The trapped field characteristics of disk-shaped YBCO bulk exposed to external AC magnetic field are investigated experimentally. The magnetic flux density on the top surface of the disk, defined as "trapped flux" in this paper, was measured in both short and long terms as functions of amplitude and frequency of the AC external magnetic field. The observed trapped flux attenuation was obviously different from that of flux creep, i.e., with no external magnetic field; this implies that the trapped flux density within the YBCO disk is reduced by a temperature rise due to AC loss. The abrupt attenuation of trapped flux density in the first several cycles was observed at frequencies of 0.1 and 1 Hz, while not observed at 10 Hz. The attenuation rate after seven minutes of applying AC magnetic field, however, became almost the same regardless of frequency. The trapped flux attenuation in one cycle of AC magnetic field decreases with the frequency and increases with the amplitude of AC magnetic field. These results imply that the characteristic of trapped flux is closely related to AC loss, especially the hysteresis loss, and the AC loss depends on the frequency and the amplitude of the AC external magnetic field. It can be considered that transient electromagnetic behavior within HTS bulk, especially supercurrent distribution, is a key factor of the relationship between the trapped flux attenuation and the AC loss.
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