Demonstration of 2D NMR Spectroscopy in a Powered Magnet at 25 T Using Cascade Field Regulation

This work describes the development and verification of a cascade feedback regulation system to reduce temporal field fluctuations to allow nuclear magnetic resonance (NMR) spectroscopy experiments in powered magnets. Powered magnets provide higher magnetic fields than persistent mode superconducting magnets, but require an external power source to achieve high fields. High magnetic field strengths provide improved sensitivity and spectral resolution for NMR spectroscopy. Unfortunately, powered magnets suffer from temporal magnetic field fluctuations due to the power supply ripple and variations in cooling water temperature and flow rate. Powered magnets also suffer from spatial inhomogeneity. The combination of field fluctuations and spatial inhomogeneity make powered magnets infeasible for NMR spectroscopy. In particular, two-dimensional NMR spectroscopy experiments which resolve off-diagonal cross peaks requires the spatial homogeneity and time-invariant field strength afforded by persistent mode magnets. Previous work has shown that feedback regulation using an inductive sensor can dramatically reduce the effect of temporal fluctuations on the magnetic field, and a combination of ferroshims and sample spinning can be used to improve spatial homogeneity. This work presents a cascade feedback regulation system that utilizes both inductive and NMR measurements. To verify the field regulation algorithm, we demonstrate a well-known two-dimensional NMR spectroscopy experiment in which cross peaks are resolved in a 16.3 MW powered magnet that provides a 25 T field. To our knowledge, this is the first 2D NMR spectroscopy results reported for a powered magnet where cross peaks are resolved.