Enhancement of critical current density in ultrafine multifilamentary Nb-Ti superconductors with Nb artificial pins for low-field a.c. applications

In order to increase critical current density J/sub c/ in ultrafine multifilamentary Nb-Ti superconducting wires for low-field a.c. applications, several kinds of Nb island-type artificial pinning centers (APC) were designed to produce high J/sub c/ in a low field region. Their pinning characteristics and a.c. losses were investigated. Effects of pin size and volume fractions on the flux pinning were also studied. APC wires containing only one artificial pin with 17 vol.% in a filament have achieved extremely higher J/sub c/ values than those of conventional a.c. wires in low fields, while they have ultrafine filaments of the order of 0.1 /spl mu/m with marked degradation of upper critical fields. Interfilamentary proximity coupling was investigated by measuring twist-pitch dependence of magnetization. The coupling was negligibly reduced by using Cu-30 wt.% Ni as a matrix until the filament spacing of 0.071 /spl mu/m. We also discuss the validity of APC wires for low-field a.c. applications.