Superconducting Properties of a Nb₃Sn Bundle Cable Made of Ultra-Fine Wires

We have been developing A15 cables made of ultra-fine wires for the high-field superconducting magnets using react-and-wind (R&W) technology. Recently, we succeeded to fabricate a bronze-processed Nb ₃ Sn ultra-fine wire with a minimum diameter of 30 μm. This wire contains 19 filaments and has a Cu/Non-Cu ratio of 0.89. In this study, we measured the transport critical current ( I _c ) of a Nb ₃ Sn bundle cable made by twisting 19 wires with a diameter of 50 μm at 4.2 K and under an external field up to 18 T. The Non-Cu critical current density ( J _c ) was also calculated from the I _c results. To replicate the R&W process, the cable was wound on glass fiber reinforced plastic cylinder (GFRP) bobbins after the heat treatment in straight geometry. To investigate the bending characteristics of the bundle cables, we prepared four GFRP bobbins with diameters of 50 mm, 30 mm, 25 mm, and 20 mm for I _c measurements. As a result, I _c degradation was observed only in the case of the 20 mm diameter bobbin. This indicates that the bending characteristics of the bundle cables are, as expected, greatly improved due to the smaller size of the wires. We also measured the critical temperature ( T _c ) and magnetization characteristics of the bundle cable by using a magnetic property measurement system of Quantum Design. The onset T _c was confirmed to be 17.1 K from the normalized magnetic moment. The low field Non-Cu J _c was also estimated from the magnetization data at 4.2 K, 6 K, 8 K, and 10 K.