Field decay rate is the key characteristic of the superconducting magnets based on closed-loop coils. However, in Maglev trains or rotating machines, closed-loop magnets work in external AC fields and will exhibit an evidently accelerated field decay resulting from dynamic resistances, which are usually much larger than joint resistance. Nevertheless, there has not been a numerical model capable of systematically studying this behaviour, which is the main topic of this work. The field decay curves of a closed-loop high-temperature-superconducting (HTS) coil in various AC fields are simulated based on H-formulation. A non-uniform external field generated by armature coils is considered. Reasonable consistence is found between experimental and simulation results. In our numerical model, the impact of current relaxation, which is a historical challenge, is analysed and subsequently eliminated with acceptable precision. Our simulation results suggest that most proportion of the field decay rate is from the innermost and outermost turns. Based on this observation, a magnetic shielding pattern is designed to reduce the field decay rate efficiently. This work has provided magnet designers an effective method to predict the field decay rate of closed-loop HTS coils in external AC fields, and explore various shielding designs.

中文翻译：

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非均匀交流场中闭环高温超导线圈的行为预测

磁场衰减率是基于闭环线圈的超导磁体的关键特性。然而，在磁悬浮列车或旋转机器中，闭环磁铁在外部交流场中工作，并且会表现出明显加速的场衰减，这是由动态电阻引起的，动态电阻通常远大于关节电阻。然而，还没有一个能够系统地研究这种行为的数值模型，这是这项工作的主要主题。基于H公式模拟了闭环高温超导（HTS）线圈在各种交流场中的场衰减曲线。考虑了由电枢线圈产生的非均匀外场。实验结果与仿真结果具有合理的一致性。在我们的数值模型中，电流松弛的影响，这是一个历史性的挑战，被分析并随后以可接受的精度消除。我们的模拟结果表明，场衰减率的大部分来自最里面和最外面的匝。基于这一观察，设计了一种磁屏蔽图案以有效降低场衰减率。这项工作为磁铁设计人员提供了一种有效的方法来预测闭环 HTS 线圈在外部交流场中的场衰减率，并探索各种屏蔽设计。