Liang Fgei, Qu Timing, Zhang Zhenyu, Sheng Jie, Yuan Weijia, Iwasa Yukikazu, Zhang Min
Department of Electronic and Electrical Engineering, University of Bath, Bath, United Kingdom.
Department of Mechanical Engineering, Tsinghua University, Beijing, People's Republic of China.
Supercond Sci Technol. 2017;30(9). doi: 10.1088/1361-6668/aa7f69. Epub 2017 Aug 11.
The second generation high temperature superconductor, specifically REBCO, has become a new research focus in the development of a new generation of high-field (>25 T) magnets. One of the main challenges in the application of the magnets is the current screening problem. Previous research shows that for magnetized superconducting stacks and bulks the application of an AC field in plane with the circulating current will lead to demagnetization due to vortex shaking, which provides a possible solution to remove the shielding current. This paper provides an in-depth study, both experimentally and numerically, to unveil the vortex shaking mechanism of REBCO stacks. A new experiment was carried out to measure the demagnetization rate of REBCO stacks exposed to an in-plane AC magnetic field. Meanwhile, 2D finite element models, based on the E-J power law, are developed for simulating the vortex shaking effect of the AC magnetic field. Qualitative agreement was obtained between the experimental and the simulation results. Our results show that the applied in-plane magnetic field leads to a sudden decay of trapped magnetic field in the first half shaking cycle, which is caused by the magnetic field dependence of critical current. Furthermore, the decline of demagnetization rate with the increase of tape number is mainly due to the cross-magnetic field being screened by the top and bottom stacks during the shaking process, which leads to lower demagnetization rate of inner layers. We also demonstrate that the frequency of the applied AC magnetic field has little impact on the demagnetization process. Our modeling tool and findings perfect the vortex shaking theory and provide helpful guidance for eliminating screening current in the new generation REBCO magnets.
第二代高温超导体，特别是稀土钡铜氧（REBCO），已成为新一代高场（>25 T）磁体开发中的一个新研究重点。磁体应用中的主要挑战之一是电流屏蔽问题。先前的研究表明，对于磁化的超导叠层和块体，在与循环电流平面内施加交流磁场会由于涡旋振荡导致退磁，这为去除屏蔽电流提供了一种可能的解决方案。本文通过实验和数值方法进行了深入研究，以揭示REBCO叠层的涡旋振荡机制。开展了一项新实验来测量暴露于平面内交流磁场的REBCO叠层的退磁率。同时，基于E-J幂律开发了二维有限元模型，用于模拟交流磁场的涡旋振荡效应。实验结果与模拟结果取得了定性一致。我们的结果表明，施加的平面内磁场会导致在第一个半振荡周期内捕获磁场突然衰减，这是由临界电流的磁场依赖性引起的。此外，退磁率随带材数量增加而下降主要是由于在振荡过程中顶部和底部叠层对交叉磁场的屏蔽，导致内层的退磁率较低。我们还证明了施加的交流磁场频率对退磁过程影响很小。我们的建模工具和研究结果完善了涡旋振荡理论，并为消除新一代REBCO磁体中的屏蔽电流提供了有益的指导。
