Abstract:

Halbach permanent magnets have potential wide application prospects in low-field nuclear magnetic resonance (LF-NMR), such as rock core relaxation analysis, because they require no iron yoke and produce small external stray fields. Compared with small rock cores, large rock cores retain the original internal structure and fluid distribution more completely, but they require a larger homogeneous region. However, the complex structure of Halbach magnets results in inadequate initial homogeneity, which makes it difficult to obtain a large homogeneous region directly. We defined a 100 mm-diameter sphere as the region of interest (ROI) and optimized the magnet structure using Halbach magnet theory and finite-element simulations. Finally, we designed a magnet, which provided a field strength of 158.4 mT and an initial homogeneity of 22 502 ppm. We applied an improved harmonic-based passive shimming method, and the field homogeneity was improved to 1 496 ppm. Then, we measured ¹H FID signals from an aqueous CuSO₄ sample (Φ 100 mm× H 100 mm), and we measured CPMG signals from a two component CuSO₄ aqueous solution, distinguished different samples through T₂. These results show that the magnet design and passive shimming method are effective for building large-bore, high homogeneity Halbach magnets.

Key words: NMR, Halbach magnet, magnet design, passive shimming, magnetic field homogeneity