A. V. Arzhannikov

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.

Super-power free electron lasers with two-dimensional distributed feedback

Abstract Using time domain analysis we have studied the processes of oscillation build-up and spatial synchronization in FELs with two-dimensional distributed feedback driven by large size sheet and tubular relativistic electron beams. It is proved that the proposed feedback mechanism ensures powerful spatially coherent radiation when the ratio between the transverse sized of the electron beam and the wavelength is up to 102–103.

Electrodynamic properties of two-dimensional Bragg resonators of planar geometry

Abstract Electrodynamic properties of two-dimensional (2-D) Bragg resonators of planar geometry realising 2-D distributed feedback are studied theoretically and experimentally. It was shown that the resonators possess high selectivity over both longitudinal and transverse mode indexes and the eigenmodes spectrum is defined by the profile of the corrugated surface of the conductors forming the resonator. Results of the theoretical analysis agree well with the data obtained in “cold” microwave measurements of the frequency dependence of the transmission, reflection and transverse scattering coefficients.

Generation of powerful narrow-band 75-GHz radiation in a free-electron maser with two-dimensional distributed feedback

A planar generator based on a free-electron maser (FEM generator) is developed employing a relativistic sheet electron beam (0.8 MeV/1 kA/4 μs) formed by an ELMI accelerator. A hybrid two-mirror resonator consisting of a two-dimensional upstream and a one-dimensional downstream Bragg reflectors is used as the electrodynamic system. The use of two-dimensional distributed feedback implemented in the upstream Bragg structure has made it possible to achieve stable well reproducible single mode generation regime with transverse dimensions of the system of ∼25 × 2.5 wavelengths. Experiments carried out at a frequency of 75 GHz have yielded narrow-band radiation with spectrum width of ∼20 MHz in pulses with a duration of ∼100–200 ns and power of 30–50 MW.

A traveling-wave ring resonator with Bragg deflectors in a two-stage terahertz free-electron laser

We propose a scheme of a two-stage terahertz free-electron laser (FEL) based on a Bragg traveling-wave ring resonator and parallel ribbon relativistic electron beams. The first beam moves in the field of a planar undulator and generates a pumping wave in the millimeter range. At the second stage, this wave transforms into terahertz radiation in the course of intraresonator backscattering on the second parallel electron beam. The ring resonator consists of four Bragg deflectors, each deflecting radiation by 90° in the vicinity of Bragg resonance, thus closing the feedback ring of the low-frequency pumping generator and simultaneously providing effective transfer of the pumping wave to the scattering stage.

Production of Powerful Spatially Coherent Radiation in Planar and Coaxial FEM Exploiting Two-Dimensional Distributed Feedback

Two-dimensional distributed feedback is an effective method of producing ultrahigh-power spatially coherent radiation from an active medium, that is spatially extended along two coordinates, including relativistic electron beams with sheet and annular geometry. This paper describes the progress in the investigations of planar and coaxial free-electron masers (FEMs) based on a novel feedback mechanism. The theoretical analysis of these FEM schemes was conducted in the frame of the coupled-wave approach and 3-D simulations and agrees well with the experimental data obtained in ldquocoldrdquo and ldquohotrdquo tests. As a result, the effective transverse (azimuthal) mode selection has been demonstrated under a transverse size of about 20-25 wavelengths, and narrow-frequency multimegawatt microwave pulses have been generated in the Ka- and W-bands.

Planar free-electron lasers with combined 1D/2D Bragg mirror resonators: A theoretical study

The operation of a free-electron laser with a combined Bragg mirror resonator is studied theoretically. The resonator comprises a pair of planar Bragg reflectors with a two-and a one-dimensional relief pattern, respectively, and is closed in the transverse direction so as to ensure unidirectional outcoupling. It is demonstrated that this design provides for a possibility of obtaining spatially coherent radiation from a sheet electron stream with the cross size exceeding the wavelength by several orders of magnitude.

The simulation of a free-electron laser amplifier with a ribbon relativistic electron beam

An amplifier based on a high-power free-electron planar laser operating at a wavelength of 4 mm is simulated. The system is built around the U-3 accelerator, which forms a ribbon relativistic electron beam with an energy of up to 1 MeV and a total operating current of up to 2 kA. The simulation uses various approaches, including the direct numerical simulation of the Maxwell equations and particle motion equations by means of the PIC-code KARAT. The approaches are shown to give close results.

Planar two-dimensional Bragg resonators with corrugated surfaces: Theory and experiment

The electrodynamic properties of a planar two-dimensional Bragg resonator with two-dimensional distributed feedback were studied. A high selectivity of the resonator with respect to the longitudinal and the transverse mode indices is demonstrated, the mode spectrum being significantly dependent on the surface relief pattern of the resonator plates. The theoretical results are confirmed by measurements under “cold” conditions.

Possible use of two-dimensional Bragg structures in an FEL amplifier powered by a sheet electron beam

It is shown that two-dimensional doubly-periodic Bragg structures can be used effectively in the amplification schemes of free electron lasers powered by sheet relativistic electron beams. The presence of such a structure ensures spatial coherence of radiation from different parts of the electron beam when the input signal propagates across the electron flux. Theoretical analysis shows that the gain in the regenerative scheme can reach 50 dB.