We propose rheological models of micro- and structurally inhomogeneous media, which explain an increase in their acoustic nonlinearity with increasing wave frequency. The equations of state for such media are obtained and the frequency dependences of the nonlinearity parameters for the processes of generation of the second harmonic and the difference frequency are determined for the media with quadratic

We consider whether it is possible in principle to retrieve the key parameters of the mixed layer in the upper ocean (its thickness, bulk eddy viscosity and the pycnocline stratification below) using a theoretical model, which assumes the surface velocity and wind stress to be known from observations. To this end we examine the dynamics of the Ekman current in the novel two-layer model of the upper

Nonlinear acoustic nondestructive evaluation (NA NDE) methods have a higher sensitivity for defect detection than standard methods. These methods use various kinds of acoustic wave interactions. In this paper, we suggest augmenting the acoustic framework and use the interaction between acoustic and electromagnetic waves. A brief review of NA NDE and medical nonlinear acoustic imaging methods is presented

We study the emergence of steady wave packets in the form of envelope solitary waves (envelope solitons) which evolve from localized pulse-type initial conditions in the long-term evolution within the framework of the Ostrovsky equation. Ostrovsky equation derived in 1978 describes weakly nonlinear oceanic waves affected by the Earth’s rotation. Subsequently it became clear that this equation is universal

In this work, we discuss the mechanisms of the ordered-pattern formation in the colonies of microorganisms, which are related to the instabilities of their collective motion. The nematic-order formation during the E.coli-bacterium growth inside an open microflow chamber is considered and intricate flower-like patterns, which emerge during the growth of the binary mixture of two types of bacteria (motile

The existence of traveling waves in a strongly inhomogeneous magnetized plasma is studied. It is shown that under certain conditions for characteristics of the medium, the waves do not reflect from inhomogeneities, although their amplitude and phase vary in space. Such non-reflective waves are found mathematically as solutions of wave equations with constant coefficients, to which the original equations

The physical reason for the difference between the small-scale self-focusing of nanosecond and femtosecond pulses is that the typical intensity of the latter is three orders of magnitude higher, i.e., TW/cm2 versus GW/cm2. This causes a significant shift of the growth-rate maximum of the Bespalov–Talanov instability to the region of high spatial frequencies. During free propagation, a decrease in the

We perform qualitative physical analysis and particle-in-cell two-dimensional numerical simulation of the Weibel mechanism of magnetic-field generation due to the self-consistent formation of filaments of the hot-electron current with anisotropic velocity distribution, which occurs in the process of decay of a strong discontinuity in a collisionless plasma with a great difference in the number density

The global navigation satellite system accuracy and the possibility to actively affect it is quite a relevant problem. Based on two experimental campaigns (2010 and 2016), we analyzed the GPS positioning accuracy with forcing from the Sura high-power HF radiation. Analysis of the positioning error variations for 14 stations at different distances from the heater (directly near the latter and more than

This paper proposes, and practically implements, a technique for finding equivalent frequencies, which are attributed to the atmospheric optical depths measured by a two-channel wideband radiometric system in the millimeter wavelength range. Establishment of the equivalent frequency (averaged over the frequency response of the system) in the corresponding atmospheric transparency window makes it possible

We present the results of experimental studies of the generation peculiarities and features of the Langmuir and ion–acoustic turbulence in the F region of the high-latitude ionosphere excited by high-power HF O-mode radio waves emitted by the EISCAT/Heating facility (Tromsø, Norway) in the direction of the Earth’s magnetic field. The experiment was carried out at frequencies fH close to the fourth

Based on a joint solution of the system of Maxwell equations and relativistic hydrodynamics equations for an electron liquid we study stationary two-dimensional electron vortices in a cold plasma with immobile ions. In particular, we consider the case of a two-parameter family of exponentially decaying electron density profiles. The main properties of a vortex as functions of its radius and thickness

We present the results of numerical simulation and experimental study of high-pow gyrotrons with recovery of the residual energy of the spent electron beam. The results of calculation of megawatt gyrotrons operated at frequencies of 110 and 170 GHz are given. A simplified method for calculating gyrotrons with energy recovery in the case of partial reflection of the electron beam is discussed. The method

Formation of radio-frequency waveforms with given spectra is the basis for controlling acousto-optic filters with synthesized transmission functions. Digital methods for synthesizing such radio signals on the basis of the discrete Fourier transform are analyzed. We propose an improved method for forming control radio signals, which allows one to improve the spectral-modulation contrast. Numerical simulation

We develop an electrodynamic method for analysis of the near- and far-zone fields of dielectric lenses, which is based on solving a high-frequency integral equation. It is shown that the accuracy of this method is high for the lens size exceeding four wavelengths. Its advantages are fast internal convergence of the solution, short computation times, and small computer resources needed. The near-zone

Optimal sounding sequences described by a pair of mutually symmetric Golomb rulers lasting from 12 to 18 pulses were synthesized within the generalization of the previously developed 13-pulse sounding sequence of SuperDARN radars. These sequences are pairs of identical sequences that are mutually symmetric in time, are separated by a lag, and do not contain an additional pulse used earlier to measure

We study the process of frequency tuning of a cyclotron resonance maser with a quasiregular sectioned cavity, which is based on gyrotron excitation of a higher longitudinal mode. It is shown that a regime can be achieved, in which the generation frequency varies fast (during one pulse) with no variation in the power of the output radiation.

We compare the values of the input impedance of a linear dipole antenna, which are obtained on the basis of solving integral equations by the exact and approximate methods. The expansions in terms of the weighted Chebyshev polynomials and trigonometric functions are used for solving the equations with exact kernels. The results obtained by our own methods are compared with those obtained using the

Using the modified algorithm for calculating the ambiguity function, we consider the method of estimation of the mutual time delay of wideband signals, which are recorded by multiposition systems for determining the radiation-source location with the help of the space segment. It is based on the extraction of narrowband channels from the received signals and their further optimal quasi-coherent processing

We consider the model of a mixture of probability distributions for background and target samples in the problem of extracting a signal from radiophysical sounding under conditions of a priori nonparametric uncertainty. Based on the study of this model, approaches relying on the risk function and selection of anomalous readouts in the sample are proposed for detection and extraction of a continuous

We discuss the relationship between the electron temperature and the potential of the body of an artificial Earth satellite in the upper ionosphere. The formula for the spacecraft potential is analyzed, and the limitations of the theoretical representation are shown. The experimental results obtained within the framework of the Sura—DEMETER and Sura—SWARM programs are considered and serve as the basis

We present the results of comparing the total electron content measurements based on GLONASS satellite signals and the EISCAT UHF incoherent scatter radar (Tromsø, Norway) during modification of the high-latitude ionosphere in the magnetic zenith direction by high-frequency radio waves of the EISCAT/Heating facility (Tromsø, Norway). The measurements were performed during two experiment campaigns in

We perform mathematical modeling of diffraction of plane electromagnetic waves by multilayer metasurfaces comprised by elements of graphene nanoribbons. The modeling is based on solving the three-dimensional boundary value diffraction problem by three methods, specifically, those of self-contained units with Floquet channels, the volume integro-differential equation, and approximate boundary conditions

We consider the characteristics of radio-signal detection using the nonlinear decision function of a threshold device. This work is aimed at estimating the influence of the difference between the nonlinear decision function of the receiver threshold device and the step function on the characteristics of detecting radio signals with a random initial phase and random initial phase and amplitude. New

The efficiency of the previously developed algorithm for predicting extreme solar flares from their geomagnetic precursors has been tested on the example of a series of class X flares recorded on September 6, 2017. Over the past few years, these events of the solar flare activity are seen as the most intense. The algorithm consists in comparing the total histograms of the normalized number of synchronization

We report on the results of studying the radio emission of the Rydberg states of atoms and molecules in the Earth’s upper atmosphere at ionospheric altitudes when the ionosphere is modified by high-power HF radio waves radiated by the Sura heating facility. Radio emission from the upper atmosphere, caused by the emission of highly excited Rydberg atoms and molecules with the principal quantum number

We propose a method for reconstruction of dynamical systems with delayed feedback, which have several hidden variables including a hidden variable with a time delay. The method is based on the original approach allowing one to significantly reduce the number of the starting guesses for a hidden variable with a delay. The method is used for reconstructing the model system of the Lang–Kobayashi equations

In this work, we study the problems of processing complex-nature signals subject to noise, which arise because of the insufficient dynamic range of these signals in the case of their digital representation. The information criteria of quantization of the complex-signal amplitude are considered. The information-analytical approach to solving the problem of coding the signal amplitudes, which is aimed

We describe the mathematical tools of the optimal estimation method for retrieval of tropospheric temperature profiles (0–10 km) from radiometric data with the desired profiles parameterized in the form of expansion over empirical orthogonal functions obtained by a singular analysis of the covariance matrix of the radiosonde measurement data. It is shown that within the framework of such an approach

We present the results of numerical simulation and comparison of characteristics of optimal spatial selection of a signal and two correlated interferences in a linear equidistant antenna array that is optimal according to the criterion of minimum standard deviation. New properties of an optimal spatial filter are revealed, namely, an additional correlation component of the interference sum at the filter

We constructed an adaptive modal rank-reduction algorithm for acoustic source localization with a vertical sensor array operating under conditions of incomplete information on the spatial variability of the waveguide propagation channel. It has been established by means of statistical modeling that the proposed estimator gives a significant advantage in the accuracy of the source position finding as

Based on the results of the experiments performed in 2005–2010 within the framework of the Sura—DEMETER program, we analyze the features of the precipitations of energetic electrons (with energies E ≈ 100 keV) from the Earth’s radiation belt. The modification of the ionospheric F2 region was conducted by means of high-power HF O-mode radio waves radiated in the CW regime. The precipitations were detected

By simulations and experiments we develop a method for generation phase locking of a Ka-band subgigawatt nonstationary relativistic backward-wave oscillator (BWO) with an external radio pulse, the duration of which is considerably greater than the pulse duration of a driven BWO. The level of input power (about 20 kW), which provided phase locking with a total spread of about 1.2 rad, is determined

The concept of Compton-type free-electron lasers (FELs) operating in short wavelength ranges with a high efficiency and power level is currently underway at the IAP RAS (Nizhny Novgorod). This concept is aimed at reducing the energy of a driving relativistic electron beam and thereby increasing the efficiency of the electron–wave interaction in FELs, as well as making the oscillator relatively compact

We demonstrate the possible implementation of the chirped pulse amplification (CPA) method, which is widely used in optics, for the microwave frequency band. This method is based on the preliminary elongation of the incident pulse in the stretcher, sequential amplification of spectral components in a wideband amplifier, and final compression in a line with negative dispersion (compressor). A circuit

The radiation-frequency locking by an external signal is experimentally studied for a megawatt gyrotron. An external signal from a magnetron is applied to the operation space of the gyrotron at a frequency of 35 GHz via a synthesized two-mirror quasioptical converter developed at the Institute of Applied Physics of the Russian Academy of Sciences. The radiation spectra are measured for both the frequency

We show that the radiation patterns of two-dimensional systems in the form of arrays depend on the duration of the emitted pulses. To study such systems, we propose to use the correlation length depending on the pulse duration and the emission direction. The obtained formula for the length of correlation in two-dimensional systems is convenient for description of distortions in the radiation patterns

We propose and study a scheme of a microwave pulse generator operating in the frequency ranges near 37 and 73 GHz, in which the electron interaction with both the (−1)st harmonic of the counterpropagating TM02 mode and the synchronous slowed-down TM01 mode is combined in a sectionalized slow-wave system with an average diameter of 2.5λ. An efficient current modulation at the input of the slow-wave

We present the results of theoretical and experimental studies of the resonant properties of a single cell of a receiving system based on cold-electron bolometers (CEBs) with a double-slot antenna and curved coplanar lines, which is intended for receiving a signal in the range 220–240 GHz with a bandwidth of 5%. The resonant properties of a CEB located on the cryostat plate with a temperature of 0

The Institute of Applied Physics of the Russian Academy of Sciences has for many years been developing sub-terahertz and terahertz large-orbit gyrotrons that permit selective oscillation at higher cyclotron harmonics than is possible in the conventional gyrotrons. Currently, experimental studies are conducted at two specialized facilities. A prototype universal sub-terahertz source for magnetic resonance

We consider the influence of an external quasi-monochromatic signal on a multimode gyrotron in the case where the signal frequency is close to the frequency of the operating mode of the gyrotron. The influence of the mode competition on the generation regime at a harmonic frequency modulation is studied. The calculations were performed for a high-power 170 GHz gyrotron developed at the Institute of

We present the results of the development and study of the field emitters for high-voltage electronic devices operating in technical vacuum. The main attention is paid to estimating the possibilities of using distributed field emitters under the conditions of intense ion bombardment.

We discuss the results of the atmospheric absorption studies on the territory of the Kara-Dag radio astronomy station in Crimea in the millimeter-wavelength transparency windows. Critical analysis of the hardware system and measurement techniques is carried out, which permits one to create a more perfect radiometric system for long-term atmospheric measurements. Connection between atmospheric absorption

The gyrotron traveling-wave tube (gyro-TWT) is a wideband version of gyrotron amplifiers, which produce pulsed or continuous-wave radiation in the millimeter-wavelength band at a power level that exceeds the powers produced by conventional TWTs with slow-wave structures and rectilinear beams by 1–2 orders of magnitude. Since 1996, researchers at the Institute of Applied Physics of the Russian Academy

We analyze the possibility to create new generation continuous-wave gyrotrons operating in the Kband (at frequencies of 28-30 GHz) at the fundamental cyclotron resonance with an output power of 25–30 kW and more. For a gyrotron with a magnetically shielded solenoid, the electron-optical and electrodynamic systems have been optimized, which ensures generation with high efficiency (about 50%) at acceptable

We have developed, manufactured, and tested a waveguide mixer in the range 211–275 GHz on the basis of the superconductor–insulator–superconductor (SIS) tunnel structures. The methods of manufacturing high-quality tunnel structures on quartz substrates have been worked out. To extend the receiver band, the Nb/AlOx/Nb and Nb/AlN/NbN tunnel junctions with a high current density of up to 20 kA/cm2 are

In this work, we consider the problem of existence of discrete Lorenz attractors in the nonholonomic model of a Celtic stone. To this end, the main local and global bifurcations leading to the appearance and destruction of the attractors are studied in two-parameter families of such models of certain types. The corresponding bifurcation diagram, in which the region of existence of a discrete Lorenz

We study the propagation of seismoacoustic waves in a three-layered medium consisting of a uniform isotropic deformable solid layer that is loaded with a uniform porous layer saturated with fluid. In turn, the porous layer covers a homogeneous isotropic solid half-space. This medium models the geological section in which the upper ground layer is separated from the deep rocks by a porous layer containing

A significant difference was found in the amplitude and polarization spectra of ULF magnetic noise at stations with a base of 120 km during periods of absence of regional thunderstorm activity. A simultaneous analysis of low-frequency data and ionosonde data allowed us to conclude that the difference in the main parameters of the polarization spectrum at two stations is due to the appearance of sporadic

We present the results of microwave observations of atmospheric radiation in the ozone line in the case of ionospheric perturbation by powerful high-frequency radio emission from the “Sura” facility in September 2016. The atmospheric-radiation spectrum in the ozone line was measured by the method of the ground-based microwave radiometry. The state of the lower ionosphere was simultaneously diagnosed

We study theoretically the possibilities of double-stage energy recuperation in a double-beam terahertz gyrotron. Numerical modeling, which was performed for a gyrotron producing radiation with a frequency of 0.78 THz and operating at the second cyclotron-frequency harmonic at the TE8.5 mode, demonstrates the possibility of spatial and energy separation of electron beams. This makes it possible to

Using the perturbation method, we study the processes of nonlinear propagation of the initially harmonic elastic waves in micro-inhomogeneous media, namely, polycrystalline solids with hysteresis-loss saturation and relaxation. The media with the basic hysteresis types, elastic and inelastic, are considered. The efficient nonlinearity parameters of such media are determined for the processes of self-action

We determine the upper limit on the correct measurement disambiguation probability in multiscale phase-measuring systems in which all scales are ambiguous. The measured value is estimated by the maximum likelihood method from the total of measured phase differences supplemented by an algorithm for rejecting (erasing) measurement results with anomalously large errors. Errors that exceed one-half of

We consider the results of the experiment on the polar ionosphere heating by a high-power modulated HF wave conducted by the AARI at the EISCAT/heating facility in October 2016. The data on joint processing of the values of the equivalent polar electrojet current above the heated ionosphere region retrieved by the IMAGE network of ground-based stations and the amplitudes of a signal of the horizontal

We briefly describe the results of the first (undedicated) experiments on studying large-scale (about 1000 km) aperiodic and quasi-periodic disturbances in the lower and middle ionosphere. The main results of modern experiments are listed. Multi-instrument radiophysical observations of large-scale (about 1000 km) aperiodic disturbances in the lower ionosphere and quasi-periodic disturbances in the

We propose a method of interperiod signal processing, which is based on the projection Doppler filterting method and used for selection of moving targets. The scheme of using the algorithm of the proposed method for signal processing is presented. The efficiency of this algorithm is assessed with regard to the optimal algorithm of matched filtering for various lengths of signal sampling.

We present the results of studying Russian synthetic crystalline quartz from the viewpoint of the possibility of its use for manufacturing optical elements of power optics. The results of experimental studies of the absorption and optical nonuniformity of crystal plates with dimensions of 12 × 8 × 1.7 cm as well as theoretical calculations of the thermooptical parameter for the disk configuration of

In the majority of modern radio telescopes, which are used for radio interferometry with very long baselines, a special signal is introduced into the signal chain for the phase calibration of the equipment. This signal is used during the correlation processing of the radio-astronomy observations for obtaining a synthesized response when it is required to match the signals of several frequency channels

We developed an algorithm for processing of the dynamic spectra of plasma emission, which makes it possible to extract chirped elements with both increasing and decreasing frequencies. The algorithm was tested on a set of experimental data obtained by studying the emission spectra of the rarefied plasma of an electron cyclotron resonant discharge in an open magnetic trap. The efficiency of the method

We propose a method of volume heating of a vertical air column by the vertically directed microwave radiation at the frequency of the spectral atmospheric-absorption line. The heating efficiency is estimated for the case of the molecular-oxygen absorption band near a frequency of 60 GHz. The proposed heating method can lead to the formation of new convection structures in an air column. Absence of