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Vol 61, No 11 (2019)
- Year: 2019
- Articles: 7
- URL: https://journal-vniispk.ru/0033-8443/issue/view/15176
Article
Millimeter-Wave Gyrotron System for Research and Application Development. Part 2. High-Temperature Processes in Polycrystalline Dielectric Materials
Abstract
Gyrotron systems operated at frequencies of 24 to 30 GHz with an output power of 3 to 15 kW have been used at the Institute of Applied Physics of the Russian Academy of Sciences for more than 20 years for the studies of high-temperature processes in polycrystalline dielectric materials under intense electromagnetic irradiation. The research has mostly been focused on the study of the physically specific features of diffusive mass transfer in solids and on the possible use of these features for applications. A distinguishing feature of the studied processes is a significant enhancement of their rates compared to similar processes performed with the use of conventional heating methods. Examples of enhanced sintering of a broad range of ceramic materials, including optical and laser ceramics and composition-graded metal–ceramic products are considered. The principles of the developed method of ultrafast sintering of oxide ceramics with rates exceeding those typical of the conventional methods by two or three orders of magnitude are described. The development of this method has resulted from a purposeful use of the functional capabilities of the gyrotron systems and the engineering solutions implemented therein.
787-796
Experimental Demonstration of the Possibility to Expand the Band of Smooth Tuning of Frequency Generation in Short-Cavity Gyrotrons
Abstract
We show experimentally the possibility to expand significantly the band of smooth tuning of the generation frequency in gyrotrons using cavities that have shorter lengths. Due to a decrease in the sensitivity of the electron-wave interaction process to the spread in the electron beam velocities, one can increase the power of generation at higher longitudinal modes up to a level comparable with the radiation power in the case of excitation of a mode with one longitudinal variation. In this case, overlapping of generation bands at the neighboring longitudinal modes is achieved by increasing the current of the electron beam. In the experiment performed in a gyrotron having an operating frequency of about 12 GHz, we demonstrated a frequency tuning band which exceeded 4% at the kilowatt level of the output radiation power in its greater part. The obtained results open up the possibility of developing tunable high-frequency moderate-power gyrotrons.
797-800
Absorbing Two-Dimensionally Periodic Structures on New Plasmonic Materials
Abstract
We study theoretically two-dimensionally periodic gratings of plasmonic strips on substrates which contain dielectric and plasmonic layers. The developed electrodynamic model is based on solving the vector integro-differential equation of diffraction by three-dimensional dielectric bodies by the Galerkin method. It is shown that at the resonant frequencies of the surface plasmon polariton, these structures absorb almost 100% of the energy of the incident radiation in a wide wavelength range. The use of nonmetal plasmonic materials makes it possible to create a wideband absorber of infrared waves.
801-805
Spectral-Dynamical Peculiarities of Polarization of the Active Medium and Space-Time Empirical Modes of a Laser with a Low-Q Cavity
Abstract
We have found a set of correlation effects, which are due to the inherent dynamics of the spectral density of polarization of an active medium with strong inhomogeneous broadening of the working transition line and occur if the rate of incoherent relaxation of optical dipole oscillations of the active centers is lower than the rate of optical-field attenuation in the laser cavity. Our analysis is based on the numerical studies of the stationary superradiant laser generation during continuous pumping with self-locking of some of the quasistationary modes. For the purposes of studying the detected effects, the methods of comparative analysis of the dynamic spectra of polarization and the field have been developed. In these methods, the time-frequency and space-time empirical modes of the spectra are used, which are determined by orthogonal eigenfunctions of special correlation matrices. The interconnection of the superradiance phenomena and mode self-locking in the considered class of lasers with low-Q cavities is discussed.
806-833
Nonlinear Dynamics of an Antiferromagnetic Spintronic Oscillator
Abstract
We study nonlinear dynamics of the spintronic nanosized antiferromagnetic terahertz oscillator consisting of an antiferromagnetic layer with easy-plane anisotropy (hematite) and a normal-metal (platinum) layer. Normal oscillation frequencies, namely, ferromagnetic and antiferromagnetic (terahertz) ones, are found. Their dependence on the value of a static magnetic field parallel to the sample plane is obtained. An approximate mathematical model in the form of the equations for the Néel-vector rotation angle in the azimuthal plane is developed for describing the oscillator dynamics. The adjustment characteristic, i.e., the dependence of the antiferromagnetic-mode frequency on the value of the direct current flowing in the platinum layer is obtained.
834-840
An Experimental Study of the Angular Superresolution of Two Correlated Signals Using the Minimum-Polynomial Method
Abstract
We present experimental results on the superresolution of two closely located signal sources, which were obtained using an automotive millimeter-wave radar. The signal sources were mutually correlated and the input process consisted of only one sample. The minimum-polynomial method (the root variant) was compared with the Minimum Description Length (MDL) criterion when determining the number of sources and the root MUltiple Signal Classification (MUSIC) method when estimating their angular location. The minimum-polynomial method is shown to have a higher efficiency compared with the MDL criterion and ensures the source superresolution for the angular distance which is a factor of 4–5 smaller than the width of the antenna-array pattern. In terms of accuracy of estimating the coordinates of the signal sources, the efficiency of this method almost coincides with that of the root-MUSIC method if the number of sources is considered to be known in the latter method.
841-852
Detection of an Ultra-Wideband Quasi Radio Signal with Unknown Duration Against the Background of White Noise
Abstract
We develop the maximum likelihood algorithm for detecting an ultra-wideband quasi radio signal with an arbitrary shape and unknown amplitude, initial phase, and duration, which is observed against the background of additive Gaussian white noise. The structure and statistical characteristics of this algorithm are found. The influence of a priori ignorance of the duration of a quasi radio signal on its detection efficiency is studied. The operation efficiencies of the maximum-likelihood and quasioptimal detectors of the ultra-wideband quasi radio signal are compared. Using computer simulation, the efficiency of the synthesized algorithm is examined and the applicability ranges of the obtained asymptotic expressions for its characteristics are determined.
853-866