Enhancing Magnetic Field Homogeneity in NMR Spectrometers Using Soft Magnetic Pole Pieces

This study focuses on improving the homogeneity of the magnetic field in a nuclear magnetic resonance (NMR) spectrometer with permanent magnets, a critical factor for enhancing the accuracy and sensitivity of NMR measurements. The proposed solution involves the use of soft magnetic pole pieces attached to the magnet poles, designed to optimize field uniformity. Three types of pole pieces were investigated: a recessed disk, a truncated cone, and a double-truncated disk. The optimization process employed experimental design and statistical analysis to determine the geometric parameters that minimize field inhomogeneity. A validated 3D model was used to simulate and analyze the magnetic field distribution. Experimental validation was conducted using a physical magnetic system, with measurements taken using a teslameter probe. The results showed that each pole piece design offered distinct advantages and trade-offs. The recessed disk provided high magnetic induction and compactness but required precise manufacturing. The truncated cone achieved the lowest field inhomogeneity (0.02%) but at the cost of reduced magnetic induction and increased size. The double-truncated disk offered a balance between field inhomogeneity (0.044%) and magnetic induction (0.68 T), making it suitable for compact and lightweight NMR systems. The study concludes that the choice of pole piece design depends on the specific requirements of the NMR system, with the truncated cone being ideal for applications prioritizing field homogeneity and the double-truncated disk offering a practical compromise for compact systems. This work provides valuable insights for the design of NMR spectrometers, particularly in applications requiring high field uniformity and portability.