Flux dynamics in current-carrying superconductors

PC controlled setup for magneto- optical (MO) investigations of current- carrying superconducting films was developed. It included a current pulse circuit synchronized with the microscope and PC software for image processing allowing us to determine field distributions across superconducting Strip using spatially-dependent calibration and iterative inversion to find current distributions. We study these distributions during and after a current pulse, with and without applied magnetic field, current re-distribution during the pulse, precursors to strip bum-out at high currents, and dendritic instability triggered by the pulse. We find critical-state behaviour accompanied by slow creep of magnetic flux at moderate currents in YBCO films, and fast avalanches related to heating effects at higher currents in YBCO and MgB2.