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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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]

We study the possibility of using Bragg resonators that implement a three-dimensional distributed feedback mechanism ensuring synchronization of radiation and mode selection under conditions of substantial oversize in three spatial coordinates. To describe the electron–wave interaction in devices of this type, a three-dimensional spatiotemporal averaged model was developed within the framework of the coupled wave approach. On this basis, a planar free-electron maser (FEM) driven by a full-scale electron beam of a sheet configuration with a particle energy of 1 MeV, a current of up to 140 kA, and a cross section of about 1 × 140 cm, which is formed using a high-current accelerating complex U-2 (BINP RAS), was simulated. The possibility of achieving a stable narrow-band oscillation regime in FEMs with a multi-gigawatt power level in the W band with transverse sizes of the proposed resonators from tens to hundreds of radiation wavelengths is demonstrated. [ABSTRACT FROM AUTHOR]

We study the possibility to advance relativistic planar Čerenkov surface-wave oscillators, which are driven by intense sheet electron beams, to the subterahertz frequency range within the framework of the performed simulation. It is shown that the use of two-dimensional periodic slow-wave structures providing the two-dimensional distributed feedback allows one to ensure high coherence of radiation at the subgigawatt power level in oscillators of this kind for transverse sizes equal to hundreds of wavelengths. The design parameters and structural elements are discussed for implementing such high-power oscillators based on the ELMI accelerator complex within the framework of the joint BINP/IAP experiments. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [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]

The project of a high-power, spatially developed, planar surface-wave W-band generator with two-dimensional distributed feedback is developed jointly by the Institute of Applied Physics of the Russian Academy of Sciences and the Institute of Nuclear Physics of the Russian Academy of Sciences on the basis of the ELMI accelerator (an electron beam with a particle energy of 1 MeV, a current of 2–5 kA, and a pulse duration of 5 μs). The electrodynamic system of the generator is based on the use of a two-dimensional Bragg structure that combines the properties of a slow-wave system, which ensures efficient ˇ Cerenkov interaction with a rectilinear ribbon electron beam, and a high-Q cavity, which ensures selective excitation of the operating mode in the conditions of a significant value of the oversize parameter. The paper discusses design elements and parameters of the generator and presents the modeling results that demonstrate the possibility to achieve a stable narrow-band generation regime with a gigawatt level of the output radiation power. To ensure a mainly one-directional output of the radiation, a planar Bragg reflector was designed, which is installed in the channel for transportation of the relativistic ribbon electron beam on the cathode side of the interaction space. [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 ways to improve the efficiency of electromagnetic waves generation in laboratory experiments with high-current relativistic electron beams injected into a magnetized plasma are discussed. It is known that such a beam can lose, in a plasma, a significant part of its energy by exciting a high level of turbulence and heating plasma electrons. Beam-excited plasma oscillations may simultaneously participate in nonlinear processes resulting in a fundamental and second harmonic emissions. It is obvious, however, that in the developed plasma turbulence the role of these emissions in the total energy balance is always negligible. In this paper, we investigate whether electromagnetic radiation generated in the beam-plasma system can be sufficiently enhanced by the direct linear conversion of resonant beam-driven modes into electromagnetic ones on preformed regular inhomogeneities of plasma density. Due to the high power of relativistic electron beams, the mechanism discussed may become the basis for the generator of powerful sub-terahertz radiation. [ABSTRACT FROM AUTHOR]

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]

In the work presented here dynamics of spiky microwave emission of a beam-heated plasma near the double plasma frequency in ∼100 GHz band was studied. The plasma is heated by 80 keV, ∼2 MW, sub-ms electron beam that is injected into the multiple-mirror trap GOL-3. The beam-heated plasma diameter is of the order of the emitted wavelength. Modulation of individual emission spikes in the microwave radiation is found. The radiation dynamics observed can be attributed to a small number of compact emitting zones that are periodically distorted. [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]

n approach was developed towards design of medium-temperature solid-oxide fuel cells based on a deformation strengthened Ni-Al alloy. Methods of sintering were described that allowed obtaining layers of complex oxides with ionic and mixed conductivity and with regulated porosity in the range of 40-1%. Power density of a fuel cell on a metallic support reaches 500 mW/cm already at 700°C when humid H was used as fuel and air was used as an oxidant. Analysis of fuel cell cross-section after tests showed absence of fractures, flaking, and new phases with low conductivity, which proves good compatibility of all materials used in fuel cell design. [ABSTRACT FROM AUTHOR]

A ribbon diode of a U-2 accelerator (800 kV, ∼30 kA) intended for generating a high-intensity electron beam for heating plasma in a GOL-3 multimirror magnetic trap (Budker Institute of Nuclear Physics, Siberian Branch, Russian Academy of Sciences) is described. The parameters of the beam characterized by a high brightness (∼7 kA/(cm2 sr)) in a magnetic field of ∼5 T resulting from a numerical simulation of the beam formation process are presented. The results of simulation of the beam transport and transformation of the profile of its cross section during movement of electrons in a curvilinear guiding magnetic field are presented. The calculated cross section is compared to the beam imprint on a target. [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]

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 "cold" and "hot" 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 Kaand W-bands. [ABSTRACT FROM AUTHOR]

The geometry of the ribbon diode of the U-2 accelerator is optimized to increase both the current density and the total current of the relativistic electron beam for its subsequent injection into the plasma of a multimirror GOL-3 trap. Beam simulation in the diode was performed using the POISSON-2 applied software modified on the basis of the results obtained using the theory of a planar diode in an inclined magnetic field. As a result of the optimization, the diode geometry and the magnetic field configuration were found that should provide a factor of 1.5–2 increase in the current density in experiments with a small angular divergence of electron velocities. [ABSTRACT FROM AUTHOR]

Results are presented from experimental studies of ion heating in the GOL-3 device. The experiments were carried out in a multimirror configuration with a local magnetic well. It was found that, during the injection of a relativistic electron beam, a decrease in the local density of the beam in a magnetic well, which is proportional to the decrease in the strength of the longitudinal magnetic field, results in the formation of a short plasma region with a low electron temperature. The measured longitudinal gradient of the plasma pressure corresponds to an electron temperature gradient of ∼2–3 keV/m. Axially nonuniform heating of the plasma electrons gives rise to the macroscopic motion of the plasma along the magnetic field in each cell of the multimirror confinement system. The mixing of the counterpropagating plasma flows inside each cell leads to fast ion heating. Under the given experimental conditions, the efficiency of this heating mechanism is higher than that due to binary electron–ion collisions. The collision and mixing of the counterpropagating plasma flows is accompanied by a neutron and γ-ray burst. The measured ratio of the plasma pressure to the vacuum magnetic field pressure in these experiments reaches 0.2. © 2005 Pleiades Publishing, Inc. [ABSTRACT FROM AUTHOR]

Results are presented from experiments on the injection of solid pellets into a plasma heated by an electron beam in the GOL-3 device. For this purpose, two pellet injectors were installed in the device. The target plasma with a density of ∼10[sup 15]cm[sup –3] was produced in a solenoid with a field of 4.8 T and was heated by a high-power electron beam with an electron energy of ∼1 MeV, a duration of ∼7μs, and a total energy of 120–150 kJ. Before heating, the pellet was injected into the center of the plasma column transversely to the magnetic field. The injection point was located at a distance of 6.5 or 2 m from the input magnetic mirror. Polyethylene pellets with a mass of 0.1–1 mg and lithium-deuteride pellets with a mass of 0.02–0.5 mg were used. A few microseconds after the electron beam starts to be injected into the plasma, a dense plasma bunch is formed. In the initial stage of expansion, the plasma bunch remains spherically symmetric. The plasma at the periphery of the bunch is then heated and becomes magnetized. Next, the dense plasma expands along the magnetic field with a velocity on the order of 300 km/s. A comparison of the measured parameters with calculations by a hydrodynamic model shows that, in order to provide such a high expansion velocity, the total energy density deposited in the pellet must be ∼1kJ/cm[sup 2]. This value substantially exceeds the energy density yielded by the target plasma; i.e., the energy is concentrated across the magnetic field onto a dense plasma bunch produced from the evaporated particle. © 2004 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

The experimental results on a multiple-mirror trap GOL-3 with a short section of reduced magnetic field ("magnetic pit") are presented. The reduced specific energy release from a relativistic electron beam in the pit brings about a region with a temperature several times lower than in the surrounding plasma. The existence of the low-temperature region directly demonstrates that the longitudinal electron heat conductivity is suppressed in the collective electron-beam interaction with plasma. [ABSTRACT FROM AUTHOR]

We suggest to use a two-dimensional distributed feedback for synchronizing the radiation of a multibeam generator representing a system of planar free-electron maser (FEM) units, each FEM being power supplied with a ribbon-shaped relativistic electron beam. It is demonstrated that various FEM units can be coupled by transverse electromagnetic energy fluxes arising in two-dimensional Bragg structures. [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]

A theoretical analysis is made of the formation of the transverse field profile in a free electron laser with an oversize planar-geometry Bragg cavity driven by a ribbon electron beam. It is shown that cw spatially coherent lasing may be achieved with an oversize parameter of the order of 50 with the excitation of supermodes comprising a set of "cold" waveguide modes. [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]