# High-Speed Frequency Modulation of a 460-GHz Gyrotron for Enhancement of 700-MHz DNP-NMR Spectroscopy

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## Abstract

The high-speed frequency modulation of a 460-GHz Gyrotron FU CW GVI (the official name in Osaka University is Gyrotron FU CW GOI) was achieved by modulation of acceleration voltage of beam electrons. The modulation speed *f* _{ m } can be increased up to 10 kHz without decreasing the modulation amplitude δ*f* of frequency. The amplitude δ*f* was increased almost linearly with the modulation amplitude of acceleration voltage Δ*V* _{a}. At the Δ*V* _{a} = 1 kV, frequency spectrum width *df* was 50 MHz in the case of *f* _{ m } < 10 kHz. The frequency modulation was observed as both the variation of the IF frequency in the heterodyne detection system measured by a high-speed oscilloscope and the widths of frequency spectra *df* measured on a frequency spectrum analyzer. Both results well agree reasonably. When *f* _{ m } exceeds 10 kHz, the amplitude δ*f* is decreased gradually with increasing *f* _{ m } because of the degradation of the used amplifier in response for high-speed modulation. The experiment was performed successfully for both a sinusoidal wave and triangle wave modulations. We can use the high-speed frequency modulation for increasing the enhancement factor of the dynamic nuclear polarization (DNP)-enhanced nuclear magnetic resonance (NMR) spectroscopy, which is one of effective and attractive methods for the high-frequency DNP-NMR spectroscopy, for example, at 700 MHz. Because the sensitivity of NMR is inversely proportional to the frequency, high-speed frequency modulation can compensate the decreasing the enhancement factor in the high-frequency DNP-NMR spectroscopy and keep the factor at high value. In addition, the high-speed frequency modulation is useful for frequency stabilization by a PID control of an acceleration voltage by feeding back of the fluctuation of frequency. The frequency stabilization in long time is also useful for application of a DNP-NMR spectroscopy to the analysis of complicated protein molecules.

## Keywords

Gyrotron Second harmonic DNP NMR Sub-THz Frequency modulation## Notes

### Acknowledgments

This work was partially supported by Grants in Aid for Scientific Research (B) (No. 24360142) and for Challenging Exploratory Research (No. 25630134) from Japan Society for Promotion of Science (JSPS) and SENTAN project of JST. This work was performed under the Cooperative Research Programs of Institute for Protein Research, Osaka University and Research Center for Development of Far Infrared Region, University of Fukui (FIR UF).

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