The manufacturing process of NbTi wires for lower field applications is optimized concerning both the number of heat treatments and the final true strain, the strain of a wire from the wire diameter at the last heat treatment to the final diameter. The sample wires were made from Nb-Mwt%Ti alloy and were manufactured to vary the number of heat treatments (0, 1,2, and 3) and the final true strain(0 to 3). One of these samples, which had one heat treatment and drew with the final true strain of 0.33, achieved Jc of 3,360 A/mm2 and nvalue of 111 at 3 T. This Jc is as good as a J c of the wires made from conventional materials and this n-value is 3 times as large as the n-value of wires mentioned above. Furthermore, the nvalue at 3 T and the minimum value of the cross-sectional area of each filament show a correlation coefficient of 0.73.
Characteristics such as the critical current density (J c ) and mechanical property of bronze-processed Nb 3 Sn superconducting wires have improved with the progress on high-field superconducting magnets such as nuclear magnetic resonance (NMR) magnets. On the other hand, for Nb 3 Sn wires used in intermediate magnetic-field applications such as fusion and particle accelerator magnets, the wire parameters have been optimized to realize high J c around 12 T. In this article, improvements in the J c and mechanical property of bronze-processed Nb 3 Sn wires based on the activities of Japan Superconductor Technology, Inc. (JASTEC) and Kobe Steel, Ltd. are described.
In order to develop bronze processed Nb Sn wire for the ITER CS coil operating under higher compressive strain, the influence of various parameters of Nb Sn wires such as filament diameter, barrier materials, barrier thickness, heat treatment pattern and Ti addition on critical current versus intrinsic strain ( 1.0% 0.1%) characteristics was investigated. The change of these parameters brought significant changes to superconducting properties such as and -value. In spite of different wire parameters, the strain dependency of normalized was almost the same, except that a Ti addition affects the upper critical field . This result suggests that assuming the same Ti-addition level, Nb Sn wire with higher performance at a certain would exhibit higher performance at any in the compressive regime. Based on the result, bronze processed Nb Sn wires with non-Cu critical current density more than 1100 A/mm at 12 T, 4.2 K, zero applied strain have been successfully developed for the CS coil.
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