Research on Large-bore and High Homogeneity Halbach Magnet for Nuclear Magnetic Resonance

Compared with small rock cores, large rock cores better preserve the original internal structure and fluid distribution, yet they demand a larger homogeneous region. However, the complex structure of Halbach magnets inherently yields inadequate initial homogeneity, making it difficult to directly obtain a sufficiently large homogeneous region.

In this study, we defined a 100-mm-diameter spherical region of interest (ROI) and optimized the magnet structure using Halbach magnet theory and finite-element simulations. The final designed magnet provides a field strength of 158.4 mT and an initial homogeneity of 22 502 ppm (1 ppm=10−6). We applied an improved harmonic-based passive shimming method and enhanced the field homogeneity to 1 496 ppm.