Control of electromagnetic phenomena in high Tc superconductor tapes to generate stable and uniform high magnetic fields by using high Tc superconductor magnets

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The shielding-current-generated magnetic field deteriorates the field quality in the NMR magnets or the MRI magnets. The magnetic field generated by a small coil wound with a Bi-2223 tape was measured to study the influence of the shielding-current-generated magnetic field on the field quality of the coil. When changing the excitation current, an apparent hysteretic behavior was observed : this is the evidence of the influence of the shielding-current-generated magnetic field, the magnetization, on the generated magnetic field. When the coil current was kept at constant for six hours, a drift was observed in the measured magnetic field. This drift should be caused by the relaxation of the tape magnetization. We considered the application of a twisted Bi-2223 tape to reduce the micro-scale electromagnetic phenomena, that is, the shielding-current-generated magnetic field, to improve the field quality. Another coil was wound using a twisted Bi-2223 tape, and the magnetic field hysteresis and the stability of the magnetic field were studied experimentally. As compared to the coil wound with the non-twisted Bi-2223 tape, the coil wound with the twisted Bi-2223 tape showed a smaller hysteresis in the generated magnetic field and showed a better temporal stability of the magnetic field. Through numerical electromagnetic field simulation, we visualized the micro-scale electromagnetic phenomena and studied the field stability of magnets. Increase in the glass-liquid transition magnetic field and increase in the glass-liquid transition temperature of high Tc superconductor tapes might be an approach to control the micro-scale electromagnetic phenomena and to improve the field stability of magnets.