The trapped field characteristics of disk-shaped YBCO bulk exposed to exter
nal AC magnetic field are investigated experimentally, The magnetic flux de
nsity an the top surface of the disk defined as "trapped flux" in this pape
r, was measured in both short and long tel ms as functions of amplitude and
frequency of the AC external magnetic field. The observed trapped flux att
enuation was obviously different from that of flux creep, i.e,, with no ext
ernal magnetic field; this Implies that the trapped flux density within the
YBCO dish is reduced by a temperature rise due to AC loss, The abrupt atte
nuation 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 r
ate after seven minutes of applying AC magnetic field, however, became almo
st the same regardless of frequency. The trapped Bur attenuation in one cyc
le of, AC magnetic field decreases with the frequency and increases with th
e amplitude of AC magnetic field. These results imply that tile characteris
tic of trapped pus 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. at can he considered that transient electromagnet
ic behavior within HTS bulk, especially supercurrent distribution, is a key
factor. of the relationship between the trapped flux attenuation and the A
C loss.