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The authors present the results of a comprehensive study of the electromagnetic behavior of high-temperature superconducting bulk YBCO exposed to a time-varying magnetic field. The characteristic of the trapped magnetic flux is one of the key factors in the application of a high-temperature superconducting bulk, such as magnetic levitation systems, rotors of motors and bulk magnets. The time inconstancy of trapped flux, such as flux creep, is especially undesirable for its application system. The relationship between a time-varying external magnetic field and trapped flux is investigated experimentally using a field-cooled (FC) YBCO disk. The magnetic flux density just above the disk is measured by Hall probes lined up along the disk radius. The magnetic field decay, which may be independent of flux creep, is observed in the experiment and is influenced significantly by the frequency and amplitude of the AC external magnetic field. The effect of changing rate of applied magnetic field in field-cool process on trapped flux is also investigated experimentally.
Date of Publication: March 2000