We report on the field-cooled magnetization (FCM) process under magnetic fields of up to 22 T for the stacked Y-Ba-Cu-O (YBCO) bulk pair reinforced by a stainless steel (SUS316) container. On the basis of the numerically simulated mechanical stress in a bulk during FCM, the SUS316 container was designed by the numerical simulation to offer the sufficient hoop stress tolerance to the bulk trapping the magnetic field of 20 T. As a result, we obtained successfully the trapped field, B T, of 15.1 T by FCM from 18 T at 28 K at the center of the YBCO bulk pair. However, the extremely large-scale vortex jumps and large temperature rise occurred suddenly during FCM from a higher field of 22 T at a lower temperature of 23 K, and then a small crack was confirmed at the periphery of "both" YBCO bulks from the distorted contour maps of B T. The simultaneous break of both YBCO bulks probably in spite of sufficient mechanical reinforcement led us to conclude that the thermal instability triggered off the large-scale vortex jumps and large heat generation. The stress concentration induced thermally at the hot spot brought about the cracking.
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