Известия ВУЗов.Радиофизика

We study the excitation of an electromagnetic field in a compact nerve fiber by a system of given filamentary electric currents, which are aligned with the fiber. In the case of a fiber consisting of several myelinated axons, equations for the scattering coefficients of the electromagnetic waves excited by such currents are obtained. On the basis of the solution of the corresponding equations, the total field due to these sources is found in the case of their location around the fiber. An approach is proposed for determining the filamentary-current amplitudes such that it ensures the formation of the required field pattern inside the nerve fiber. Numerical results demonstrating the possibility of selective field excitation in individual axons are presented.

We present the results of experimental studies of the oscillation regimes of the pulse signals, which are analogs of the temporal solitons, in a distributed active ring resonator with a multicavity klystron amplifier and a spin-wave transmission line. The soliton-like pulses are formed under conditions of three-wave parametric decay of a surface magnetostatic spin wave when the klystron amplifier operates in a small-signal regime. Parametric turbulence is shown to cause the chaotic nature of generated patterns, which is confirmed by the performed estimations of the senior Lyapunov exponent from experimental time series. In addition, the possibility of controlling the phase coherence, the autocorrelation time, and the noise-to-signal ratio of the generated chaotic soliton-like pulses by varying the beam current and the acceleration voltage of the klystron amplifier is demonstrated.

We propose a method allowing one to improve the accuracy of retrieving the temperature distribution over the object surface by taking into account the influence of the antenna patterns of a multichannel radiometer on the measurement results at the nodal points. The accuracy improvement that is equivalent to an increase in the number of the measuring channels of the system from 9 to 16 is achieved. The positive effect is enhanced with increasing antenna-pattern overlapping degree.

We study the dynamics of the spectra of multiband sporadic magnetic pulsations in the Pc1 range (0.2–5.0 Hz) during the event of 5–6 March 2011 by using ground-based magnetic measurements at stations largely spaced from each other in latitude and longitude. The event is characterized by a long duration (about 16 h), the presence of several bands with varying frequencies, splitting of these bands into narrower subbands, significant variations in the amplitude and polarization of the signals on the ground, and their observation in a wide range of latitudes and longitudes. On the basis of a joint analysis of the Pc1 pulsation properties and the data of low-orbiting spacecraft detecting localized precipitations of energetic protons into the ionosphere, we infer the possible generation regions of these waves in the magnetosphere and conclude that they are multiple. The results of analysis allowed us to determine the mechanisms of broadening and splitting of Pc1 frequency bands even in the absence of direct wave observations in the magnetosphere. We also propose an explanation of the atypical (for ground-based detection) character of two-band Pc1 spectra when the signal at frequencies above the helium ion gyrofrequency has a higher amplitude than at lower frequencies. We also explain the inhomogeneous frequency profile of polarization in different frequency bands. Possible variations in the magnetospheric plasma parameters that resulted in the observed dynamics of amplitude and polarization spectra of Pc1 pulsations are revealed by using calculations of the wave cyclotron amplification by energetic protons in the magnetosphere.

We model the evolution of radiation harmonics in a single-pass free-electron laser using a phenomenological model, in which the main losses are taken into account separately for each of the harmonics. The modeling results are compared with the corresponding FEL experiments and the simulations, which we performed using the PERSEO code. A wide wavelength range (0.15–500 nm) is considered. The phenomenological description based on the use of several basic FEL parameters, such as the electron beam current, average energy, energy spread, and emittance of electrons, yields the results that correspond well to the LEUTL, SPARC, and LCLS experiments in various conditions. The evolution of the radiation power, bunching, and electron energy spread in the Spring 8 FEL at the follow-up stage is also considered. The necessity to improve electron beam parameters for generation of harmonics in this FEL is shown. The phenomenological model allows assessing the operation of the available and future FELs fast. Using it, one can model FELs with nearly any undulator with allowance for higher harmonics of their magnetic fields. Moreover, this model makes it possible to describe easily various effects in designed FELs, e.g., those with dephasing of electrons and photons, filtering of harmonics, and other features.

We present the results of experimental investigation of a laboratory prototype of the electron-wave microwave amplifier for the case of great values of the inhomogeneity parameters of electron beam velocities. The influence of several factors on the gain of such systems in the case under consideration is demonstrated experimentally. It is found that at great values of the inhomogeneity parameters a significant increase in the power of the input signals leads to an increase in the gain of the system. The proposed scheme of operation of the electron-wave amplifier allows improving its stability, increasing the gain to about 20 dB, and reducing its size. The significance of the obtained experimental results and studied phenomena for advancement of the operating frequencies of electron-wave amplifiers into a shorter-wavelength band is shown.

We propose a new method for compensating the thermally induced depolarization of laser radiation, which is based on using spatial light modulators. It is shown that using one modulator, the integral depolarization degree can be reduced by several times, whereas two modulators make it possible to almost completely rule out depolarization, regardless of its nature. The phase front of the laser beam is analyzed and it is shown that this phase front remains qualitatively unchanged when depolarization is compensated by one modulator, while in the case of compensation by two modulators the phase nonuniformity decreases, which leads to an increase in the far-field intensity.

We propose a new approach to reconstructing complex, spatially distributed systems on the basis of the time series generated by such systems. It allows one to combine two basic steps of such a reconstruction, namely, the choice of a set of phase variables of the system using the observed time series and the development of the evolution operator acting in the chosen phase space with the help of an artificial neural network with special topology. This network, first, maps the initial high-dimensional data onto the lower-dimension space and, second, specifies the evolution operator in this space. The efficiency of this approach is demonstrated by an example of reconstructing the Lorenz system representing a high-dimensional model of atmospheric dynamics.

We present the results of prediction and extrapolation of the maximum usable frequency (MUF) on the subauroral and midlatitude paths of the Euro-Asian region by adapting the reference ionosphere model IRI-2007 based on the results of the oblique chirp sounding of the ionosphere on the reference and working radio links. It is shown that under conditions of a quiet ionosphere where the adapted ionospheric model is used, the errors of the MUF prediction on the reference path and the error of the MUF extrapolation to the neighboring paths not equipped with diagnostic tools are 2–8%, which is significantly less than the long-term prediction data which amount to 9–20% [1–3].

We consider the problem of energy transmission from one aperture to another by the wave fields in smoothly inhomogeneous media. The study is performed in the quasioptical and scalar approximations when the aperture sizes significantly exceed the wavelength and there exists a ray connecting the aperture centers. The transmission coefficient is determined using the well-known parabolic equation, which describes the wave-beam propagation in smoothly inhomogeneous media. As in the homogeneous media, the maximum transmission coefficient is shown to be reached when the field structures at the aperture are specified in the form of special functions, namely, the prolate spheroidal angular functions. The maximum achievable transmission coefficient is determined.

Ionograms of the oblique ionospheric sounding radio paths Cyprus—Lovozero and Lovozero—Gor’kovskaya (near St. Petersburg) are analyzed using magnetometers and riometers. The behavior of the maximum observed frequencies (MOF) of the F2 and Es layers during a magnetic storm of September 7–8, 2017 is considered. The storm has a specific, not classical form and can be divided into two parts, namely, the first and the second storms. The main results are as follows. 1) The manifestation of the first storm is accompanied by a significant increase in absorption and frequent absence of signals on the paths. The second storm is characterized by increased values of MOF-Es and the presence of signals. 2) The signals on the midlatitude Cyprus—Lovozero path propagate both as 1F2, 2F2, and 3F2 modes and as 2Es and 3Es modes. 3) On the subpolar Lovozero—Gor’kovskaya path, during the disturbance, the signals mainly propagate via reflection from the sporadic Es layer of very high intensity. 4) Modeling of the propagation of HF radio waves using the IRI-2016 model adapted to the vertical sounding data made it possible to match the model ionograms with the real oblique sounding ionograms, including the cases of presence of the M and N modes, as well as triplets, in both quiet and disturbed conditions, which is a new result.

We have synthesized elliptic bandpass waveguide filters. The resonators of the filter are thin plane-transverse resonant irises with an aperture in the form of a rectangular window with two metal L-shaped ridges. The electrodynamic problem of calculating the complex conductance of the iris is solved. An equivalent circuit of the resonant iris is proposed, and the correspondence of the geometric size of its aperture to the parameters of the circuit is established. The results of synthesis of an elliptic filter for the WR137 rectangular waveguide with a passband of 6.35–6.83 GHz at a level of −3 dB are presented. The longitudinal size of the filter is 34 mm, which ensures the small size of the developed device.

We propose a simple method for estimation of the probabilities of elementary processes of quantum electrodynamics in a strong field. It is shown that quantum-electrodynamic processes run in the quasiclassical or quantum regime depending on the ratio of the characteristic scales of the time of particle acceleration by the field (a quasiclassical effect) and energy absorption by charged particles from the field directly during the process (a purely quantum effect). Using simple kinematic considerations and the uncertainty principle for these regimes, it is possible to reproduce the probabilities of the known processes in a strong field, in particular, spontaneous pair creation, photon emission by an electron, pair photoproduction, and radiation mass correction.

We study theoretically two-dimensionally periodic gratings of plasmonic strips on substrates which contain dielectric and plasmonic layers. The developed electrodynamic model is based on solving the vector integro-differential equation of diffraction by three-dimensional dielectric bodies by the Galerkin method. It is shown that at the resonant frequencies of the surface plasmon polariton, these structures absorb almost 100% of the energy of the incident radiation in a wide wavelength range. The use of nonmetal plasmonic materials makes it possible to create a wideband absorber of infrared waves.

We show experimentally the possibility to expand significantly the band of smooth tuning of the generation frequency in gyrotrons using cavities that have shorter lengths. Due to a decrease in the sensitivity of the electron-wave interaction process to the spread in the electron beam velocities, one can increase the power of generation at higher longitudinal modes up to a level comparable with the radiation power in the case of excitation of a mode with one longitudinal variation. In this case, overlapping of generation bands at the neighboring longitudinal modes is achieved by increasing the current of the electron beam. In the experiment performed in a gyrotron having an operating frequency of about 12 GHz, we demonstrated a frequency tuning band which exceeded 4% at the kilowatt level of the output radiation power in its greater part. The obtained results open up the possibility of developing tunable high-frequency moderate-power gyrotrons.

We develop the maximum likelihood algorithm for detecting an ultra-wideband quasi radio signal with an arbitrary shape and unknown amplitude, initial phase, and duration, which is observed against the background of additive Gaussian white noise. The structure and statistical characteristics of this algorithm are found. The influence of a priori ignorance of the duration of a quasi radio signal on its detection efficiency is studied. The operation efficiencies of the maximum-likelihood and quasioptimal detectors of the ultra-wideband quasi radio signal are compared. Using computer simulation, the efficiency of the synthesized algorithm is examined and the applicability ranges of the obtained asymptotic expressions for its characteristics are determined.

We study nonlinear dynamics of the spintronic nanosized antiferromagnetic terahertz oscillator consisting of an antiferromagnetic layer with easy-plane anisotropy (hematite) and a normal-metal (platinum) layer. Normal oscillation frequencies, namely, ferromagnetic and antiferromagnetic (terahertz) ones, are found. Their dependence on the value of a static magnetic field parallel to the sample plane is obtained. An approximate mathematical model in the form of the equations for the Néel-vector rotation angle in the azimuthal plane is developed for describing the oscillator dynamics. The adjustment characteristic, i.e., the dependence of the antiferromagnetic-mode frequency on the value of the direct current flowing in the platinum layer is obtained.

Gyrotron systems operated at frequencies of 24 to 30 GHz with an output power of 3 to 15 kW have been used at the Institute of Applied Physics of the Russian Academy of Sciences for more than 20 years for the studies of high-temperature processes in polycrystalline dielectric materials under intense electromagnetic irradiation. The research has mostly been focused on the study of the physically specific features of diffusive mass transfer in solids and on the possible use of these features for applications. A distinguishing feature of the studied processes is a significant enhancement of their rates compared to similar processes performed with the use of conventional heating methods. Examples of enhanced sintering of a broad range of ceramic materials, including optical and laser ceramics and composition-graded metal–ceramic products are considered. The principles of the developed method of ultrafast sintering of oxide ceramics with rates exceeding those typical of the conventional methods by two or three orders of magnitude are described. The development of this method has resulted from a purposeful use of the functional capabilities of the gyrotron systems and the engineering solutions implemented therein.

We have found a set of correlation effects, which are due to the inherent dynamics of the spectral density of polarization of an active medium with strong inhomogeneous broadening of the working transition line and occur if the rate of incoherent relaxation of optical dipole oscillations of the active centers is lower than the rate of optical-field attenuation in the laser cavity. Our analysis is based on the numerical studies of the stationary superradiant laser generation during continuous pumping with self-locking of some of the quasistationary modes. For the purposes of studying the detected effects, the methods of comparative analysis of the dynamic spectra of polarization and the field have been developed. In these methods, the time-frequency and space-time empirical modes of the spectra are used, which are determined by orthogonal eigenfunctions of special correlation matrices. The interconnection of the superradiance phenomena and mode self-locking in the considered class of lasers with low-Q cavities is discussed.

We present experimental results on the superresolution of two closely located signal sources, which were obtained using an automotive millimeter-wave radar. The signal sources were mutually correlated and the input process consisted of only one sample. The minimum-polynomial method (the root variant) was compared with the Minimum Description Length (MDL) criterion when determining the number of sources and the root MUltiple Signal Classification (MUSIC) method when estimating their angular location. The minimum-polynomial method is shown to have a higher efficiency compared with the MDL criterion and ensures the source superresolution for the angular distance which is a factor of 4–5 smaller than the width of the antenna-array pattern. In terms of accuracy of estimating the coordinates of the signal sources, the efficiency of this method almost coincides with that of the root-MUSIC method if the number of sources is considered to be known in the latter method.