Normal transition and stability characteristics of YBCO coated conductors under AC / pulse operation

对单芯/复芯YBCO导体和平行导体的S/N转变过程进行了实验和数值研究，得到了以下结果：1)对单根YBCO导体施加升压电流和交流过电流，研究其S/N转变过程。结果表明，外加电流的时间导数不影响YBCO单导体的S/N相变过程。在交流过电流试验中，随着电流大小的减小，在电源电流峰值附近产生的正常区域消失。2)将两个YBCO涂层导体并联，并提供上升电流，以研究它们的均流特性。由于对电流引线的接触电阻不同，在两根导线的电流中观察到了一些电流不平衡。当载流较多的导体的电流大于其临界电流时，会产生一个正常区，并将电流转移到其他导体，影响其稳定性。3)采用电磁场分析和有限元热分析相结合的方法，对单芯/复芯YBCO导体的S/N相变过程进行了数值研究。研究发现，外加交流/脉冲磁场对YBCO导体上的电流分布有很大影响，但对温度分布和失超特性影响不大。临界电流的纵向分布不会对失超特性产生实质性影响。

S/N transition processes of monofilament / multifilament YBCO conductors as well as parallel conductors were studied experimentally and numerically, and the following results were obtained.1)Ramp-up currents and AC over-currents were supplied to single YBCO conductors to study their S/N transition processes. It was found that the time-derivative of the supplied currents does not influence the S/N transition processes of single YBCO conductors. In case of AC over-current tests, a normal zone generated near the peak of the supplied current disappeared when the magnitude of the current decreased.2)Two YBCO coated conductors were connected in parallel, and ramp-up currents were supplied to study their current sharing characteristics. Due to different contact resistances to the current lead, some current imbalance was observed in currents of two conductors. When the current of the conductor carrying more current became larger than its critical current, a normal zone was produced, and the current transferred to other conductor to influence their stability property.3)S/N transition processes of monofilament / multifilament YBCO conductors were studied numerically by the combination of the electromagnetic field analysis and thermal analysis using FEM. It was found that the externally applied AC/pulse magnetic field substantially influences the current distribution on YBCO conductors but not substantially influences the temperature distribution and quench characteristics. Longitudinal distributions of critical currents do not substantially influence quench characteristics.