FLUX PINNING IN TUBES OF A CERAMIC SUPERCONDUCTOR

The onset of magnetic flux penetration through the wall of a tube of sintered YBa2Cu3O7 has been measured in the temperature range of 4 to 90 K. These measurements have been used to calculate the temperature dependence of the intergranular critical transport current employing three critical-state models. Good agreement was obtained between this calculated critical current and direct critical current measurements on rod-shaped samples of the same material. The critical-state magnetic field profiles were used to explain the observed voltage waveforms induced in a pick-up coil inside the bore of tubular samples when excited by an AC field. Best agreement was obtained using a model in which the critical current (j(c)) has a magnetic field dependence given by j(c) = j0H0(2)/(H-2 + H0(2)) where j0 and H0 depend only on temperature.