Application of prebending effect to high strength Nb_3Sn strands

摘要

It is reported that the repeated bending loads at room temperature for Nb_3Sn wires, which show the strain dependence of superconducting properties, cause the large enhancement of a critical current, an upper critical field and a critical temperature. Those phenomena are so called "prebending effect". It is considered that the enhancement effect is associated with the reduction of the residual strain state for Nb_3Sn wires. In order to apply the prebending treatment to a react-and-wind processed Nb_3Sn superconducting magnet, repeatedly bent Nb_3Sn wires with bending strains up to 1.0% were fabricated by using pulleys, and were confirmed to demonstrate the large enhancement of the critical current in spite of the bending strain of 0.5%. From these experimental results, we intended to fabricate the high strength Nb_3Sn strands applying the prebending effect for the cabling conductor. As a first step, we prepared triplet cables using prebent CuNb/Nb_3Sn and Cu/Nb_3Sn strands and evaluated critical currents and n-values of each strand. The prebent Nb_3Sn strands show the large enhancement of critical current I_c even after the cabling process. Hence, it is expected that the prebent high strength Nb_3Sn strands may be useful for the cabling conductors.