Magnetic shielding properties of a bulk Bi-2223 superconducting hollow cylinder subjected to an inhomogeneous magnetic field

Bulk superconducting materials are well suited for magnetic shielding applications. At low frequencies, the performances of superconductors are higher than those of conventional ferromagnetic materials. In shielding applications, two situations may be encountered. The first one corresponds to the case where the screen has to shield a volume from the magnetic field in its environment; this corresponds to an “immunity” problem. In the second situation, the screen has to prevent the magnetic field generated by an electronic device from perturbing its environment; this is an “emission” problem. So far, superconducting screens have been extensively studied in “immunity” and were subjected to uniform magnetic fields. In “emission”, the magnetic field is no longer uniform because a local magnetic source has to be placed inside the screen. In this work, we have studied experimentally at 77K the magnetic flux penetration in a Bi-2223 superconducting hollow cylinder subjected to a non-uniform quasi-static magnetic field generated by a small coil placed inside the sample. Two configurations were investigated: axial and transverse; corresponding respectively to the situation where the axis of the coil is coaxial or perpendicular to the axis of tube. We also investigate the influence of the sweep rate of the magnetic field on the magnetic shielding performances. Planar and circular (i.e. at constant distance of the tube) mappings of the magnetic field at proximity of the external surface of the tube were obtained thanks to a bespoke experimental setup using a three axes miniature Hall probe. It was observed that the three components of the magnetic field measured outside are affected differently by the superconducting screen. A simple one-dimensional model based on the conservation of magnetic flux and the Bean model was developed for the axial configuration. It was found to be in accordance with experimental data. It allows one to predict the maximal magnetic flux that can be generated inside the coil before the tube is fully penetrated and a magnetic field is measured outside the tube. Finally, it was observed that the inner surface of the tube is subjected to a magnetic field higher than the one at the same place without the tube. This concentration phenomenon arises because of the diamagnetic behaviour of the superconductor