Radiophysics and Quantum Electronics最新文献

We use the method of auxiliary sources to solve the problem of scattering of an electromagnetic wave in a thin perfectly conducting cylinder located inside a dielectric body. The specially developed computer code is briefly described. The results of calculating the scattering cross sections are presented for cylinders of different lengths located inside a dielectric sphere or a dielectric spheroid.

We present the results of spontaneous parametric down-conversion in a single-resonant optical parametric oscillator with PPLN crystal. The possibility of producing narrow-band single photons at a wavelength of 867 nm with a bandwidth of 19 MHz is shown. The latter makes this single-resonant parametric photon source compatible with solid-state quantum memories based on crystals doped with Nd³⁺ ions.

We consider the problem of passive acoustic direction finding with a linear microphone array in the presence of intense interference. It is assumed that a useful signal, as well as interference, is broadband and has an arbitrary time shape. It is also assumed that interference at various microphones differs only by the delay and amplitude. Such a scenario is realized when detecting a weak acoustic source against the background of intense local source (interference). The former and the latter can be represented by a small unmanned aerial vehicle and, e.g., a helicopter, respectively. For the considered scenario, we propose an algorithm, which is based on the maximum-likelihood method in the frequency domain. The proposed algorithm is studied on the basis of numerical models using the spectra of the experimentally obtained useful signal and interference. The achievable accuracies of the noise direction finding for various parameters of the problem are obtained.

We propose a wave mechanism for explaining “fast breakdown” in thunderstorm clouds and the generation of powerful pulses of electromagnetic radiation of very high frequencies (VHF, from 30 to 300 MHz). A polarization wave moves along the discharge gap; an area of increased electric field is formed at the wave front, inducing the transition of corona micro-discharges into streamers. The mechanism does not require an electron-initiating source to start streamers, as in the case of direct avalanche-streamer transitions. It is assumed that the concentration of corona micro-discharges before the front is much greater than that of the streamers, since the threshold number of lengths of the electron avalanche growth for the ignition of the corona discharge is much smaller than that to start the streamer. The mechanism makes it possible to reduce the field strength necessary to initiate micro-discharges before the wave front. The wave is a self-consistent process of the birth of streamers, polarization of the medium, and the motion of the amplified-field area. It is a macroscopic analog of the ionization wave in a streamer: the elementary acts of ionization of molecules correspond to the emerging of streamers, such that the wave velocity ismuch greater than the speed of moving particles. For this reason, to denote a wave of fast breakdown, we introduce the term “streamerization,” which briefly and accurately reveals the essence of the process. The emerging streamers are sources of radio-frequency radiation. Calculations have shown the possibility of the front traveling at a speed of fast breakdown.

In recent years, significant progress has been made in generating ultrashort electromagnetic pulses of single-cycle and subcycle duration. Unipolar pulses contain one half-cycle of the field and have a nonzero electric area. The conventional concepts of interaction of electromagnetic radiation with matter (in particular, interference) used in the case of multicycle pulses are not applicable to unipolar ones. This minireview discusses the latest results on the effects of extremely short low-amplitude pulses (when the perturbation theory is valid) on resonant media and individual quantum systems (atoms, molecules, and nanostructures) from the viewpoint of the recently introduced concept of “interference” of the areas of short light pulses (electric and envelope areas). We provide a simple relation showing that in order to compare the effects of multicycle bipolar and subcycle unipolar pulses on micro-objects, one should compare their areas, not energies. By numerically solving the Maxwell–Bloch equations, we study the features of area interference are studied beyond the limits of perturbation theory. It is shown that, after the collision of a pair of π-like ultrashort pulses, polarization structures and population difference gratings with non-harmonic multipeak structures are formed inside the medium. The possibility of experimentally determining the electric area of unipolar pulses through interference of their areas is discussed for the first time.

We present a mathematical formalization and the results of computer simulation of an angular phase discriminator, which employs a linear antenna array with adaptation according to the Howells–Applebaum algorithm. A method of improving the direction-finding accuracy of the radio-emission sources, which provides the measurement unambiguity with a considerable increase in the measurement baseline, is proposed. The limiting values of the measurement baseline are determined with allowance for the unambiguity preservation in the case of an increase in the element number in the antenna array and a comparison of the slope of the direction-finding characteristics of the standard and proposed phase direction finders is performed with an admissible increase in the unambiguous-baseline sizes.

We have compared the conditions of stability and the properties of topological scalar and vector (polarized) dissipative solitons in wide-aperture lasers with saturable absorber. In numerical simulations, vector solitons were formed by scalar solitons and even unstable scalar localized structures. Topological singularities and features of the dynamic regimes in lasers with isotropic and anisotropic intracavity media have been revealed.

We develop a new concept for the formation of behavior features of inviscid incompressible fluids on the rigid boundary due to breaking of slipping flows. The breaking possibility is related to the compressibility of such flows due to the boundary. For two- and three-dimensional inviscid Prandtl equations, we analytically obtain the criteria for a gradient catastrophe for slipping flows. For the two-dimensional Prandtl equations, breaking occurs for both the velocity component parallel to the boundary and the vorticity gradient. The explosive growth of the vorticity gradient correlates with the appearance of a jet in the direction perpendicular to the boundary. For the three-dimensional Prandtl flows, breaking (fold formation) leads to an explosive growth for both the symmetric part of the velocity-gradient tensor and its antisymmetric part, i.e., vorticity. The blow-up generation of vorticity is possible due to the fluid suction from the slipping flow with simultaneous formation of a jet perpendicular to the boundary. These factors can be considered as a tornado-formation mechanism. Within the framework of the two-dimensional Euler equations, we numerically study the problem of the formation of increasing velocity gradients for the flows between two parallel plates. It is revealed that on the rigid boundary, the maximum velocity gradient exponentially increases with time simultaneously with an increase in the vorticity gradient according to the double exponential law. This process is also accompanied by a jet formation in the direction perpendicular to the boundary.

We study the forced synchronization of cyclic stochastic oscillations with a finite lifetime (metastable oscillations). A mathematical model of the battle of the sexes evolutionary game is considered as an example. The model describes the competition between the strategies for selecting a partner and raising offsprings in two populations of the opposite gender. For a system under the influence of a multiplicative harmonic signal, the frequency and the phase synchronizations are found in a certain range of the frequency detuning. As a result of studying the features of changes in the characteristic amplitude and the lifetime of metastable oscillations, depending on the amplitude and the frequency of periodic modulation of the parameters which correspond to some game payoffs (which simulates seasonal changes in the partner choice preferences), we show the possibility of controlling the properties of such oscillations, in the synchronization mode and outside it.

We discuss the question of the stability of the single-mode generation regime in the gyrotron. An example of a gyrotron with the fixed Gaussian structure of the high-frequency field is used to analyze theoretically the instability mechanism during the interaction of modes with the quasiequidistant spectra on the assumption that the amplitudes of the spurious modes are low. The boundaries of the stability domain for the operating mode on the parameter plane “cyclotron resonance mismatch—normalized current parameter” are plotted at various densities of the mode spectrum. It is shown that at growing mode spectrum density, the structure of the stability domain of the operating mode becomes complicated and the domain dimensions become smaller. The boundaries obtained theoretically agree well with the results of the calculations within the framework of the multimode gyrotron theory.