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NONLINEAR AND TERAHERTS PHOTONICS
THz-I-7
Development of High-Power Frequency Tunable THz Band Gyrotrons for
Spectroscopy Applications
Development of a medium power (0.1-1 kW) frequency tunable sub-THz and THz band sources is highly demand for a number of spectroscpy applications including direct measurement of positronium hyperfine structure (Ps-HFS) [1] and sensitivity enhancement of DNP/NMR and RAD spectroscopy [2]. Nowadays, such level of sub-THz radiation power can only be achieved based on gyrotrons [3,4]. Significant widening of the tuning band can be provided in a gy-rotron with a specially shortened normalized cavity length. In this case, a number of axial modes can be excited at the high power level due to the weaker sensitivity to the electron velocity spread. The experimental tests of such operation regime was successfully demonstrated at the 0.5 THz tube [5]. Record value of RAD spectromenter sensitivity has been obtained [6]. For Ps-HFS measurement gyrotron with output power 0.5-1 kW at the frequency of 203.4 GHz with the possibility of fine tuning in a band of 3-10 GHz has been developed [7]. Step by step frequency tuning based on the excitation of different operating mode has been obtained at 0.25 THz tube with output power level up to 100 kW in a pulse operation regime [8].
Other attractive method for tunable generation in gyrotrons is related to using an open confocal resonator when smooth frequency tuning is provided by mechanical variation of the distance between mirrors. However, the transverse structure of the operating mode in this case is rather non-uniform. As a result, when the gyrotron is conventionally powered by a tubular electron beam, the large part of electrons is located outside the caustic bounding the field area. Thus, the interaction efficiency is low. As a method of increasing the efficiency of confocal gyrotrons, it is possible to use the modified electron-optical systems injecting an electron beam in the form of two circular arcs. However, a simpler approach is related with using, for example, a double-confocal configuration proposed in [9].
We demonstrate that the number of confocal resonators can be increased to three while maintaining the selectivity of the operating mode excitation. Such a system provides high selective properties for the operating mode. Thus, the rather high efficiency can be achieved with using conventional tubular rotating beams. As shown in 3D PIC simulations, a 6-mirror (triple-confocal) gyrotron can provide 6-10 kW output power in the frequency range of 200-207 GHz. Indicated parameters is highly demand for applications including Ps-HFS. The work is supported by the IAP RASproject #0030-2019-0027
[1] T. Yamazaki, A. Miyazaki, T. Suehara, et al., «Direct Observation of the Hyperfine Transition of Ground-State Positronium», Phys. Rev. Lett. 108, 253401 (2012); doi: 10.1103/PhysRevLett.108.253401
[2] M Koshelev, A. Tsvetkov, M. Morozkin et al., «Molecular gas spectroscopy using radioacoustic detection and high-power coherent subterahertz radiation sources», Journal of Molecular Spectroscopy, 331, 9-16 (2017); doi:10.1016/j.jms.2016.10.014
[3] T. Idehara, S. Sabchevski, M. Glyavin, S. Mitsudo. «The Gyrotrons as Promising Radiation Sources for THz Sensing and Imaging», Appl. Sci. 10, 980, 2020; doi:10.3390/app10030980
[4] A. Litvak, G. Denisov and M. Glyavin, «Russian Gyrotrons: Achievements and Trends», IEEE Journal of Microwaves, 1, 1, 260-268 (2021); doi: 10.1109/JMW.2020.3030917
[5] M. Glyavin, A. Kuftin, M. Morozkin et al., «A 250-Watts, 0.5-THz Continuous-Wave Second-Harmonic Gyrotron», IEEE Electron Device Letters, 42, 11, 1666-1669 (2021); doi: 10.1109/LED.2021.3113022
[6] G. Golubiatnikov, A. Koshelev, A. Tsvetkov et al., «Sub-Terahertz High-Sensitivity High-Resolution Molecular Spectroscopy With a Gyrotron», IEEE Transactions on Terahertz Science and Technology, 10, 5, 502-512 (2020), doi: 10.1109/TTHZ.2020.2984459
[7] A. Fedotov, R. Rozental, I. Zotova et al., «Frequency Tunable sub-THz Gyrotron for Direct Measurements of Positronium Hyperfine Structure», J Infrared Milli Terahz Waves 39, 975-983 (2018); doi:10.1007/s10762-018-0522-2
[8] A. Zuev, A. Fokin, A. Ananichev et al. «Realization of an Octave Frequency Step-Tuning of Sub-terahertz Gyrotron for Advanced Fusion Research», J Infrared Milli Terahz Waves, 42, 1131-1141 (2021); doi:10.1007/s10762-021-00832-4
[9] W. Fu, X. Guan and Y. Yana, «Harmonic terahertz gyrotron with a double confocal quasi-optical cavity», Physics of Plasmas 26, 043109 (2019) ; doi: 10.1063/1.5090471
M. Glyavin, A. Fokin, A. Tsvetkov, M. Morozkin, M. Proyavin, A. Fedotov, I. Zotova
Institute of Applied Physics RAS (IAP RAS), 46 Ul'yanov str., Nizhny Novgorod, Russia
