Dynamic resistance leads to the demagnetization of high-T _c superconducting (HTS) permanent magnets (such as closed-loop coils and tape stacks) when exposed to AC fields. This letter reports on the systematic study of an unexpected dynamic resistance occurring in first cycle AC field, which can be induced by any weak AC field—even below the threshold value ‘conventionally’ defined for generating dynamic resistance. This resistance is predicted by a critical-state analytical calculation, and its physical origin is shown to be from the asymmetrical flux penetration. Numerically, this resistance is verified, and simulated for a closed-loop HTS coil, showing a non-negligible demagnetization effect. Experimentally, the current decay (i.e. demagnetization) characteristics of a closed-loop HTS coil in a sufficient range of AC fields are measured. This resistance manifests itself in the closed-loop coil as a relatively sharp current decay in the first cycle of the applied field; this observation is in qualitative agreement with the simulation. A comparative experiment is performed to eliminate the contribution from the index loss or ‘conventional’ dynamic resistance that also possibly results in a greater demagnetization in the first cycle owing to the larger coil current. As the result, this unexpected dynamic resistance is verified. The unexpected, sharp demagnetization caused by this resistance might need to be considered when designing HTS closed-loop coil applications.

动态电阻导致高的退磁Ť _Ç暴露于交流场时的超导 (HTS) 永磁体（例如闭环线圈和磁带堆栈）。这封信报告了对发生在第一周期交流场中的意外动态电阻的系统研究，该动态电阻可以由任何弱交流场引起 - 甚至低于“传统”定义的用于产生动态电阻的阈值。该电阻是通过临界状态分析计算预测的，其物理起源表明来自不对称的通量渗透。在数值上，该电阻经过验证，并针对闭环 HTS 线圈进行模拟，显示出不可忽略的退磁效应。通过实验，测量了在足够的交流场范围内闭环 HTS 线圈的电流衰减（即退磁）特性。这种电阻在闭环线圈中表现为在外加场的第一个周期中相对急剧的电流衰减；这一观察结果与模拟定性一致。进行比较实验以消除指数损失或“传统”动态电阻的影响，由于较大的线圈电流，这些因素也可能导致第一个周期中更大的退磁。结果，这种意想不到的动态电阻得到了验证。在设计 HTS 闭环线圈应用时，可能需要考虑由这种电阻引起的意外的急剧退磁。进行比较实验以消除指数损失或“传统”动态电阻的影响，由于较大的线圈电流，这些因素也可能导致第一个周期中更大的退磁。结果，这种意想不到的动态电阻得到了验证。在设计 HTS 闭环线圈应用时，可能需要考虑由这种电阻引起的意外的急剧退磁。进行比较实验以消除指数损失或“传统”动态电阻的影响，由于较大的线圈电流，这些因素也可能导致第一个周期中更大的退磁。结果，这种意想不到的动态电阻得到了验证。在设计 HTS 闭环线圈应用时，可能需要考虑由这种电阻引起的意外的急剧退磁。