Damping effects and flux kinetics in a type II superconducting disk

Magnetooptic techniques coupled with high-speed cinephotography have been used to study flux jumps in a Nb-25% Zr superconducting disk triggered by the application of an external magnetic field. It is shown that the exponential time constant Τ_Μ which characterizes the penetration of the magnetic flux and the flux jump field H _fj are independent of the sweep rate of the applied magnetic field over three orders of magnitude. It is also demonstrated that magnetic coupling between the disk-shaped superconductor and a high-purity normal metal beneath it can significantly damp the motion of magnetic flux into the superconductor, thus enhancing its stability against flux jumps.