Radiophysics and Quantum Electronics最新文献

We present the first experimental results obtained with a setup created on the basis of the PEARL laser facility for studying the processes of generating terahertz radiation from laser wake fields which are formed during the propagation of a high-power femtosecond laser pulse in a rarefied plasma. In particular, the occurrence of terahertz generation in the case where the laser–plasma interaction region is located between a pair of dielectric prisms of total internal reflection is demonstrated. The dependence of the terahertz radiation energy on the energy of a femtosecond laser pulse and on the plasma density is studied.

We consider propagation and absorption of a narrow-band signal in a homogeneous dispersive medium. It is shown that all signals can be divided into two qualitatively different classes, namely, holonomic and piecewise-holonomic signals. When propagating in an absorbing medium, the holonomic signals are attenuated obeying an exponential law and, therefore, almost disappear at a sufficient long path lengths. The piecewise-holonomic signals are attenuated significantly slower (obeying a hyperbolic law) and, therefore, continue to exist even at substantially longer paths.

We study generation of Čerenkov radiation by an ultrashort laser pulse propagating in the magnetized plasma across an external magnetic field basing on the analysis of the dispersion relation and numerical simulation in the case where the cyclotron frequency exceeds the plasma frequency. It is shown that the radiation with a frequency in the range from the plasma frequency to the cyclotron one and with an angle between the wavevector and propagation direction below 45° can escape the plasma. The radiation power increases with the magnetic field before reaching the maximum at the cyclotron frequency being of the order of the inverse of the laser pulse duration and starts decreasing afterwards. It is also observed that, with an increase in the field, the pattern of the radiation escaping from the plasma narrows, the generated pulse is shortened, and the pulse spectrum widens.

We analyze the possibility of existence of electromagnetic-field pulses with a non-zero electric area (time integral of the electric field) within the framework of the Maxwell equations. It is demonstrated that in the absence of charges and currents in vacuum, the non-zero electric area of a pulse would lead to its infinite energy. A pulse with the non-zero area is shown to form already in the case of uniform and rectilinear motion of the charge. The conditions for formation of such pulses by a localized charge system are found, and an example of such a system is presented. The asymptotics of the electric area far from the system of charges is obtained.

We consider the problem of simultaneous detection of close and distant targets in an automotive millimeter-wave radar. Two approaches for solving the problem are proposed. The first approach is based on the subtraction of more powerful signals in the time domain and the subsequent detection of less powerful signals. The second approach is based on the most plausible estimate of the impulse response of the radar channel. The methods have different computational complexity. Numerical simulation of the proposed methods for processing of signals reflected from targets in terms of the false-alarm and missed-detection probabilities has been carried out. Simulations have shown that both close and distant targets can correctly be detected simultaneously with a low predetermined false-alarm probability.

We propose a method of multisignal adaptive spatial filtering, which generalizes the classical method of adaptive spatial filtering of a single signal with a known arrival direction. Additionally, the formulas for the modified Capon direction-finding method are found. Reliability of the relationships, which are obtained on the basis of the maximum-likelihood method, is confirmed by the limiting transitions and the results of numerical simulation for typical signal-reception conditions in radio-monitoring and passive radar systems.

We consider the problem of acoustic source localization in a shallow water channel, in which the prevailing mechanism of sound scattering is developed wind seas. For the given scenario, an adaptive multi-rank Capon algorithm, which is capable of processing in both the conventional and mode space and localizes the source under conditions of incomplete information on a random propagation channel, is constructed. This method of solving the inverse problem uses the worst-case principle and provides greater stability of the estimation procedure to a mismatch caused by the discrepancy between the true coherence matrix of the signal field and its computational model. The results of statistical modeling, which demonstrate the probabilities of correct source localization depending on the input signal-to-noise ratio and the sample size, are presented. The method is validated using the experimental data observed on a stationary path in the Barents Sea. It has been shown that under real conditions, the presented approach is rather efficient and ensures an acceptable source reconstruction quality without using a computationally intense joint search for both the source coordinates and unknown waveguide parameters.

We study experimentally the possibility of creating a HF and VHF radio communication channel at frequencies above the hop-by-hop MUF through an artificial ionospheric repeater, which is formed by as scatterers in the form of magnetic field-aligned small-scale artificial ionospheric irregularities, created at the altitudes of reflection of a high-power O-mode pump wave. The experiments were carried out on the Dymer (Kyiv region)—Sura—Kachalinskaya (Volgograd region) path in the HF sounding regime at fixed frequencies and in the chirp sounding regime on the IZMIRAN—Sura—Rostov-on-Don path. On the Novocherkassk (Rostov region)—Sura— Kachalinskaya path, the measurements were carried out at a frequency of 45.8 MHz. Comparison with simulation results is performed. To ensure stable HF and VHF radio communication through an artificial ionospheric repeater, it is necessary to take into account the state of the ionosphere, the frequency of the communication signal, and the height of the location of scattering irregularities. To adapt communication systems to the current ionospheric environment under conditions of insufficient accuracy in predicting the state of the ionosphere, it is necessary to monitor the ionospheric channel using technical tools for broadband oblique HF and VHF sounding.

We present a description of a plasma-chemical facility designed for synthesis of micro- and nanoparticles using the radiation of a high-power gyrotron. The facility has been developed at the Plasma Physics Division of A.M.Prokhorov General Physics Institute of the Russian Academy of Sciences. The facility includes a plasma-chemical reactor, a gyrotron unit, an in-line calorimeter, a diagnostic system, which ensures video and spectroscopic measurements, balance measurements of the microwave radiation, and thermal imagery measurements, a chemical unit for specimen preparation and analysis of synthesis products, an electronic logging unit, and a specimen labeling system for organization and storage of a large amount of experimental data and specimens.

We consider a quasi-stationary approximation for turbulent electrical processes in the atmospheric boundary layer and their recording by a passive horizontal ring antenna acting as a collector of atmospheric electric current. On the basis of large eddy simulation, variations in electrical variables are calculated and spatial correlation functions and the spectral density of fluctuations of the atmospheric-current density components are estimated along with the response of the collector current to local turbulent perturbations of the conduction-current density and an instantaneous change in the global potential difference between the ionosphere and the Earth’s surface. Based on the simulation results, we estimate the ratio of the total vertical atmospheric electric current density and the conduction current density at the height of the antenna location, which can be used to extrapolate the results of ground-based measurements beyond the atmospheric boundary layer. The results of direct observations of the atmospheric electric current density by the collector are compared with the results of synchronous observations of the atmospheric electric field and the electrical conductivity in the surface layer. The frequency spectra of the collector-measured and calculated fluctuations of electric current have a power-law decay with increasing frequency with indices that are consistent for measurements and model calculations in the range from −1.0 to −1.5.