Achievement of very high critical currents in type II superconductors

Persistent currents are induced to flow along a ribbon of a type II superconductor placed in a static longitudinal field H/sub /// by the sweep of a transverse field component directed along the flat faces. The induced magnetic moments, hence the induced critical current densities, are considerably enhanced when a longitudinal field is present and maximum magnitudes are encountered when H/sub /// approximately=0.6 H c2 . Exploiting the critical state concept together with the idea that pinning force densities depend on the magnetic induction B and display a broad peak when B is a significant fraction of H c2 , families of magnetization curves are calculated which reproduce the general behaviour of the observations. Further, when a static longitudinal field permeates the ribbon, a magnetic moment appears along the length of the sample as the transverse field is varied. This phenomenon is studied and attributed to the adoption by the induced currents of patterns of circulation which tend to be force free. Introducing this feature in calculations of the transverse moments leads to a better fit with the observations.