Field replication and flux shielding in annular superconductors

The phenomenon of flux trapping in type II superconductors can be exploited by fabricating simple, tubular assemblies of bulk or layered superconductors which then accurately preserve a wide variety of field configurations. The associated flux shielding properties can be used where field free regions are required. The results of flux trapping and shielding experiments on samples of vapor deposited Nb3Sn, bulk Nb-Ti, Nb-Ti/Cu sheet-composite, and bulk Pb-Bi are reported. Measurements of the magnetic field in the vicinity of the samples as a function of position and applied field produce field profiles and magnetization-like curves. The experimental curves are related to critical state models and together with an appropriate model give estimates of critical currents in the materials. The occurrence and extent of the flux jumps, which often limit flux trapping and shielding capabilities, are compared with predictions of stability models. Heat- and surface-treatments which increase pinning strengths and trapped flux are described.