High-strength CuNb/Nb3Sn strand cables with residual strain controlled by the repeated bending treatment

We have developed practical multifilamentary Nb3Sn wires with CuNb composite reinforcement (CuNb/Nb3Sn). In order to fabricate a CuNb/Nb3Sn superconducting magnet by a react-and-wind method, the bending strain effect was investigated in detail for CuNb/Nb3Sn wires. We found that the repeated bending treatment enhances Tc from 17.4 to 17.9 K and Bc2 from 24.0 to 25.3 T at 4.2 K for CuNb/Nb3Sn wire. As a result, the repeated bending treatment for CuNb/Nb3Sn wire outstandingly enhances the critical current in high magnetic fields. A Nb3Sn filament was prepared by chemically solving practical multifilamentary CuNb/Nb3Sn wires. After removing Cu stabilizer, CuNb reinforcement, Nb barrier, and Bronze, Bc2 of a Nb3Sn filament was measured, and the Bc2 value of 25.9 T at 4.2 K was obtained. This means that the bending treatment for CuNb/Nb3Sn wires extremely reduces the residual strain close to the strain free state. To decrease the residual strain, it is important to control the three-dimensional strain distribution of CuNb/Nb3Sn wires. We intended to apply the bending effect to the cabling process of CuNb/Nb3Sn strands by a react-and-wind method. High strength Nb3Sn cables consisting of CuNb/Nb3Sn strands and stainless steel reinforcement strands were developed for a performance test of a next phase superconductor with a large critical current and a strong mechanical property at a high magnetic field of 20 T.