Bang Jeseok, Kim Jaemin, Jang Jae Young, Ahn Minchul, Hwang Young Jin, Kim Kwangmin, Kim Youngil, Ku Myunghwan, Lee Hunju, In Sehwan, Hong Yong-Ju, Yeom Hankil, Lee Jung Tae, Yang Hongmin, Hahn Seungyong, Lee SangGap
Department of Electrical and Computer Engineering, Seoul National University, Seoul, 08826, South Korea.
National High Magnetic Field Laboratory, Tallahassee, FL, 32310, USA.
Sci Rep. 2024 Aug 19;14(1):19146. doi: 10.1038/s41598-024-68607-0.
REBa Cu O (REBCO, RE = rare earth)-coated conductor is a competitive option in terms of current-carrying capacity and high-stress durability in developing high-field magnets for nuclear magnetic resonance (NMR) research. Meanwhile, a technical challenge in utilizing a stand-alone REBCO NMR magnet is an unexpected difference in the field uniformity between the designed and measured values after being constructed and charged, i.e., harmonic errors. Bortot et al., and Li et al., reported analytic evidence of the related issue. However, sufficient research has not yet been conducted, so evidence should be supplemented further. Here we report harmonic errors due to screening current and inconsistent conductor thickness, confirmed by a 400 MHz H NMR magnet development project. The magnet was first charged up to its operating current, and then multiple overcharge-discharge cycles were applied, which was an empirically optimized operation protocol. A field mapping device obtained magnetic fields at designated locations in the room-temperature bore. The result showed over 100 ppm field uniformity difference between designed and measured values. A simulation model was developed considering screening current and inconsistent conductor thickness for reproducing the field distribution. Comparison of voltages and fields between simulation and measurement validated the model. Further analysis of the overcharge-discharge effect on harmonic errors demonstrated that even and odd-order harmonics are mainly attributed to screening current and geometric inconsistency while confirming the limitation of the screening current mitigation effect. Hence, we concluded that the desirable requirement of the sub-ppm level field uniformity generation might be barely possible with the current REBCO NMR magnet design approach.
稀土钡铜氧化物（REBCO，RE = 稀土）涂层导体在开发用于核磁共振（NMR）研究的高场磁体时，在载流能力和高应力耐久性方面是一种具有竞争力的选择。与此同时，使用独立的REBCO NMR磁体的一个技术挑战是，在构建和充电后，设计值与测量值之间的场均匀性存在意外差异，即谐波误差。博托特等人以及李等人报告了相关问题的分析证据。然而，尚未进行充分的研究，因此证据应进一步补充。在此，我们报告了由屏蔽电流和导体厚度不一致导致的谐波误差，这是通过一个400 MHz氢核磁共振磁体开发项目得到证实的。该磁体首先充电至其工作电流，然后进行多次过充 - 放电循环，这是一个经验优化的操作协议。一个场测绘装置在室温孔的指定位置获取磁场。结果表明，设计值与测量值之间的场均匀性差异超过100 ppm。开发了一个考虑屏蔽电流和导体厚度不一致的模拟模型来再现场分布。模拟与测量之间的电压和场的比较验证了该模型。对过充 - 放电对谐波误差影响的进一步分析表明，偶数和奇数阶谐波主要归因于屏蔽电流和几何不一致，同时证实了屏蔽电流缓解效果的局限性。因此，我们得出结论，使用当前的REBCO NMR磁体设计方法，几乎不可能实现亚ppm级场均匀性的理想要求。
