Flux-flow voltages and faraday induction during guided entry or exit of flux in hollow superconducting cylinders

Abstract

The instantaneous and integrated voltages detected between pairs of diametrically opposite contacts attached on the periphery of superconducting Nb tubes of different wall thicknesses were monitored as flux threading the walls and the hole is caused to change. For a chosen change of flux, the signals across a pair of contacts were found to depend dramatically on the configuration of leads to the contacts relative to the direction of flux motion. The persistent currents induced to flow azimuthally around the walls of the cylinder are made to collapse partially or completely and the flux retained in or shielded from the hole and the wall is made to exit or enter by applying heat at a narrow strip along the length of the tube. The numerous observations are consistent with the following picture. Changes of flux in the hole or in the walls generate a voltage only across the leads which embrace the region where entry or exit of flux occurs (only changes of magnetic flux in the measuring circuit excluding the superconducting volume contribute).