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The purpose of the research presented in the review is to analyze a new feedback mechanism Џ two-dimensional (2D) distributed feedback, and to study the possibility of using this mechanism for generation of powerful spatially-coherent radiation. Such 2D distributed feedback is implemented by applying 2D Bragg structures, which represent sections of planar or coaxial waveguides with the 2D-periodical corrugation and can be considered as an analog of the 2D photonic crystals. Methods. Theoretical analysis of 2D Bragg structures and powerful relativistic masers based on them was carried out in the framework of the coupled-waves method using geometrical and quasi-optical approximations. High selectivity of 2D Bragg structures was confirmed by the simulations and «cold» electrodynamic tests. Results. Simulations of the dynamics of oscillators with 2D distributed feedback shows that the transverse (relative to the electron beam propagation) wave-fluxes that occur in 2D Bragg structures lead to synchronization of the radiation of different parts of wide electron beams and establishment of a stable single-mode generation at the transverse sizes of the systems of up to ∼ 102 ...103 wavelengths. Operability of novel feedback mechanism has been demonstrated experimentally in planar and coaxial Free-Electron Masers realized in the 4-mm and 8-mm wavelength bands, where a stable narrow-band generation with a record output power of up to 100 MW level is achieved at the transverse sizes of the system of about 50 wavelengths. Conclusion. Theoretical and experimental studies have shown that the 2D distributed feedback is an effective method for obtaining coherent radiation of millimeter and submillimeter ranges with a power level of ∼ 108 ...1010 W from relativistic electron beams of sheet and tubular configurations formed by the high-current accelerators. From a practical point of view, it also attractive to use the 2D feedback mechanism for synchronization of a laser active media, in particular, semiconductor heterolasers.

The authors describe the current stage of the development of a powerful long-pulse free electron laser (FEL) of the terahertz band at the Budker Institute of Nuclear Physics in cooperation with Institute of Applied Physics on the basis of the LIU induction accelerator with an energy of 5–10 MeV, a current of up to 2 kA, and a pulse duration of up to 200 ns. It is proposed that two alternative types of electrodynamic systems may be used to ensure a regime of stable narrow band generation: advanced Bragg resonators and quasi-optical resonators of the Talbot type. FEL parameters calculated on the basis of these resonators are presented, along with results from modeling under conditions of substantial oversize. The operability of the new types of resonators is verified in cold electrodynamic tests. [ABSTRACT FROM AUTHOR]

A project for a submillimeter free electron laser (FEL) based on a relativistic electron beam (REB) generated in a linear induction accelerator (LIA) is proposed for the Budker Institute of Nuclear Physics, Siberian Branch, Russian Academy of Sciences, and the Institute of Applied Physics, Russian Academy of Sciences. A theoretical analysis shows that the electron beam generated in the LIA (energy MeV, current kA, normalized emittance ~ 1100 π mm mrad) is a suitable driver for generating subgigawatt pulses of coherent EM radiation in the submillimeter range of wavelengths (0.3–1 THz). The main proposals for developing a FEL based on the electron beam generated in a linear induction accelerator are presented, the main project tasks are outlined, and proposed ways of solving them are described. Results from electron-optical experiments on the formation of an electron beam intended for FEL applications are presented. [ABSTRACT FROM AUTHOR]

A diagnostic has been developed to study the focusing dynamics of a high-power electron beam on a target. The diagnostic uses a pinhole camera to observe bremsstrahlung from the converter target. A data acquisition system yields images of the beam focal spot with a frame duration of 20 ns and an almost unlimited recording duration. The focal spot dynamics of a linear induction accelerator with an energy of 1.5 MeV, a current of 1.2 kA and a pulse duration of 350 ns were studied. The focal spot was found to be disrupted within the first 100 ns of the pulse. Defocusing has a complex 3-D character. The feasibility of additional cleaning of the target using an accelerator pre-pulse was shown. Stable beam focusing for up to 200 ns was demonstrated. [ABSTRACT FROM AUTHOR]

The power of electromagnetic emission near the plasma frequency during collective electron beam-plasma interaction is found to be significantly increased in a plasma with preformed large-amplitude density perturbations. Laboratory experiments at the GOL-PET facility show that injection of a kiloampere relativistic electron beam into a magnetized plasma with strong radial density gradients is accompanied with an order of magnitude more intense generation of sub-millimeter waves than in the case of smooth density profile. As a possible mechanism for the enhanced generation of the longitudinal radiation flux observed in these experiments, we discuss the direct beam pumping of longitudinally propagating electromagnetic plasma modes due to their coupling with the Doppler shifted beam branch in the presence of oblique modulation of plasma density. [ABSTRACT FROM AUTHOR]

Presents a study that investigated the generation and transport of microsecond ribbon electron beam in an insulated diode. Methodology; Examination of the efficiency of the ribbon electron beam; Analysis of the difference between the beam current and diode current.

The paper presents results of measurements of sub-terahertz electromagnetic emission from magnetized plasma during injection of a powerful relativistic electron beam of microsecond duration in plasma with the density of 3 x 1014 cm-3 . It was found that the spectrum of the radiation concentrated in three distinct regions with high level of spectral power density. The first region is located near ƒ1 = 100 GHz; the second one is in the vicinity of 190 GHz, and the third region is in the frequency interval ƒ3 = 280-340 GHz. Polarization vectors of the emission in the first and third regions ƒ1 and ƒ3 ) are directed mainly perpendicular to the magnetic field in the plasma. At the same time, the polarization of the radiation in the vicinity of ƒ2 = 190 GHz is parallel to the magnetic field. The most likely mechanism of electromagnetic wave generation in the frequency regions ƒ1 and ƒ2 is the linear conversion of the plasma oscillations into the electromagnetic waves on strong gradients of the plasma density. The third region is situated in the vicinity of second harmonic of electron plasma frequency, and we explain this emission by the coalescence of the upper-hybrid oscillations at high level turbulence in plasma. [ABSTRACT FROM AUTHOR]

To advance relativistic masers into the short wavelength region of the millimeter and sub-millimeter wavelength bands, advanced Bragg reflectors based on coupling of propagating and quasi-cutoff waves were proposed in Ginzburg et al., Appl. Phys. Lett. 95, 043504 (2009). In this paper, we present experimental examination of the reflectors of such type, which was carried out in the 4-mm wavelength band. A power reflection coefficient of ∼0.8 was measured in the bandwidth of ∼200-300 MHz. Tunability of the reflection zone by varying the distance between the plates in the frequency band of 3%-5% was demonstrated. Experimental data agree well with the result of 3D simulations. [ABSTRACT FROM AUTHOR]

Electrodynamic properties of two-dimensional (2D) planar Bragg resonators realising 2D distributed feedback were studied. It is shown that high selectivity of a 2D Bragg resonator originates from the specificity of the dispersion diagrams of normal waves in a double-periodic corrugated unbounded planar waveguide resulting in formation of high-Q eigenmodes in the vicinity of the Bragg resonance frequency. Results of theoretical analysis in the frame of geometric-optical approach of coupled-wave theory coincide well with 3D simulations using the code CST 'Microwave Studio' and are corroborated by data obtained in 'cold' microwave tests. Frequency dependences of scattering coefficients were measured for incident wave-beams of different structures and excitation of the high-Q eigenmodes has been demonstrated. [ABSTRACT FROM AUTHOR]

Electrodynamical properties of two-dimensional Bragg resonators of planar geometry are studied in the frame of the geometrical-optical approach and in three-dimensional simulations. Results of the theoretical analysis are corroborated by data obtained in “cold” microwave measurements. Formation of the high-Q eigenmodes in the vicinity of the Bragg resonance frequency has been demonstrated. [ABSTRACT FROM AUTHOR]

A spatially extended planar 75 GHz free-electron maser with a hybrid two-mirror resonator consisting of two-dimensional upstream and traditional one-dimensional downstream Bragg reflectors and driven by two parallel-sheet electron beams 0.8 MeV/1 kA has been elaborated. For the highly oversized interaction space (cross section 45?2.5 vacuum wavelengths), the two-dimensional distributed feedback allowed realization of stable narrow-band generation that includes synchronization of emission from both electron beams. As a result, spatially coherent radiation with the output power of 30-50 MW and a pulse duration of ~100 ns was obtained in each channel. [ABSTRACT FROM AUTHOR]

The use of two-dimensional Bragg resonators of planar geometry, realizing two-dimensional (2D) distributed feedback, is considered as a method of producing spatially coherent radiation from a large sheet electron beam. The spectrum of eigenmodes is found for a 2D Bragg resonator when the sides of the resonator are open and also when they are closed. The higher selectivity of the open resonator in comparison with the closed one is shown. A time-domain analysis of the excitation of an open 2D Bragg resonator by a sheet electron beam demonstrates that a single-mode steady-state oscillation regime may be obtained for a sheet electron beam of width 100-1000 wavelengths. Nevertheless, for a free-electron maser (FEM) with a closed 2D Bragg resonator, a steady-state regime can also be realized if the beam width does not exceed 50-100 wavelengths. The parameters for a FEM with a 2D planar Bragg resonator driven by a sheet electron beam based on the U-2 accelerator (INP RAS, Novosibirsk) are estimated and the project is described.