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To cover the so-called terahertz gap in powerful sources of coherent electromagnetic waves, a second-harmonic gyrotron operating in a 10 T liquid helium-free superconducting magnet has been designed, manufactured, and tested. With an operating voltage of 15 kV and a beam current of 0.6 A, the gyrotron generated coherent continuous-wave radiation at a frequency of 0.526 Terahertz with an output power of 250 Watts and an efficiency of 2.7%.

The influence of the magnetic-field profile in the gyrotron interaction space on the generation efficiency and frequency was experimentally studied. A profiled magnetic field was created by a system of several low-inductance coils. It is shown that it is possible to increase the generation efficiency by ~1.3 times, which is comparable with the results that are obtained using single-stage recuperation of the residual electron-beam energy. In this case, the proposed method is easier to implement and can be used jointly with the recuperation. It is also demonstrated that it is possible to control the generation frequency within a range of at least twice the width of the radiation spectrum with characteristic times of the order of milliseconds. This allows one to believe that the gyrotron frequency-stabilization circuit can be successfully performed using small changes in the magnetic field.

We analyze the motion of the electron beam in the collector of a gyrotron with the shielded magnetic system. The methods of optimization of the collector are proposed, depending on the limitations imposed on its transverse dimensions. Calculations are performed for collectors of the technological gyrotrons capable of operating in a wide range of magnetic fields (1.0–1.8 T), which allows generating radiation at a frequency of 24 to 48 GHz at the fundamental cyclotron resonance and, correspondingly, up to 100 GHz when exciting oscillations at the second gyrofrequency harmonic.

We study the possibility of using the technology of 3D photopolymer printing with subsequent metallization of working surfaces for developing electrodynamic components of microwave devices. The technology applicability is considered by examples of mockups of a profiled helical waveguide for a microwave undulator of a Compton free-electron laser and a periodic slow-wave system of a backward-wave oscillator with a ribbon electron beam. The results of “cold” measurements of the parameters of these electrodynamic systems are presented and good agreement with the simulation results is obtained, which confirms the prospects of using additive technologies for developing components of the vacuum microwave electronic devices.

A magneto-armored system for a new generation of technological complexes based on CW K-band gyrotrons (28–50 GHz), which operate on the main cyclotron resonance with an output power of 25–30 kW or higher, was developed and experimentally tested. The possibility of creating a homogeneous (better than 0.5%) sector of the magnetic field in the region of the electron–wave interaction with a length of several tens of millimeters has been demonstrated. Optimization of the configuration, shapes, and material that were used for the ferromagnetic screens for solenoids with copper windings and working-hole diameters of several tens of millimeters made it possible to obtain record values of constant magnetic fields with respect to the power consumption. For the operating mode with a uniform section of 65 mm and a magnetic-field induction of 1.03 T, the power consumption of the solenoid was 12.5 kW, which makes it possible to significantly (by ~1.5 times) increase the efficiency of the complex as a whole when using a gyrotron with a full radiation efficiency of nearly 50%, which is significantly superior to the existing analogues.

We present the results of studying experimentally the expansion of laser plasma in a strong external magnetic field (with a magnetic flux density of 13.5 T) at various sizes of the region of plasma formation on the surface of a solid-state target. It is shown that when the size of the plasma formation region is smaller than the classical plasma braking radius, a nearly identical topology of plasma flows is observed, which is characterized by the formation of a thin plasma sheet directed along the external magnetic field. If the width of the plasma formation region is comparable with the classical plasma braking radius, an additional plasma sheet starts to be formed.

International audience; An up to 15 T pulsed magnetic field generator in a volume of a few cubic centimeters has been developed for experiments with magnetized laser plasma. The magnetic field is created by a pair of coils placed in a sealed reservoir with liquid nitrogen, installed in a vacuum chamber with a laser target. The bearing body provides the mechanical strength of the system both in the case of co-directional and oppositely connected coils. The configuration of the housing allows laser radiation to be introduced into the working area between the coils in a wide range of directions and focusing angles, place targets away from the symmetry axis of the magnetic system, and irradiate several targets simultaneously.

The main trends in gyrotron development are escalation of the radiated power and increasing the frequency of coherent radiation. For both trends, it is beneficial to develop gyrotrons with wide emitters because this allows one to use cryomagnets with smaller inner bore sizes. For analyzing and optimizing the operation of gyrotrons with wide emitters, it is proposed to represent such emitters as a superposition of thin rings and analyze the properties of electron beams emitted by each of these rings. The present paper consists of two parts. In the first part, the peak values of the orbital velocities and their spread are determined in all fractions of an electron beam in a gyrotron with the standard and widened emitters; also, the effect of profiling the anode on characteristics of these electron beam fractions is considered. In the second part, the interaction efficiency of electron beams produced by thin emitter rings is described and the relationship between these efficiencies and orbital-to-axial velocity ratios in these beams is discussed.

We analyze the possibility to create new generation continuous-wave gyrotrons operating in the Kband (at frequencies of 28-30 GHz) at the fundamental cyclotron resonance with an output power of 25–30 kW and more. For a gyrotron with a magnetically shielded solenoid, the electron-optical and electrodynamic systems have been optimized, which ensures generation with high efficiency (about 50%) at acceptable thermal loads on the key device elements. The proposed design of the gyrotron ensures long-term continuous-wave operation of the technological complex with a total system efficiency of about 35%, which is significantly superior to the existing analogs.

Spatially-extended high-power Cherenkov masers intended to be used in powering systems of Compton-type FELs are under development in IAP RAS (Nizhny Novgorod) at the "Sinus-6" accelerator 0.5 MeV / 5 kA / 20 ns. Electrodynamic systems of such oscillators are based on 2D periodical slow-wave structures realizing 2D distributed feedback mechanism, allowing for provision of stable narrow-band oscillation regime under conditions of substantial oversize. In the proof-of-principal experiments at the Ka-band under the transverse oversize (perimeter) of ~ 16 wavelength, narrow-band generation was obtained with ~ 0.2 - 0.3 MW output power level. Project to advance this oscillator into W-band with the oversized parameter of ~ 30 is in progress.

In the gyrotron operating at the second harmonic of the gyrofrequency at a frequency of 526 GHz, radiation power of 250 W was obtained in the continuous wave generation mode. The indicated value is several times higher than all known world analogs. Experimental results open new perspectives of gyrotron application for spectroscopy, where both high frequency and high-power are needed.

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).

Conception of Compton FEL-scattrons is under development at IAP RAS and aimed at reducing energy of a driving relativistic electron beam, increasing efficiency of the electron-wave interaction and achieving relative compactness of the generator. The basis of this concept is RF-undulator of a new type - the so-called "flying" undulator, which represent short microwave pulse that synchronously propagates with driving electron bunch and provides pumping of its oscillations. Profiling parameters of "flying" undulator allows realization of the non-resonant trapping regime resulting in further enhancement of the FEL efficiency. Present paper is devoted to the design parameters, results of simulations and "cold" tests of regular and profiled "flying" undulators realized in the Ka and W bands.

Using analytical approaches and CST Microwave Studio 3D simulation, the electrodynamic characteristics of 2D Bragg resonators based on planar dielectric waveguides with doubly periodic corrugation are theoretically analyzed. Such resonators are of great interest to obtain directed narrow-band radiation in heterolasers with large active regions. Model electrodynamic experiments on the “cold” testing of such structures in the millimeter range are performed. Good agreement between the experimental and simulation results is shown, including the existence of the highest-Q mode within the Bragg reflection band in the absence of defects in the periodicity.

We show experimentally the possibility to expand significantly the band of smooth tuning of the generation frequency in gyrotrons using cavities that have shorter lengths. Due to a decrease in the sensitivity of the electron-wave interaction process to the spread in the electron beam velocities, one can increase the power of generation at higher longitudinal modes up to a level comparable with the radiation power in the case of excitation of a mode with one longitudinal variation. In this case, overlapping of generation bands at the neighboring longitudinal modes is achieved by increasing the current of the electron beam. In the experiment performed in a gyrotron having an operating frequency of about 12 GHz, we demonstrated a frequency tuning band which exceeded 4% at the kilowatt level of the output radiation power in its greater part. The obtained results open up the possibility of developing tunable high-frequency moderate-power gyrotrons.

We describe a series of gyrotron facilities developed at the Institute of Applied Physics of the Russian Academy of Sciences for studying physical processes during interaction of millimeter-wave electromagnetic radiation and matter. This paper presents the universal principle of designing such systems on the basis of a facility having an output radiation power of 5 kW at a frequency of 24 GHz. The main components of the facility and their technical parameters are described. Design of high-efficiency radiation sources and radiation transmission lines for various research applications is a sophisticated radiophysical problem, and the need for long-term stable operation with automatic adjustment of the parameters of the generation regime requires unique engineering solutions. Application of multimode electrodynamic devices in the radiation transmission line allows one to treat materials with significantly different dielectric properties, in particular, heat them up to temperatures of about (and exceeding) 2000°C. The vacuum-tight working chamber of the facility is a high-Q untuned cavity resonator having a volume of about 0.1 m3, in which microwave heating of items with characteristic dimensions of more than 10 cm can be performed. The automatic control system of the facility ensures its reliable and long-term failure-free operation.

Novel schemes of a trapping regime in free-electron devices of different types were studied. Theoretical analysis of various schemes of the so-called multi-stage “non-resonant” trapping regime, which is implemented in a sectioned system of profiled undulators, was performed for the short-wavelength FELs operating up to X-rays. The use of such trapping regimes allows significant enhance in the FEL efficiency compared with the “traditional” schemes based on the regular undulators and reduction of their sensitivity to the quality (energy and velocity spread) of driving electron beams. Such regimes not require phase matching of the pumping wave in different undulator sections, and, thus, can be powered by independent powerful pulsed microwave generators.Operability of the “non-resonant” trapping regime has been demonstrated in the proof-of-principal experiments carried out at the Ka-band in collaboration between IAP RAS (N.Novgorod) and JINR (Dubna). In these experiments, FEM-amplifier was realized with an output power of ~ 25 – 28 MW (gain ~ 40 dB) and an ultra-wide amplification band.

Due to a growing interest for microwave radiation sources with a radiation frequency of several tens of GHz and output power of several tens of kW, we developed a new generation of technological gyrotron setups. The key feature of these systems is a novel magnetic system based on magnetically shielded solenoids (MSS). This magnetic system is several times more energy efficient than the common warm solenoids and it also allows to control the properties of the helical electron beam to make it interact with various cavity modes. The developed MSS provides a high field induction in the interaction space and a small stray field, so the gyrotron can operate in wide range of magnetic fields (from 1 T to 1.8 T in relatively low-consumption regime). It allows a gyrotron complex to operate within a wide frequency range (from 24 to 50 GHz on main cyclotron resonance). A total system efficiency expected to be about 35%, which is significantly higher than the existing analogs.

The main trends in gyrotron development are escalation of the radiated power and increasing the frequency of coherent radiation. For both trends it is beneficial to develop gyrotrons with wide emitters because this allows one to use cryomagnets with smaller inner bore sizes. For analyzing and optimizing the operation of gyrotrons with wide emitters it is proposed to represent such emitters as a superposition of thin rings and analyze the properties of electron beams emitted by each of these rings. The analysis of electron beam properties, for electron optical systems with different emitters is presented. The possibility to reduce velocity spread by anode profiling is discussed. The dynamics of electron beam and interaction efficiency for different emitters are calculated.

3D printing technologies offer significant advantages over conventional manufacturing technologies for objects with complicated shapes. This technology provides the potential to easily manufacture barrier windows with a low reflection in a wide frequency band. Several 3D printing methods were examined for this purpose, and the dielectric properties of the various types of materials used for 3D printing were experimentally studied in the frequency range 26–190 GHz. These measurements show that the styrene-butadiene-styrene and polyamide plastics are suitable for broadband low-reflection windows for low-to-medium-power microwave applications. Two barrier windows with optimized surface shapes were printed and tested. Results demonstrate that the studied technique can fabricate windows with a reflection level below −18 dB in the frequency band up to 160 GHz. Studied windows can be used for spectroscopic tasks and other wideband microwave applications.

It is shown that it is possible to use warm solenoids with copper winding screened with ferromagnetic shields in order to reduce the solenoid supply power by a factor of 1.8–2 while retaining the strength of the magnetic field in the working space for continuous gyrotrons operating in a frequency range of 24–30 GHz. The configuration of the shields as well as the shape and location of the correcting solenoids providing an extended (up to 10 wavelengths long) section of a uniform field in the region of the gyrotron cavity have been determined. It has been shown that introduction of an additional cathode coil into the magnetic system reduces the degree of nonadiabaticity of the magnetic field approximately by an order of magnitude and thereby makes it possible to use the magnetron injection gun for formation of a helical electron beam. Optimization of the configuration of the gun electrodes based on the trajectory analysis makes it possible to obtain an electron beam whose parameters are as good as parameters of the electron beams formed in classical adiabatic electron–optical systems of gyrotrons.

A 527 GHz continuous-wave second harmonic gyrotron is in development at the IAP RAS jointly with GYCOM Ltd. as a microwave source for spectrometry and diagnostics of various media. We present first results of experimental measurements of this gyrotron including output power, power losses in the cavity, wave beam profile, frequency, and spectrum.

The present article demonstrates the recent results of investigations devoted to development of a new generation of a high-current ion sources and its application in a scheme of a neutron generator capable to generate neutron fluxes that surpass the state-of-the-art devices by an order of magnitude. The recent results at GISMO experimental facility on light and heavy ion beams production will be reported. Measurements results of an ion beam current, its emittance and charge state distribution will be presented.

The overview of recent results and trends of the development of K-band (24-30 GHz) technological gyrotrons are given. The high-efficient second harmonic CW gyrotrons with output power up to 15 kW has been developed and tested. The gyrotron efficiency of 60% has been obtained by magnetic field profile optimization and single-stage energy recovery of the spent electron beam. The double-frequency CW 10 kW gyrotron with 2% fast frequency tuning has been realized. It is shown that promising technologies needs power increasing up to several tens kW. Due to the problem of suppression of high harmonics by fundamental one, the project of powerful CW first harmonic gyrotron is proposed. The reduction of total power consumption is realized by the magnetic system shielded by ferromagnetic screens. The report presented the 28 GHz CW technological gyrotron with the output power 25-30 kW and total (including the power consumption of the main coil) efficiency 35%. The characteristic features of the magnet, electron beam optics and electrodynamics system are described.

Results of the experimental investigation of the discharge front propagation in the beams of powerful sub-terahertz radiation are presented. The front propagation velocity is measured in the wide range of the gas (argon, krypton) pressure (0.1 – 1 atm). It is demonstrated that the discharge front runs against the incident sub-terahertz radiation beam into the region with the electric field value well below the threshold. Discharge propagation slowing down in correspondence with the electric field strength decrease. Characteristic propagation velocity value is in the range of $10^{5}$–$10^{6}$ cm/s.

The project of a first harmonic CW gyrotron with the operating frequency of 28 GHz and an output power more than 20 kW, intended for various technological applications is described. Both magnetic system and the system of the beam-wave interaction are optimized in such a manner as to provide very high (close to 50%) electronic efficiency and at the same time small energy consumption of the “hot” coils. For the last purpose the magnetically shielded coils are used. It allowed both to increase the operating magnetic field up to 1.024 T and so to operate on the first cyclotron harmonic and at the same time to provide reasonable (13 kW) power consumption of the main coil. Operation on the first cyclotron harmonic ensures absence of the spurious modes within the cyclotron frequency band and so allows to increase the output power in some times in comparison with existing at the moment technological gyrotrons operating on the second cyclotron harmonic. Results of optimization of the key gyrotron subsystems such as electron gun, cavity and collector allowing to operate in CW regime are presented.

A 250 GHz continuous-wave (CW) gyrotron has been developed at the IAP RAS jointly with GYCOM Ltd., as a prototype of the microwave source for the envisaged prospective nuclear fusion power plants (DEMO). The main applications of such a tube are electron cyclotron resonance heating and electron cyclotron resonance current drive of magnetically confined plasma as well as its diagnostics based on collective Thomson scattering in various reactors for controlled thermonuclear fusion (e.g., tokamaks and stellarators). The results of the preliminary experimental tests in a pulsed mode of operation are presented. The microwave power of up to 330 kW with an efficiency of 30% without collector depression was obtained. At an accelerating voltage of 55 kV and an electron beam current of 12.5 A (which corresponds to the design parameters for CW operation), the measured output power was about 200 kW. The TEM00 mode content evaluated at the tube output is not less than 98.6%.

We demonstrate analytically that for short-cavity gyrotrons posses weaker sensitivity to electron velocity spread that allows us to use such a system for making the frequency tunable range wider through the excitation of a series of axial modes. Based on this approach the proof-of-principle experiment was performed using low-frequency gyrotron, and the 4 % tuning band was demonstrated at the 1-kW power level which is in a good agreement with simulations. The method of overlapping the power dips between the excitation zones of neighboring axial modes due to the shift of the tube position inside the solenoid is proposed and tested. The obtained results open the possibility of developing tunable high-frequency gyrotrons of medium power for a number of promising applications.

The report presents review of technological gyrotron systems developed and made by Institute of Applied Physics of the Russian Academy of Sciences (lAP RAS) jointly with GYCOM Ltd. (Nizhny Novgorod, Russia). Typical microwave power of these systems is several tens kW with efficiency up to 0.4 (0.65 in the case of depressed collector) at the frequency range 24–45 GHz. The output power of gyrotrons can be smoothly regulated from about 5% to 100% of the full power. All CW gyrotrons can operate also in pulse regime with the same or slightly higher output power.

In the last few years, gyrotron and other gyrodevises complexes based on “warm” magnets that do not require cooling to liquid-helium temperatures have been successfully created and investigated at many institutions [1]–[4]. A further step to improve the operation of such complexes is the development and design of gyrotrons operating on gyrofrequency harmonics. However, for such systems, the problem of selection of the working type of oscillations is exacerbated, especially when using oversized resonators whose ohmic losses are much lower than in the case of modes with relatively small indices. At present, a number of methods are proposed that provide high selection, both with the help of electron-optical systems and with the use of noncanonical electrodynamic systems. In this work, investigation the possibilities of using non-traditional electrodynamic systems with annular diaphragm to increase the selectivity of the working type of oscillations was done. Based on the results of the work, it is planned to develop the electrodynamic system for a W-band gyrotron operating on the third harmonic (in this case solenoid with water or oil cooling, which costs much less than a cryomagnet can be used).

An optoelectronic gyrotron system that makes it possible to efficiently heat a cathode by electrons reflected from a magnetic mirror has been developed. The parameters of the primary electron beam are retained at a level that is acceptable for efficient microwave power generation. The results of a trajectory analysis of electrons and the cathode temperature conditions are reported. The corresponding decrease in power and temperature of the cathode filament suggests a significant (severalfold) increase in the filament lifetime, which is one of the main parameters determining the device lifetime.

The concept of a CW THz range gyrotrons operated at the second or third cyclotron harmonic is presented. To suppress the mode competition effects a double beams scheme is applied. The helical electron beams are formed in a double-beam triode magnetron-injection gun (MIG), where both emitters of the electron beams are located on a common cathode of the conventional MIG. Results of numerical simulation of electron beams formation and mode excitation have been presented. The concept makes possible high power (several hundred Watts) single-mode gyrotron operation at 0.7–1.0 THz frequency range at high harmonics.

A 263 GHz continuous-wave (CW) gyrotron was developed at the IAP RAS for future applications as a microwave power source in Dynamic Nuclear Polarization / Nuclear magnetic resonance (DNP/NMR) spectrometers. A new experimental facility with a computerized control was built to test this and subsequent gyrotrons. We obtained the maximum CW power up to 1 kW in the 15 kV/0.4 A operation regime. The power about 10 W, which is sufficient for many spectroscopic applications, was realized in the low current 14 kV/0.02 A regime. The possibility of frequency tuning by variation of the coolant temperature about 4 MHz/1 °C was demonstrated. The spectral width of the gyrotron radiation was about 10(-6).