Effect of Size of Superconductor on Estimation of Critical Current Density Using AC Inductive Method

Applicability of the ac inductive measuring method for estimation of the critical current density, J_c, in superconductors of relatively small size is theoretically investigated in terms of the Campbell model for pinning force density vs the displacement of fluxoids. It is shown that J_c is significantly overestimated by the conventional method due to reversible fluxoid motion for superconductors comparable to or smaller than Campbell’s ac penetration depth, λ₀'. Only the recently proposed analysis of imaginary ac susceptibility yields relatively accurate values even for superconductors much smaller than λ₀'.