MAGNETIC-FLUX DIFFUSION THROUGH HTS SHIELDS

Slow field leakage in a polycrystalline superconducting cupshield plac ed in an external axial field (H-ext) much weaker than H-C1 shows a di ffusive time dependence with a time scale of 10(2) s. As the field str ength increases but is still less than H-C1, a logarithmic time depend ence of 10(2) s time scale is observed. It is shown that this logarith mic time dependence is a result of flux line diffusion between superco nducting grains rather than fluxoid avalanches or flux bundle creep.