforcements and Nb/sub 3/Sn layers. Such a compressive strain also decreases the critical current of the wire. An intermetallic compound was selected as a reinforcement for Nb/sub 3/Sn superconducting wire because such a compound is expected to have a high Youngs modulus and a similar thermal contraction to the Nb/sub 3/Sn compound. Fortunately, the Nb-Ti-Cu compound can be formed from the Nb-Ti and copper composite during reaction heat treatment for Nb/sub 3/Sn layers. Therefore, (Nb,Ti)/sub 3/Sn superconducting wire reinforced with the Nb-Ti-Cu compound has been fabricated. The wire shows a 0.2% proof stress of 400 MPa and maintains a critical current density equal to the unreinforced wire.

Development of high-strength Nb/sub 3/Sn conductor

Nb/sub 3/Sn superconducting wires reinforced with Cu-Ni/Nb-Ti composite have been developed. Nb/sub 3/Sn wires reinforced with Cu-Ni/Nb-Ti showed good mechanical and electrical properties. In this study, 1.2 mm-diameter (Nb, Ti)/sub 3/Sn superconducting wire of 13 km in length reinforced with Cu-Ni/Nb-Ti was successfully fabricated. Moreover, the effect of copper fraction to the strength of the wire was studied.

Development of Nb-Rod-Method Cu–Nb Reinforced Nb 3 Sn Rutherford Cables for React-and-Wind Processed Wide-Bore High Magnetic Field Coils

We developed Cu-Nb reinforced Nb3Sn Rutherford cables for wide-bore high magnetic field coils that were able to be produced by React-and-Wind technique. The Cu-Nb/Nb3Sn strands were reinforced by internal Cu-20 vol%Nb composites fabricated by the Nb-rod-method and were manufactured through the bronze-process. The Rutherford cables composed of sixteen un-reacted Cu-Nb/Nb3Sn strands of 0.8-mm diameter were heat-treated at 670 °C for 96 hours. After that, repeated bending strains (pre-bending strains) were applied to the flatwise direction of the reacted Nb3Sn Rutherford cables at room temperature. Critical current (Ic) values of Cu-Nb/Nb3Sn strands extracted from the Rutherford cables were compared to those of the single strands that had undergone the same processing as the cables. Ic values under the applied tensile stress of the Rutherford cable were increased by the pre-bending treatment as similar to the single strand. The performances of Cu-Nb/Nb3Sn Rutherford cables were improved by utilization of the pre-bending process.

The Prebending Strain Effect on

The prebending strain effect is that the repeated bending load at room temperature enhances superconducting properties of practical wires. The authors are now investigating the application of the effect to practical superconducting cabling conductors for high field superconducting magnets. Large current capacity and high strength are required for superconducting cabling conductors to make large scale and high field magnets. The superconducting cabling conductor with high strength wires will be useful for making high field magnets. The prebending strain effect was applied to a cabling technique with wires. High strength wires reinforced with CuNb composite and conventional wires without reinforcement were prepared, which were heat-treated at 943 K for 345.6 ks. Both wires were bent by 10 pulleys to give 0.8% prebending strain. After the prebending treatment the wires were assembled and fabricated 3-strand cables and 7-strand cables. The cables with high strength wires showed the enhancement of critical currents even after the cabling process. The results imply that the prebending treatment is applicable to the fabrication of cabling conductors for making a high field superconducting magnet.

\hbox{Nb}_{3}\hbox{Sn}

Nb-rod-method Cu-Nb reinforced Nb3Sn (Cu-Nb/ Nb3Sn) wires have been successfully developed for commercialization of the react-and-wind (R&W) processed Nb3Sn coils. Superconducting performance of the Cu-Nb/Nb3Sn wires is improved by applying the prebending treatment at room temperature, because of releasing the residual stress of Nb3Sn filaments (prebending strain effects). The purpose of this study is to confirm the efficient use of the prebending strain effects for optimizing critical current (Ic) characteristics under pure bending strain on the R&W coil design. The Cu-Nb/Nb3Sn strands of 0.8-mm diameter were heat treated at 670°C for 96 h on stainless steel grooved cylindrical holders of different radii of 10.1-32.1 mm. After half of the reacted strands were applied repeating prebending strain of ±0.5%, each of the prebent strands and the as-reacted (without prebending) strands was transferred to a Ti-6Al-4ν alloy grooved cylindrical holder of 15.6-mm radius. Then, those Ic, measurements were carried out under applied pure bending strains ranging from -0.9% to +0.84% at 11-17 T, 4.2 K. As a result, the prebent strand showed no deterioration of Ic, in the applied pure bending strain ranging from -0.5% to +0.5% and had larger Ic, than that of the as-reacted strand at no pure bending strain. In this paper, the Ic, enhancements of the prebent Cu-Nb/Nb3Sn strands under pure bending strains are analyzed by using the Ic, characteristics under axial strains.