Enhancement of the superconducting critical current from saturation in Nb-Ti wire. I.

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Abstract
The flux pinning characteristic of processed Nb‐Ti monofilamentary wires was investigated at 4.2 K and higher temperatures by an ac measuring method. Saturation of the global pinning force, as in Nb3Sn, was observed at high fields for specimens heat treated after initial heavy cold work. A transition from the saturation to a strongly pinned nonsaturation occurred after the specimen was finally cold worked. This transition directly demonstrates that the saturation is not caused by a shearing deformation of the fluxoid lattice assumed in Kramer’s model [J. Appl. Phys. 4 4, 1360 (1973)], because this proposed mechanism primarily depends on intrinsic superconducting properties which are not changed by the final cold working. From the measurements of the elastic and plastic behavior of the fluxoid lattice by using the ac method, it was also found that it was not the shearing deformation of the fluxoid lattice but rather the depinning that occurred in the saturation: The saturation is expected to be caused by a catastrophic avalanching flow of the brittle fluxoid lattice. The transition can be explained by an increase of the elementary pinning force of normal Ti precipitates due to elongation by the cold work. The present result and existing results on various materials suggest that the nonsaturation is attained by increasing the elementary pinning force and/or the pin concentration.