Powerful Cherenkov oscillators with 2D distributed feedback

The feasibility of using 2D distributed feedback based on 2D planar and coaxial Bragg structures for generating spatially coherent radiation from rectilinear ribbon and tubular electron beams is studied. One-section and sectional Cherenkov masers are analyzed. In the former design, a 2D Bragg structure acts as a resonator and a periodic slow-wave system simultaneously. In the latter (sectional) design, radiation is synchronized in a 2D Bragg structure that is placed at the cathode end of the interaction space and couples longitudinal and transverse (azimuthal) wave flows. The wave is amplified by the electron beam mainly in the fairly long middle section. The output (collector) part contains a standard 1D Bragg structure that partially reflects the amplified radiation toward the cathode and closes the feedback circuit. It is shown that dissipation introduced into the 2D Bragg structure of the sectional design makes it possible to increase one of the transverse sizes of the system to ∼10³ wavelengths with the energy exchange efficiency and one-frequency masing mode stability remaining the same. With such an overdimension, the millimeter-wave radiation integral power may reach a gigawatt level.     

Generation of terahertz radiation by the optical breakdown induced by a bichromatic laser pulse

Experimental results are presented on generation of terahertz radiation by the air breakdown induced by a high-intensity laser pulse having not only a fundamental component, but also a second-harmonic one. A theoretical explanation of the experimental data is proposed, based on a model of field ionization of a gas by a bichromatic laser.     

Development of a compact hardware/software package for noninvasive diagnostics of skin diseases in the THz frequency range

The key elements of a mobile hardware/software package for noninvasive diagnostics of skin diseases in the THz frequency range have been designed, produced, and approved in model experiments. These elements are a compact THz oscillator based on an all-fiber femtosecond laser system and a unit for recovering electrodynamic characteristics of layered objects from scattered THz radiation spectra. Generation of 250-fs optical pulses at a wavelength of 1.03 µm with energy of 0.3 µJ and a repetition frequency of 1MHz is demonstrated and the efficiency of optical-THz conversion is found to be 5×10^?6. The proposed algorithm is constructed based on an iterative procedure and can be used for dispersive and absorbing media. It has higher operating speed in comparison with the algorithms for solving inverse problems, which are based on functional minimization methods.     

Chaotic generation in a megawatt backward-wave tube

Experimental and theoretical studies on the self-modulation mode of generation in a high-power BWT with the electrodynamic system representing a slightly corrugated waveguide are presented. The BWT is fed by an electron beam with the energy 150 keV and the current 150 A. The system operates at the E ₀₁ mode with the mean frequency 8.7 GHz. Dynamic chaos is obtained by a three-fold increase in the length of the interaction space in comparison with the prototype exhibiting stationary generation. The stationary generation was changed to periodic sinusoidal self-modulation and then to chaotic self-modulation as the current increases from 6 to 60 A. The generation mode is simplified when the current ranges from 70 to 90 A and becomes complicated again for the current exceeding 100 A. Experimental observations are in good agreement with the results of simulation predicting a certain simplification of the self-modulation mode at the currents 70–90 A owing to the effect of high-frequency space charge. Under the conditions of chaotic generation, the mean power was as high as 2 MW at the relative spectral width of the signal 4% and the total duration of the microwave pulse 10 μs.     

Experimental study of the possibility to increase the pitch factor of an intense relativistic helical electron beam

We present the results of experiments on the formation of a high-quality relativistic helical electron beam (HEB) in a magnetron-injection gun. It is shown that suppression of parasitic excitation of microwaves in the input part of the transportation channel allows eliminating high-voltage breakdowns in the gun and achieving greater beam compression. A modulation of the electron beam current at the frequency of longitudinal electron oscillations between the cathode and the magnetic mirror in the trap, which is related to the instability of the helical electron beam, has been observed for the first time. The modulation depth can reach tens percent. Pickup of reflected electrons by a special diaphragm makes it possible to increase the achievable pitch factor, eliminate the beam-current modulation and, as a result, form although with a current loss on the diaphragm, an HEB with record-breaking values of the pitch factor, which exceed 2. For a moderate HEB compression, when the portion of reflected electrons is relatively low, their pickup by the diaphragm allows one to form a beam in which the total energy of the transverse motion of electrons conserves despite the loss of part of the current. After the optimization, a beam with an electron energy of 300 keV, a current of 100 A, a pitch factor of 1.5, and a velocity spread of 20% is obtained for a 15% loss of the current on the diaphragm. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 10, pp. 855–863, October 2008.     

Relativistic carcinotron with a thermionic injector of electrons

An experimental study directed to the determination of a mechanism for microwave breakdown in a relativistic X-band carcinatron is presented. An electron beam was generated using a thermionic cathode, which provided a stable beam geometry. The use of this cathode decreased the probability of breakdown caused by electron bombardment of the slow-wave structure. An important part played in microwave breakdown are molecules absorbed on the slow-wave structure surface. It is shown that the presence of these molecules, an implementation of conditions for secondary-electron resonant discharge (SERD), can result in a very fast (during 10-20 ns) limitation of the radiation pulsewidth. To remove the adsorbed molecules, heat degassing of the slow-wave structure and a collector of electrons was applied, going on continuously, during a working day of the device. By degassing and choice of slow-wave structure material, output radiation power of the device was increased by a factor of ten (up to 5 MW at the pulsewidth of 10 μs)     

Experimental study of a saturated multipactor in crossed fields

We propose an experimental scheme of and develop a method for studying a one-sided multipactor discharge in crossed fields in the saturation regime. Basic parameters of the discharge have been measured. In the experiment performed in a 3-cm wavelength band, the discharge develops in microwave fields with intensities from 8 kV/cm (near the lower threshold) to 20 kV/cm and became stabilized during at least a hundred cycles of multiplication of secondary electrons on the discharge surface. In this case, the maximum power absorbed in the discharge amounts up to 4 kW/cm², and the corresponding density of the saturated-discharge current, up to 10 A/cm². The measured time of discharge development is equal to 15–30 ns. These parameters agree well with estimating calculations based on approximate analytical models.     

Terahertz emission from a metallic surface induced by a femtosecond optic pulse

Results of experimental and theoretical investigations on generation of terahertz radiation at the interaction of femtosecond laser pulses with a metal surface are presented. Investigations are performed with the laser pulse intensities higher compared with that used in papers [Opt. Lett.29, 2674 (2004); Opt. Lett.30, 1402 (2005)]. The most effective generation is observed for p-polarized optical pulses with incidence angles in the range 5°-10° (from the surface), depending on the kind of metal. For the copper, the exponential growth of terahertz pulse energy with the increase of optical pulse energy was registered. Theoretical interpretation for some of the experimental results is proposed based on the model of free electrons in metal.     

Generation of terahertz radiation by the axicon focusing of ionizing laser pulses

JETP Letters - Experimental results on the generation of terahertz radiation in the gas breakdown by high-power quasi-monochromatic laser pulses focused by an axicon lens are presented. A...