Development of high J_c Nb-Ti multifilamentary superconducting wires for a.c. use by artificially introduced pins

To achieve further increases in the critical current densities of Nb-Ti ultrafine multi-filamentary superconducting wires for a.c. applications, wires were fabricated in which Nb island-type artificial pinning centres had been designed and introduced into filaments according to flux pinning theory. The changes in pin parameters which accompanied the diameter reduction increased the pinning force density F_p over a wide range of magnetic field, and oversaturation phenomena were observed, with the position of the F_p peak shifting towards the high-field side. High critical current densities J_c were achieved: 1.42 × 10¹⁰ A m^-2 at 1 T for 0.1 μm filaments, and 8.7 × 10⁹ A m^-2 at 2 T and 6.1 × 10⁹ A m^-2 at 3 T for 0.4 μm filaments. Furthermore, it was determined that where the correlation length I₆₆ and fluxoid interval a_f are such that 2I₆₆ < a_f, the magnetic field at the F_p peak is determined by the pin interval d_s. This discovery has given us useful pointers for the design of Nb-Ti superconducting a.c. wires, in terms of J_c, through the introduction of artificial pins.