ac losses and low-field superconducting behavior of Nb3Sn

Abstract
Measurements have been made of the low‐field ac and dc superconducting properties of thin films of electron‐beam‐evaporated Nb3Sn. Comparison of the 50‐Hz hysteretic loss and flux entry waveforms with the measured field‐dependent critical current indicates that a modification of the usual critical state model with a uniform surface barrier is necessary. A model based upon a nonuniform flux distribution, consistent with the observed morphology, is presented. Its success in describing the low‐field properties of this sytem and its importance to related materials in which the superconducting properties are strongly dependent upon the microstructure are discussed.