RELAXATION OF MAGNETIC SHIELDING TUBES OF SINTERED YBCO

Magnetic shielding properties have been studied for sintered YBaCuO tubes of different size. The maximal dc shielding field reached 3.7 mT. It is found that the total shielding effect is not simply additive for two tubes put together. The shielding effect depends on temperature and the position in the extermal field, and the trapping field was found damping linearly with the logarithm of time, exhibiting flux creep behaviour. The average persistent current density was about 10(2) A/cm2. The potential barrier was estimated to be 0.18 eV and 0.76 eV for weak-link region and grain, respectively, according to Anderson's model of thermal activation from the damping rate of persistent current.