Effect of proximity length on flux pinning in APC composites: An overview of APC

The lack of adequate high-field pinning in APC composites is an intrinsic property of a dominant magnetic pinning mechanism and the long proximity length of Nb pins. Recent work on APC composites has uncovered trends in the flux-pinning and microstructural data, which hold despite the variety of their designs. By comparing these trends to those of conventional Nb-Ti composites, evidence for magnetic pinning being the dominant flux-pinning mechanism is found. This model departs from the widely accepted view that core-pinning is the dominant pinning mechanism. The parameter that controls the optimization of flux-pinning in the microstructure is the proximity length ξN instead of the diameter of the fluxon core. The optimum bulk pinning force occurs for the best balance of a strong elementary pinning force fp and a high number density of pins. However, since fp is maximum when the pin thickness t ∼ ξN, the number density of pins can be made to be on the order of the fluxon number density by using artificial pins which have very short proximity lengths. Such pins are most desirable to achieve high Jc at high fields