Axial Tensile/Transverse Compressive Stress Characteristics of Advanced Cu-Nb Reinforced Nb3Sn Wires for 33 T Cryogen-Free Superconducting Magnet

Nb-rod method Cu-Nb reinforced Nb₃Sn wires (Cu-Nb/Nb₃Sn wires) have been applied in various applications as one of high strength Nb₃Sn wires and 25 T cryogen-free superconducting magnet (25 T-CSM) at Tohoku University is one of the successful examples. To further strengthen the Cu-Nb/Nb₃Sn strands for a 33 T cryogen-free superconducting magnet (33 T-CSM), currently under construction at Tohoku university, we implemented the following modifications: increasing Cu-Nb fraction, niobium content in Cu-Nb and tin content in the bronze material. The strands were cabled into 16- or 18-strands Rutherford cables and pre-bent with -0.30% to +0.31% strain after heat treatment for Nb₃Sn reaction. The heat treatment conditions were 670 °C × 96 h (HT-A) or 575 °C × 100 h +670 °C × 50 h (HT-B), depending on the critical current (I_c) requirement of the magnets. To evaluate the performance of the Cu-Nb/Nb₃Sn wires, we measured the I_c of both non-cabled strands and strands extracted from the cables under axial tensile stress and transverse compression stress at 14.5 T and 4.2 K, respectively. Under tensile stress, the I_c of the 33 T-CSM wire with HT-A reached a peak at 251 MPa and became the same I_c as zero applied stress/strain at 345 MPa. This is significantly higher than the previous Cu-Nb/Nb₃Sn wires for the 25 T-CSM, which had a peak I_c at 215 MPa and the same I_c as zero applied stress/strain at 300 MPa. An increase of residual strain by 0.049% was also observed. Regarding transverse compression characteristics, the 33 T-CSM strands maintained their initial I_c value under higher compression stress compared to the 25 T-CSM wires and cabling process caused no degradation in compressive stress performance in 33 T-CSM strands.