THE MAGNETIC-FIELD STEP RESPONSE OF A TYPE-II SUPERCONDUCTOR AS A SIMPLE TEST OF THE VORTEX BULK PINNING

A slab in parallel field is submitted to a step-like magnetic-field ex citation, of amplitude 1-10 G, in the same direction as the applied fi eld. The slab has been rolled or spark cut, without special care, from a conventional superconductor ingot (lead-indium alloy) and its criti cal currents have standard values. The transitory electrical field ind uced on a face of the slab has been measured. Thus, we obtain informat ion about the magnetic flux penetrating and vortices entering the samp le through this face. The only observed deviation from the ideal respo nse is that magnetic-field diffusion is limited by surface screening, associated with a superficial critical current density. Otherwise, the vortex array inside the slab responds exactly as if bulk pinning were ineffective.