Active shimming for a 25 T NMR superconducting magnet by spectrum convergence method

In the design of ultrahigh field nuclear magnetic resonance (NMR) superconducting magnets, it typically requires a high homogeneous magnetic field in the diameter of spherical volume (DSV) to obtain high spectrum resolution. However, shimming technique presents challenges due to the magnet bore space limitations, as accurate measurement of magnetic field distribution is very difficult, especially for customized micro-bore magnets. In this study, we introduced an active shimming method that utilized iterative adjustment of shim coil currents to improve the magnetic field homogeneity based on the full width at half maximum (FWHM) of the spectrum. The proposed method can determine the optimal set of currents for shim coils, effectively enhancing spatial field homogeneity by converging the FWHM. Experimental validation on a 25 T NMR superconducting magnet demonstrated the efficacy of the proposed method. Specifically, the active shimming method improved the field homogeneity of a 10 mm DSV from 7.09 ppm to 2.27 ppm with only four shim coils, providing a superior magnetic field environment for solid NMR and further magnetic resonance imaging (MRI) experiment. Furthermore, the proposed method can be promoted to more customized micro-bore magnets that require high magnetic field homogeneity.