Transport current distribution in (Bi,Pb)-2223/Ag tapes

We have measured the local distribution of the magnetic field component H-z (x) perpendicular to the plane of (Bi,Pb)-2223/Ag tape conductors carrying transport currents at T = 77 K using scanning Hall probe magnetometry. The sheet current density distribution J(y)(x) across the tape width was determ ined via a magnetic inversion scheme based on a Fourier transform/spatial f iltering method. With increasing current, dramatic changes in J(y)(x) clear ly define a characteristic current I-star which marks the onset of homogene ous distribution of additional current over the entire cross section of the superconductor and corresponds to the intrinsic critical current in the co ntext of the critical state model. The behaviour of a single-filament tape with a slowly varying ac transport current flowing in it is found to follow closely the predictions of the critical state model. For multifilament tap es, magnetic coupling between neighboring filaments results in a current di stribution corresponding to an "effective" large individual filament.