We study longitudinal nonlinear solitary deformation waves in an elastic cylindrical shell completely filled with a viscous fluid. In the case under consideration, the state of the shell is assumed to be momentless, and the stress–strain relation for its material has the form of a generalized Hooke’s law which additionally allows for the dependence of the components of the stress tensor on the strain tensor components to the power of 3/2. Moreover, the quadratic nonlinearity of the initial equations of motion of the shell element is also taken into account. The problem of hydroelasticity for the specified shell is formulated, and asymptotic analysis by the method of two-scale expansions yields an evolution equation that generalizes the Schamel–Korteweg–de Vries equation by incorporating the dissipation of the energy of the wave process in the material of the shell and the fluid. The evolution of hydroelastic solitary waves in a shell is studied numerically with a proposed difference scheme similar to the Crank–Nicolson scheme for the heat equation. The scheme is verified with an exact particular solution for the case where the influence of the fluid in the shell and dissipation in its material are excluded from consideration. It is shown that for this case, the speed of deformation waves is supersonic, and they are solitons. On the other hand, it has been established that when the dissipative properties of the shell material and the fluid, as well as the inertia of the latter, are taken into account, the speed of deformation solitons is subsonic, and their destruction is observed.
We present the results of optimizing numerically a small-size cylindrical multipass cell based on a modified Herriott optical scheme. The cell has a separate THz beam input/output system and a common reflective cylindrical element that forms a closed measuring volume. Using numerical modeling methods and the geometric optics approach, the geometry of the cylindrical multipass cell was optimized. It is shown that the use of a reflective element with a parabolic profile allows one to minimize beam deformations during multiple reflections of the beam. The optimized geometry of the input/output system of the radiation and a reflective element having an internal radius of 65 mm and a height of 40 mm ensures the possibility to vary the optical path length in the range from 0.78 m to 2.61 m.
We study the possibility of using double spatial field processing based on the double weighted Fourier transform (DWFT) method to increase the resolution of satellite diagnostics of an inhomogeneous ionospheric plasma. The case of synthesizing a line of sources of the probing signal by a low-orbiting satellite of the “Ionosphere-M” type is considered. As a model of the ionosphere, an approximation is used in the form of a background medium represented by a simple Chapman layer, with the addition of horizontally moving local plasma irregularities having dielectric permittivity variations in the form of Gaussian functions. Computer calculations of the phase of the field of a wave that has passed through an inhomogeneous medium showed that for the chosen parameters of the problem, the use of double spatial field processing based on the DWFT method makes it possible to overcome the Fresnel resolution limit for diagnosing small-scale ionospheric irregularities moving with horizontal drift velocities less than 200 and 380 m/s for probing-signal frequencies of 150 and 400 MHz, respectively.
We derive a mode basis of aberration components of the Kolmogorov wavefront. The basis is characterized by the maximum contribution to the geometric transverse aberration of a uniformly intense circular monochromatic laser beam. The result represents the Karhunen—Loève functions derived from the Lukosz polynomials. With modal successive correction, subtraction of these components from the wavefront results in the strongest decrease of the RMS focal spot radius, allowing for statistical improvement of the focusing effect of laser radiation. The result obtained within the geometric optics approximation is also valid for the diffraction focal spot. Results of numerical simulation show that the characteristic focal spot size depending on the turbulence level can be reduced by several percent or tens of percent, while the Strehl ratio and the power in the bucket can be increased from several percent to more than a hundred percent (as compared to the modal correction of the same number of the known aberration components). It is also proposed to use modified Karhunen—Loève functions as aberration components, which in most cases allow for higher focusing parameters than those with the newly derived Karhunen—Loève—Lukosz modes. These results can be generalized for any given model of atmospheric phase distortions, an arbitrary aperture shape, and a given laser beam apodization.
We measure the dielectric parameters of the samples of new Aristar and Aristid composite materials in the frequency range 125–180 GHz. The measurements are performed using a quasioptical resonator spectrometer developed at the Institute of Applied Physics of the Russian Academy of Sciences on the basis of a Fabry–Perot open resonator. The values of the refractive index and the dielectric loss tangent of the composites studied are presented.
We present the results of studying the thermal effects produced by a backward-wave oscillator signal at a frequency of 1.0–1.1 THz on a mixing element based on an Nb–AlN–NbN superconductor–insulator–superconductor (SIS) tunnel junction included in a NbTiN–SiO2–Al microstrip transmission line. The developed receiving system consists of a slot antenna, matched microstrip transmission lines, and two SIS mixers with junction areas of about 1 μm2. Under exposure to a signal at a frequency around 1 THz, a significant change in the current–voltage characteristic of the SIS junctions was observed, which manifested as a decrease in the gap voltage and indicated heating of the mixing element, significantly affecting the characteristics of the receiving system. An analytical model describing the studied structure exposed to a high-frequency signal is proposed. The mechanisms leading to direct heating of the tunnel junction and to a change in the distribution function of quasiparticles in the electrode material are analyzed. The relevant effects are identified and their influence on the heating of the junction and, as a result, on the decrease in the gap voltage is numerically estimated. The total heat generation budget and the spatial temperature distribution in the integrated circuit are calculated.
We obtain a system of differential equations, whose solutions represent the equations of the electric field lines of a moving charge. The initial conditions necessary for solving this system are presented. Examples of graphic representation of fields are given, which illustrate the capabilities of the proposed method. The field is considered for a uniformly accelerated rectilinear motion of the charge and a uniform circular motion.
We consider numerical models for generating sequences of subgigawatt microwave superradiance (SR) peaks in a relativistic Ka-band backward-wave oscillator (BWO) due to a deep periodic modulation of the electron beam current. In the first model, this regime is achieved by applying a sequence of trapezoidal accelerating pulses to the explosive-emission cathode with a certain delay. The delay duration determines the influence of the residual background of slow spent electrons on the dynamics of formation of the subsequent SR peak and the peak amplitude. In an alternative model, the injected beam current is continuous, but modulated by the preceding SR pulse, which generates strong electric fields in the radial gap near a coaxial insert used in the anode unit of the BWO instead of the resonant reflector. Here, the beam is periodically dumped onto the wall. In this case, unlike the first model, the electrons at the leading edge of the next beam segment have the maximum energy. Therefore, the excitation of subsequent SR pulses is less sensitive to residual slow electrons and occurs under reproducible conditions. The pulses have a short delay and close amplitudes.
We simulated an electron-optical system, based on a thermionic cathode, capable of forming a helical electron beam with an electron energy of 500 keV, a current of 200 A, and a pitch factor of 1.3 at a guiding magnetic field of up to 5.6 T. The optimization of the gyrotron cavity profile with an operating frequency of 90.6 GHz was carried out, and the possibility of achieving a generation efficiency of up to 34
We present experimental results on recognition of propeller-modulated radar signals based on the eigenvalue analysis of a sample correlation matrix for a Yak-52 propeller-driven sports and training aircraft, a Robinson R44 light multipurpose commercial helicopter, and a turbofan passenger aircraft. A case is considered where methods based on the discrete Fourier transform and superresolution for the low reflectivity of aircraft propellers fail to identify the spectral components of propeller modulation.
For most practical applications of superconducting oscillators, it is necessary to implement terahertz generation with the frequency tuning over a wide range and a narrow radiation line in a single device. Until now, this has been achieved only for systems based on distributed Josephson junctions. We give a brief overview of the main achievements in the field of creating superconducting integrated oscillators and present the results of developing a new type of oscillators based on arrays of Josephson junctions included in a superconducting coplanar line. The implementation of a new type of Josephson junctions has provided the possibility of developing an oscillator with operating frequencies of up to 700 GHz, which are limited only by the energy gap of niobium. The use of a superconducting harmonic mixer based on a superconductor–insulator–superconductor tunnel junction made it possible not only to determine the operating frequency range of the oscillator and estimate its power, but also to measure the radiation linewidth of a superconducting oscillator at frequencies of up to 700 GHz in the frequency-stabilization mode and to implement the phase-locked loop mode. The possibility of developing terahertz-superconducting oscillators for integrated receiving systems of ground-based and space radio-telescopes is discussed.
We consider the resonant interaction of a whistler-mode wave packet with energetic electrons. The wave packet parameters are based on Cluster spacecraft data. Both model profiles of the packet amplitude and frequency (Gaussian and linear, respectively) and profiles taking into account variations in amplitude and frequency, directly based on spacecraft data, are used. The effect of the wave packet amplitude and frequency modulation on the acceleration of electrons and their precipitation into the loss cone as a result of the interaction is analyzed by using test particle simulations. It is shown that both modulations have a similar effect on the interaction characteristics: the presence of modulation (amplitude and/or frequency) generally significantly increases the probability of particle trapping by the wave field, but the maximum energy gain for trapped particles decreases. However, there are regions of the initial electron parameters in which, even in the presence of modulation, significant acceleration of individual groups of particles is possible. As a result of the interaction considered, electrons with energies of 20 to 200 keV can effectively precipitate into the loss cone. As the packet propagates away from the equator, less effective precipitation of particles with energies of 200 to 600 keV is possible. The precipitation characteristics depend only weakly on the presence of amplitude and/or frequency modulation of the wave packet.
We present the results of theoretical and experimental studies of a relativistic backward-wave oscillator in the long-wavelength band of the centimeter-wave range (λ ≈ 10 cm). Using a nonlinear nonstationary model, weshow that the choice of the reduced high-frequency space charge parameter plays an important role in all options for increasing the oscillator efficiency. Numerical simulation using the macro-particle method and the experiments confirm the need to minimize the intense space charge fields. This key parameter decreases due not only to a decrease in the electron beam current, but also to an increase in the maximum transport current through the slow-wave structure of the backward-wave oscillator. This effect is achieved by bringing the electron beam closer to the structure walls in a nonuniform magnetic field. In this approach, partial current deposition on the slow-wave structure in the electron deceleration region has proven beneficial. The experiment employed the pulse-periodic regime of operation of the relativistic backward-wave oscillator with a clock frequency of up to 250 Hz at a carrier frequency of 3 GHz and a power of 1.0±0.1 GW in pulses with a duration of 23±1 ns. The corresponding degree of the beam–to–wave power conversion can exceed 50
We propose a method for designing a mechanical modulator for an uncooled subterahertz radiometer capable of operating under prolonged expeditionary conditions. The modulator enables periodic modulation of thermal radiation from the observed object directly in front of the radiometer’s receiving antenna and provides mechanical scanning within an angular range from 0° to 90°. As a matched reference load, an absorber fabricated via 3D printing from a composite polymer with high electrical conductivity is used.
We have developed a method for calculating modes in axially symmetric open cavity resonators. The method is based on a weak formulation of the equations of electrodynamics, a finite element method, and a perfectly matched layer method for limitation of the computational domain. Using a “non-canonical” echelette cavity as an example, TE modes are simulated. It is shown that such a cavity has groups of longitudinal modes whose properties are similar to those of the longitudinal modes in “classical” cavities, which have the form of weakly nonuniform cylindrical waveguides loaded on both sides with tapering and expanding horns. The frequencies and the diffraction and ohmic Q-factors of the echelette-cavity modes are calculated. It is found that the ohmic Q-factor significantly exceeds the diffraction Q-factor, which agrees qualitatively with the results presented in the literature. It is shown that there exist optimal values of the corrugation radius and amplitude, at which the diffraction Q-factor is maximum.
We propose a scheme for a new artificial atom suitable for constructing quantum simulators of photonic topological models. The design consists of four capacitively shunted Josephson junctions, controlled by an external magnetic flux, in a diamond-shaped configuration. It maintains arbitrary polarization states through two degenerate excited states with mutually orthogonal electric field distributions. Furthermore, we show that a high operating ratio of the Josephson energy to the charge energy suppresses the sensitivity to fluctuating background charges far from the degeneracy points.
We present the formulation and solution of a hydroelasticity problem to determine the nonlinear response for the wall of a wedge-shaped channel filled with a pulsating viscous fluid. A plane problem is considered for the channel formed by rigid walls that are rectangular in plan view. The upper wall is fixed and has a wedge shape, while the bottom one has a nonlinear elastic fixation at the ends and performs steady forced oscillations due to pressure pulsation at the end of the channel. Nonlinear-elastic fixation of the bottom wall is represented by weightless springs with a symmetric hardening stiffness characteristic with cubic nonlinearity. For the fluid in the channel, a model of a Newtonian fluid of constant density is adopted, and its motion is studied as creeping one. A boundary-value problem of mathematical physics was formulated, including the equations of creeping motion for a viscous fluid, the equation of motion of the rigid wall on a nonlinear elastic suspension, as well as boundary conditions for fluid pressure at the ends of the channel and velocities at the contact boundaries between the channel walls and the fluid. Asymptotic analysis of this problem allowed us to find the laws of distribution for the velocities and pressure of a viscous fluid in a wedge-shaped channel and reduce the initial problem to considering the generalized Duffing equation describing nonlinear hydroelastic oscillations of the channel wall. The solution of this equation, carried out using the Krylov–Bogolyubov averaging method for the primary resonance, made it possible to determine the nonlinear hydroelastic amplitude response and phase response, which are implicit functions of the amplitude of oscillations and frequency. A numerical study of these responses was carried out, showing a significant influence of the wedge shape of the channel, the thickness of the fluid layer, and the amplitude of pressure pulsation at the channel end on the amplitude of oscillations, resonance frequencies, and the possibility of unstable oscillations of the bottom wall of the wedge-shaped channel.
We present an review of the resonance acoustic spectroscopy method and its use in acoustic diagnostics. Potential applications of the method in the materials science problems is discussed. A brief description of the method is provided along with particular examples of its use in studying the microstructure of materials, assessing fracture density, and estimating the history of loading (plastic deformation). Experimental techniques are also described, which ensure precise measurements and open up new possibilities for nondestructive testing of material strength.
We consider transverse vibrations of a semi-bounded string lying on an elastic foundation with a damper on the boundary. The simplest mechanical models of a viscoelastic body used as a damper (Maxwell, Voigt—Kelvin, Poynting—Thomson and Burgers models) are analyzed allowing for its inertial properties, where such a body is a set of series- and parallel-connected springs and dampers. The conditions are found that must be satisfied by the parameters of the damper eliminating reflected waves and providing the lowest intensity of oscillations of the “string + elastic foundation” system.
