Limitation of flux bundle volume and its influence on irreversibility fields

The irreversibility fields of multifilamentary Nb–Ti wires with different filament size were measured and compared with the theoretical prediction based on the flux creep theory. It was found that the irreversibility fields were seriously suppressed due to the flux creep at high temperatures while the critical current density of multifilamentary Nb–Ti wires was fairly large. This can be explained from the small pinning potential compared with the bulk specimen because the pinning potential is limited by the size of the filaments. In comparison with the theoretical prediction, quantitative differences occurred in the vicinity of the critical temperature. These can fit well by assuming the Gaussian distribution of the critical temperature for the multifilamentary Nb–Ti wires.