The work reports the achievement of an energy content of 10 J per microsecond pulse in a directed flux of electromagnetic radiation in the frequency range of ~ 0.2–0.3 THz. The flux is generated by a fundamentally new method, which is realized through the pumping of upper-hybrid plasma oscillations in a magnetized plasma column with a relativistic electron beam (REB) and their subsequent transformation into a flux of electromagnetic radiation. In the described experiments at the GOL-PET facility, this method to generate THz radiation is implemented in the following way a beam of electrons with energy E ~ 0.5 MeV with a current density of (1–2) kA/cm2 is passing through a magnetized (4 T) plasma column with a density of 1014–1015 cm–3. By comparing the experimentally measured spectral composition of the radiation flux with the calculated spectrum, it is proved that this process is realized through resonant pumping of the branch of upper-hybrid plasma waves by such beam. A coordinated increase in plasma density and beam current density opens up the prospect of advancement in the generation of multi-megawatt radiation fluxes in the region of one terahertz.

One of the possible applications of high-current relativistic electron beams (REBs) is to generate electromagnetic waves at plasma frequencies due to the propagation of a beam through a magnetized plasma column. Research work in this direction, aimed at creating terahertz radiation sources at the BINP, is underway using the GOL–PET facility. We study the relaxation of a REB beam with a current density of (1–2) kA/cm2 in a magnetized plasma column with a density of 5 · 1014 cm–3. The purpose of these studies is to create a pulse radiation source with a power of tens of megawatts in the frequency range 0.1–1 THz. To date, a radiation flux with a power level of 10 MW and a maximum power spectral density in the frequency range 150–200 GHz has been achieved in the experiments. Further progress in these studies was related to the experimental establishment of the dependence of the power and spectral composition of the radiation flux on the parameters of the injected beam, in particular, its current density. The current density of the injected beam was varied due to the different compression of the beam cross section by the magnetic field. The results of measuring the characteristics of the radiation flux are presented in correlation with the results of measurements of the beam current density and plasma density.

This paper reports on the generation of a directed flux of electromagnetic radiation with an energy content of 10 J in the frequency range of 0.2–0.3 THz at a microsecond pulse duration in a beam–plasma system. The flux is generated when a relativistic electron beam (REB) pumps electron plasma waves in a magnetized plasma column. In the described experiments, this fundamentally new approach to generate terahertz radiation was carried out at the GOL-PET facility in the conditions of varying the beam current density and the plasma density in the appropriate ranges of 1–2 kA/cm2 and 1014–1015 cm–3. From the comparison of the flux energy spectrum measured experimentally in the frequency range 0.15–0.45 THz with the calculated one obtained using the previously proposed model of radiation generation in a beam–plasma system it was shown that this process occurs through resonant pumping by REB of precisely the branch of upper-hybrid plasma waves. Mastering this new method to generate terahertz radiation opens the prospect of its use to obtain multi-megawatt radiation fluxes in the frequency range up to 1 terahertz and higher. For such a development approach the most promising beam for pumping plasma oscillations seems to be a kiloampere REB generated in a linear induction accelerator.

One of the possible approaches to generate high power (up to 100 MW) submm radiation flux is to apply mechanism of the plasma wave conversion into electromagnetic (EM) ones at intense beam-plasma interaction. Given report is devoted to study of such transformation process in case of generation radiation fluxes with multimegawatt power in the frequency range 0.1-0.6 THz. Experimental study on this approach is carried out at the GOL-PET facility when a relativistic electron beam (REB) with parameters 0.6MeV/10kA/6us is propagated in a plasma column. The plasma column with the density $\mathrm{n}\sim 10^{15}\text{cm}^{-3}$ is created and confined in a multi-mirror magnetic trap (B = 4 T). In recent experiments, the energy content and power of a submm radiation flux of microsecond pulse duration achieved 7 J and 10 MW, respectively. Results of the spectral composition measurements of the flux generated by this radiation source will be presented and discussed in our report.

BINP SB RAS together with RFNC-VNIITF carry out a research in the field of creating new sources of electromagnetic radiation in the THz range. Within the framework of this article, a project of the THz beam-plasma generator based on an electron beam generated by a linear induction accelerator is presented. The article provides generator scheme and describes the main elements of the electron beam formation system. In addition, the results of modeling the beam transport and its cross-section compression are presented. These calculations were performed under the current up to 1 kA and energy up to 1 MeV for the subsequent injection of the beam into the plasma section with plasma density up to 1015 –1016 cm–3. The article also contains the analysis of previous experimental studies results which are connected with the beam-plasma interaction for various beam and plasma parameters. Based on this analysis, a requirement for the ratio of the beam and plasma electron densities was formulated. This requirement should be satisfied for creation of the beam-plasma generator of EM radiation for the range of 0.1–1 THz with a pulse power of several MW.

Research on one of the methods for generation of high-power radiation flows in the subterahertz/terahertz frequency range using the GOL–PET facility is underway at the G. I.Budker Institute of Nuclear Physics of the Siberian Branch of the Russian Academy of Sciences (BINP SB RAS). This method consists in using the mechanisms of conversion of the plasma waves excited by a relativistic electron beam (REB) into an electromagnetic radiation flow. In the course of research, a number of mechanisms by which plasma oscillations are converted into radiation were revealed, and a power level of 10 MW was experimentally achieved in a directed submillimeter radiation flow with a duration of a few microseconds. This paper is devoted to the study of radiation generation as a function of the degree of compression of the REB cross section. The compression degree was varied in the experiments by varying the magnetic field distribution along the axis of the facility. The magnetic field was varied uniformly along the entire facility axis. In addition, the configuration with an increased ratio of fields in the accelerator diode and in the plasma section of the facility was considered. As a result of the experiments, it was shown that a decrease in the strength of the guiding magnetic field by a factor of 1.6 does not lead to a change in the radiation generation mechanism.

This article presents the results of experimental studies on radiation flux generation in the submillimeter wavelength range due to a strong beam–plasma interaction. A relativistic electron beam (REB) with parameters 0.5 MeV/12 kA/ $6 \mu \text{s}$ pumps plasma waves in a plasma column with a length of ~2 m at a plasma density of $\sim 10^{15} {\mathrm{ cm}}^{-3}$ in a magnetic field of ~4 T. In the presence of density gradients in the plasma column, direct measurements of the energy content of the radiation flux 18 cm in diameter leaving the plasma column into the atmosphere showed that its value reaches 5–7 J. The pulse duration of the flux at half of its amplitude was about $0.5 \mu \text{s}$ , and therefore, the pulse power was at the level of ~10 MW. In this series of experiments, the spectral composition of the radiation flux in the frequency range of 0.1–0.5 THz and the energy distribution function of the beam electrons passed through the plasma have been measured.

A powerful spatially extended planar Cherenkov maser project operating in ${W}$ -band is under development in collaboration between Budker Institute of Nuclear Physics of the Russian Academy of Sciences (BINP RAS) (Novosibirsk) and Institute of Applied Physics of the Russian Academy of Sciences (IAP RAS) (Nizhny Novgorod). The ELMI accelerator (1 MeV/5–7 kA/ $3~\mu \text{s}$ ) forms a sheet electron beam with the transverse size (width) of up to 18 cm, which serves to drive the oscillator. The electrodynamic system of this Cherenkov maser is based on doubly periodical structure, which combines the properties of a slow wave system that realizes conditions for an effective Cherenkov interaction with a high-current rectilinear sheet electron beam and a high- ${Q}$ resonator that implements the mechanism of two-dimensional distributed feedback and provides selective excitation of the operating mode in the strongly oversized interaction space. In this article, the crucial elements and design parameters of the oscillator are discussed, and the results of simulations are presented to demonstrate the possibility of achieving a stable narrow-band generation regime for a transverse system size reaching about 50 wavelengths and a gigawatt output power level. To provide a single-directed output of radiation, a planar Bragg reflector is elaborated for installation at the cathode side of the interaction space. A mode converter formed by a slowly tapered waveguide section for transformation of the radiated wavebeam into a Gaussian-type wavebeam was designed to be installed at the oscillator output. In the electron-optical experiments, the formation of a large-width sheet electron beam with parameters acceptable to drive designed ${W}$ -band Cherenkov maser is demonstrated.

In Budker Institute of Nuclear Physics (BINP), experimental and theoretical studies of generation of submillimeter electromagnetic radiation with characteristic frequencies 0.1–0.5 THz under conditions of intense plasma–beam interaction are carried out. Generation of radiation occurs in the specialized G-OL‑PET facility during collective relaxation of pulsed relativistic electron beam (REB) with the following parameters: electron energy 0.6 MeV, beam current 15 kA, and pulse duration 5 µs in a plasma column with plasma density (0.5–1) × 1015 cm–3, under conditions of its confinement in a vacuum chamber with a corrugated magnetic field (the ratio of the maximum and minimum values of magnetic induction in the corrugations is Bmax/Bmin = 4.5/3.2 T). An important feature of the carried out studies is the presence of specifically created inhomogeneities both in its cross section and along its length. In experiments carried out earlier under the same conditions, a high level of power (about 10 MW) was reached in the flux of submillimeter radiation going out into the atmosphere. Radiation energy of 7 J was reached at pulse duration of 0.5 µs, which is limited by the high-frequency breakdown at the vacuum side of the output window. This article is devoted to the problem of increasing the duration of the pulse of generated submillimeter radiation of such a high power. This is attained by suppressing high-frequency (HF) breakdown near the surface of the window through which the radiation flux is going out into the atmosphere and by increasing the duration of megavolt radiation pulse applied to the accelerator diode in which REB is generated.

Experiments at the multimirror open trap as plasma emitter of THz radiation (GOL-PET) facility have shown that the injection of a relativistic electron beam (REB) with a kiloampere current into a magnetized plasma column is accompanied by the generation of a THz radiation flux. Our experiments have revealed three-generation mechanisms. The first mechanism is the scattering of upper hybrid waves on plasma density gradients, the second is the direct pumping of electromagnetic waves by an electron beam, and the last one is the merging of two upper hybrid waves into an electromagnetic one. Both theoretical and experimental studies have shown the possibility to achieve a megawatt power of the flux at the upper hybrid frequency in the presence of significant plasma density gradients. Resent experiments have shown that a strong density decrease at the end of the plasma column is necessary for high-efficiency emission of the generated flux along its axis. This THz radiation flux is emitted from the column into a vacuum chamber and then through an output window to the atmosphere. The measurement results of the spatial and angular properties of such a megawatt flux are described in our article.

Experiments on pulse generation of submm electromagnetic waves are carried out at GOL-PET facility in Budker Institute of Nuclear Physics. The generation is realized in the process of relativistic electron beam interaction with a plasma column. Recently, the experiments have demonstrated the multi-megawatt power generation of 1 mm wavelength radiation at pulse duration up to 3 µs [1] . Evolution of the energy distribution function of the beam electrons as a result of the beam-plasma interaction is very important characteristic of this process. Firstly, measuring this function permits to evaluate the efficiency of the beam energy deposition in plasma. Secondly, analyzing the evolution of the function in correlation with the time behavior of the power and the spectral composition of the radiation outgoing from the plasma column during the E-beam pulse at various experimental conditions allows one to understand the physics mechanism of the wave generation.

Spectral measurement of a flux of electromagnetic waves in a frequency range from 0.1 to 0.6 THz is a complex and demanded task. A special device called a polychromator was developed in order to provide measurements of spectral power density distribution in the given spectral range with high time resolution. This paper presents the description of the units of the polychromator and the results of conducting the calibration procedure for its frequency selective channels.

Submillimeter wave generation due to the intense interaction of a relativistic electron beam (REB) with magnetized plasma is studied at GOL-PET facility. In a plasma column with density ~ 1015 cm-3 the e-beam (0.5 MeV/15kA/6μs) generates a well-directed 1 mm wavelength flux of ~ 10 MW power. Paper presents measurement results of the radiation spectral composition in 0.1 – 0.6 THz band and the energy distribution function of the beam electrons at various plasma density distributions. The obtained results are discussed in the consideration of theoretical models.

Studies on the generation of electromagnetic radiation during beam-plasma interaction are of considerable scientific interest from the standpoint of analyzing the results of astrophysical observations [1] . Such studies look no less important from the standpoint of solving a wide range of practical problems using high-powerful submillimeter radiation. Experimental and theoretical studies on multimegawatt power submillimeter wave generation due to intense interaction of a relativistic electron beam (REB) with a magnetized plasma column are carried out in BINP SB RAS [2] . In the experiments performed at the GOL-PET facility, the beam with parameters 0.5 MeV/15 kA/6 μs was propagated in a magnetized (induction up to 4.7 T) plasma column with the diameter 6 cm, length 2 m and density (1÷2)x10 15 cm -3 . A radiation flux generated due to beam-plasma interaction was extracted in atmosphere through an output fluoroplastic window of 18 cm diameter. We measured the power and the spectral power density of this flux in the frequency band 0.1÷0.8 THz for various plasma density distributions. The radiation angular distribution of the output flux propagated in the hall atmosphere was also measured. The experiments have shown that the maximal spectral power density of the flux power is located in the frequency range 0.2÷0.35 THz that is the area of the upper-hybrid frequency of plasma oscillations. The value of the spectral power density in this frequency interval depends on the plasma density distribution over the plasma column diameter and on the law of density decrease along the axis at the beam output from the column. The experimental results are discussed in comparison with analytical and computer modeling.

Project of powerful submillimeter wave generator based on intense interaction of a relativistic electron beam with a magnetized plasma is developed at BINP RAS in collaboration with NSU. In presented experiments at the GOL-PET facility, the beam with parameters 0.8 MeV/15 kA/6 μs is injected into a magnetized (induction up to 4.7 T) plasma column with diameter 6 cm, length 2.5 m and the density 8x1014 ÷2 x1015 cm−3 [1]. Experiments showed that at this plasma density, the emission from the beam-plasma system is concentrated in a flux directed along the axis of the column. We measured the power and the spectral composition of this flux in the frequency band 0.1÷0.8 THz. Our studies were focused on the role of strong plasma density gradients in generation of the flux.