Development of Super-High-Field NMR Operated Beyond 1 GHz Using High-Temperature Superconducting Coils

Abstract

Conventional low-temperature superconducting (LTS) NMR magnets cannot exceed 23.5 T, as the critical current density decreases steeply beyond 23 T. The high-temperature superconducting (HTS) magnet provides a high current density even at the field above 23 T, enabling 1H NMR at a frequency above 1 GHz. Thus, super-high-field NMR magnets are possible if we employ LTS outer coils and HTS inner coils; that is, an LTS/HTS NMR magnet. We firstly conducted basic experiments and made a precise comparison between two 500 MHz-class LTS/HTS NMR spectrometers operated in driven mode; one with a Bi-2223 innermost coil and another with an REBCO innermost coil. We then evaluated the effect of screening current induced in the HTS coil, the NMR sensitivity and NMR signal resolution, and the quality of protein NMR spectra. Based on these results, we constructed the world's first beyond 1 GHz LTS/Bi-2223 NMR spectrometer; the magnet was successfully charged to 1.02 GHz (24.0 T). It was demonstrated that the quality of the 2-D solid-state NMR spectrum of a membrane protein was considerably better than that obtained by a conventional 700 MHz NMR magnet. Finally, a perspective on the super-high-field NMR spectrometers (1H at 1.2 GHz), is discussed. Keywords: super high field NMR magnet; high temperature superconductor (HTS); driven mode operation; internal/external lock; screening current induced magnetic field; temporal magnetic field drift; magnetic shielding due to screening current; NMR magnet operated beyond 1 GHz; membrane protein; quadrupolar nuclei