Development of advanced magnetically-levitated high-temperature superconducting coils with YBCO coated conductors

使用最先进的YBCO涂层导体检测了高温超导线圈的磁悬浮。制作了三个直径为90~140 mm的线圈，并用液氮在外加磁场作用下产生持续电流(场冷法)进行了测试。这些线圈虽然体积很小，但已经成为世界上第一个磁悬浮YBCO线圈。由于线圈薄片之间采用了复杂的连接结构，持续电流的衰减时间常数足够长(2-7小时)。当外磁场以特殊方式施加时，发现初始激励后持续电流甚至增加。这种现象可以用超导层中的局部感应屏蔽电流与通过线圈绕组的传输电流之间的磁耦合来解释。使用稳定的持续电流，在有反馈控制和没有反馈控制的情况下进行了磁悬浮实验。在特定的条件下观察到了自稳定磁悬浮，这可以用超导浮动线圈中的磁通守恒来解释。这些结果对于改进目前的Mini-RT内等离子体约束装置的高场版本具有一定的参考价值。此外，还进行了将YBCO高温超导体应用于聚变机的可行性研究。结果表明，用于大型螺旋装置等螺旋机的磁场校正线圈可由高温超导线圈提供。它们应该比目前的铜线圈效率高得多，很快就会出现在不久的将来的技术中。

Magnetic levitations of high-temperature superconducting coils were examined using state-of-the-art YBCO coated conductors. Three coils (diameter: 90-140 mm) were fabricated and tested by inducing persistent currents using liquid nitrogen under external magnetic field (field-cooling method). These coils, despite their small sizes, have become the world first magnetically-levitated YBCO coils. The decay time constant of persistent currents were found to be long enough (2-7 hours) owing to the application of sophisticated joint structures between pancakes of the coils. When the external magnetic field was applied in a special way, it was found that the persistent current even increased after the initial excitation. This phenomenon can be explained by the magnetic coupling between the locally induced shielding currents in the superconducting layers and the transport current through the coil windings. By using stable persistent currents, magnetic levitation experiments were carried out with and without feedback controls. Self-stabilized magnetic levitations were observed with specified conditions, which could be explained by conservation of magnetic flux in the superconducting floating coils. These results are supposed to be valuable for designing the upgraded high-field version of the present Mini-RT device for internal plasma confinement. Furthermore, feasibility studies for applying YBCO high-temperature superconductors for fusion machines have been carried out. It is shown that field correction coils used for helical machines, such as the Large Helical Device, can be provided by high-temperature superconducting coils. They are supposed to be much more efficient than the present copper coils and will soon be available with the near future technology.