Time constant measurements and effective permeability of a stack of High-T-c tapes

Filament cross-sections of High-T-C tapes have generally a large aspect rat io. The magnetic moment of the filaments which determines the magnetic prop erties of a stack of tapes therefore depends strongly on the magnetic field orientation, The stack can be treated as a material with an effective perm eability mu (ef). For large field amplitudes mu (ef) approaches unity, whil e for low amplitudes it can be considerably smaller than unity, namely in t he case of perpendicular external field. We derived expressions for mu (ef) for parallel and perpendicular field amplitudes well below the filament pe netration field. Under this assumption the filaments are supposed to be per fectly diamagnetic. The theoretical predictions for mu (ef) are compared wi th the values extracted from direct time constant measurements. The experim ents were performed with samples stacks of twisted and untwisted Bi-2223 ta pes. The effective transverse matrix resistivity was found to be highly ani sotropic which is understood as the presence of a boundary layer resistance between filaments and silver matrix.