Decreasing Magnetic-Field Inhomogeneity by Inlaying the RF Coil Into a Tubular Bracket

The homogeneous magnetic field is highly desired for achieving high-resolution nuclear magnetic resonance (NMR) spectra. Inevitably, however, nonzero magnetic susceptibility (non-ZMS) structures near the detection region, such as radio-frequency (RF) coils and brackets among others, accentuate the magnetic field inhomogeneity ( $\beta $ ) and broaden NMR spectral lines. In this study, we discover that when cylindrical sensors are perpendicular to the applied magnetostatic field, inlaying the RF coil into a tubular bracket that coaxially overlays the tube can reduce $\beta $ within the detection region of this RF coil. This inlay depth can be optimized by enforcing magnetic dipole fields induced by the RF coil to equal those of the materials replaced by this RF coil. Based on this mechanism, we have developed an inlaid-coil sensor for a lab-built compact NMR spectrometer, which is merely additionally installed with a tubular Teflon bracket. Simulated results demonstrate that, as this depth approaches the optimal theoretical value, $\beta $ within the detection region gradually decreases, supporting our identification of such a mechanism. Furthermore, experimental results reveal that the inlaid-coil sensor can acquire higher-resolution NMR spectra than the bracketless sensor can, validating the compensation mode based on this mechanism. This approach can achieve magnetostatic-field compatibility among different materials inside cylindrical sensors perpendicular to the imposed magnetostatic field. Hopefully, our study can help provide a guide for the magnetostatic-field design of complex cylindrical sensors in emerging NMR technologies such as microsystems, hyperpolarization, and in situ detection.