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Our group has been developing an Nb-rod-processed CuNb-reinforced Nb3Sn Rutherford cable for developments in high-field superconducting magnets, such as a 25-T cryogen-free superconducting magnet. The Rutherford cable has good mechanical, electrical, and superconducting properties in high fields. In this paper, we investigated the influence of winding coil diameters on the transport properties of the Rutherford cable for the coil application wound with a react-and-wind method. We prepared two Rutherford coils with diameters of 110 mm (0.2% pure bending strain) and 70 mm (0.6% pure bending strain) with 0.7% prebending treatment in order to measure the critical current at various temperatures in fields up to 18 T. We found that the Rutherford coil with a diameter of 110 mm has good transport properties. The critical current of the Rutherford coil with a diameter of 70 mm exhibited a deterioration of about 14% in comparison to the coil having a diameter of 110 mm. Although the tensile strain dependence of the critical current $(I_{c})$ suggests a large deterioration for the 0.6% pure bent coil, the Rutherford coil can keep the high $I_{c}$ under a large pure bending strain.

We have developed a CuNb-reinforced Nb3Sn (CuNb/Nb3Sn) wire. Critical current $(I_{c})$ of prebent CuNb/Nb3Sn coils with various diameters was investigated in order to develop a Nb3Sn superconducting magnet by a react-and-wind (R&W) method. It was found that the prebending treatment enhances $I_{c}$ of the small pure bending coil. In the small pure bending strain region, $I_{c}$ of the prebent coil with over 96-mm outer diameter is larger than that of the as-reacted coil. However, the as-reacted coils exhibited large $I_{c}$ in the large pure bending strain region. Calculated results on the basis of the perfect current transfer model with a neutral axis shift successfully indicated the enhancement behavior of $I_{c}$ by pure bending strains. Hence, we can understand the relationship between coil diameter and prebending treatment for the R&W method using CuNb/Nb3Sn wires.

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 $(I_{c})$ 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-4V alloy grooved cylindrical holder of 15.6-mm radius. Then, those $I_{c}$ 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 $I_{c}$ in the applied pure bending strain ranging from −0.5% to +0.5% and had larger $I_{c}$ than that of the as-reacted strand at no pure bending strain. In this paper, the $I_{c}$ enhancements of the prebent Cu-Nb/Nb3Sn strands under pure bending strains are analyzed by using the $I_{c}$ characteristics under axial strains.

We developed Cu-Nb reinforced Nb 3 Sn Rutherford cables for wide-bore high magnetic field coils that were able to be produced by React-and-Wind technique. The Cu-Nb/Nb 3 Sn 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/Nb 3 Sn 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 Nb 3 Sn Rutherford cables at room temperature. Critical current (Ic) values of Cu-Nb/Nb 3 Sn strands extracted from the Rutherford cables were compared to those of the single strands that had undergone the same processing as the cables. I c 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/Nb 3 Sn Rutherford cables were improved by utilization of the pre-bending process.

We are planning to develop three large superconducting magnets, using CuNb-reinforced Nb3Sn Rutherford cables with the prebending treatment. CuNb-reinforced Nb3Sn wires using a new Nb-rod-processed CuNb composite were developed for the Rutherford cables. In this study, the prebending effects for the mechanical and the superconducting properties of the Nb-rod-processed CuNb/Nb3Sn wire were investigated. We found that the CuNb/Nb3Sn wire with 0.8% prebending treatment had largely enhanced critical current at 4.2 K. The reduction of the n-value was observed around 0.8% prebending strain. The mechanical properties of the prebent wires were enhanced. Young's modulus and 0.2% proof stress of the wire with prebending treatment at 4.2 K are 180 GPa and 270 MPa, respectively. The residual strain was released from 0.30% to 0.15% by 0.8% prebending treatment.

A 7-strand Nb3Sn superconducting cable, which consists of three pre-bent CuNb/Nb3Sn strands, three stainless-steel (SS) strands, and a center SS strand, was fabricated. 2.5 turns of the cable was wound on a 262-mm diameter GFRP bobbin A triplet consisting of three pre-bent CuNb/Nb3Sn strands was also tested for comparison. The reinforcement effect of stainless-steel strands in a cable was investigated to develop a high-strength large-current superconducting cable. The hoop stress test result indicated that the stainless-steel reinforced CuNb/Nb3Sn cable showed better performance than the CuNb/Nb3Sn triplet under the hoop stress, because the stainless-steel strands reduced the hoop stress on Nb3Sn strands. Furthermore, the solder impregnation of the cable, which made the seven strands a monolithic conductor, improved its performance.

An application of the prebending effect to a cabling process using Nb3Sn strands was demonstrated. The prebending effect is the enhancement effect of superconducting properties due to the repeated prebending treatment for practical bronze-route Nb3Sn wires. CuNb/Nb3Sn and Cu/Nb3Sn strands were applied prebending treatment using 10 fixed pulleys with 0.8% prebending strain. Four kinds of triplets, i.e., prebent CuNb/Nb3Sn, no-prebent CuNb/Nb3Sn, prebent Cu/Nb3Sn and no-prebent Cu/Nb3Sn triplets were fabricated. Critical currents were measured for the four kinds of triplets in magnetic fields up to 11 T at 4.2 K. The obvious critical current enhancement due to the prebending effect was maintained for the prebent CuNb/Nb3Sn triplet. The results imply that the prebending treatment for high-strength Nb3Sn strands is applicable to the cable conductor fabrication.

New process to compose a reacted Nb3Sn superconducting cable with aluminum-alloys has been developed by using friction stir welding (FSW) technique. The FSW joining process, which takes place in the solid phase below the melting point of the materials, has considerable advantages such as a high strength of welding zone and much lower distortion due to both small heat affects and reliable control of joining depth as compared with conventional welding method. In order to realize practical aluminum-alloy jacketed Nb3Sn superconductors, we have carried out elemental researches by using FSW technique. For a fabrication of aluminum-alloy jacketed Nb3Sn superconductor of a dimension 17 mmw × 4.9 mmt a Rutherford type superconducting cable having 18 reacted Nb3Sn strands of diameter 1.0 mm was assembled into a set of channel and fitting-cover made of aluminum-alloy A6061-T6 with filling material indium. Then, their joining parts were connected by using the FSW technique. The measurement on critical current (Ic) of the fabricated Nb3Sn superconductor achieved over 9.5 kA at 8 T, 4.26 K even after bending with a radius of 150 mm in a flatwise direction, which showed no degradation in comparison with Ic values multiplied by number of strands in the cable. As the results, we succeeded in developing novel type of aluminum-alloy jacketed Nb3Sn superconductors manufactured by FSW technique, which have large current capacity, sufficient allowable bending strains for a coil winding and improved contact between a cable and an aluminum-alloy jacket. The developed superconductors will be applicable to react-and-wind superconducting magnets having very large current at high magnetic field such as nuclear fusion reactors and detectors for high energy physics of the next generation.