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18THz-I-1
Terahertz gyrotrons and their applications: resent results
M. Glyavin
Institute of Applied Physics RAS (IAP RAS) 603950, 46 Ulyanov str., Nizhny Novgorod, Russia glyavin@ipfran. ru
The last decade has contributed to the rapid progress in the gyrotron development. The paper describes the main features of high frequency gyrotrons [1,2]. The most impressive data about pulsed and CW tubes, working in the terahertz frequency range, are given. These gyrotrons operate (in some specific combinations) at very low voltage and beam current, demonstrate an extremely narrow frequency spectrum or wide frequency tuning. Although in comparison with the classical microwave tubes the gyrotrons are characterized by greater volume and weight due to the presence of bulky parts (such as superconducting magnets) they can easily be embedded in a sophisticated laboratory equipment. All these features have opened the road to many novel and prospective applications of gyrotrons.
At IAP RAS, the development of the terahertz band by the methods of vacuum electronics, as well as by using conventional gyrodevices, which employ extremely strong magnetic fields at the main cyclotron resonance and the second cyclotron harmonic, is supported by the development of gyrodevices operated at higher cyclotron harmonics (large-orbit gyrotrons (LOGs). Pioneering work of IAP RAS demonstrated the fundamental possibility of obtaining high-power continuous-wave and pulsed radiation of the frequencies from 0.33 to 0.65 THz using gyrotrons. In recent experiments, a pulse solenoid with a magnetic field of up to 50 T generated a power of 5 - 0.5 kW at the fundamental cyclotron resonance in single pulses with a duration of 50 microseconds at record-breaking frequencies of 1 - 1.3 THz. IAP RAS developed a technology which ensures the creation of relatively simple pulse solenoids with a magnetic field of up to 30 T and a pulse repetition rate of up to 0.1 Hz. Based on such coils the gyrotron with an operating frequency of 0.67 THz and a power of 200 kW has been realized. This gyrotron can be used for recording of ionization sources from distances of several hundreds of meters. The continues wave (CW) gyrotron developed at IAP RAS jointly with GYCOM Ltd. provides CW radiation with a kW range power in the frequency range from 0.26 to 0.53 THz. The maximum power measured by a calorimeter is 1 kW at 0.26 THz and 0.24 kW at 0.53 THz. Such power is approximately five times higher than in the worldwide reported gyrotrons at the same frequency range. It is important that power values up to 10W required for spectroscopy applications can be obtained at low voltage of about 1.5 kV or low beam current of 20 mA. The phase-lock loop control of the anode voltage result in the width of the frequency spectrum from 0.5 MHz for a free-running gyrotron down to 1 Hz for the stabilized gyrotron, which corresponds to long-term stability up to S f / f ~ 10-12. Recently, the phase-lock loop stabilized gyrotron was used in experiments for recording characteristic lines in pure gases and gas mixtures. It is interesting that the same scheme can be used for quick power modulation and data transmitting. High quality transmission of sound and pseudo random bit sequence (with a speed up to 1.5 Mbit/s) has been obtained. Second-harmonic CW gyrotron with improved mode selection based on double electron beam has been tested. Wide step tuning of the frequency by excitation of various modes was demonstrated in the range of 0.4-0.78 THz with a power level of about few Watts, which is useful for modern NMR/DNP spectroscopy applications. A complex cavity is under discussion for high harmonic excitation and preliminary tests confirmed operation with the frequency near 1.2 THz. The project of gyrotrons with field emission cathode are under development. Feasibility of a high-power sub-THz gyrotron with smooth wideband frequency tuning suitable for spectroscopy has been studied. Simulations and preliminary experiments demonstrated a possibility of wide-band (about 10%) fine frequency tuning. An output power of 0.5 to 1 kW can be obtained at a frequency of about 0.2 THz within a 10 GHz band, which are the parameters needed for testing of quantum electrodynamics predictions in positronium spectroscopy measurements.
[1] A.Litvak, G.Denisov, M.Glyavin. Russian gyrotrons: achievements and trends. IEEE Journal of Microwaves, 1,1, 260-268 (2021)
[2] S.Sabchevski, M.Glyavin, S.Mitsudo, Y.Tatematsu, T.Idehara. Novel and Emerging Applications of the Gyrotrons Worldwide: Current Status and Prospects. Journal of Infrared, Millimeter, and Terahertz Waves (2021) https://doi.org/10.1007/s10762-021-00804-8
