We have found that prebending treatment, which is repeated bending load at room temperature, greatly enhances the critical current, upper critical field and critical temperature of practical Nb 3 Sn superconducting wires. In this paper, we focus on the application of the prebending strain effect to practical superconducting coils fabricated by a react-and-wind method.To demonstrate the prebending strain effect on the react-and-wind coil, we prepared two kinds of CuNb/Nb 3 Sn superconducting monolayer coils. For one of the coils, the superconducting wire of 1.0 mm diameter was repeatedly bent by using ten fixed pulleys before the winding process, resulting in a prebending strain value of 1.0%. The final winding diameter of both coils was 200 mm and the number of turns was 49.In the compressive stress condition, the critical current of the coil with 1.0% prebending strain was 296 A at 11 T, which was larger than that of the coil with 0% prebending strain. These values were much larger than the critical current of the witness sample. The results indicate that the prebending treatment enhanced the coil critical current.In the case of the hoop stress condition, the coil critical currents were approximately 230 A at 11 T for both coils. The maximum electromagnetic hoop stress was calculated to be 360 MPa. The short sample tensile test results qualitatively explained the critical current deterioration of the coil. Furthermore, the 360 MPa hoop stress did not deteriorate the critical current irreversibly.
The superconducting wire with approximately 10 pm diameter filaments of' Nb-4lwt%Ti-28wt%Ta alloy was fabricated from our standard production billet. Critical current densities and upper critical fields aRer various heat treatments were measured in the temperature range of 4.2 K to 1.6 K. Thermo-mechanical treatments for a-Ti precipitates were optimized to improve critical current densities. A 27-strand Rutherford type cable having a large keystone angle was trial-fabricated using the optimized-processed wires. The achieved J,'s were 1383 A/mmz (10 T and 1.9 K) and 602 A /mm2 (12 T and 1.9 K). The fabricated wire was proved to have high homogeneity and good workability.
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