Abstract:

In recent years, there has been a significant utilization of Halbach magnet in the field of miniaturized NMR spectrometry. However, the lack of magnetic field homogeneity presents huge demands on shimming method. In this paper, we conducted a passive shimming study of Halbach permanent magnet array structure which is mechanically adjustable. This work optimization modeled the relationship between the radial position of the magnetic blocks and magnetic field homogeneity. Then, an optimization algorithm combining the Levenberg-Marquardt method and the Quasi-Newton method was utilized to optimize the magnetic field homogeneity by adjusting the radial positions of the magnetic blocks. With this approach, the homogeneity of a 1.03 T Halbach magnet was improved from 7 391 ppm to 154.23 ppm in a sphere with a radius of 2.5 mm. This work provides a flexible and convenient passive shimming method for compact Halbach magnet, and the method has potential applications in NMR spectrometers and other instruments that require high magnetic field homogeneity.

Key words: nuclear magnetic resonance spectroscopy, Halbach magnet, passive shimming, Levenberg-Marquardt method, Quasi-Newton method