Towards a beyond 1 GHz solid-state nuclear magnetic resonance: External lock operation in an external current mode for a 500 MHz nuclear magnetic resonance

Abstract

Achieving a higher magnetic field is important for solid-state nuclear magnetic resonance (NMR). But a conventional low temperature superconducting (LTS) magnet cannot exceed 1 GHz (23.5 T) due to the critical magnetic field. Thus, we started a project to replace the Nb(3)Sn innermost coil of an existing 920 MHz NMR (21.6 T) with a Bi-2223 high temperature superconducting (HTS) innermost coil. Unfortunately, the HTS magnet cannot be operated in persistent current mode; an external dc power supply is required to operate the NMR magnet, causing magnetic field fluctuations. These fluctuations can be stabilized by a field-frequency lock system based on an external NMR detection coil. We demonstrate here such a field-frequency lock system in a 500 MHz LTS NMR magnet operated in an external current mode. The system uses a (7)Li sample in a microcoil as external NMR detection system. The required field compensation is calculated from the frequency of the FID as measured with a frequency counter. The system detects the FID signal, determining the FID frequency, and calculates the required compensation coil current to stabilize the sample magnetic field. The magnetic field was stabilized at 0.05 ppm∕3 h for magnetic field fluctuations of around 10 ppm. This method is especially effective for a magnet with large magnetic field fluctuations. The magnetic field of the compensation coil is relatively inhomogeneous in these cases and the inhomogeneity of the compensation coil can be taken into account.

Correction of field instabilities in biomolecular solid-state NMR by simultaneous acquisition of a frequency reference

“…Malär et al (2021) describe a spectrometer equipped with a magnet-bore heater system that prevents major long-term field drifts due to temperature changes inside the magnet bore, which greatly increases stability and shortens time constants to reach a stable field after a disturbance. External locks can monitor the 2 D or 7 Li resonance frequency of an auxiliary sample located in the proximity of the main sample, which is then utilized for the lock (Paulson and Zilm, 2005;Takahashi et al, 2012). On benchtop low-field NMR spectrometers, a "SoftLock" system monitoring the non-deuterated solvent signal is available (Minkler et al, 2020).…”

Achievement of 1020 MHz NMR

“…The spectrometer is equipped with a 2 H lock system, sufficient to stabilize the magnetic field fluctuation caused by the DC power supply for solution NMR. A laboratory made external lock is used for solid-state NMR; a 2 H frequency detection coil was installed beside the NMR sample, connected to the lock system of the spectrometer [14]. Z 0 coil of a room-Temperature (RT) shim was used for the lock operation.…”

NMR of Macromolecular Assemblies and Machines at 1 GHz and Beyond: New Transformative Opportunities for Molecular Structural Biology

“…Due to residual resistance in the HTS coils and further advancements still needed in HTS/LTS jointing, for stable operation of the 1.02 GHz NMR spectrometer at NIMS, the magnet needed be to run in external current or driven mode (27). Field fluctuations generated by the DC power supply had to be corrected with an external 2 H lock, not typically required for solid state NMR experiments (28). Further, use of a DC power supply required development of a new safety mechanism in case of power failure (29).…”