Frequency dependent AC loss in degraded Bi-2223/Ag tape

The low-frequency (f) AC loss per cycle of a high-temperature superconducting Bi-2223/Ag tape is frequency independent when the measurements are performed at 77 K with current amplitude Im below the critical current Ic. Its loss vs. Im/Ic is consistent with the results derived from the critical state model with a Kim type field dependent critical current density Jc. After Ic of the sample is degraded from 32 to 16.5 A by repeated cooling and warming, the loss becomes strongly f dependent and it increases by a factor of 2-3 with decreasing f from 405 to 15 Hz. It is found that by defining an effective critical current Ic1, which increases from 14 to 25 A with increasing f from 15 to 405 Hz, the Norris result for an elliptic bar with a Bean type Jc fits the data for all values of f. This behavior indicates that the Ic degradation results from the division of as-obtained wide Josephson junctions into fine ones, and the thermally activated creep of Josephson vortices formed in the system of fine junctions leads to a frequency dependent Ic1. © 2003 Published by Elsevier Science B.V.