Magnetic flux diffusion through HTS shields

Slow field leakage in a polycrystalline superconducting cupshield placed in an external axial field (H/sub ext/) much weaker than H/sub C1/ shows a diffusive time dependence with a time scale of 10/sup 2/ s. As the field strength increases but is still less than H/sub C1/, a logarithmic time dependence of 10/sup 2/ s time scale is observed. It is shown that this logarithmic time dependence is a result of flux line diffusion between superconducting grains rather than fluxoid avalanches or flux bundle creep.