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

屏蔽电流产生的磁场使NMR磁体或MRI磁体中的场质量劣化。测量了用Bi-2223带材绕制的小型线圈产生的磁场，研究了屏蔽电流产生的磁场对线圈磁场质量的影响。当改变励磁电流时，观察到明显的滞后行为：这是屏蔽电流产生的磁场、磁化对产生的磁场的影响的证据。当线圈电流保持恒定六小时时，在测量的磁场中观察到漂移。这种漂移应该是由磁带磁化的弛豫引起的。我们考虑了一个扭曲的Bi-2223磁带的应用，以减少微尺度的电磁现象，即屏蔽电流产生的磁场，以改善场质量。另一个线圈是使用扭曲的Bi-2223磁带缠绕，和磁场的磁滞和磁场的稳定性进行了实验研究。与缠绕有非扭转Bi-2223带的线圈相比，缠绕有扭转Bi-2223带的线圈在所产生的磁场中表现出较小的磁滞，并且表现出更好的磁场的时间稳定性。通过电磁场数值模拟，可视化了微尺度电磁现象，研究了磁体的场稳定性。提高高温超导带材的玻璃-液体转变磁场和玻璃-液体转变温度可能是控制微尺度电磁现象和提高磁体场稳定性的一种途径。

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.