Anisotropic pinning in macroscopic electrodynamics of superconductors

Abstract

Anisotropy of critical currents and electric fields in superconductors with strong pinning has been ascribed in the macroscopic model to features of the material equation system relating the electric field to the current density in a superconductor. The anisotropy of the pinning proper is described by an operator relating the pinning force density to the vectors of magnetic induction and Lorentz force. In the approximation of an extended critical state model, a feasible expression of this operator is given in the form of an algorithm based on the concept of a collective anisotropic potential well containing fluxoids. The current-carrying capacity of a strongly anisotropic niobium-titanium foil as a function of the orientation of the current density and applied field with respect to the principal axes of the material has been investigated in detail. Given measurements of the transverse electric fields in the foil under magnetic fields normal to the foil plane, we can plot cross sections of surfaces describing the pinning force density in the space of magnetic induction and Lorentz force.