Abstract
We have experimentally studied the magnetic shielding properties of a cylindrical shell ofBiPbSrCaCuO subjected to low frequency AC axial magnetic fields. The magneticresponse has been investigated as a function of the dimensions of the tube, themagnitude of the applied field and the frequency. These results are explainedquantitatively by employing the method of Brandt (1998 Phys. Rev. B 58 6506) with aJc(B) law appropriate for a polycrystalline material. Specifically, we observe that the applied fieldcan sweep into the central region either through the thickness of the shield or through theopening ends, the latter mechanism being suppressed for long tubes. For the first time, wesystematically detail the spatial variation of the shielding factor (the ratio of theapplied field over the internal magnetic field) along the axis of a high-temperaturesuperconducting tube. The shielding factor is shown to be constant in a region around thecentre of the tube, and to decrease as an exponential in the vicinity of the ends.This spatial dependence comes from the competition between two mechanismsof field penetration. The frequency dependence of the shielding factor is alsodiscussed and shown to follow a power law arising from the finite creep exponentn.
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