Study on High Jc and Low Loss Experimental NbTi/Cu0.5Mn/Cu Superconducting Wires for Accelerator Magnet in WST

With the development of Chinese medical industry, medical proton therapy instrument accelerator magnets were gradually developed to miniaturization, which puts forward high requirements for the performance of low loss NbTi superconducting wires. A new experimental structure of high critical current density and low loss NbTi/Cu0.5Mn/Cu superconducting wire has been developed in this paper. The microstructure and hardness of high purity oxygen-free copper and Cu-0.5%Mn alloy were characterized, and the resistivity at 300 K, 10 K and 4.2 K were compared. The resistivity of Cu-0.5%Mn alloy at 300 K, 10 K and 4.2 K is 3.8 µΩ·cm, 2.2 µΩ·cm and 2.2 µΩ·cm. The residual resistance ratio is only 1.7. The Cu-0.5%Mn alloy has a large resistance at low temperature, which could prevent the coupling between NbTi filament effectively. The experimental wire of Φ0.8 mm and 44 064 filaments was successfully prepared by a double stacking method. The filaments of the wire were uniformly deformed and the diameter of the filament was only 2.5 µm. With the increase of aging heat treatment times from 0 to 4 times, the critical current density increased from 492 A/mm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> to 2615 A/mm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 5 T&4.2 K. The critical current density of the wire has a weak correlation with the twist pitch of 14.7 mm to 5.3 mm. The hysteresis loss of the wire is 41.6 mJ/cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> at ±3 T and the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$D_{eff}$</tex-math></inline-formula> is 2.62 µm. The RRR(300 K/10K) of the wire is more than 200, which could improve the thermal stability of the magnet and protect the magnet from quench.