Известия ВУЗов.Радиофизика

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Bremsstrahlung at Low-Energy Electron–Nucleus Collisions in the Quantizing Magnetic Field. I. Distant Collisions

Sat, 05/04/2019 - 00:00

We analytically calculate the spectral power of bremsstrahlung from a slow electron colliding with motionless nuclei in a strong quantizing magnetic field, in which the energy of the Coulomb interaction between particles at a distance of the order of the Larmor radius exceeds the mechanical energy of the system in absolute value. In this case, the electron motion becomes quasibound in sufficiently close collisions. In this part of research, we consider bremsstrahlung at low frequencies, which is stipulated by distant flybys without quasibound motion: an electron can spread over many Landau levels as a result of the collision, but keeps the direction of its motion along the magnetic field. We prove that the transition from the classical to quantum cyclotron gyration of an electron does not manifest itself in the spectral emission power of the waves with arbitrary polarization at the considered frequencies. This property stems from the fact that the low-frequency emission is due to the longitudinal motion and electric drift in the crossed Coulomb and magnetic fields which remain quasiclassical. Thus, we confirm that the bleaching of the photosphere of a magnetic white dwarf, which was discovered in the classical consideration with respect to collisional absorption of the extraordinary wave (polarized across the external magnetic field), is also preserved in the quantum limit—for stars of this spectral type with the strongest magnetic field.

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Millimeter-Wave Gyrotron Research System. I. Description of the Facility

Sat, 05/04/2019 - 00:00

We describe a series of gyrotron facilities developed at the Institute of Applied Physics of the Russian Academy of Sciences for studying physical processes during interaction of millimeter-wave electromagnetic radiation and matter. This paper presents the universal principle of designing such systems on the basis of a facility having an output radiation power of 5 kW at a frequency of 24 GHz. The main components of the facility and their technical parameters are described. Design of high-efficiency radiation sources and radiation transmission lines for various research applications is a sophisticated radiophysical problem, and the need for long-term stable operation with automatic adjustment of the parameters of the generation regime requires unique engineering solutions. Application of multimode electrodynamic devices in the radiation transmission line allows one to treat materials with significantly different dielectric properties, in particular, heat them up to temperatures of about (and exceeding) 2000°C. The vacuum-tight working chamber of the facility is a high-Q untuned cavity resonator having a volume of about 0.1 m3, in which microwave heating of items with characteristic dimensions of more than 10 cm can be performed. The automatic control system of the facility ensures its reliable and long-term failure-free operation.

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Outshifted Plasma Lines Observed in Heating Experiments in the High-Latitude Ionosphere at Pump Frequencies Near Electron Gyroharmonics

Sat, 05/04/2019 - 00:00

We present experimental results concerning the features and origin of the outshifted plasma lines in the high-latitude ionospheric F region, which are excited by high-power HF O-mode radio waves radiated by the EISCAT/Heating facility at pump-wave frequencies near the fourth and fifth gyroharmonics. HF pump waves were transmitted towards the magnetic zenith during long-lasting (2–20 min) heater-on cycles. The studies were carried out using the measurement data obtained by the EISCAT incoherent scatter radar in Tromsø (the radar operating frequency 930 MHz). The analysis of the radar data has demonstrated the simultaneous excitation of several plasma lines at frequencies close to the pump frequency and at frequencies upshifted by 0.15–0.45 MHz on long-lasting (longer than 30 s) time intervals. The spectral width of the outshifted plasma lines was 0.10–0.15 MHz. The features of their occurrence were analyzed as functions of the relation between the pump-wave frequency and gyroharmonics and the proximity of the pump-wave frequency to the cutoff frequency of the ionospheric F2 layer. Plausible excitation mechanisms of the outshifted plasma lines are discussed. Comparison between the experimental data and calculation results concerning the features of the outshifted plasma lines for different pump-wave frequencies was made.

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Development of Wireless Communication Systems in the Subterahertz Frequency Range

Sat, 05/04/2019 - 00:00

The subterahertz and terahertz frequency ranges are promising for the creation of high-speed wireless communication systems because of the possibility to achieve a bandwidth of about several dozen gigahertz, which ensures a high channel capacity. However, rapid attenuation of a signal during its propagation in the atmosphere complicates the operation of communication systems in these ranges. The use of fixed narrow-beam antennas with a high power gain provides a direct surface communication distance of up to a few kilometers. The communication distance limitation can be partially removed by decreasing the frequency down to 200 GHz and narrowing the channel bandwidth down to a few gigahertz. In this paper, we present a radically new approach that was developed to create a wideband (up to one gigahertz) communication channel based on rapid modulation of a centimeter wavelength signal followed by the carrier-frequency multiplication up to 230 GHz without the modulating-signal distortion. This approach was not used in the previous communication systems. The model of a transmit–receive system (200–220 GHz) based on modern semiconductor devices is described in detail. The possibility of digital signal transmission at a speed of up to 1 Gbit/s is experimentally shown. According to calculations, an output power of the transmitter about several hundreds of microwatt is enough for data transmission to a distance of up to 1.5 km with an antenna gain of no less than 50 dB.

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Influence of the Choice of Boundary Conditions on the Distribution of the Electric Field in Models of the Global Electric Circuit

Sat, 05/04/2019 - 00:00

We obtain a new analytical representation of the solution for the classical model of the Roble—Hays global electric circuit, where the connection between the values of the electric potential and the current at magnetically conjugate points of the upper boundary of the atmosphere is allowed for in the boundary conditions. Using this representation, we analyze the influence of various boundary conditions at the upper boundary of the atmosphere on the potential distribution and present an estimate of perturbations of the electric field by thunderstorm sources at magnetically conjugate points.

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Chaotic Dynamics and Multistability in the Nonholonomic Model of a Celtic Stone

Sat, 05/04/2019 - 00:00

We study dynamic properties of a Celtic stone moving along a plane. We consider two-parameter families of the corresponding nonholonomic models in which bifurcations leading to changing the types of stable motions of the stone, as well as the chaotic-dynamics onset are analyzed. It shown that the multistability phenomena are observed in such models when stable regimes various types (regular and chaotic) can coexist in the phase space of the system. We also show that chaotic dynamics of the nonholonomic model of a Celtic stone can be rather diverse. In this model, in the corresponding parameter regions, one can observe both spiral strange attractors various types, including the so-called discrete Shilnikov attractors, and mixed dynamics, when an attractor and a repeller intersect and almost coincide. A new scenario of instantaneous transition to the mixed dynamics as a result of the reversible bifurcation of merging of the stable and unstable limit cycles is found.

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