Pinning characteristics in multifilamentary NbTi superconducting wires with submicrometre filaments introduced artificial pinning centres

Multifilamentary NbTi superconducting wires for a.c. use were developed by introducing ultrafine Nb island-type artificial pinning centres. The maximum J_c value obtained for wire with a filament diameter of 0.4 μm reached as high as 1.07 × 10¹⁰ A m^-2 at 1 T and 4.44 × 10⁹ A m^-2 at 3 T, which are more than twice the J_c values for conventional wires for a.c. use. For these wires, the constants of the scaling rule for global pinning force, F_p α b^γ(1-b)^δ, became γ = 0.8 and δ = 1.6. This indicates that the pinning characteristics of the present wires were shifted to the oversaturation-type by the introduction of artificial pinning centres, while they are of the saturation-type for conventional a.c. wires. As a result, the global pinning force at low and medium magnetic fields can be explained fairly well by the direct summation model which is available only for strong pins. A helpful guideline in designing J_c values for a.c. wires is also discussed.