Enhancement of upper critical field and critical temperature by prebending process for practical Nb₃Sn wires

It is recognized that the strain in Nb₃Sn conductors is a serious problem, because it suppresses superconducting properties. We have proposed that prebending treatment, which means repeated bending loads, is one of the ways to control the strain state. The upper critical field and the critical temperature were examined for the prebent Nb₃Sn wires. We found that the enhancement of the upper critical field from 23.7 T to 25.2 T at 4.5 K and the enhancement of the critical temperature from 17.4 K to 17.9 K were achieved for the CuNb-reinforced Nb₃Sn wire by applying the prebending strain. Since these values are similar to those of the strain-free Nb₃Sn bulk, the strain state of the wires is definitely optimized by the prebending treatment.