Clarification of electromagnetic properties of high temperature superconducting tapes used for superconducting power machines cooled by

用液氮冷却超导电机的实现一直是人们关注的问题。然而，高温超导（HTS）带中产生的大量交流损耗和由于施加在其表面的定向磁场而导致的带临界电流的降低已经成为实现高温超导的严重障碍。前者来自于整个细丝作为一个厚单细丝，传输电流在外层细丝局部流动的电磁现象。后者是Bi-2223磁带的特性，它是最有希望作为电源应用的真正磁带的候选者。对Bi-2223带电流分布与交流损耗的关系进行了理论分析，探讨了降低损耗的适当方法。由于以往的损耗测量方法不适合本研究的目的，因此，开发了一种新的系统来测量实际应用条件下的损耗，即传输电流和外部横向磁场同时扫描。我们称这个系统为“坡印廷矢量法”。在Bi-2223短直胶带上成功地证明了该方法的有效性和实用性。然后对其进行改进，使其能够测量用高温超导胶带缠绕的单层螺线管线圈的损耗。用Bi-2223带绕制双盘饼式线圈，在液氮条件下对其交流损耗进行了详细测量。结果表明，在实际应用条件下，高频超导带的电磁特性在较宽的频率范围内得到了澄清。此外，本文还从理论上和实验上研究了一种新的线圈缠绕方法，以获得优异的性能，并证明了这种方法对液氮冷却超导电机的实现是有用的。

Realization of superconducting power machines cooled with liquid nitrogen has been concerned. However, a large amount of ac losses produced in high temperature superconducting (HTS) tapes and degradation of critical currents of the tapes due to the face-on orientated magnetic field applied in to the tapes have been serious barrier to the realization. The former comes from the electromagnetic phenomena that whole filaments act as a thick mono-filament, and transport currents locally flow in the outer filament. The latter is the characteristic property of the Bi-2223 tape that is the most hopeful candidate as a real tape for power applications.Theoretical analyses were carried out on the relation between current distributions and ac losses in Bi-2223 tapes, and an adequate method to reduce the loss was discussed. Because the previous loss-measuring method is not adequate with purpose of this study, a new system to measure losses of the tape under the real application conditions, i.e., simultaneous sweep of transport currents and external transverse magnetic fields, was developed. We call this system the 'Poynting vector method.' The validity and usefulness of it were successfully demonstrated for the short and straight Bi-2223 tape. Then it was improved to be able to measure losses of the single-layered, solenoidal coil wound with HTS tapes. The ac loss produced in the double pan-cake type coil wound with Bi-2223 tapes was also measured in detail in liquid nitrogen. As the result, the electromagnetic properties of the HTS tapes were clarified for the wide range of frequencies of the real condition applied to the tape. Moreover, a new coil winding method to get excellent performances was studied theoretically and experimentally, and showed this is useful for realization of the superconducting power machines cooled with liquid nitrogen.