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Abstract: We consider oblique incidence of a vortex polarized beam on an inhomogeneous anisotropic plane layer of an artificial metamaterial. A matrix model of beam reflection from the layer boundaries and of its propagation in an inhomogeneous anisotropic medium is developed on the basis of the classical equations of electrodynamics. Using the forward Fourier transform, the beam is represented as a set of plane monochromatic waves, which have different directions of the wave vector and different directions and amplitudes of the electric fields. The obtained matrix solution allows one to calculate projections of the vectors of the electromagnetic wave inside an inhomogeneous anisotropic medium. The reflection and transmission matrices for an inhomogeneous anisotropic layer are found for the plane waves which have different propagation directions and constitute the beam. The reflected beam is reconstructed by means of the inverse Fourier transform. The distributions of the components of the electric field over the cross section of the reflected beam at oblique incidence and the phases of these components are calculated. The cross correlation functions are calculated for the components of the electric field of the beam. The Fourier series of the cross correlation function shows that components with a variation of the topological charge by ±1 appear in the reflected beam. PubDate: 2023-02-15
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Abstract: We calculate the profile of a phase plate for an operating frequency of 100 GHz, which allows one to obtain a beam with the most uniform power distribution in the region 384 × 6 mm at a distance of 50 cm, using the Gerchberg–Saxton iterative algorithm. A series of phase plates made of polyamide was 3D-printed. The parameters of the phase plate measured experimentally agree well with the theoretical calculations. The depth resolution of the image and the dependence of the beam profile on the frequency of the incident radiation are also studied. The obtained results are of practical use in the field of nondestructive control for the creation of linear terahertz scanners. PubDate: 2023-02-15
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Abstract: We study the observed response of microwave radiation to the perturbation of a transversely inhomogeneous plasma slab, which is stretched along a uniform magnetic field, by a symmetric (sausage-mode) fast magnetosonic wave guided by the slab. Two-dimensional modeling was carried out within the framework of the analytical solution of a system of linearized magnetohydrodynamic equations. The accelerated electrons filling only part of the slab were considered as the source of gyrosynchrotron radiation. It is shown that for gyrosynchrotron sources with a transverse size significantly smaller than the slab width and for all angles of sight there is enhancement of the microwave response to a fast magnetosonic wave. Namely, the modulation depth of the radiation is an order of magnitude higher than the initial wave amplitude, while an opposite effect was detected for gyrosynchrotron sources larger than the slab width. Contrast of the modulation depth of the radiation from narrow and wide sources increases with increasing difference of the densities outside and inside the slab. It was found that the microwave response of the slab to a fast magnetosonic wave is nonlinear, which may lead to a complete disappearance of the periodic response or its frequency doubling for certain combinations of the model parameters. PubDate: 2023-02-15
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Abstract: The problem of simultaneous positioning of several radio-frequency radiation sources is complicated by the necessity of unambiguous determination of a set of navigation parameters for each source. In this work, we propose an algorithm for eliminating the ambiguity of correspondence of the mutual time delays of the signal propagation. The algorithm is based on the criterion of consistency of the time delays. We consider mutual time delays, which are obtained using and analyzing the mutual uncertainty function of the signals. The conditions of applicability of the algorithm and its resistance to the noise level in the studied channels are analyzed. PubDate: 2023-02-14
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Abstract: In this paper, the method of transforming the Mössbauer gamma radiation into a train of ultrashort pulses in a resonant nuclear absorber, which harmonically oscillates as a whole along the direction of radiation propagation [1], was generalized to the case of multifrequency absorber oscillations. Based on the example of the Mössbauer absorber 57Fe and the radioactive Mössbauer source of 14.4 keV photons 57Co, it is shown that under optimal conditions, adding new harmonics of the fundamental frequency to the law of absorber motion makes it possible to reduce the duration and increase the intensity of the generated pulses while maintaining their repetition period. An optimal law of absorber motion leading to the formation of the shortest and most intense pulses is derived. It corresponds to periodic fast displacements of the absorber relative to the source by the wavelength of resonant radiation. In this case, the duration of each individual pulse of the output radiation is determined by the duration of the constructive interference that appears and disappears with a rapid change in the relative phase between the incident field and the field coherently forward-scattered by the nuclei of the moving absorber due to the Doppler effect. The possibility of transforming quasi-monochromatic radiation into a single intense short pulse, the duration and instant of the formation of which are determined by the duration and instant of the onset of the absorber displacement, is shown. PubDate: 2023-02-14
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Abstract: We analyze the resonant interaction of energetic electrons with radio emission in the magnetospheres of exoplanets using the planet Tau Bootis b as an example. The conditions of this interaction and its impact on the parameters of electrons are considered for the radio emission which is detectable on the Earth. Specific features of the interaction are studied in comparison with the interaction of energetic electrons with auroral kilometric radiation in the Earth’s magnetosphere. It is shown that for the considered parameters of the magnetosphere of Tau Bootis b and radio emission in it, the resonant interaction of electrons with radio emission is strongly nonlinear. For a wave packet of finite duration, the interaction can cause a significant acceleration of particles in a wide range of energies (from tens to hundreds of kiloelectronvolts). PubDate: 2023-02-14
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Abstract: We present the results of studying experimentally gyroresonance K-band traveling-wave tubes having kilowatt power levels in the output radiation, in which the transition to multi-frequency generation regimes is implemented by the formation of relections of the output signal. It is shown that there exist regimes of significant interest for the problems of multi-frequency plasma heating, where the level of the second strongest spectral component is comparable with the level of the fundamental component. The problem of vacuum-window overheating during operation under the conditions of strong output-radiation reflections formed by a wideband Bragg reflector is discussed. PubDate: 2023-02-02
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Abstract: We give a brief review of the results of using gyrotrons to study molecular spectra by means of a radioacoustic absorption detection (RAD) spectrometer. Examples of recording molecular spectral lines of sulfur dioxide (SO2), carbon sulfide (OCS), and methane (CH4) are presented. The achievement of a record-breaking sensitivity for the sub-terahertz frequency range in terms of the absorption coefficient is demonstrated when observing weak methane lines. The possibility of spectroscopy at harmonics of the gyrotron radiation with a frequency of up to 1 THz is shown. Spectroscopic estimates of the radiation power at the second and third harmonics with respect to the power at the fundamental frequency are given. The possibilities of using the methods of nonlinear spectroscopy, namely, two-photon spectroscopy, which extend the range of potential spectroscopic studies using intense gyrotron radiation, are demonstrated. The results of observations with a RAD spectrometer and a direct absorption spectrometer for the lines of two-photon rotational transitions of OCS and SO2 molecules, which have well-studied spectra and a large dipole moment, are presented. Promising opportunities for using gyrotrons to solve fundamental and practical problems are discussed. PubDate: 2023-02-02
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Abstract: Using the method of sintering by microwave heating at a rate of 10–100°C/min to a temperature of 1250 °C without isothermal hold, we obtain ceramic samples with the composition Ba0.75Sr0.25TiO3. The experiments are performed on a gyrotron complex for the high-temperature microwave treatment of materials, which is operated at a frequency of 24 GHz with a maximum power of 5 kW. The features of the formation of a solid solution in the sintered samples are analyzed. The dependence of the intensity of recrystallization growth of grains on the microwave-heating rate during the sintering is demonstrated. The permittivity of the samples obtained by microwave sintering with heating rates of 10 and 30 °C/min amount to 3600–4400 in the frequency range 25Hz–3MHz at room temperature. PubDate: 2023-02-02
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Abstract: We analyze theoretically and model a generator of a periodic sequence of ultrashort microwave pulses in a scheme with two coaxial cylindrical electron beams formed by the “Saturn” accelerator (Institute of Applied Physics of the Russian Academy of Sciences). An external cylindrical beam, as it moves rectilinearly along a periodic slow-wave system, ensures amplification of the radiation, whereas the internal near-axis beam ensures nonlinear cyclotron absorption. The modeling includes both the electron wave interaction and the formation of two electron beams in a common cathode unit, as well as their transportation in a common vacuum volume. It is shown that at a voltage of 210 kV and currents of 6.0 A and 4.4 A of the amplifying and absorbing beams, respectively, generation of 8-mm pulses with a duration of 650 ps, a peak power of 540 kW, and a repetition rate of 3.3 ns is possible. PubDate: 2023-02-02
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Abstract: We present the concept of creating sources of gigawatt-level powers in the subterahertz and terahertz frequency ranges, which are based on excitation of high-Q cavities by high-current relativistic electron beams. In order to ensure selective generation in a cavity with a great oversize parameter, we propose to use a high-Q supermode formed by a set of partial eigenmodes of the system. The results of theoretical analysis and numerical modeling are presented both for the “cold” problem (finding the supermode structure in the absence of the beam), and for the “hot” one, where the supermode is excited by an electron beam of small “seed” noises having a random spatiotemporal structure. PubDate: 2023-02-02
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Abstract: We propose a method for considering the spread of initial electron velocities caused by the roughness of the cathode surface while performing a three-dimensional trajectory analysis in the gyrotron electron–optical system. The effect of the size of roughness on the velocity characteristics of the electron beam entering the gyrotron cavity with a frequency of 74.2 GHz and an output power of approximately 100 kW was determined in the simulations. Comparison of simulation data with experimental results showed that the average size of cathode inhomogeneities for this gyrotron is approximately 14 μm for the model of a cathode with regular hemispheres on its surface. PubDate: 2023-02-02
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Abstract: We analyze the coherence degree of active noise interference in radar systems. The entropy of the distribution of eigenvalues of the interference correlation matrix is used as a quantitative measure of the coherence degree of interference. The main factors influencing the coherence of active noise interference are analyzed, namely, the interference spectrum width and the interferenceenvironment nonstationarity due to variation in the angular relations between the antenna-array beam and the interference source in the process of the radar observation. It is shown that the analysis of coherence of an active noise interference allows one to quantitatively estimate the notions of wideband and narrowband interference. PubDate: 2022-12-06 DOI: 10.1007/s11141-022-10201-3
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Abstract: Increasing requirements to modern synthetic-aperture radars leads to a complication of both the radars themselves and the tools for monitoring of the radar parameters. Reflected-signal simulator is designed to check through characteristics of such radars at various stages of the life cycle, starting with the development of components and including regular operation, using the same methods. We describe a device for converting the sounding signals of synthetic-aperture radars to simulated reflected ones. A distinctive feature of the presented device is the ability to simulate point and spatially distributed targets and an arbitrary target environment (a set of observation conditions) for test, calibration, and validation. PubDate: 2022-12-06 DOI: 10.1007/s11141-022-10199-8
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Abstract: We perform a comparative analysis of the radiation of X-ray free-electron lasers (FELs) LCLS, PAL-XFEL, SwissFEL, SACLA, FLASH2, and European XFEL, as well as the visible radiation of the LEUTL FEL. The spectral characteristics of the considered FELs with account of all the main losses due to the electron energy spread, diffraction, emittance, and the beam diameter are compared. The results of the theoretical studies agree well with the experimental data available for all the FELs under consideration. The possibility to use and amplify harmonics in X-ray FELs with an adjustable dipole parameter (SwissFEL, LCLS-II, SACLA, FLASH2, and European XFEL) is studied. The advantages of amplifying the self-seed radiation of HLSS harmonics are demonstrated, at which the FEL can be made shorter due to efficient bunching at the harmonic wavelengths in a buncher with a great dipole parameter k of the undulator. The possibility of forced bunching discontinuity between the LCLS-II undulators and its influence on the third harmonic amplification are studied theoretically. Comparative analysis of the parameters of radiation, electron beams, and undulators of the considered FELs in various operation regimes is performed. PubDate: 2022-12-06 DOI: 10.1007/s11141-022-10197-w
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Abstract: We develop models of the formation of additive sea noise and surface reverberation at the output of a phased vertical antenna placed in a plane-layered waveguide with a two-dimensional variabledepth bottom. The study is performed within the framework of geometrical acoustics. The noise calculations are performed for various types of refractive waveguides under conditions of coastal waters. The results of the studies allow one to conclude that the maximum contribution is ensured by the shallow-water areas concentrated near the radiating and receiving antennas. The directions of the main lobes of the antenna patterns also play a significant role during the noise formation. PubDate: 2022-12-06 DOI: 10.1007/s11141-022-10200-4
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Abstract: We generalize a variant of the auxiliary-source method, which was proposed earlier for solving the problem of electromagnetic-wave scattering by a structure consisting of several dielectric finitelength thin cylinders to the case where, along with the dielectric thin cylinders, the structure contains several perfectly conducting thin cylinders. The specially developed computer code is briefly described. The results of calculating the scattering cross sections of certain particular structures are presented. PubDate: 2022-12-06 DOI: 10.1007/s11141-022-10198-9
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Abstract: We study the dependence of the parameters of large-scale plasma inhomogeneities upon HF ionosphere heating on the frequency and power of HF radiation by using a modified SAMI2 model for numerical simulations. It is shown that the variation of the plasma density profile in the F region due to heating results in a strong dependence of plasma density disturbances on the frequency and power of HF radiation. We also analyze the effect of the Joule absorption in the lower ionosphere on the formation of large-scale inhomogeneities. PubDate: 2022-12-06 DOI: 10.1007/s11141-022-10196-x
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Abstract: We present the experimental results of measuring the reflection coefficient of a microwave output window made of radiation-modified fluoroplast (Raflon™) with antireflection coating made by using surface machining. The quarter-wave antireflection coating was created by drilling shallow holes in a high-density hexagonal pattern over the plate surface. The Maxwell Garnett formula was used to estimate the value of the effective dielectric permittivity of the periodic structure, since this formula is the most suitable for approximation of effective properties of cylindrically shaped inhomogeneities. The dielectric permittivity of such a structure in the transverse direction corresponds to the value optimal for increasing the window transparency in the specified frequency range. It is found experimentally that the reflection coefficient in the band 120–140 GHz does not exceed −20 dB. The obtained results are compared with the theoretical calculation and numerical analysis in the CST Studio Suite. PubDate: 2022-11-01 DOI: 10.1007/s11141-022-10194-z
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