The implementation of sub-terahertz gyrotrons operating at high cyclotron harmonics inevitably involves the use of oversized operating cavities having dense mode spectra. This leads to a problem of the mode selectivity. This article is based on our work aimed to creation of a sub-THz CW gyrotron operating at the second cyclotron harmonic. We give a detailed picture of competition between the operating wave and a parasitic fundamental-harmonic wave in the case when these two waves have similar starting currents and operating magnetic fields, so that the operating mode can be excited in a fairly narrow range of parameters. Two-mode simulations show that in such a situation the parasitic wave can be excited at currents below the starting threshold predicted by the single-wave theory.

In short-wavelength gyrotrons operating at high cyclotron harmonics, long cavities are required to provide the start of operating oscillations. In this situation, high diffraction Q -factors lead to a problem of a high share of ohmic losses. In this article, a cavity consisting of two sections separated by a short cutoff narrowing is proposed as a way to decrease the share of ohmic losses. As compared to the regular cavity of the same length, in the sectioned cavity, the operating wave has a shorter effective length of its field and, therefore, a lower diffraction Q -factor. In a definite range of parameters, this wave wins in a competition with another, parasitic axial mode, which has a relatively low starting current and starts first, but it is suppressed by the operating mode at the steady-state generation regime.

Recently, a pulsed third-cyclotron-harmonic large-orbit gyrotron (LOG) operating with a radiation power of 1.3 kW at a frequency of 1 THz was implemented on the basis of an 80 kV/0.7 A/10 mu s version. A modified cavity with a reduced diffraction Q-factor and improved mode selection was used to ensure the kilowatt level of the output power, as well as to expand the range of parameters where the selective excitation of the operating third-cyclotron-harmonic wave was provided. In this article, we describe a new series of experiments, where the same power level was achieved for a longer (30 mu s) electron beam pulse. Stable single-mode generation of various axial modes excited at different stages of the electron beam pulse was observed. Such regimes were reproduced in numerical simulations.

We describe our works aimed to realization of frequency-tunable gyrotrons based on the use of narrow-frequency-band reflections from an output mechanically-tuned mirrors. Simulations for both fundamental-harmonic and second-harmonic gyrotrons are discussed. Experimental setup of the fundamental-harmonic gyrotron is described. In addition, we describe results of an analysis of the problem of mode competition in this system.

The possibility of creating gyrotrons operating at the frequency of 1 THz with the output wave power at the level of several kilowatts is being investigated. Such gyrotrons should be used to provide a gas-discharge plasma; this is a way to create a stable point-like source of extreme ultraviolet radiation. In this work, we discuss possibilities for experimental realization of pulsed 1 THz third-cyclotron-harmonic large-orbit gyrotrons with the kW level of the output wave power. In particular, the use of complicated schemes of operating gyrotron cavities aimed to both improve the selectivity of the high-harmonic operating and to increase of the output power is described.

In this report we present various electrodynamic approaches aimed to improve selection excitation of high harmonics in low-power sub-terahertz gyrotrons. These are fixing the frequency of the gyrotron operation by means of the use of an external rf signal, as well as the use of cavities with resonant selective element. In particular, a project of a 0.8 THz continuous-wave second-harmonic tubes with a kW power level is described.

We describe a concept of a sub-terahertz frequency-tunable gyrotron based on a combination of a low-Q irregular cavity and a frequency-tunable external reflector. Simulations predict possibilities for creation of a gyrotron with high (tens of percent) efficiencies provided in a wide ($\sim10$%) frequency band.

For terahertz-frequency-range gyrotrons, irregular cavities are proposed, in which improved selectivity of the high-harmonic operating mode generation is achieved. The use of the method allows to significantly increase the efficiency of gyrotrons designed for plasma applications. It is also shown that the use of such electrodynamic systems can be a promising way for implementation of gyrotrons with broad bands of frequency tuning.

An overview is presented of research performed at two experimental facilities with large-orbit gyrotrons (LOGs) operating in the subterahertz frequency range at the high harmonics of the frequency of electron cyclotrons. The pulsed LOG facility (80–100 keV/0.7–1.0 A) is developed as a source of radiation with a frequency of 1 THz with a kilowatt level of output power intended for use in plasma applications. A continuous subterahertz LOG (30 keV/0.7 A) is developed as a prototype of a universal multifrequency source for spectroscopic applications. Complicated electrodynamic systems intended to improve the selectivity and efficiency of high cyclotron harmonics excitation in these devices and allow tuning of the frequency of generation are described as well.

A pulsed third-cyclotron-harmonic large-orbit gyrotron (LOG) operating with a radiation power of 0.4 kW at a frequency of 1 THz was implemented in 2008 in the 80 kV/0.7A/ $10~\mu \text{s}$ version. In this letter, we describe a new experiment carried out on the basis of this LOG, where a modified cavity with a reduced diffraction Q-factor and improved mode selectivity was used. This modification allowed to increase the output radiation power up to 1.3 kW, as well as to expand the range of parameters of the selective excitation of the operating high-harmonic wave.

In this article, we describe a concept of a sub-terahertz frequency-tunable gyrotron based on a combination of a low-Q irregular cavity and a frequency-tunable external reflector. Simulations predict possibilities for the creation of gyrotrons with high (10% and higher) efficiency provided in a wide (~10%) frequency band.

In the radar remote sensing of sea ice, the main informative parameter is the backscattering radar cross section (RCS), which does not always make it possible to unambiguously determine the kind of scattering surface (ice/sea waves) and therefore leads to errors in estimating the area of the ice cover. This paper continues a line of articles that explore the possibility of using the Doppler spectrum (DS) of the reflected signal at near-nadir probing for ocean remote sensing. Experimental DS of the microwave signal reflected from water surface and ice cover are compared. The experiment took place on the Nizhny Novgorod cable car. Pulsed X-band radar was installed on a technological trolley and performed measurements in two modes that worked sequentially while moving. In the Doppler mode the DS of the reflected signal was measured. It contains information about the statistical parameters of the surface. Data processing was carried out and the first results confirmed the assumption that the DS can be an effective tool for classifying the type of the underlying surface according to the «ice/water» criterion. Comparison with model of DS for sea ice has shown that the general behavior of the DS for ice and waves in experiment is in agreement with theory. A development of the theoretical description of the DS for freshwater ice and comparison with experimental data is planned for the future research.

We propose an electromechanical method of continuous frequency tuning of the gyrotrons operating in the subterahertz frequency range. The method is based on the use of a quasisymmetric operating mode excited in a cavity with an azimuthally asymmetric cross section. Variations in the eigenfrequency of the near-cutoff wave in such a cavity are ensured by changing its cross section mechanically.

In this article, the possibility of obtaining an automodulation output signal in a powerful (megawatt-level) gyrotron is investigated. Self-modulation of the output power can be provided in a highly efficient (tens of percent) gyrotron-type regime using complicated operating cavities with special axial irregularities (phase correctors).

We show numerically that the gyrotron cavity with specially designed rectangular grooves provides a selective gyrotron excitation at the second cyclotron harmonic and allows frequency tuning. Simulations predict that the output power higher than 10 W can be obtained in the 0.3% band near the frequency of 400 GHz for a 15-kV, 0.5-A electron beam. Results obtained by analytical model calculations and particle-in-cell (PIC)-code are in good agreement.

In the experiments with a 30 keV/0.7 A sub-terahertz (THz) large-orbit gyrotron operating at the third cyclotron harmonic in the continuous-wave regime, it was found that the presence of reflections of the output signal from an external load leads to the excitation of parasitic oscillations at the fundamental cyclotron harmonic. Theoretical analysis shows that the excitation of parasitic oscillations occurs in the smooth-irregular output horn section of the microwave system, where the parasitic wave is close to the cyclotron resonance with the electron beam. Moreover, the fundamental-harmonic parasitic wave excited first in the output horn section can “penetrate” into the operating cavity, which can lead to suppression of the operating oscillation excited there at a high cyclotron harmonic.

We describe a concept of a sub-terahertz frequency-tunable gyrotron based on a combination of a low-Q irregular cavity and a frequency-tunable external reflector. Simulations predict possibilities for creation of gyrotrons with high (20% and higher) efficiencies provided in a wide (~10%)frequency band.

Novel additive technology of the Chemical Metallization of Photopolymer-based Structures (CMPS) is under active elaboration currently at the IAP RAS (Nizhny Novgorod). The use of this technology has made it possible to implement components of electron–optical and electrodynamic systems for high-power microwave vacuum tubes, such as a gyrotron and a relativistic Cherenkov maser, the design and experimental studies of which are described in this paper. Within the framework of the gyrotron developments, we carried out a simulation of the distribution of the heat load on the collector of high-power technological gyrotron taking into account secondary emission. The prospect of a significant reduction in the maximum power density of the deposited electron beam was shown. The experimental study of the gyrotron collector module manufactured using CMPS technology demonstrated high potential for its further implementation. Recent results of theoretical and experimental studies of a spatially extended Ka-band Cherenkov maser are presented. In this oscillator, the 2D-periodical slow-wave structure made by the proposed technology was applied and a narrow-band generation regime was observed with a sub-GW power level. The design and simulations of a novel selective electrodynamic system for a high-harmonic gyrotron with the planned application of the CMPS technology are discussed.

We describe an electrodynamic method based on azimuthally asymmetric cavities with a “close-to-circular” cross-section to provide a continuous frequency tuning in subterahertz gyrotrons operating on symmetric modes. A small mechanical change in the rotation angle of the moving “visor” changes the effective volume of the cavity and provides a relatively wide (several percent) bandwidth of continuous tuning of the gyrotron frequency.