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A microwave system in the form of a quasi-optical (QO) transmission line is proposed as an interaction circuit of an amplifier or an oscillator based on the cyclotron resonance maser (CRM) instability. The circuit consists of periodically spaced along the horizontal ${z}$ -axis focusing mirrors, which transport the Gaussian wave beam along a zigzag path with vertical and inclined segments. A beam of electrons having sufficiently high perpendicular velocities is guided by a static B-field along the ${z}$ -axis so that the e-beam periodically crosses the wave beam. The CRM instability occurs in regions where an electromagnetic wave having a Bz-component, as in a gyrotron, propagates strictly across the e-beam resulting in the interaction with minimal sensitivity to particle velocity spread. Three-dimensional Particle-In-Cell (3-D PIC) simulations demonstrate the attractiveness of this “zigzag” CRM for the implementation of relatively broadband amplifiers or oscillators with unique (more than an octave bandwidth) frequency tunability at wavelengths of 1–2 mm.

We present results of experiments where Ka-band chaotic oscillation regimes with a bandwidth of about 1 GHz have been obtained in a pulsed gyrotron backward wave oscillator (gyro-BWO) using a helically corrugated waveguide. The averaged radiated power reached 40 kW with a pulse duration of about $120~\mu \text{s}$ and a repetition rate of up to 10 Hz. The variation of magnetic field allows for tuning of the central frequency of the radiation in the 32.5–35.5 GHz range.

By means of reflecting the output radiation from a wideband Bragg reflector, a multifrequency radiation source was experimentally implemented based on a continuous wave (CW) helical gyroresonance traveling-wave tube (gyro-TWT) with an operating frequency of about 24 GHz. Oscillation regimes were obtained with distances between the spectral component at 93–122, 193–239, 311–376, 496, 624, and 736 MHz, corresponding to excitation of various combinations of longitudinal modes. Existence of regimes with the second spectral component in intensity at the same level as the intensity at the central frequency was demonstrated, which are of significant interest for multifrequency plasma heating applications.

We study experimentally the electron distribution functions in longitudinal velocities in actual (non-modeled) operating regimes of a two-cavity gyroklystron. It is shown that as the beam’s magnetic compression factor α increases, the distribution function is transformed smoothly from the quasi-Gaussian to a multi-peak one. The main region of this evolution is longitudinal bunching of the beam, which is affected by low-frequency oscillations of the space charge accumulated in the trap between the cathode and the magnetic mirror, with the growing α. At the stage of well-developed instability, the electron beam is a sequence of electron bunches with greatly different longitudinal velocities. It is demonstrated experimentally that the maximum gyroklystron efficiency is achieved at a certain α = αopt, when the electron distribution function in longitudinal velocities is not quasi-Gaussian already, but has only one extremum yet. At a greater magnetic compression factor (α > αopt), the gyroklystron efficiency decreases sharply, which is connected with the modulation of the electron beam density. In the regimes with a multi-peak distribution function, the bunches, which are isolated in the electron beam, excite high-frequency oscillations in the output cavity independently of each other as individual generation zones with respect to the magnetic field that have identical frequencies coinciding with the cavity frequency. The results of calculating the positions of the generation zones with respect to the magnetic field coincide with the experimental data with sufficient accuracy.

Experimental details are discussed and millimeter wave measurements presented of a W-band gyrotron traveling-wave tube (gyro-TWT) that uses a helically corrugated interaction waveguide operating at the second electron cyclotron harmonic. A method for increasing the gain by using an interaction circuit with two sections separated by a long sub-cutoff drift section has been experimentally verified. A 10-mW driving source was sufficient to enable an amplified signal output power of 3 kWs. The continuous-wave (CW) operation with a single-stage depressed collector and deposited beam power of about 20 kW is believed to be the first demonstration of its kind for a W-band gyro-TWT.

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.

The gyrotron traveling-wave tube (gyro-TWT) is a wideband version of gyrotron amplifiers, which produce pulsed or continuous-wave radiation in the millimeter-wavelength band at a power level that exceeds the powers produced by conventional TWTs with slow-wave structures and rectilinear beams by 1–2 orders of magnitude. Since 1996, researchers at the Institute of Applied Physics of the Russian Academy of Sciences (IAP RAS) have been developing the concept of the gyro-TWT based on the use of a waveguide with a helically corrugated surface, which changes the dispersion of one of the eigenmodes in a circular waveguide radically. In this paper, we discuss several problems that arise in implementation of such devices, which many years of experimental studies have revealed, present the parameters of gyro-TWTs developed with allowance for specific applications of their end users, and discuss lines of further perfection of such amplifiers.

We present the results of first experiments on generation of trains of sub-nanosecond Ka-band pulses in the electron oscillator consisting of a helical-waveguide gyro-TWT and a saturable absorber based on cyclotron resonance interaction of radiation with initially rectilinear electron beam. The process of pulse formation is associated with the effect of passive mode locking which is well known in laser physics but has not yet been employed in high-power microwave electronics. In good agreement with the theoretical predictions, periodical trains of coherent (in-phase) 0.5 ns pulses with peak power of 100 kW and repetition period of 2.5 ns were measured.

The paper presents the results of calculations of the output microwave systems of broadband gyro-TWTs of the 3 mm wavelength range united in the common cascade. To achieve a high output power (300 kW), it is proposed to use a cascade of two gyro-TWTs, the first of which has a relatively high gain (40–60 dB), and the second has a high output power with a moderate gain (15–20 dB). The radiation output of the first stage is performed along the axis of the tube through the waveguide depressed collector. The microwave system of the second stage has a transverse radiation input-output through a single barrier window with the signal separation by their polarizations. The transformation of wave fields is carried out by waveguide converters and a system of external quasi-optical mirrors. The calculated efficiency of the output system in the frequency range 90–100 GHz exceeds 95%. The paper compares the methods and approaches used to model the microwave systems.

An overview of the main principles of operation and design for major components of the gyrotron travelling-wave tube (gyro-TWT) based on a helically corrugated waveguide is presented.

Design, simulation results and first experimental results are presented for a W-band gyro-TWT using helically corrugated waveguides in its microwave circuit. The specific of this gyro-TWT is a sectioning (2 helical-waveguide sections separated by a sub-cutoff drift channel) which enables its operation at the second cyclotron harmonic with a high gain (45–50 dB) and high stability to the spurious oscillations. Using an available superconducting magnet with B-field of 1.9 T the tube was designed to be powered by a 40-kV 0.7-A electron beam with a pitch-factor of 1.3. The 3D PIC simulations predict the maximum output power of 3 kW at about 95 GHz and l-kW-level bandwidth of 7.2 GHz when driving by a 100-mW input signal.

In order to realize a high-power (100 s of kilowatts at the W-band) wideband microwave amplifier, we suggest to use a cascade of two gyrotron traveling-wave tubes (gyro-TWTs), one of which possesses a relatively high gain (40–50 dB) while the other provides a high output power at a moderate (15–20 dB) gain. Both gyro-TWTs are assumed to use helically corrugated waveguides and operate at the second cyclotron harmonic. A principal scheme of such an amplifier chain is discussed, including issues on the microwave input, output, and transmission between the tubes. Computer modeling of the beam-wave interaction using CST Particle Studio PIC Solver shows the possibility of achieving of 200–370-kW output power with 8–10-GHz bandwidth when driving by a 50-mW input source at the W-band.

We consider in detail the method allowing one to input and output the microwave radiation produced by an elecrovacuum amplifier through the same barrier window, which was proposed earlier, in the context of its application in a traveling-wave tube based on a waveguide with a helically corrugated surface. Special attention is given to the splitter of differently polarized radiation, and the results of studying this splitter at wavelengths of about 6 and 1 mm theoretically and experimentally are presented.

We present experimental results on a high-power transmission line from the broadband pulsed Ka-band gyro-TWT to the phased antenna array. The transmission line is designed to operate in a pulse-periodic regime with a pulse width of up to 250 μs, a duty factor of 8, and an average output power of up to 15 kW. Amplitude–frequency and phase–frequency characteristics of the transmission line were measured at a low power level. It is shown that the nonlinearity of the phase–frequency characteristic does not exceed ±10° in the 34 ± 0.5 GHz frequency band.

A cascade of two gyrotron travelling-wave tubes (gyro-TWTs), one of those possesses a relatively high gain (46 dB) while the other provides high output power (350 kW) at the moderate (22 dB) gain, is under design and modeling. Both gyro-TWTs use helically corrugated waveguides and operate at the second cyclotron harmonic. A principal scheme of such an amplifier chain is discussed including issues of microwave input, output and transmission between the tubes. Computer modeling of the beam-wave interaction using CST Particle Studio PIC Solver shows feasibility of achieving of 350 kW pulse output power with 6-8 GHz bandwidth when driving by a 50-mW input source at W-band.

The results of experimental investigation of two Ka-band gyrotron traveling-wave tube (gyro-TWT) amplifiers with helically corrugated waveguides are presented. The first tube produces pulsed output power of 130-160 kW within the frequency range of 33.1-35.5 GHz and is capable of operating with a 10% duty factor. Reliability of its major components in the high average power operation regime (about 10 kW) was proven in a continuous-wave (CW) experiment. The second gyro-TWT amplifier delivered CW power of up to 7.7 kW with -3-dB bandwidth of 2.6 GHz and -1-dB bandwidth of 2.1 GHz. Effective implementation of single-stage depressed collectors (to the best of our knowledge, for the first time for gyro-TWTs) enabled the electron efficiencies as high as 36% for the pulsed tube and 33% for the CW tube to be achieved at operation at the second cyclotron harmonic.

A brief review of research activity on gyrotron traveling-wave tube amplifiers (gyro-TWTs) with helically corrugated waveguides developing by the Institute of Applied Physics and GYCOM (Nizhny Novgorod, Russia) will be presented.

Research activity on development of gyrotron travelling-wave tubes (gyro-TWTs) at the Institute of Applied Physics of the Russian Academy of Sciences (IAP RAS) is presented. In the gyro-TWTs under discussion a helically corrugated waveguide is used as a principal part of their microwave circuits which ensures a number of advantages over other configurations of the gyro-TWTs.

We present experimental results for a 93.2-GHz gyroklystron amplifier operated in the high-order TE021 cavity mode in a cryomagnet. In a three-cavity gyroklystron, a peak output power of 340 kW with 27% efficiency, 23-dB saturated gain, and 0.41% (380 MHz) bandwidth was obtained with a 75-kV, 17-A electron beam. The output-power and efficiency restriction was due to the selfexcitation of the TE021 operating mode in the output cavity. The influence of the electron beam current and intermediate cavity Q-factor on output characteristics of a three-cavity gyroklystron has been studied experimentally.

We have developed and tested a gyroklystron operating with the second harmonic of the electron cyclotron frequency at a frequency of 32.3 GHz in the field of a permanent magnet. In the two-and three-cavity versions of the gyroklystron, the peak power of the output radiation reached 320 kW with an efficiency of 30%, an amplification coefficient of 20–25 dB, and an operating frequency bandwidth of 0.05%. In the wide-band version of the gyroklystron, the amplification bandwidth was equal to 0.27% for an output power of 200 kW and an amplification coefficient of 13 dB.

The results of numerical analysis of the electron beam parameters influencing on the efficiency of energy recovery in gyrotrons are given. The weak dependence of minimal electron beam energy in collector region versus velocity distribution function is demonstrated. This fact may simplify the electron-optical system design of modern gyrotrons.

One of the problems encountered in the design of high-power gyrotrons is the need to ensure stable single-mode generation. The conditions of spurious self-excitation in a gyrotron resonator have been studied fairly completely, and methods for its suppression have been developed (see Petelin [1] and Nusionvich [2], for example). At the same time, stable gyrotron operation can also be disrupted by the generation of spurious oscillations in the transition region between the electron gun and the resonator, where the electromagnetic field is nonuniform. A theoretical basis for this assumption is provided by results [1, 3, 4] indicating a fairly high efficiency of interaction of a high-frequency field with an electron current guided by a nonuniform magnetostatic field.

New projects and experimental results on low-relativistic second-harmonic gyro-devices using a microwave system in the form of a helically corrugated waveguide will be presented. A CW 30 kV/0.7 A gyro-TWT with designed output power up to 4 kW and frequency bandwidth 29.8–31.3 GHz (5%) will be discussed in more detail.

A number of various possibilities to realize frequency multiplication in gyro-devices is investigated and several methods of enhancing the characteristics of gyro-multipliers are proposed. Some of proposed schemes are in close-to-realization stage.

A two-cavity gyrotron under study consists of two cavities separated by a drift area. The first cavity is excited at the fundamental cyclotron harmonic, the second one operates at the higher harmonic. The way is promising for production of high frequency (up to THz) microwaves by gyrotrons. In the work a "low-frequency" prototype of such a device is studied. The first cavity is excited at 95 GHz and the second cavity is optimized for radiation of the same electron beam at the third harmonic. With low power of the electron beam - 23 kV/0.3A - the CW radiation at 285 GHz frequency has power about 30W.

Recent results on low-relativistic second-harmonic gyro-devices using a microwave system in the form of a helically corrugated waveguide are presented. New Ka-band gyro-TWT is designed to operate with peak power of 100-120 kW, average power up to 10 kW and frequency bandwidth of about 10%. In the first experimental test a peak power of 130 kW, efficiency of 22-24% and bandwidth at least 2 GHz were achieved for 70-kV electron beam with pulse length up to 100 mus and repetition frequency up to 5 Hz. Frequency-tunable gyro-BWOs, attractive for some technological applications, enable CW output power of several kW at 24 GHz with smooth frequency tuning of 4-6%.