V. Yu. Zaslavskii

Generation of spatially coherent radiation in free-electron masers with two-dimensional distributed feedback

The results of theoretical and experimental studies on the generation of spatially coherent electromagnetic radiation in a planar free-electron maser with two-dimensional distributed feedback are reported. A two-dimensional Bragg structure is used at the initial part of the interaction space to ensure the transverse synchronization of the radiation. The possibility of the narrowband generation in the 75-GHz frequency band is demonstrated experimentally for a sheet kiloampere electron beam whose width is 20 times larger than the wavelength.

Quasi-optical model of relativistic surface-wave generators for millimeter and submillimeter range

A nonlinear, nonstationary theory of relativistic surface-wave generators for the microwave range has been constructed in the framework of a quasi-optical approach. The radiation field is represented by two counter-propagating quasi-optical waves that are coupled at a corrugated surface so as to form an evanescent wave mode. A synchronous interaction with a straight sheet electron beam takes place with allowance for the surface wave retardation. An experimentally implemented prototype of the relativistic surface-wave generator operating in the millimeter range has been simulated and it shown that generators of this type can be developed so as to operate in the submillimeter range.

Cherenkov masers with two-dimensional distributed feedback

The possibility of using a two-dimensional (2D) distributed feedback to generate spatially coherent radiation by straight sheet and annular electron beams is studied. The schemes of sectional Cherenkov masers with the transverse synchronization of radiation performed in a 2D Bragg structure, which is arranged at the cathode end of the interaction space and conjugates the longitudinal and transverse (azimuthal) wave flows, is analyzed. By introducing dissipation into this structure, it is possible to increase the transverse dimensions of masers to a level on the order of 103 wavelengths with the corresponding increase in the total output radiation power without changing spectral characteristics and decreasing the energy conversion efficiency.

Terahertz free-electron lasers with bragg structures based on the coupling between traveling and quasicritical waves

The possibility of creating high-power tunable planar terahertz free-electron lasers based on modified Bragg structures with the coupling of paraxial and transverse (with respect to the velocity of particles) wave fluxes has been demonstrated. In addition to the compatibility with the transport channels of intense electron beams, the advantage of the proposed structures is the spatial coherence of radiation at a large oversize factor of the interaction region in two transverse coordinates.

Relativistic surface-wave generators based on two-dimensional periodic structures

In order to increase the integral output radiation power of relativistic surface-wave generators, it is suggested to use two-dimensional (2D) periodic slow-wave systems. Numerical simulations of dynamics of a new variant of these generators showed that additional wave beams, which appear in the 2D structure and propagate in the transverse direction, are capable of synchronizing radiation from a wide sheet electron beam. This circumstance makes it possible to implement surface-wave generators operating in the millimeter wavelength range at a gigawatt power level—e.g., based on the ELMI high-current accelerator at the Institute of Nuclear Physics (Novosibirsk).

Generation of a spatially coherent field structure in free-electron masers with 2D distributed feedback

Nonlinear dynamics of free-electron masers (FEMs) is studied in the planar geometry with 2D distributed feedback (DFB). As is distinct from previous works, the field structure is not fixed with respect to the three spatial coordinates including the coordinate that is orthogonal to surfaces of the plates of the 2D Bragg resonator. Conditions on the allowed oversize parameter (ratio of the gap between the resonator plates to wavelength) under which the steady-state generation remains stable upon variation in electron-beam parameters are derived. It is demonstrated that, at a relatively large gap, variations in the mismatch lead to the jumps of oscillation frequency that correspond to the excitation of bunches of modes with different transverse indices of partial waves. The results of simulation using a particle-in-cell method are presented for a FEM prototype with 2D DFB that is created using an ELMI accelerator at the Institute of Nuclear Physics, Siberian Branch, Russian Academy of Sciences. The simulated results show that narrow-band spatially coherent radiation can be generated at experimental parameters of the electron beam and electrodynamic system. The advantages of 2D Bragg structures in comparison with conventional 1D structures are demonstrated for FEMs.

Development of the 75-GHz planar gyrotron with transverse energy extraction

The opportunity of generating ribbon-shaped polyhelical electron beams intended to excite oscillations in the new type of gyrotrons is investigated via particle-in-cell simulation. Its distinctive features are the planar geometry of an interaction space and transverse energy extraction (with respect to the direction of translational motion of a helical electron beam). The electrode configuration has been found in the gyrotron with an operating frequency 75 GHz, where the formed electron beam has a velocity spread of ∼10%, the rotational energy fraction is 50%, and the beam profile is weakly distorted. When the voltage is 12 kV and the beam current is 1 A, the electron beam under study is demonstrated to excite the single-mode generation with an efficiency of up to 15% in the gyrotron’s operating space produced by the planar waveguide.

Submillimeter planar gyrotrons with transverse diffraction output of radiation

In order to increase the integral output power of short-wave gyrotrons, it is suggested to use a planar scheme with the transverse (relative to the direction of electron translation) diffraction output of radiation. An advantage of the planar design in comparison to the traditional cylindrical gyrotron geometry is the possibility to ensure the coherence of radiation at a greater oversized factor by using a diffraction mechanism of mode selection with respect to the transverse coordinate. The results of simulation of the nonlinear dynamics of a planar gyrotron with a polyhelical ribbon electron beam show that it is possible to reach an output power of several hundred kilowatt at frequencies up to 1 THz. An additional advantage of the proposed scheme is the possibility of frequency tuning by changing the distance between plates.

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.

Free-electron maser with high-selectivity Bragg resonator using coupled propagating and trapped modes

A free-electron maser (FEM) with a double-mirror resonator involving a new modification of Bragg structures operating on coupled propagating and quasi-cutoff (trapped) modes has been studied. The presence of trapped waves in the feedback chain improves the selectivity of Bragg resonators and ensures stable single-mode generation regime at a considerable superdimensionality of the interaction space. The possibility of using the new feedback mechanism has been confirmed by experiments with a 30-GHz FEM pumped by the electron beam of LIU-3000 (JINR) linear induction accelerator, in which narrow-band generation was obtained at a power of ∼10 MW and a frequency close to the cutoff frequency of the trapped mode excited in the input Bragg reflector.