The paper presents the results of theoretical and experimental studies of a G-band surface-wave oscillators with one-dimensional and two-dimensional planar slow-wave structures energized by a sheet high-current relativistic electron beam produced by SINUKI accelerator (IAP RAS, Nizhny Novgorod, 1 kA / 650 keV / 17 ns). The advantages of using a two-dimensional structure in an oscillator of this type for providing of a stable single-mode radiation regime with both significant oversize factor $l_x^2 / \lambda l_z \approx 10$ and instability of the electron beam parameters are demonstrated. Based on the theoretical predictions, stable generation regime was obtained in the experiment at a central frequency of 160 GHz with a pulse duration up to 5 ns. The output power measured by the calorimetric method was not less than 30 MW, which corresponded to an efficiency of 5%.

Experiments on the propagation of powerful high-voltage pulses with a duration on the order of a nanosecond or less in large-scale transmission lines in an atmosphere of various gases make it possible to solve a wide range of fundamental and applied problems, from modeling the passage of electromagnetic pulses (EMPs) through the atmosphere to estimating the electrical strength of elements of pulsed high-voltage gas-filled systems. At the Institute of Applied Physics, Russian Academy of Sciences, experiments with strip lines with a length of up to 5 m were conducted using the Krot and Sprait setups. When EMP propagates with an electric field strength level of up to 100 kV/m in a rarefied gas, effects such as the formation of inhomogeneous and significantly nonstationary glow structures, nonlinear absorption, and dispersive transformation of the shape of high-power pulses caused by gas ionization in the EMP field are studied.

This report presents the results of theoretical and experimental studies of G-band relativistic surface-wave oscillator, the electrodynamic system of which uses a two-dimensional slow-wave structure that provides mode selection along a wide transverse coordinate. Theoretical analysis based on quasi-optical approach is carried out and demonstrate efficient single-mode operation in this scheme. Stable oscillation regime is obtained at G-band with output power level of about 50 MW. The theoretical predictions are in good agreement with full-scale 3D PIC (particle-in-cell) simulations. The main components of the oscillator have been manufactured and testing has begun. The possibility of further advancement of this oscillators class in the terahertz range up to a frequency of 1 THz is being discussed.

Based on three-dimensional particle-in-cell modeling results experimental studies of a W-band surface-wave oscillator powered by a ribbon high-current relativistic electron beam produced by SINUKI accelerator (Federal Research Center Institute of Applied Physics of the Russian Academy of Sciences, Nizhny Novgorod, 1 kA / 650 keV / 17 ns) were carried out. Efficient mode selection along a wide transverse coordinate was demonstrated by using an open planar waveguide with Fresnel parameter of 5. A stable regime of single-mode narrow-band generation is experimentally obtained. The output power measured by the calorimetric method reaches 25 MW. The radiation spectrum is measured based on the heterodyne scheme and the central frequency is 75 GHz.

This paper presents the results of theoretical and experimental studies of a surface-wave oscillator (SWO) of planar geometry excited by a ribbon high-current relativistic electron beam. Within the framework of the quasi-optical approach and direct three-dimensional particle-in-cell modeling, we demonstrate the advantages of open transverse edged configuration against the closed one for effective mode selection at a fairly large oversize factor. In the experiments carried out on the basis of the SINUKI accelerator (Institute of Applied Physics of the Russian Academy of Sciences, Nizhny Novgorod, 1 kA/650 keV/17 ns), we form a magnetically guided ribbon electron beam with a cross-section of 0.3 × 20 mm2, which moves parallel to a slow-wave structure with a period of 1.75 mm. A planar W-band SWO both of open and closed edged configuration has been experimentally tested. In full agreement with modeling, the open configuration exhibits much more stable operation, where at a frequency of 75 GHz, we observe pulse generation with duration of about 7 ns. The output power measured by the calorimetric method reaches 25 MW.

We present the results of simulations and experimental studies of a W-band surface-wave oscillator powered by a ribbon high-current relativistic electron beam produced by SINUKI accelerator (IAP RAS, Nizhny Novgorod, 1 kA / 650 keV / 17 ns). Planar geometry of the interaction space facilitates efficient mode selection over wide transverse coordinate by using open waveguide at fairly large oversize factor (the width of the waveguide is ~8 wavelengths). Stable oscillation regime is obtained at frequency of 75 GHz with a pulse duration of about 5 ns. The output power measured by the calorimetric method reaches 25 MW.

The concept of a large-scale (gigantic) coaxial line, filled with partially ionized plasma and excited by a nanosecond-pulse generator, is proposed and implemented for laboratory simulation of the phenomena arising under propagation of ultrawideband electromagnetic pulses (UWB EMPs) in the Earth’s atmosphere and ionosphere. The line makes it possible to study the waveform transformation effects of UWB EMPs due to dispersion and nonlinear effects in plasma (including in the presence of an external magnetic field). The line characteristics are demonstrated in a vacuum chamber of the large-scale plasma facility KROT.

A large-scale coaxial line filled with the plasma of RF discharge has been developed for laboratory modeling of the effects of the interaction of ultrashort electromagnetic pulses (EMPs) with the atmosphere and the ionosphere in the KROT facility. The oversized coaxial line ensures pulse transmission through an ionized medium in the TEM mode, which corresponds to the polarization of the transverse electromagnetic wave in free space, and in uniform isotropic plasma. The coaxial line has a length of 10 m and a diameter of 140 cm. The processes of propagation of the nanosecond and subnanosecond pulses in this line, in vacuum and with plasma, have been simulated numerically.

The paper presents the results of theoretical and experimental studies of W-band planar surface-wave oscillator driven by sheet high-current relativistic electron beam with experimentally realized parameters on the basis of the “SINUKI” accelerator (IAP RAS, N. Novgorod, 1 kА / 650 keV / 17 ns). In simulation of the oscillator nonlinear dynamics, we used both advanced quasi-optical approach and direct 3D PIC modeling. In the experiment, the microwave generation with a frequency of about 75 GHz was registered by microwave detector; measured pulse duration was about 4 ns. The output power measured by the calorimetric method was about 25 MW which is in a satisfactory agreement with the theoretical predictions. The important specific of the experimental set-up is the use of highly efficient side wall absorber for the implementation of a single-mode single-frequency generation regime.

Relativistic surface-wave oscillator driven by 600 kV, 1 kA sheet electron beam is designed and preliminary experiments were carried out. 2-D simulations based both on quasi-optical theory and on the PIC code predict that the device can operate at the π-mode of the rectangular grating providing the output power of up to 70-100 MW at the frequency of 75 GHz. In the initial experiments, a thin sheet electron beam was injected from a blade-shaped explosive cathode and guided very close to the rectangular corrugation resulting the 80-GHz radiation pulses with 3 ns duration.

A method of moments (MoM) with mode matching is proposed for numerical simulation of waveguide components including cavity resonators. Waveguide Ka-band notch filters with cavity resonators were simulated by the proposed method and manufactured; their simulated and measured characteristics were in a good agreement. Using proposed method the notch filter with pyramidal shape resonator was simulated and designed. Attenuation of 52 dB was achieved at notch frequency of 240 GHz, which makes the filter to be promising in plasma diagnostics systems in a prototype of an industrial fusion reactor of the next generation (DEMO).

A design of a tunable notch filter based on prismatic and pyramidal resonators is proposed. In Ka frequency band, two samples of the filter with a prismatic resonator were manufactured, one of them by mechanical processing, and another using polymer 3D printing technology, followed by surface metallization. A good agreement was shown between simulated and measured characteristics of the fabricated filter samples, including a 20% stopband tuning. The attenuation of the single-cavity Ka band notch filter at the central frequency of stopband was -25 dB. Numerical simulations showed that when the number of resonators is increased to four, the attenuation increased to -60 dB in the absence of parasitic stopbands in the frequency range from 26 to 47 GHz. For a three-cavity filter with a pyramidal cavity in the numerical simulation, attenuation of -92 dB at a frequency of 240 GHz was obtained, which makes this filter attractive for plasma diagnostics systems in a promising prototype of an industrial fusion reactor of the next generation (DEMO).

We analyze theoretically millimeter-wave surface-wave generators based on relativistic ribbon electron beams within the framework of the averaged quasioptical model, as well as using direct numerical modeling by the Particle-in-Cell method. The regime of excitation of π modes and the regime of the backward-wave oscillator based on a surface wave are studied. The possibility to achieve generation of pulsed radiation at a frequency of 75 GHz with a power of up to 100 MW on the basis of the SINUKI acceleration (IAP RAS) is demonstrated. Experiments have been performed to test experimentally the formation of a relativistic ribbon electron beam with a particle energy of 600 keV, a current of up to 1 kA, and a width of up to 2 cm, which will be used to feed the developed generator.

The multi-mode dynamics of pulsed magnetic field gyrotron due to excitation of different modes at the fronts of high voltage pulse and slight magnetic field variation near the flat region of the coil current pulse was investigated. Careful measurements of radiation frequency have been made. Such information looks useful for future development of pulsed magnetic field systems and for analysis of present experimental data.

We show that the radiation patterns of two-dimensional systems in the form of arrays depend on the duration of the emitted pulses. To study such systems, we propose to use the correlation length depending on the pulse duration and the emission direction. The obtained formula for the length of correlation in two-dimensional systems is convenient for description of distortions in the radiation patterns, which are caused by shortening the duration of the emitted pulses. In this paper, we consider the effect of suppression of side lobes in radiation patterns. This is a general effect, since it is related to the delay of signals from different parts of the emitting system. In particular, it is observed in optical systems of the diffraction grating type, in which a decrease in the intensity of diffraction maxima in the case of pulse duration shortening can be used as the basis for a new method of estimation (or measurement) of the duration of optical pulses.

Directivity patters of some antenna systems depend on duration of radiated microwave pulses. It is shown that for large enough twodimensional systems the shortening of pulses leads to suppression of side lobe radiation. We propose to use a novel for two-dimensional antenna systems parameter - a correlation length which is useful for estimation of pulse shortening influence on antenna pattern. Correlation length depends both on the pulse duration and direction of radiation. The effect of side lobes suppression can be used for estimation of pulse duration.

A novel design of notch filter was proposed and tested. Waveguide filter consists of a prismatic cavity coupled to waveguide by a slot and a frequency tuning plunger. We intend to apply this filter for measuring the output radiation of Ka-band gyrotron operating in regime of rogue wave generation and for microwave plasma diagnostics. X-band and Ka-band prototypes of the filter were manufactured and their characteristics were measured. Four-cavity filter was designed and its characteristics were numerically simulated.

A rising interest to the generation of short pulses is observed lately [1-3]. In relativistic backward wave oscillators (BWO) operating in superradiance regime the peak power of output radiation may exceed the power of electron beam [1,2]. Such BWOs are effectively synchronized by the leading edge of accelerating voltage pulse so that the multi-channel systems can be built [1,2]. Radiation patterns of these systems consist of overlapping wave beams. The structure of each wave beam is usually close to the fundamental Gaussian beam with linear polarization. Dimensions of the output window of each channel exceed wavelength λ because of microwave breakdown on smaller apertures. It means that the distance between channels d is greater (typically much greater) than wavelength λ  d (1) and far field radiation pattern has many maximums. In the report we analyze theoretically the peculiarities of such multi-channel systems in case of radiating the phase synchronized short pulses. Superposition of 2D scalar Gaussian beams (see Fig. 1) has been considered:  

The possible formation of an extended low-energy state of electron beam in a coaxial diode with homogeneous cylindrical anode and moderate magnetic field with inhomogeneous profile is demonstrated for the first time. It is established that, depending on the magnetic field configuration, virtual cathodes (VCs) of two types can be formed: (i) a stationary VC with a localized reflection plane and (ii) a moving VC with a two-stream low-energy state of the electron beam.

We have developed equivalent schemes of high-frequency hot-carrier detectors, which allow one to measure the parameters of output radiation of relativistic microwave generators with higher accuracy, as well as to solve the problems of optimization of the design of such detectors and the ways of connecting them into external circuits.