Dynamic Evolutions of Flux Distributions in a Superconductor by a Pulsed Current

We observed time--evolution of flux--density distribution in a superconductor by a pulsed current with the magneto--optical microscopy. The flux density distribution in a NbN film was measured up to 10000 frame per second by a high--speed camera. The voltage induced by the motion of vortices was simultaneously measured. Local reconfiguration of vortices occurred even below the critical current, which was qualitatively explained within the critical state model. However, some deviations between experiments and the theory were observed, indicating that the current flowing the sample was less than the critical current in the zero--field--cooled state.