Internal Strain Measurement by Neutron Diffraction Under Transverse Compressive Stress for Nb₃Sn Wires With and Without Cu-Nb Reinforcement

For an accelerator magnet, a certain mechanical strength is required to sustain against a transverse compression stress due to Lorentz force. A bronze-route Nb ₃ Sn wire with Cu-Nb reinforcement was developed by Tohoku University and Furukawa Electric to enhance the strength against axial tension. The Cu-Nb reinforcement wire also exhibited some indication of strength improvement against transverse compression; however, the details of a reinforcement mechanism for the transverse compression stress have not been clarified. In this study, the internal strains of Nb ₃ Sn bronze-route wires with and without the Cu-Nb reinforcement under transverse compression stress were evaluated by neutron diffraction at BL19 (TAKUMI) in J-PARC. The samples were attached to jig with solder only at the ends and compression was applied at the center of the samples with 30-mm anvil with 5-mm wide and 8- to 15-mm high beam. Since a critical current, I_c of a superconducting wire depends on the three-dimensional strain, internal strain of Nb ₃ Sn along the axial and two orthogonal radial directions were evaluated at room temperature (RT). In the different setup, I_c measurements of the wires under transverse compression stresses were also performed at 4.2 K and 14.5 T. Using 3-mm wide anvil, the transverse compression was applied at 4.2 K or RT. The neutron diffraction results indicated no significant differences in the internal strains of Nb ₃ Sn under transverse compression between the samples with and without Cu-Nb reinforcement, while the I_c measurements showed potential increase in the irreversible stress ( σ_irr ) for Cu-Nb reinforced wires. The reason for this discrepancy was discussed based on the difference in the experimental setups for each measurement.