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We consider the project of a sub-gigawatt free-electron laser (FEL) in the THz range based on a high-current electron beam proposed in 2020 by a scientific collaboration team from G. I.Budker Institute of Nuclear Physics of the Siberian Branch of the Russian Academy of Sciences (BINP SB RAS, Novosibirsk) and the Institute of Applied Physics of the Russian Academy of Sciences (Nizhny Novgorod). A new generation of linear induction accelerators (LIA) with a kiloampere current level and an energy of up to 10 MeV, which are capable of forming beams with a high current density and low normalized emittance, is developed at the BINP SB RAS and can be used as a source of an electron beam for such a FEL generator. The objective of our research is to develop and create a FEL generator producing pulses of coherent radiation in the THz range with a sub-GW power level and a record-breaking energy content in a pulse of about 10–100 J. Combination of a high current density of the beam and its long pulse duration (about 100 ns) together with a small spread in the longitudinal electron velocities of the beam opens up the possibility of implementing the FEL scheme in two different types of oversized electrodynamic systems. The first is based on a two-mirror Bragg resonator, in which waves are reflected due to the coupling of the traveling and quasi-critical waves on a corrugated surface. In the second type of the electrodynamic system, a quasi-optical resonator based on the Talbot effect is used. According to the theory, the simulation results, and the data of the cold experiments, both schemes make it possible to ensure a stable regime of narrow-band generation of THz radiation under the conditions of significant cavity oversize, i.e., the ratio of the cavity diameter and the radiation wavelength (ϕ/⋋ > 30–40). The main structural elements of the developed section of the FEL generator and their design parameters are discussed within the framework of this article. When developing the magnetic system of this section, we calculated the time dependence of the spatial configurations of pulsed magnetic field in a helix undulator with a period of d = 10 cm and a length of 2 m, as well as in the solenoid of a quasi-homogeneous magnetic field of the same length intended for compression of the beam cross section before its input in the vacuum channel of the FEL section and for consequent transport of the beam inside it. The presented results of modeling and testing of the manufactured elements for the FEL section will become the basis for the design of a high power FEL generator operated in the frequency range from 0.3 to 1.2 THz. [ABSTRACT FROM AUTHOR]

The high-power long-pulse submillimeter-wave free-electron laser (FEL) is developed jointly by the Institute of Nuclear Physics and the Institute of Applied Physics on the basis of the linear induction accelerator complex, which forms electron beams with a particle energy of 5–20 MeV, a current of 2 kA, and a duration of 200 ns. The studies are aimed at achieving power levels of 0.1–1.0 GW and an energy of 10–100 J in pulses of radiation in the indicated range. We present the results of electron-optical experiments, in which electron beams with parameters acceptable for their efficient application in FELs were formed. Helical pulse undulators have been developed for the build-up of operating transverse oscillations of electrons. The possibility to use modified Bragg cavities, which are based on the coupling of propagating and quasi-critical waves and capable of ensuring stable narrow-band generation at significantly large (as compared with the wavelength) transverse dimensions of the interaction space, is analyzed as the key component of the electrodynamic system of a generator. The results of the simulating and cold testing of this type of cavities for operation in the submillimeter-wave range with a channel diameter exceeding 20 wavelengths are presented. [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]

Experimental and theoretical investigations on simultaneous generation and transport of two microsecond E-beams with an electron energy of 0.8 MeV and beam currents of up to 3 kA at a high current density of ∼1-1.5 kA/cm2 are described in this paper. Peculiarities of using such beams for' a generation of millimeter and submillimeter radiation in a free-electron maser are discussed. Parameters of the beams that are suitable for these applications are experimentally demonstrated. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]