DC and AC shielding properties of bulk high-Tc superconducting tubes

In this work we have studied numerically and experimentally the magnetic flux penetration in high-Tc superconducting tube subjected to a uniform magnetic field parallel to its long axis. This study is carried in view of designing low-frequency magnetic shields by exploiting the diamagnetic properties of high-Tc superconducting ceramics. We have measured the field attenuation for DC and AC applied fields (40 Hz < f < 400 Hz) and we have confronted data with computer modeling of the field distribution in the hollow of the tube. The numerical model is based on an algorithm proposed by Brandt (Phys. Rev. B 58, 6506, 1998). We have studied the spatial variation of the shielding factor (ratio between the external applied field and the internal applied field) along the axis of the tube. The agreement between experimental and simulated data was found to be very satisfactory. The data enabled us to distinguish between two penetration routes for the magnetic flux: (i) from the external surface of the tube (ii) and from the opening ends. We have also determined the phase shift between the internal and the applied field. Finally, we have studied the frequency dependence of the AC magnetic shielding factor. The shielding factor was found to exhibit a power law which can be explained quantitatively from scaling laws arising from the E = Ec(J/Jc)^n dependence.