Prebending effects in bronze route Nb3Sn wires

Control and optimization of the residual strain are one of the most important issues in the development of Nb3Sn superconducting magnets. We found that the repeated bending loads at room temperature change the prestrain of Nb3Sn wires and result in the enhancement of J(c), B-c2 and T-c. We call this repeated bending strain 'prebending strain'. In order to understand the prebending effect, superconducting properties were measured as functions of temperature, field, axial tensile stress and strain. In addition, the three-dimensional strain state was also evaluated by the neutron diffraction. Those obtained results strongly suggest that the prebending process changes the radial residual strain as well as the axial one independently. Hence, it is considered that the prebending effect is effective for the control and optimization of the three-dimensional strain state on the react and wind process.