Internal-space-free passive shimming for Halbach magnets via axial displacement of magnetic blocks

To address the issue of conventional passive shimming (PS) occupying the internal space of Halbach magnets, this study explores a PS method that improves magnetic-field homogeneity by controlling the axial displacement of the magnet blocks composing the Halbach magnet. Based on the spherical harmonic decomposition method (SHDM), the Axial-Moving Array of Magnetic Blocks configurations (AAMBs) capable of generating the corresponding spherical harmonic terms (SHs) is constructed. By optimizing the movement step size of each AAMBs, the inhomogeneous magnetic-field components of the Halbach magnet are compensated. The method is experimentally validated on a home-built 90 mT dual-layer Halbach magnet. The results show a 17-fold improvement in field homogeneity and significant suppression of SHs. The proposed approach offers simple operation and high PS efficiency, making it suitable for the design of high-homogeneity Halbach magnets for portable and miniaturized MR instruments. This work provides a structurally integrated solution to the longstanding trade-off between magnet compactness and field homogeneity. Moreover, it has potential for future extension to high-field, high-homogeneity Halbach designs for benchtop NMR spectrometers.