This paper presents a warm bore ferromagnetic shimming design for a high resolution NMR magnet based on spherical harmonic coefficient reduction techniques. The passive ferromagnetic shimming along with the active shimming is a critically important step to improve magnetic field homogeneity for an NMR Magnet. Here, the technique is applied to an NMR magnet already designed and built at the MIT's Francis Bitter Magnet Lab. Based on the actual magnetic field measurement data, a total of twenty-two low order spherical harmonic coefficients is derived. Another set of spherical harmonic coefficients was calculated for iron pieces attached to a 54 mm diameter and 72 mm high tube. To improve the homogeneity of the magnet, a multiple objective linear programming method was applied to minimize unwanted spherical harmonic coefficients. A ferromagnetic shimming set with seventy-four iron pieces was presented. Analytical comparisons are made for the expected magnetic field after Ferromagnetic shimming. The theoretically reconstructed magnetic field plot after ferromagnetic shimming has shown that the magnetic field homogeneity was significantly improved.

中文翻译：

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高分辨率 NMR 磁体被动铁磁匀场设计的分析方法


本文提出了一种基于球谐系数降低技术的高分辨率核磁共振磁体的热孔铁磁匀场设计。被动铁磁匀场与主动匀场是提高 NMR 磁体磁场均匀性的至关重要的一步。在这里，该技术应用于麻省理工学院弗朗西斯·比特磁体实验室设计和制造的核磁共振磁体。根据实际磁场测量数据，总共推导出22个低阶球谐系数。另一组球谐系数是针对连接到直径 54 毫米、高 72 毫米的管子上的铁件计算的。为了提高磁体的均匀性，采用多目标线性规划方法来最小化不需要的球谐系数。提出了一套由七十四块铁片组成的铁磁匀场装置。对铁磁匀场后的预期磁场进行了分析比较。铁磁匀场后理论上重构的磁场图表明，磁场均匀性显着改善。