Vol 59, No 8-9 (2017)

We study theoretically the mechanism of the formation of high-power ultrashort microwave pulses, which is based on the effect of passive mode locking in electron generators, whose feedback circuit includes a nonlinear saturable absorber (amplitude filter). The mechanism under consideration is an analog of the well-known method of generation of ultrashort pulses, which is used widely in laser physics. Nevertheless, realization of this method in electron generators has certain specific features connected with the motion of the active medium (electron flow). It is shown that being applied to vacuum electronics, this method is sufficiently universal, and various types of both relativistic and subrelativistic amplifiers operated in wavelength ranges from the centimeter to submillimeter ones can be used for actualization of ultrashort pulse generators as active elements. In the additional section, which acts as a saturable absorber, one can use either the Kompfner absorption effect, or the effect of cyclotron resonance absorption of radiation by a rectilinear electron beam. It is demonstrated on the basis of numerical simulation that the peak power of ultrashort pulses in electron generators with passive mode locking can exceed the power achieved in the stationary generation regimes significantly and, in some cases, the power of the electron flow.

We study frequency locking of the gyrotron operating mode by a monochromatic external signal under conditions of competition of several modes. The calculations were performed for a megawatt-power 170-GHz gyrotron with the operating TE_28.12 mode. Mode interaction of both equidistant and nonequidistant spectra is considered. Frequency detuning of the modes is comparable to the cyclotron resonance bandwidth. Influence of the external-signal power on the boundaries of the frequency locking zones is studied. The possibility to achieve a high orbital efficiency and broadband frequency tuning in the frequency-locking regime is considered.

We discuss the possibilities of using quasiregular resonator cavities with short irregularities, which ensure correction of the wave phase, in low-power gyrotrons operated at higher cyclotronfrequency harmonics. The use of such phase correctors can help with solving two problems, namely, increasing the selectivity of excitation of a higher cyclotron harmonic and decreasing the diffraction Q-factor of the gyrotron wave excited in an extended cavity.

We develop a high-power wideband amplifier based on a free-electron maser for particle acceleration, which will be operated in the 30 GHz frequency band, on the basis of the LIU-3000 linear induction accelerator forming an electron beam with an electron energy of 0.8 MeV, a current of 250 A, and a pulse duration of 200 ns. As the operating regime, we chose the regime of grazing of dispersion curves, since, according to the modeling performed, it allows one to ensure an instantaneous amplification band of about 5–7% in an undulator with regular winding for an output radiation power at a level of 20 MW and a gain of 30–35 dB. The results of the first experiments studying this FEM-based scheme are presented, in which the specified power level is achieved in the range around 30 GHz, and fast tuning of ±0.5 GHz in the band of variations in the frequency of the master magnetron is demonstrated. Modeling shows that the use of the non-resonance trapping/braking regime, which is realized in an undulator with profiled parameters, allows one to expect an increase in the radiation power of up to 35–40 MW with simultaneous widening of the amplification band up to 30% under the conditions of the LIU-3000 experiments.

We have developed a method for production of high-density MgAl₂O₄ ceramics characterized by low dielectric losses. The method is based on sintering of ultrafine, high-purity powder materials in the electromagnetic field of intense microwave radiation. The initial nanodimensional magnesium-aluminate powder was synthesized by the sol-gel method and sintered in a gyrotron complex operated at a frequency of 24 GHz. The results of studying the conditions and regimes of microwave sintering of MgAl₂O₄ ceramics are presented, along with its phase features, microstructural peculiarities, and mechanical and dielectric characteristics. The possibility of controlling the microstructure of the sintered material by varying the regime of RF heating is demonstrated.

We have experimentally discovered an instability, which manifests itself as precipitations of hot electrons occurring synchronously with generation of bursts of electromagnetic radiation, in the plasma of an electron cyclotron resonance discharge maintained by a high-power, continuous-wave radiation of a 24 GHz gyrotron, for the first time. The observed instability has the kinetic nature and is determined by the formation of the non-equilibrium velocity distribution of hot particles. Two possible explanations are proposed for the mechanism of wave excitation in a two-component plasma with a stationary source of non-equilibrium particles. The results of the studies performed are of interest for modeling of the dynamics of magnetospheric cyclotron masers.

We briefly consider the method of the frequency (phase) modulation and signal detection at the second harmonic of the modulation frequency for recording and analyzing the spectral-line shapes. The precision sub-Doppler spectrometer in the millimeter- and submillimeter-wave ranges, which operated in the regime of nonlinear saturation of the spectral transitions in a standing wave (the Lamb-dip method), was used during the measurements. The influence of the saturation degree on the value and shape of the recorded frequency-modulated signals in the quadrature channels during the synchronous detection is demonstrated. Variation in the relationships among the signals determined by dispersion and absorption was observed. The necessity of allowance for the influence of the group-velocity dispersion and coherent effects on the shape of the recorded spectral lines is experimentally shown.

We describe a microwave radiometric complex intended for remote passive monitoring of the atmospheric temperatures from the Earth’s surface. The complex consists of three spectroradiometers operating in a frequency range of 50–60 GHz, which covers the central part of the absorption band of molecular oxygen and its low-frequency slope. The radiometers have different spectral resolutions and allow one to simultaneously study the thermal structure of the surface air, the free troposphere, and the stratosphere. To ensure internal calibration of the intensity of the received atmospheric radiation, a built-in device of the modulator-calibrator type on the basis of the Schottky-barrier GaAs diodes is used. The complex is equipped with an automated system to control the measurement process, calibration, and preliminary data processing. Using the microwave sensing results, we intend to retrieve the atmospheric temperature profiles in an altitude interval of 0.05–55 km on the basis on the Bayesian approach to solving ill-posed inverse problems.

We perform experimental and theoretical studies of the series-parallel arrays of the cold-electron bolometers integrated into a cross-slot antenna and composed with an immersion silicon lens. This work is aimed at determining the efficiency of radiation absorption by bolometers, their volt-watt sensitivity, and equivalent noise power. The absorbed power was found using two independent methods, which ensured a better reliability of the results. The first method is based on comparing the experimental current-voltage characteristics of bolometers with the model based on the heat-balance equation. The second approach involves simulation of the electromagnetic properties of the system including the antenna, the lens, the bandpass filters, and the radiation source. The discrepancy among the results obtained using various methods does not exceed 30%. Optimization of the experimental setup is proposed to reach the photon-noise detection regime.