Time constant measurements and effective permeability of a stack of high-Tc tapes

Filament cross-sections of high-T_c tapes have generally a large aspect ratio. The magnetic moment of the filaments which determines the magnetic properties of a stack of tapes therefore depends strongly on the magnetic field orientation. The stack can be treated as a material with an effective permeability ν_ef. For large field amplitudes μ_ef approaches unity, while for low amplitudes it can be considerably smaller than unity, namely in the case of perpendicular external field. We derived expressions for μ_ef for parallel and perpendicular field amplitudes well below the filament penetration field. Under this assumption the filaments are supposed to be perfectly diamagnetic. The theoretical predictions for μ_ef are compared with the values extracted from direct time constant measurements. The experiments were performed with samples stacks of twisted and untwisted Bi-2223 tapes. The effective transverse matrix resistivity was found to be highly anisotropic which is understood as the presence of a boundary layer resistance between the filaments and the silver matrix