Prebending strain effect on CuNb/Nb₃Sn superconducting wire during practical react-and-wind process

To demonstrate the applicability of the prebending strain effect to the R&W coil winding process, we have developed the prebent react-and-wind (R&W) process, which is the combination of the conventional R&W process and the prebending treatment. The superconducting and mechanical characteristics were investigated for CuNb/Nb₃Sn superconducting wires which were picked up from the prebent R&W coil winding process. The results were compared with that of CuNb/Nb₃Sn wires which were prebent manually using a prebending former. The prebending treatment using pulleys enhanced I_c more than the manual prebending treatment. This is explained qualitatively by a difference of the prebending strain distribution. The prebending strain distribution is plain symmetric and axis symmetric in cases of the manual prebending and the pulleyed prebending, respectively. The axis symmetrically distributed prebending strain reduces the residual strain more than the plain symmetrical one. The mechanical property was also enhanced. The slope of the stress-strain curve for the pulleyed prebent samples were steeper than that of as-heat-treated sample. The maximum I_c of the I _c-strain curve was enhanced and the strain value corresponding to the maximum I_c shifted to smaller strain by the pulleyed prebending treatment. Though in the large stress region over 200 MPa, the I _c-stress curves of pulleyed prebent samples and as-heat-treated sample were in good agreement, suggesting that the I_c enhancement effect by the prebending treatment was not beneficial, in the small stress region, it is quite beneficial.