Magnetic shielding by superconducting Y-Ba-Cu-O hollow cylinders

The authors have measured the magnetic field H´ at the center of a hollow of sintered Y-Ba-Cu-O superconductor as a function of applied field H, temperature, tube length, and tube wall thickness. The maximum field that can be shielded H_sh agrees with the value calculated from the critical state model using the measured critical-current density J _c. The maximum trapped field H_tr in the tube, on decreasing H to zero, exceeds H_sh by as much as a factor of two, and large enhancements in J _c are observed in the decreasing field. These phenomena are identified with intragranular flux pinning present only after H has exceeded H_c1 of the grains. Finite tube length does not affect the H_tr/H_sh ratio appreciably. H_sh depends approximately on the square root of the wall thickness