Artificial pinning centre Nb–Ti superconducting wire for AC applications

The present work presents the steps for the manufacture of an Nb-Ti artificial pinning centre (APC) multifilament wire for AC applications. The wire production starts from a Ti bar inserted in a Nb tube. The superconductor Nb-Ti alloy was obtained using heat treatments during the wire production steps through the growth of the diffusion layer between the pure Nb and Ti regions. The final wire is composed of many filaments dispersed in a Cu-Ni matrix. The transport critical current density was measured in samples for different conditions of heat treatment, followed by wire diameter reduction. Magnetization hysteresis loops were measured to extract the hysteretic losses in wires with filament diameters of 76 and 92 mu m and compared to the results in commercial wires with comparable filament diameters. We obtained an Nb-Ti APC wire with hysteretic losses of the same order of magnitude as the commercial wires. However, critical current densities J(c) are higher for fields lower than 2 T, where AC applications are most important. The J(c) results confirmed a previous result that the decrease of normal region sizes is a less important pinning source in this material than the pinning from variations in the composition of the diffusion layer.