Radiophysics and Quantum Electronics最新文献

We study the possibility to advance relativistic planar Čerenkov surface-wave oscillators, which are driven by intense sheet electron beams, to the subterahertz frequency range within the framework of the performed simulation. It is shown that the use of two-dimensional periodic slow-wave structures providing the two-dimensional distributed feedback allows one to ensure high coherence of radiation at the subgigawatt power level in oscillators of this kind for transverse sizes equal to hundreds of wavelengths. The design parameters and structural elements are discussed for implementing such high-power oscillators based on the ELMI accelerator complex within the framework of the joint BINP/IAP experiments.

We study theoretically the possibility of creating sources of terahertz radiation of high (up to several watts) power based on excitation of the fifth cyclotron harmonic in the frequency multiplication regime in high-power gyrotrons intended for plasma applications. It was previously shown that effective excitation of isolated (s = 4n + 1, n = 1, 2, 3, . . . ) cyclotron harmonics in gyrotrons is due to the specific property of the eigenmodes of cylindrical waveguides, according to which the conditions for simultaneous resonance of an electron beam with two TE modes having asymptotically multiple cutoff frequencies can be fulfilled. For the fifth cyclotron harmonic, this method was tested in experiments with a low-frequency kilowatt power level gyrotron. The use of gyrotrons for plasma applications will significantly increase the power and frequency of radiation based on the observed effect. To suppress spurious oscillation at the main cyclotron resonance, which occurs in the region of optimal magnetic fields for the multiplication effect, it is proposed to use frequency locking of the gyrotron by an external signal. Simulations performed in this work based on an averaged self-consistent gyrotron model shows the possibility of oscillation in the described radiation scheme with a power of several watts at a frequency of 1.25 THz with radiation at the fifth harmonic of the gyrofrequency in a recently developed sub-MW/0.25 THz gyrotron with TE_{19, 8} operating mode.

We present the results of studying the impact of high-intensity terahertz pulses on the differentiation of induced human neural progenitor cells (drNPCs). The differentiation was estimated using the immunocytochemical analysis, i.e., the marker of the undifferentiated cells (SOX2) and the markers of the neuronal (β-III-tubulin and MAP2b) and glial (GFAP) phenotype. The cell exposure was performed by terahertz pulses with an intensity of 21 GW/cm² and an electric field of 2.8 MV/cm for 30 min. As a result of exposure, the phenotype of induced neural progenitor cells did not differ from that of unexposed cells and the appearance of mature neurons or glial cells was not detected. The ability of the terahertz radiation to cause effects in neural cell cultures apparently requires further studies for higher exposure intensity or duration.

We develop the spatiotemporal theory of a gyrotron backward-wave oscillator (gyro-BWO) which allows broad frequency tuning and is based on the use of a sectioned quasi-optical system with a zigzag path of the signal wave beam. The theory explains the characteristic features of the dependences of the frequency and power of the output signal on the magnitude of the operating magnetic field. We study the competition between various modes of the system during the excitation of the auto-oscillator, predict the possibility of achieving auto-modulation generation regimes (including the regime with a short powerful superradiant pulse formed), and consider the hysteresis effect of frequency characteristics (when the operating magnetic field is varied over an extended generated pulse). Additionally, an external feedback system is proposed as a way to increase the generation efficiency.

We present the results of studies aimed at creation of accelerating structures for a photoinjector accelerator that is currently under construction at the IAP RAS. Based on the general theory of coupled oscillators, a simple method for diagnosing the resonant properties of two-cell accelerating structures is given. Electrodynamic measurements of the manufactured photogun at a low power level confirm the ability to achieve the required acceleration gradient, although they show quite a notable difference between its resonant properties and the design. These measurements are in good agreement with analytical estimates and numerical simulations. The project of an additional accelerating structure designed to further accelerate the photoinjector electron bunches up to the energy providing their efficient injection into a wakefield plasma accelerator is also presented.

The paper is devoted to a comparison of the results of an experimental study of a 28 GHz/25 kW gyrotron with a magnetically shielded system and the results of calculating the electron–wave interaction with the use of the ANGEL software code on the basis of a self-consistent model with an unfixed radio-frequency field structure. The good agreement between the results confirms the validity of using the developed code for creation of new gyrotrons. Comparison of various experimental data with the calculation results for the electron–wave interaction as well as for electron optics makes possible implicit reevaluation of the actual values of some important gyrotron parameters (the electron pitch factor, transverse velocity spread, etc.).

We consider the problem of Rayleigh wave scattering by an inclusion of small wave sizes in the form of an oblate ellipsoid located under the free surface of an elastic isotropic half-space. The material parameters of the filling of the inclusion can have an arbitrary contrast with respect to the material parameters of the elastic medium containing the inhomogeneity. The solution of the problem is based on the formalism of the Green’s functions of a force source in a half-space and the reciprocity theorem for an elastic medium. The examples demonstrate the possibility of diagnosing the presence of a cavity, as well as estimating its sizes and orientation by analyzing the features of the distribution of the displacement vector projection amplitudes in the scattered field.

We consider specific features of highly efficient generation of the second (eo-e and oe-o synchronism types) and third (oo-e synchronism type) harmonics using laser pulses with opposite frequency chirps for further application in electron photoinjectors. The relationships of the frequency chirps of the pulses interacting in a nonlinear crystal are obtained for the three-wave frequency summation process, which allow one to ensure high energy efficiency of the conversion and reproduce the spatiotemporal distribution of the intensity of the initial pulses in the generated signal.

We have studied the spectral properties of the inhomogeneities in the surface profile of a quartz plate in the spatial range from 10 μm to 100 mm on the basis of precision interferometric measurements. Analytical integral relations connecting one- and two-dimensional spectral densities of an isotropic random function, such as surface profile or spatial phase distribution of the laser beam, were obtained. It is shown that if the one-dimensional spectral density in a certain range of spatial frequencies decreases in a power-law manner, then the two-dimensional spectral density in the same range also decreases in a power-law manner with an exponent greater by one. The experimental data are in good agreement with the theoretical ones.

We study the vertical propagation of acoustic waves in a strongly inhomogeneous atmosphere. The class of monotonically varying profiles for the sound velocity, gas density, and gas pressure, for which acoustic waves can propagate with no reflection outside the framework of the classical WKB approximation, is found. This class of solutions, called reflectionless, is obtained by transforming the levels for waves in an ideal gas located in the gravity field into the Euler—Darboux—Poisson equation of a special kind, which has a solution in the form of a superposition of non-interacting waves propagating in opposite directions. It is noted that with this stratification, acoustic waves can have any frequency, in contrast to the exponential atmosphere, where the cutoff frequency is present.