Search

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.

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%.

Nonlinear dynamics of a gyrotron with high-order operating mode, driven by an external signal with harmonic or stepwise modulation of its parameters or by two monochromatic signals, was studied theoretically. Conditions for locking and control of the radiation frequency were found. Exit from the locking regime is accompanied by the beats of the operating mode or excitation of spurious modes. Simulations were performed using a megawatt-power TE28.12 mode gyrotron as the example (Chirkov et al., Appl Phys Lett 106(26), 2015).

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 study the oscillation regimes for the model of a gyrotron with a fixed Gaussian longitudinal field structure and three modes of an equidistant spectrum (an operation mode and two satellites symmetrically spaced by frequency) during autonomous operation and under the action of an external monochromatic signal. The two-parameter (dimensionless current and the cyclotronresonance mismatch for an operating mode) plane is partitioned into the regions corresponding to various regimes. It is shown that the presence of a maximum at the leading edge of the voltage pulse can lead to excitation of the parasitic modes in the process of the power-source switch-on and to quenching of oscillation of the operating mode. These undesirable effects can be suppressed by locking the operation-mode frequency by an external monochromatic signal, which provides oscillation at the operation mode.

As part of the creation of a unique T-15MD tokamak with a magnetic field of 2 T and an aspect ratio of 2.2, the first one of a series (8 pieces) of microwave setups of a megawatt power level for electron–cyclotron plasma heating and current drive was developed and successfully tested. The setup includes a gyrotron, power supplies, a microwave radiation transmission line, and a fast (response time is no more than 10 μs) gyrotron emergency protection system. The generation regime was experimentally demonstrated with the parameters 1 MW/30 s/82.6 GHz for an efficiency of 57%.

We show that during phase locking of the oscillations of a megawatt-level gyrotron by a relatively weak external monochromatic signal, the phase and frequency modulation of the gyrotron radiation due to the low-frequency fluctuations of the accelerating voltage and current decreases severalfold compared to the autonomous regime. The calculations were performed using the example of two megawatt-level gyrotrons, namely, a gyrotron developed at the Institute of Applied Physics of the Russian Academy of Sciences and operated at the TE28,12 mode with a frequency of 170 GHz [1] and a gyrotron with twice as high frequency and operation-mode indices (345 GHz and TE56,24), whose ability to generate a megawatt power in the case of frequency and phase locking by an external signal is shown theoretically [2].

Based on the analysis of electron–wave interaction, we study the possibility of developing a gyrotron that is able to provide a megawatt output power in quasi-continuous (long-pulse) oscillations at both first and second gyrofrequency harmonics. For selective excitation at the second cyclotron harmonic when the operating current is significantly higher than its starting value, we propose to lock the gyrotron by a weak (several percent power) monochromatic incoming signal which suppresses spurious oscillations at the fundamental cyclotron resonance in the gyrotron start-up process. Simulations of a two-frequency gyrotron with an output power of 1 MW at the first cyclotron harmonic (19 GHz) and 0.8 MW at the second one (38 GHz) based on a nonstationary self-consistent system of equations are presented. Such a source of radiation is of interest for plasma-heating systems in spherical tokamaks.

Influence of the density of mode spectrum in the gyrotron cavity on the possibility of stable single-mode operation has been investigated for high-order modes. Interaction of the operating mode wit...

We propose a robust way of advancement of the gyrotrons into the region of terahertz frequencies and MW-level power of radiation while retaining high efficiency and high stability of the frequency. These gyrotrons may be used for electron cyclotron resonance heating and current drive systems (ECRH&CD) in future fusion setups, in large complexes of coherently radiating gyrotrons for THz data and energy transmission, for THz particles acceleration. We at first demonstrate the advantages of this way at the example of the gyrotron with very-high-order operating mode TE56.24, driven by a relatively weak external monochromatic signal, and show that in the phase- and frequency-locking mode a significant enhancement of radiation parameters – frequency, power and efficiency – can be achieved.

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.

We consider the influence of an external quasi-monochromatic signal on a multimode gyrotron in the case where the signal frequency is close to the frequency of the operating mode of the gyrotron. The influence of the mode competition on the generation regime at a harmonic frequency modulation is studied. The calculations were performed for a high-power 170 GHz gyrotron developed at the Institute of Applied Physics of the Russian Academy of Sciences (IAP RAS). Depending on the parameters of the gyrotron and the external signal, various generation regimes were determined, including locking of the operating-mode frequency when the radiation frequency is close to the frequency of the external signal, beats of the frequency and amplitude of the operating mode, and excitation of spurious modes.

The radiation-frequency locking by an external signal is experimentally studied for a megawatt gyrotron. An external signal from a magnetron is applied to the operation space of the gyrotron at a frequency of 35 GHz via a synthesized two-mirror quasioptical converter developed at the Institute of Applied Physics of the Russian Academy of Sciences. The radiation spectra are measured for both the frequency locking regime and the frequency beats. The locking region is constructed on the plane of two parameters, namely, the external-signal power and frequency. The experimental and theoretical results are compared to show a good agreement.

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 theoretically study the possibility of high-power (MW-level) generation in second-harmonic gyrotrons based on the frequency-locking by a weak (several percent of the power) external monochromatic signal. Such a signal can suppress the generation of the parasitic fundamental harmonic during the start-up process. Within the frame of multimode self-consistent non-stationary model, we demonstrate the applicability of the described approach to provide 0.65 MW output radiation in a 230 GHz 2nd-harmonic gyrotron with TE 31,8 operating mode.

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.

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.

In the present work we compare oscillation regimes in a gyrotron under the influence of a signal with harmonically, stepwise, and randomly varying frequency and amplitude. At stepwise (regular or random) modulation of the signal parameters threshold values of these modulations, corresponding to the excitation of parasitic modes, are lower than at harmonic modulation.

This report aims to bring together information about the most striking experimental results, new trends in gyrotron development, modern remarkable applications, new demands in parameter enhancement and future goals. The paper separated into two parts: first, related to progress in MW class gyrotron development for nuclear fusion and second, focused on the development of terahertz band gyrotrons. The data about pulsed and CW tubes, working in both specified frequencies ranges, are given. In particular, the series of 1MW/170 GHz/CW tubes with efficiency more than 50% has been developed successfully for ITER project. Same time, despite the requirement for strong magnetic fields, the problem of high ohmic losses and electron beam formation, the gyrotrons go through magic 1 THz mark with kW power level and demonstrate (in some specific combinations) operation at extremely low voltage and beam current, narrow frequency spectrum, wide frequency tuning. Novel schemes of high-frequency gyrotrons are analyzed. The novel quasioptical mode converters opened the possibility of phase-locking a number of tubes, which makes maximum power almost "unlimited"

We present the results of recent IAP RAS investigations, which were aimed at controlling the radiation frequency and spectrum of sub-THz medium power gyrotrons. Different methods of extending the smooth frequency tuning band and providing high frequency stability in CW regimes are discussed. The development of gyrotrons with stated parameters are of interest for many modern applications, including DNP/NMR and RAD spectroscopy, direct measurements of positronium hyperfine structure, diagnostic of various media, etc.

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.

We consider nanosecond subterahertz waveguide switches based on a 3D resonator with an active element made of a semiconductor, whose conductivity is controlled by a laser. Recently discovered possibilities to use these switches to obtain pulses with very long durations (up to tens of seconds) along with nanosecond pulses in one and the same device prototype are discussed. Switching with no distortion of the coherent radiation of promising subterahertz gyrotrons, which have powers of about several watts and pulse durations of up to ten seconds are demonstrated experimentally. The theoretical estimate of limiting powers of the switched subterahertz waves, which was proposed earlier, is confirmed and generalized. For this purpose, we perform a measurement of the powers by reducing it to a trivial measurement of the power of radiation of an industrial IR laser. Improvement of the resonance characteristics of the developed switch after switching several sequential long subterahertz pulses has been revealed. Most probably, it is due to “burning-off” of microscopic manufacturing defects and the approach of the actual frequency-amplitude characteristic to the calculated one. It has been predicted theoretically and partially confirmed experimentally that it is not possible to disable the switch being in the fundamental equilibrium state upon switching arbitrarily high powers of subteraheratz waves near the resonance band.

A well-known principle for periodical production of high-power ultrashort optical pulses is related to passive mode locking realized when a saturable absorber is incorporated into the laser resonator. A similar effect can be implemented in two-section electronic oscillators comprising an amplifier and a saturable absorber, which should be applicable in the microwave frequency band and suitable for high-power operation. In this paper, we analyze in detail the possibility of realizing a saturable absorber based on cyclotron resonance interaction in the regular waveguide with initially rectilinear electron beam, when saturation is caused by the relativistic dependence of the gyrofrequency on electron energy. Maximum contrast between the absorption of a high-power signal and of a weak signal is achieved in the group synchronism regime when the particle velocity is close to the wave's group velocity. Within the frame of a time-domain self-consistent model, we demonstrate that when such an absorber is installed in the feedback loop of an electron generator powered by existing Ka-band helical-waveguide gyrotron traveling-wave tube, periodical generation of subnanosecond pulses can be provided with peak power up to several hundred kW and sub-GHz repetition frequency. We discuss the influence of deviation from the group synchronism regime on the parameters of generated pulses. Experimental feasibility of the considered scheme is verified based on three-dimensional particle-in-cell simulations.

Three different designs of an external microwave system for feeding and extracting power to and from a gyrotron traveling-wave tube (gyro-TWT) through one oversized port are presented and discussed. As it shown, such a microwave system should operate as a splitter which splits two wavebeams with mutually orthogonal polarizations incoming through one port into two spatially separated ports or transmission lines transporting each wavebeam individually. The orthomode splitters presented in this paper were designed to be used with a gyro-TWT producing 350- and 50-kW maximum pulsed and average powers, respectively, with a 3-dB bandwidth of 10 GHz at the center frequency of 95 GHz. Such a high-power level and wide bandwidth at short millimeter waves make quasi-optical components to be the most appropriate solutions in contrast to low-mode standard waveguide circuits.

A 45-GHz gyrotron-based microwave oscillator setup has been purposefully developed for powering of a superconducting electron cyclotron resonance (ECR) ion source. The setup provides a smoothly regulated output power in the 0.1–20-kW range (up to 26 kW in manual mode) with an efficiency of up to 50% and collector depression in both continuous wave (CW) and pulse modes. Pulse mode specifications: duration 5–200 ms, leading edge less than ${2}~\mu \text{s}$ , trailing edge less than ${1}~\mu \text{s}$ , repetition rate 1–10 Hz, and triggered operation using an external reference oscillator. The set of high-voltage power supplies (HVPSs) is equipped with a fast electronic protective system to ensure trouble-free operation of the gyrotron. The functional structure of the setup corresponds to the general design method for feedback-controlled microwave sources. The feedback loop consists of a power meter (or a detector unit, to the user’s choice), a control system, and an anode HVPS, the regulated voltage of which controls the output power of the gyrotron. The feedback characteristic time is less than 1 s. Feedback control is provided for both CW and pulse modes. The setup includes a unique quasi-optical line that transmits the microwave beam through a 2-m air gap, as well as a synthesized mode converter from the Gaussian beam to the TE01 or TE11 mode (to the user’s choice) installed on a 300-kV platform. During the tests, the developed setup proved to be a reliable and user-friendly tool applicable for long-term powering of the fourth-generation ECR ion sources.

We are considering the possibility of ultrawideband millimeter-wave generation in microwave oscillators consisting of two coupled helical-waveguide gyro-TWTs, one of which operating as an amplifier whereas the other acting as a saturable absorber or as a nonlinear converter. In the first case, the regime of passive mode-locking is realized when the radiation represents a periodical train of ultrashort electromagnetic pulses. The spectral domain corresponds to the formation of a microwave “frequency comb.” On the other hand, if the second gyro-TWT acts as a nonlinear converter with a fast variation of the phase characteristic, it leads to the generation of chaotic signals with quasi-continuous spectrum and a bandwidth of up to ~10% which is defined by the bandwidth of the gyro-TWTs.

An effective five-channel quasi-optical multiplexer, which distributes microwave signals of the frequency range of 12–90 GHz on five frequency channels, Ku, K, Ka, U, and E (frequency ranges of standard single-mode waveguides), or combines these channels into a single wave stream, has been developed for the first time. A relatively homogeneous characteristic with a total loss level of about 1.5 dB is achieved in 90% of each range band. A detailed analysis of diffraction and ohmic losses in the frequency channels of the multiplexer has been carried out.

A method for determining the rms deviation of the heights of surface inhomogeneities based on an algorithm for calculating the spectral density of the one-dimensional correlation function over a wide range of spatial frequencies is developed and justified scientifically. The methodological errors of the algorithm are analyzed using experimental data from studies on a test stand under production conditions.

The electron cyclotron resonance heating (ECRH) system of the ASDEX Upgrade tokomak has been upgraded over the last 15 years from a 2 MW, 2 s, 140 GHz system to an 8 MW, 10 s, dual frequency system (105/140 GHz). The power exceeds the L/H power threshold by at least a factor of two, even for high densities, and roughly equals the installed ion cyclotron range of frequencies power. The power of both wave heating systems together (>10 MW in the plasma) is about half of the available neutral beam injection (NBI) power, allowing significant variations of torque input, of the shape of the heating profile and of Qe/Qi, even at high heating power. For applications at a low magnetic field an X3-heating scheme is routinely in use. Such a scenario is now also forseen for ITER to study the first H-modes at one third of the full field. This versatile system allows one to address important issues fundamental to a fusion reactor: H-mode operation with dominant electron heating, accessing low collisionalities in full metal devices (also related to suppression of edge localized modes with resonant magnetic perturbations), influence of Te/Ti and rotational shear on transport, and dependence of impurity accumulation on heating profiles. Experiments on all these subjects have been carried out over the last few years and will be presented in this contribution. The adjustable localized current drive capability of ECRH allows dedicated variations of the shape of the q-profile and the study of their influence on non-inductive tokamak operation (so far at q 95 > 5.3). The ultimate goal of these experiments is to use the experimental findings to refine theoretical models such that they allow a reliable design of operational schemes for reactor size devices. In this respect, recent studies comparing a quasi-linear approach (TGLF) with fully non-linear modeling (GENE) of non-inductive high-beta plasmas will be reported.

The frequency capture of radiation by an external signal was experimentally studied for the first time for a megawatt-power gyrotron. An external signal from the magnetron at a frequency of 35 GHz entered the working space of the gyrotron through a synthesized two-mirror quasi-optical converter that allowed transforming the input signal into a working mode. This converter has been recently developed and implemented in the Institute of Applied Physics, Russian Academy of Sciences. The radiation spectra were experimentally obtained in modes of frequency capture and frequency beats. The dependence of the frequency capture bandwidth on the power of the external signal was found. Comparison of experimental results with the data of numerical modelling of gyrotron equations with an external signal showed their good agreement.

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.

Oscillation regimes in a multimode gyrotron under the influence of quasi-monochromatic external signal with a frequency close to the operating frequency are studied. The calculations were performed for a powerful 170 GHz gyrotron perspective prototype developed at IAP RAS for ITER. The parameter regions were found where the frequency of operating mode is locked by external signal and follows the variations in the external signal frequency.

Presently, Ka-band ultrashort pulse (USP) generator is under development at Institute of Applied Physics RAS. Based on a time-domain model and direct PIC simulations we demonstrate that a periodic train of ultrashort microwave pulses can be generated in an electron oscillator consisting of a helically corrugated gyro-TWT and a saturable absorber based on cyclotron resonance interaction of radiation with an initially rectilinear electron beam. The gyro-TWT operates at the second cyclotron harmonic while in the absorber interaction at the fundamental harmonic should be realized. According to simulations with parameters of existing Ka-band gyro-TWT, the peak power of generated pulses with a duration of 200 ps and repletion frequency 1 GHz is about 400 kW.

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.

We study theoretically the mechanism of the formation of high-power ultrashort microwave pulses, which is based on the effect of passive mode locking in electron generators, whose feedback circuit includes a nonlinear saturable absorber (amplitude filter). The mechanism under consideration is an analog of the well-known method of generation of ultrashort pulses, which is used widely in laser physics. Nevertheless, realization of this method in electron generators has certain specific features connected with the motion of the active medium (electron flow). It is shown that being applied to vacuum electronics, this method is sufficiently universal, and various types of both relativistic and subrelativistic amplifiers operated in wavelength ranges from the centimeter to submillimeter ones can be used for actualization of ultrashort pulse generators as active elements. In the additional section, which acts as a saturable absorber, one can use either the Kompfner absorption effect, or the effect of cyclotron resonance absorption of radiation by a rectilinear electron beam. It is demonstrated on the basis of numerical simulation that the peak power of ultrashort pulses in electron generators with passive mode locking can exceed the power achieved in the stationary generation regimes significantly and, in some cases, the power of the electron flow.

In the analysis of propagation of wave beams in quasi-optical transmission lines, the loss rate and transformation of modes by various elements of the transmission line are of great importance. The effects occurring when a spatially bounded, obliquely incident wave beam is reflected from a Bragg resonator being a periodic, plane-layered medium with varied dielectric permittivity are considered. Such a reflector can be used as a frequency-selective element of the transmission line. Since the reflection coefficient of a plane wave from the Bragg reflector depends on its incidence angle, there occurs a transformation of the reflected beam. Analytical expressions for the beam displacement, the angle of deflection of the beam from the mirror reflection direction, and the wavefront curvature variation have been obtained. The influence of the mentioned effects on characteristics of the reflected wave beam in the main band of the Bragg reflector is numerically analyzed. The possibility of compensation for these distortions is considered.

We study frequency locking of the gyrotron operating mode by a monochromatic external signal under conditions of competition of several modes. The calculations were performed for a megawatt-power 170-GHz gyrotron with the operating TE28.12 mode. Mode interaction of both equidistant and nonequidistant spectra is considered. Frequency detuning of the modes is comparable to the cyclotron resonance bandwidth. Influence of the external-signal power on the boundaries of the frequency locking zones is studied. The possibility to achieve a high orbital efficiency and broadband frequency tuning in the frequency-locking regime is considered.

We study locking of the oscillation frequency of the operating TE28,12 mode by an external monochromatic signal in a multimode gyrotron operated at a frequency of 170 GHz in the switch-on regime close to the real one. Locking zones, i.e., regions of single-mode generation at the external-signal frequency are found on the “current—detuning” plane of parameters. It is shown that as the number of competing modes increases, the maximum achievable current decreases, and the locking zones contract at sufficiently high currents.

Previously, relativistic gyrotrons with thermionic magnetron-injection electron guns have been demonstrated successfully to produce radiation in the cm and long-mm wavebands with the pulse power exceeding 10 MW. This brief presents a new step toward generating higher-frequency microwaves with a relativistic gyrotron operated in the 3-mm wavelength band. The gyrotron employed with a 250 kV, 100 A, $1~\mu \text{s}$ helical electron beam. The operating wave was the rotating TE12,5,1 mode of a circular waveguide cavity. The mode was transformed into the Gaussian wave beam by a built-in mode converter. In the optimized single-mode regime, the gyrotron delivered 5.6 MW of the peak power in pulses with durations of about 0.5 $\mu \text{s}$ .

A high-quality flat wave front is usually used to calibrate the Shach-Hartmann wave-front sensors. The article discusses the possibility of calibrating sensors with spherical wave fronts. Special attention is paid to the consideration of calibration in standard laboratory conditions. The mathematical apparatus and the scheme of the experiment are considered.

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%.

A brief summary of current developments in Russia of megawatt power gyrotrons is presented. The gyrotron developed are applied in experiments at several modern tokamaks. New demands to gyrotron parameters are discussed.

Based on a time-domain model we demonstrate that a periodic train of powerful ultrashort microwave pulses can be generated in an electron oscillator consisting of helically corrugated gyrotron travelling wave tube (gyro-Twt)and saturable absorber based on cyclotron resonance interaction of radiation with initially rectilinear electron beam. The gyro-TWT operates at the second cyclotron harmonic while in the absorber interaction at the fundamental harmonic should be realized. According to simulations with the parameters of an experimentally realized Ka-band gyro-TWT, the peak power of generated pulses with duration of 200 ps is about 400 kW.

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.