THEORY OF AC LOSSES IN TYPE-II SUPERCONDUCTORS WITH A FIELD-DEPENDENT SURFACE-BARRIER

The irreversible magnetic behavior of a type-II superconductor in a periodically varying longitudinal applied magnetic field is theoretically examined. Bulk pinning is characterized by a critical current density J c, and surface-barrier effects are characterized by critical entry and exit fields Hen and Hex. Theoretical expressions for magnetic field and flux-density profiles, voltage waveforms, hysteretic-bulk and surface-energy losses, and the ac permeability are derived for the arbitrary flux-density (B) dependence of Jc, Hen, Hex, and the equilibrium magnetic field Heq. Numerical model calculations are performed to demonstrate the influence of the B dependence of the latter functions upon the measurable quantities.