Application of Continuously Frequency-Tunable 0.4 THz Gyrotron to Dynamic Nuclear Polarization for 600 MHz Solid-State NMR

Abstract

In this paper we present results that demonstrate the utility of a continuously frequency-tunable 0.4 THz-gyrotron in a dynamic nuclear polarization (DNP)-enhanced solidstate NMR (SSNMR) spectroscopy at one of the highest magnetic fields, B 0 014.1 T (600 MHz for 1 H Larmor frequency). Our gyrotron called FU CW VI generates sub-mm wave at a frequency near 0.4 THz with an output power of 4-25 W and a tunability over a range of more than 1 GHz by sweeping the magnetic field at the gyrotron cavity. We observed overall down shifting of the central frequency by up to~1 GHz at high radiation duty factors and beam current, presumably due to the cavity thermal expansion by a heating, but the tunable range was not significantly changed. The frequency tunability facilitated the optimization of the DNP resonance condition without time-consuming field-sweep of the high-resolution NMR magnet, and enabled us to observe substantial enhancement of the SSNMR signal (ε DNP 012 at 90 K).

Helium-cooling and -spinning dynamic nuclear polarization for sensitivity-enhanced solid-state NMR at 14T and 30K

“…The frequency was tunable over a range of 1 GHz around the central frequency by adjusting the field strength at the gyrotron cavity [44]. The gyrotron's operation characteristics and its utility in DNP SSNMR measurement are described in a separate publication [45].…”

“…In our measurements, indeed, the maximum positive enhancements were obtained in the tunable frequency range at all temperatures. The output microwave power was not flat over the tunable range [45] since we employed a highest allowed beam current (100 mA) throughout the frequency sweep. Alternatively, one can flatten the power profile over the tunable range by adjusting the beam current.…”

Recent Advances in Magic Angle Spinning‐Dynamic Nuclear Polarization Methodology

“…The commercial state‐of‐the‐art DNP gyrotron is a single‐frequency, continuous‐wave device capable of delivering 20–50 watts of power. Although the commercial MAS‐DNP instruments use single‐frequency gyrotrons, devices that allow adjustment of the output frequency have been built . In a recent development by the Barnes group, a frequency‐agile gyrotron operating at 198 GHz ( 1 H 300 MHz) was introduced (Figure b).…”

Development of the Multifrequency Gyrotron FU CW GV with Gaussian Beam Output

“…The gyrotrons of the FU CW G-series are equipped with an internal mode converter that converts the electromagnetic (EM) wave that is oscillating in the cavity into a Gaussian beam. Applications based on these gyrotrons have begun to be implemented, including sensitivity enhancement of nuclear magnetic resonance (NMR) spectroscopy with the dynamic nuclear polarization (DNP) method [10,11], measurements of positronium hyperfine splitting [12][13][14], and ceramic sintering [15,16]. FIR UF is thus becoming both a domestic and an international center for joint research into the far-infrared region by providing many advanced gyrotrons.…”