Terahertz high-harmonic gyrotrons with irregular microwave systems
- Prof. Andrei SAVILOV
- Prof. Andrei SAVILOV (Institute of Applied Physics)
- Dr. Ilya BANDURKIN (Institute of Applied Physics of Russian Academy of Sciences, Nizhny Novgorod)
- Dr. Yuriy KALYNOV (Institute of Applied Physics of Russian Academy of Sciences, Nizhny Novgorod)
- Yulia OPARINA (Institute of Applied Physics of RAS)
- Mr. Ivan OSHARIN (Institute of Applied Physics of Russian Academy of Sciences, Nizhny Novgorod)
- Dr. Nikolai ZAVOLSKY (Institute of Applied Physics of Russian Academy of Sciences, Nizhny Novgorod)
I.V. Bandurkin, Yu.K. Kalynov, Yu.S. Oparina, I.V. Osharin, A.V. Savilov, and N.A.Zavolsky
The conventional low-harmonic gyrotrons operating in the terahertz frequency range require very strong magnetic fields. The fields can be decreased by using high-cyclotron harmonic operation of the gyrotron performed in the configuration of the so-called large-orbit gyrotron (LOG) with an axis-encircling electron beam. This configuration improves both the electron-wave coupling and the mode selectivity significantly. In the experiment , a 80 keV/0.7 A LOG was realized. Stable single-mode third-harmonic operation was observed at frequencies up to 1 THz with an output efficiency of ~1%. Recent experiments on this installation are aimed to improving this system by using irregular cavities with decreased diffraction Q-factors . The next our step in LOG development is aimed to creation of a 30 keV/0.7A CW gyrotron. The experimental setup is based on the use of a 5 T cryomagnet and a cusp gun forming axis-encircling electron beam with a pitch-factor of 1.5. The main scope is to provide gyrotron operation at the second-third-fourth cyclotron harmonics at the frequencies 0.26 THz, 0.39 THz, and 0.52 THz, respectively, with the output power level of hundreds of Watts for DNP/NMR applications. An increase in operating magnetic field up to 6.3 T together with an increase in the voltage up to 45 keV should allow achieving frequencies up to 0.65 THz at the fourth cyclotron harmonic.
In order to decrease Ohmic losses in the 80 keV/0.7 A LOG, we tested a sectioned system with a klystron-like electron-wave interaction . In the sectioned-cavity experiment, the operating current (0.3-0.5 A) was slightly lower as compared to earlier experiments. Selective excitation at the third harmonic was achieved at a magnetic field close to 10.2 T. The output rf signal at a frequency of 0.74 THz corresponded to the transverse mode TE3,5. The detected output rf power was 100–250 W. According to simulations , the share of the ohmic losses in this experiment was relatively low (20%–25% of the rf wave power emitted from the electron beam) as compared to the first experiment , where the losses were as high as 85%.
The 30 keV/0.7 A gyrotron based on a 5 T cryomagnet and an axis-encircling electron beam with a pitch-factor of 1.5 was tested in first pulsed experiments. The same regular cavity was used to provide excitation of the mode TE 2,5 at the second cyclotron harmonic (0.267 THz) and of the mode TE 3,7 at the third harmonic (0.394 THz) at slightly different magnetic fields. In order to increase the efficiency of the third-harmonic gyrotron, as well as to provide operation of the fourth-harmonic gyrotron at a frequency of 0.52 THz with the power level of ~ 100 W, special cavities with a decreased diffraction Q-factors are required. It was proposed [5,6] to use a cavity consisting of several regular sections, which are separated by short non-regularities providing the -shift of the wave phase between the sections. Such a configuration ensures the “gyrotron-like regime” of the electron-wave interaction for a far-from-cutoff mode possessing a relatively low diffraction Q-factor. This approach is used to design operating cavities for pulsed gyrotron (80 keV, 1.0-1.3 THz) operating at third and fourth cyclotron harmonics, as well as for the CW gyrotron (30 keV, 0.38-0.65 THz) operating at the second and third harmonics. Several methods for improving selectivity of excitation of high harmonics based on the use of cavities with short irregularities are proposed and tested in experiments.
The work is supported by the Russian Science Foundation, project # 17-19-01605.
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