Radiophysics and Quantum Electronics最新文献

We have studied the influence of non-ideal standard loads of the probe station on the accuracy of microwave impedance spectrum measurements in the process of testing electronic devices and materials. Using the Cascade Microtech station as an example, it is shown that the calibration procedure based on standard loads leads to significant measurement errors. Prospects for improving load characteristics in order to eliminate errors are given. An original technique of impedance measurement, which is free of these problems, is considered.

We present the results of experiments on detection of carbon monoxide (CO molecules) in atmospheric and exhaled air using a microwave spectrometer with sensitivity 1000 times lower than the CO detection threshold in the atmosphere. A modification of the spectrometer by adding a simple and easy-to-use device (preconcentrator) allowed us to increase the sensitivity of gas analysis by 5 orders of magnitude and confidently observe CO in the atmosphere and exhaled air. Combined with a filter performing deep dehumidification of an analyzed air sample without noticeable distortion of the composition of the polar impurity gases, the preconcentrator may be used to increase the sensitivity of many if not all currently known gas analysis methods.

Models of the Earth climate system, along with the physical components of the climate (atmosphere, ocean, sea ice, active land layer), contain modules for describing (bio)geochemical processes in the Earth system, as well as the socio-economic processes in some cases. At the top of the hierarchy of such models one can see general-circulation models which are able to represent each of the considered components in detail, but are characterized by high computational cost. The simplest models of the Earth climate system are the energy-balance models and radiative-convective models characterized by low spatial resolution and allowance for only a small number of the most important climate-forming processes. Nevertheless, these models are characterized by a number of advantages, primarily, simple and understandable physics. Moreover, radiative-convective models are useful for studying a number of the processes allowed for in general-circulation models and tuning appropriate modules. In addition, there is a class of models of the Earth climate system of intermediate complexity, which take into account most of the processes presented in the general-circulation models (and sometimes the processes unaccounted for in the latters), but with a number of simplifications. The advantage of this class is related to an opportunity of integrating the model for the periods of tens of thousands years or even more. The review deals with all these classes of models with discussions of their features, including the conservation laws explicitly taken into account in them, as well as the classes of problems to which it is advisable to apply the models of the Earth climate system of different types. Additionally, the projects for comparing the models of the Earth climate system in which models of different classes are used simultaneously are discussed.

Using numerical simulation methods, we find that in the generalized Morse–van der Pol model for a chain of active particles with account of their nonlocal interactions, the emergence and stable propagation of dissipative kinks and solitons are possible. Switching from local to nonlocal interactions results in a decrease in the maximum particle velocities and an increase in the general width of both the kink and the soliton. It is shown that the approximate analytical model describing the leading front of the solitary wave based on two-particle collisions loses its relevance in the generalized chain model. We propose an efficient numerical procedure for determining the shape of the traveling wave in a conservative Morse chain with nonlocal interactions; the procedure is based on solving an advanced-delay differential equation.

We perform theoretical and numerical studies of nonlinear propagation and distortion of elastic initially harmonic waves in crystalline solids with Granato–Lucke dislocation hysteresis. We obtain analytical solutions describing nonlinear propagation and evolution of waves in such media. The amplitude dependences of the nonlinear decay decrements and changes in the propagation velocity of the fundamental-frequency wave, as well as the amplitudes of its higher harmonics are determined. A graphical analysis of the solutions obtained is carried out.

We study the reflection of plane waves and radar signals with synthesized apertures from statistically rough surfaces with an exponential height correlation function. Approximations of the correlation function are proposed using superpositions of the Gaussian functions. This makes it possible to develop an effective practical algorithm for simulating radar echoes. The results are verified against known asymptotic laws for diffuse wave reflection from random surfaces. The proposed computational approach can be applied in a wide range of situations where the stationary and transient wave fields from wave sources of different configurations are reflected from rough surfaces of the types considered here.

We present an example of calculating the nonlinear stage of the formation of small-scale variations of the electric field in a multidisperse plasma-like medium consisting of a fraction of relatively heavy particles and a weakly conducting convective flow around them in a constant background electric field. A general system of equations describing the indicated medium in a one-dimensional approximation and consisting of the equations of dynamics of heavy particles, continuity equations for the flow of heavy particles and total charge density, as well as the Poisson equation, is given. Calculations of the instability were performed for conditions at altitudes of about 6 km above the Earth’s surface in the region of strong thundercloud convection. The results of calculating the initial stage of instability correspond to the estimates following from the analysis of the dispersion relation. It is shown that due to the development of the instability, multilayer structures of the electric field, density of heavy particles, and charge density with a characteristic vertical scale of the order of 10 m are being formed and are shifted with the flow. The maximum value of the electric field can significantly exceed the initial (background) value.

We consider surface-wave resonators made as a planar corrugated structure with the operating π-mode and an additional double-period corrugation, which ensures radiation output in the form of a Gaussian beam in the direction transverse to the direction of the structure. The dependences of the intensity of the transverse energy output and the corresponding mode Q-factor on the parameters of the main and additional rectangular corrugations are studied. Analytical expressions are obtained in two limiting cases, specifically, one for a shallow corrugation with a period close to half the wavelength and a depth much smaller than the period, and the other, for a corrugation with a period much smaller than the wavelength and a depth close to quarter the wavelength. Such resonators can be used in relativistic surface-wave oscillators and non-relativistic clynotrons, respectively. The Q-factor of the surface-wave resonator with a rectangular corrugation having an arbitrary ratio of the period and the depth is calculated using direct numerical simulation. It is shown that the Q-factor related to the transverse radiation output decreases in inverse proportion to the squared depth of the additional corrugation and increases with decreasing corrugation depth for a fixed wavelength.

We present the averaged gyrotron equations derived within a model with a fixed longitudinal field structure and an arbitrary cross section of the cavity resonator. The presented set of equations allows one to describe and simulate the dependences of the starting currents and the electronwave interaction efficiency on the parameters of the electron beam and the characteristics of the electrodynamic system in promising gyrotrons having resonators with improved mode selection. A gyrotron with an operating frequency of 263 GHz based on a two-mirror confocal resonator with cylindrical mirrors is considered as an example.

We study experimentally an atmospheric pressure discharge sustained by the millimeter-wave gyrotron radiation in an argon flow. The spatial distribution of gas temperature in the plasma jet emerging from the conical nozzle of a waveguide plasmatron has been measured by two independent methods. Using the method of optical interferometry and the method of thermosondes, it is shown that even with a microwave heating power of several tens of watts, it is possible to create a weakly ionized plasma at the nozzle exit, in which the gas temperature reaches about 1000 K and declines uniformly in the postdischarge area. The interferometric method applied in this work makes it possible to obtain an instantaneous complete spatial picture of the gas temperature distribution in the plasma torch during a single passage of a probing laser beam through the medium under study. This permits one to observe the complex dynamics of convective processes during the outflow of plasma and hot gas from the hole in the plasmatron.