Effective volume of voids for flux pinning in superconductors

Flux pinning by voids in Nb and vanadium has been investigated. How the collective pinning theory of Larkin and Ovchinnikov may be applied in the case where plastic defects mainly determine the disorder in the flux-line lattice is discussed. It is argued that the macroscopic pinning force above its max. close to the upper critical field is described by the amorphous limit of the collective pinning theory. This provides a means for comparing the elementary interaction, deduced from experiment, with the recent nonlocal theory of Thuneberg, et al . that predicts an appreciable enhancement of the effective void volume for pinning. Good agreement is obtained for Nb; the results of V are satisfactory. 52 ref.--AA