FACTORS CONTROLLING THE ONSET OF PROXIMITY EFFECT COUPLING (PEC) AND THE INFLUENCE OF PINNING-ASSISTED PEC ON M-H LOOP ASYMMETRY IN MULTIFILAMENTARY COMPOSITES

摘要

An expression is developed for the onset of proximity effect coupling (PEC) in multifilamentary (MF) strands, that depends on intrinsic proximity effect (PE) properties, in particular: the normal/superconducting (N/S) boundary conditions and ζ_N, the coherence length in N, also the MF strand geometry as well as a measurement-sensitivity criterion. This expression is applied to experimentally measured data on various NbTi/Cu and NbTi/CuMn MF composite strands and its use in the determination of N-metal coherence lengths is described. A transcendental equation for determining the PEC-onset interfilament spacing, d_c, is derived. The so-called three-dimensional pinning properties of the PE coupling currents are then invoked to explain the asymmetry commonly seen out to relatively high magnetic fields in the magnetization loops of PEC MF composites as well as the "dips" or "bites" that occur in the low-field region under appropriate conditions. The results of model calculations are presented.