Study on Fine Filament, Ultra-Low Loss Experimental NbTi/Cu5Ni/Cu Superconducting Wires for Accelerator Magnet in WST

A new type of superconducting wire with high critical current density, fine filament, ultra-low loss experimental NbTi/Cu5Ni/Cu has been developed in this paper for the miniaturized heavy ion therapy device of China. The wire uses Cu-5%Ni alloy instead of conventional high-purity OFC as the matrix, which could prevent the coupling between filaments at low temperature and reduce the eddy current loss effectively. Meanwhile, the Cu5Ni matrix and NbTi filament are separated by high-purity Nb. Nb, as a barrier layer, could isolate the diffusion reaction between Cu and Ti in the process of high-temperature and long-time aging heat treatment. In order to reduce the hysteresis loss of the wire, the filament inside the wire is increased to 75276. The filament diameter is reduced to 1.9 µm under wire diameter of Φ0.8 mm. Due to large number of filaments of the wire, filament breakage and wire breakage phenomena occur during processing of the wire. The J_c (4.2 K, 5 T) of the wire is only 2410 A/mm², and the “n” value is only 17.9. The hysteresis loss (4.2 K, ±3 T) of the wire is 19.9 mJ/cm³, which reached a new higher level in history.