Superconducting Properties of Wires With Radially Arranged Filaments

An internal-tin route Nb₃Sn superconducting wire that has both remarkably low hysteresis loss (Q_h) and high critical current density (J_c) was developed according to a new design idea. The wire was constructed by arranging the filaments in a radial layout, enlarging the outer filaments along the radial direction, intentionally narrowing the filament spacing in the radial direction, and enlarging the filament spacing in the tangential direction. Thus, the electromagnetic coupling among the filaments in the tangential direction due to the bridging and/or proximity effect was suppressed without decreasing the volume fraction of Nb. As a result, excellent properties such as J_c(12 T)=1.15times10³ A/mm² Q_h=301 mJ/cm³ (for 1 cycle of B=plusmn3 T), T_c=17.3 K, and B_c2=24.1 T were obtained. Scanning electron microscope observations confirmed that the cause of experimental Q_h values being lower than expected is that the majority of the filaments in the second and outermost rows were not completely combined. Electron probe micro-analyses confirmed that the good T_c and B_c2 values resulted from the qualitative improvement of the Nb₃Sn compound. This wire is promising for use with conduction-cooled high-field magnets and for the strands of fusion coils