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Metallic helically corrugated structures play an important role in microwave vacuum electron devices serving as wave converters, wave compressors, and interaction circuits. Thermal analysis of such structures operating in a high-power regime is important for their design and development. In this paper, an algorithm for an efficient analysis of temperature distribution in helically corrugated metallic structures caused by Ohmic losses in their walls is presented. The algorithm is based on the coordinates transformation with the subsequent expansion of the equations to the Fourier series with respect to the longitudinal coordinate. The advantages and limitations of the algorithm are discussed. Comparison with a 3-D finite-element simulation was performed, and a sufficiently good agreement was demonstrated.

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.

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.

Results of numerical simulation of a miniaturized sub-THz traveling-wave tube amplifier with sheet electron beam and planar grating slow wave structure (SWS) are presented. Cold characteristics of the SWS are calculated. Small-signal and large-signal gain regimes are studied by the 1-D parametric frequency-domain code. The results are verified by simulations using 3-D particle-in-cell codes.

The evaluation, design, and simulations of two different electron guns generating the beams for W-band second cyclotron harmonic gyro-TWTs forming a high-gain powerful amplifier cascade are presented. The optimum configurations of the systems creating nearly axis-encircling electron beams having velocity pitch-factor up to 1.5, voltage/current of 40 kV/0.5 A, and 100 kV/13 A with acceptable velocity spreads have been found and are presented.

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.

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.

In this paper, we present a design of a broadband powerful $W$ -band gyroamplifier with an interaction circuit in the form of a helically corrugated waveguide operating at the third cyclotron harmonic. An analysis of stability of the gyrotron traveling wave tube (gyro-TWT) with respect to parasitic wave-beam interactions at the operating and nonoperating cyclotron harmonics has been performed, and the range of parameters at which the gyro-TWT is zero-drive stable is found. Moreover, a comparison with a traditional smooth-waveguide gyro-TWT revealed that the helical-waveguide gyro-TWT possess superior stability characteristics at higher cyclotron harmonics. For a set of parameters attractive for possible applications, the 3-D particle-in-cell simulations were performed, according to which the proposed gyro-TWT yielded a 6% amplification bandwidth with the maximum output power of 80 kW and gain of 25 dB for a 10-A, 70-kV electron beam.

Backward Wave Oscillators (BWO's) utilizing moderately relativistic (~550kV), high-current (~10 kA) electron beams are capable of producing hundreds of MWs of pulsed radiation in the centimeter wavelength range. Such relativistic BWOs (RBWOs) allow for broadband, smooth, frequency-tuning via adjustment of the accelerating potential; making them an attractive source for use in frequency-swept pulse compression. This paper presents results of a 2.86m long 5-fold helically corrugated, dispersive pulse compressor where a maximum power compression ratio of 25 was achieved by compressing an input microwave pulse of 80 ns duration swept from 9.65 GHz to 9.05 GHz into a 1.6ns Gaussian-envelope pulse. For an average input power of 5.8 kW generated by a conventional traveling wave tube amplifier, a peak pulse output power of 145 kW was measured corresponding to an energy efficiency of 66%. An X-band relativistic BWO, designed to drive a 0.97m long 5-fold compressor, was built and tested using the accelerating potential generated by a SINUS-6 highcurrent accelerator. The experimental RBWO operated close to the predicted power of 700MW with its oscillation frequency varied from 10 to 9.6GHz via the falling edge of the voltage pulse. It was demonstrated that the ~15ns duration frequency-swept part of the RBWO pulse was effectively compressed resulting in a 4.5-fold peak power increase with a maximum power of 3.2 GW generated. The potential for a 5-fold helical waveguide to compress longer duration pulses generated by a RBWO is discussed.

For the design of microwave tubes, precise electromagnetic simulations of the cold structures are of particular importance. In this paper, we present the importance of full-wave vector simulations for the analysis of components in microwave tubes, particularly in high-power gyro-devices. Full-wave vector simulations based on surface integral equations allow an accurate analysis of various structures. A fast solution of surface integral equations is provided by the recently developed advanced acceleration techniques. Simulations of quasi-optical mode converters in high-power gyrotrons, calculations of dispersion relations of gyrotron traveling-wave tubes with helical traveling wave structure and simulations of mirror systems are shown as examples for the wide range of applications of full wave vector simulations in the development of microwave tubes.

A periodic train of powerful ultrashort microwave pulses can be generated in a two-section electron oscillators consisting of an amplifier and a saturable absorber installed in the feedback loop. This method of pulse formation is similar to the passive mode-locking effect well-known in the laser physics. In the electronic scheme, the wide class of amplifiers can be used including gyro-TWTs with an interaction space in the form of a multi-fold helically corrugated waveguide. As a saturable absorber we suggest to use the similar section in which the regime of Kompfner dip is realized by proper matching of parameters. It was experimentally demonstrated with using of a microwave magnetron as a source of the input signal.

Project of high-power cyclotron autoresonance maser (CARM), aimed at DEMO ECRH system, is under development at ENEA (Frascati). Since the competition of designed mode with spurious gyrotron modes is a serious problem in CARM implementation, we propose highly-selective two-mirror open cavity which do not support near-cutoff modes.

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.

Results of 3D modeling of the 0.2 THz double-grating half-period staggered slow-wave structure are presented. A fast and accurate finite edge elements technique for simulation of broadband input/output couplers is developed. Beam focusing in a uniform magnetic field is simulated showing very good beam transmission. Small-signal and large-signal gain regimes are simulated using CST Particle Studio 3D code.

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.

We present a waveguide system which makes it possible to input/output radiation to and from a low-magnetic-field-operating W-band gyro-TWT being currently under development. The system makes it possible to use one barrier window at the collector's end both for radiation inputting and outputting.

In this paper the result of experimental investigation of one of the key elements of a sub-millimeter wave (260 GHz) broadband gyro-TWT's radiation input/output system is presented. A quasi-optical waveguide device based on the Talbot's effect effectively separates power flows of input and output radiation in the required frequency range.

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.

Hollow metallic circular waveguides with helical corrugation of the inner surface (helical waveguides) attract considerable interest for various applications such as interaction regions in gyrotron traveling-wave tubes and as a dispersive medium for passive microwave pulse compression. In this paper, we employ a vector finite-element method in 2-D formulation for computation of the dispersion, attenuation characteristics, and the distribution of electromagnetic fields of eigenwaves of helical waveguides. As shown by comparison with the results of previously performed 3-D calculations, the presented method provides sufficient accuracy of calculating the characteristics of helical waveguides for a number of practically important cases at much lesser consumption of computer resources.

We present a method for calculating the dispersion characteristics of eigenmodes of metal waveguides with helical corrugations on the inner surface, which is based on the transition to a new nonorthogonal system of coordinates. In the new coordinate system, the problem of finding helical-waveguide modes is rigorously equivalent to the problem of finding the modes of a circular unitradius waveguide which has an anisotropic filling and is homogenous in the longitudinal direction. To solve the equivalent problem, we expand the field of the desired eigenmode in the modes of an empty circular unit-radius waveguide. As a result, the problem is reduced to solving a generalized algebraic eigenvalue problem. The comparison with the results of earlier three-dimensional calculations shows that the developed method allows one to determine characteristics of helical-waveguides with sufficient accuracy for many important applications and requires much lower calculation costs.

The effect of passive compression of frequency-modulated pulses in dispersive media is used to raise the microwave radiation peak power to a multigigawatt level. A waveguide with a helically corrugated surface is applied as a dispersive medium, and a relativistic 3-cm backward-wave oscillator with an accelerating voltage decaying within the pulse duration serves as a source of frequency-modulated pulses. The compression of pulses to an FWHM of 2.2 ns attended by a rise in the peak power by a factor of 4.5 (to 3.2 GW) is demonstrated with a SINUS-6 accelerator.

A method, which enables feeding and extraction of microwave radiation to and from gyrotron traveling wave tubes (gyro-TWT) using a single highly oversized barrier window is proposed. The essential conditions for effective implementation of this method are as follows: 1) the operating mode of the amplifier should be a circularly polarized one and 2) the “cold” losses of its interaction circuit should be sufficiently low. A detailed explanation of the method and discussion of the major microwave components are presented.

A relativistic backward wave oscillator for the 10-GHz range with the oscillation frequency tuned within about 5% by changing the accelerating voltage from 600 to 350 kV has been developed. Discrete variations in the voltage and the corresponding frequency tuning from pulse to pulse is rapidly performed by changing the anode-cathode distance in the vacuum diode without breaking vacuum in the working volume. During this, the electron beam power remains almost constant, while the output microwave power varies within 0.4–0.8 GW. The introduction of a dielectric cylinder into the accelerating gap provides a smooth voltage drop from 600 to 350 kV with the corresponding frequency tuning during a 20-ns pulse.

A method of considerably elevating (by 20–30 times) the peak power of frequency-modulated microwave pulses by passive compression in a waveguide with a helically corrugated surface used as a dispersive medium is considered. It is shown that the use of a sectioned compression waveguide (i.e., a waveguide consisting of several sections connected to each other with slightly differing parameters) considerably improves precise tuning of the dispersion characteristic of the system to a desired law of frequency modulation of an input pulse. An algorithm for selecting optimal parameters of the sections is developed, and a high efficiency of the method is demonstrated by several examples.

For 3d waveguiding system calculations we used the Electric Field Integral Equation (EFIE) model, applied to infinitely thin, perfectly conducting waveguide surfaces. The solution represents induced current, from which the field in the device is obtained. With the help of this method an asymmetric mode converter TE01-TE11 was simulated and optimized for maximum efficiency. We have also studied a helically corrugated structure, which is used in pulse compressors, and analyzed the structures dispersion curves and compared it with measurement results.

We use the well-known method of analysis of oversized electrodynamic systems, which is based on numerical solution of the integral equation for the surface current and is employed to calculate diffraction of an electromagnetic field by arbitrary conducting surfaces, for calculation of the characteristics of waveguide structures. In contrast to the finite-difference algorithms for straightforward solution of the Maxwell equations, which are dealt with the field components in the calculation space divided into cells with small sizes (compared with the wavelength), the desired quantity in this method is the current on the conducting surface bounding the structure. This reduces the required RAM capacity and speeds up calculations by several orders of magnitude in the case of oversized systems. In this work, the integral equation method is realized in the form of a computer code and is used to calculate three-dimensional multimode waveguide systems. The calculation results are compared with the results of perturbation theory and the experimental data.

Metal waveguide can be used as a dispersive medium to convert long duration, lower power pulses into short, higher peak power pulses. This provides an advanced method to generate radiation with gigawatts power in the millimeter and sub-millimeter wavelength range by compressing a megawatt level long duration pulse. In this paper, a five-fold helically corrugated waveguide operating in X-band was designed and constructed. The experiments conducted show that a 5.75 kW average power microwave pulse with a 6% bandwidth and duration of 80 ns can be compressed into a 144.8 kW, 1.6 ns pulse with a power compression factor of 25.2.

In this paper we present design of a broadband third-harmonic operating gyro-amplifier with helically corrugated interaction circuit. An analysis of stability of the gyro-TWT with respect to parasitic wave-beam interactions has been performed and the range of parameters at which the gyro-TWT is zero-drive stable is found. According to 3D simulations, the amplifier yields a 6% amplification bandwidth with the maximum output power/gain of 80 kW / 25 dB for a 10-A, 70-kV electron beam.

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 two coupled helically corrugated gyrotron travelling wave tubes (gyro-TWTs) operating in regimes of amplification and saturable absorption, respectively. The mechanism of pulse formation in such an oscillator is based on the effect of passive mode-locking widely used in laser physics. Saturable absorption can be implemented in a gyro-TWT in the Kompfner dip regime by a proper matching of the guiding magnetic field. According to simulations with the parameters of an experimentally realized Ka-band gyro-TWT, the peak power of generated pulses with a duration of 200 ps can achieve 400 kW.

Microwave pulse compression results from a five-fold helically corrugated waveguide are presented. In these experiments an X-band microwave pulse of duration 80 ns and a 6% frequency sweep was compressed from 5.8 kW to 128kW into a 1.6 ns pulse with a compression factor of 22 in the peak power.

Summary form only given. Microwave pulse compression has applications in radar technology and plasma diagnostics. The pulse compressor requires a dispersion curve with a monotonic group velocity and low losses in a reasonable wide frequency band. It can be achieved by the helically corrugated waveguide (HCW) [1–3] with both axial and azimuthal periodicity that couples modes in the circular waveguide to generate new operating eigen waves. The HCW has successfully been used in the gyro-TWA [4,5], gyro-BWO [6,7] and pulse compressor[8–10]. Following the success of a pulse compressor using a 3-fold HCW, the dispersion characteristic of the 5-fold HCW has been analytically, numericall y studied and measured. The compression factor of the compressor was also measured.

A high microwave pulse compressor based on a large diameter 5-fold helical waveguide structure was studied. The eigenwave dispersion was calculated using numerical and analytical techniques and compared with experimental measurements. The results of 5-fold helically corrugated waveguide microwave pulse compression experiments will be presented.

The results of 3-fold helically corrugated waveguide microwave pulse compression experiments are presented. A pulse of 80 ns duration having a power of 5.6 kW is compressed into a 140 kW peak power pulse of 1.5 ns duration. To enhance the power capabilities of the microwave pulse compressor a larger diameter 5fold helical waveguide structure was studied; the eigenwave dispersion is discussed using numerical, analytical, and experimental techniques.

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.

Waveguide with a helical corrugation of the inner surface has the desirable properties of optimised dispersion, operation far from cut-off and high breakdown field strength which make it an excellent medium for passive pulse compression. New compression measurements at both milli-watt and kilowatt powers using an optimised frequency modulation of the input pulse will be presented and discussed.

Several examples of numerical calculations of waveguiding structures for microwave devices are presented. In the calculations we used the electric field integral equation (EFIE) model, applied to infinitely thin, perfectly conducting waveguide surfaces. The method of moments (MoM) is used to transform this model into an algebraic system of linear equations. It is solved by means of an iterative procedure with fast multipole method (FMM). The solution represents induced surface current, from which the electromagnetic field in the device is obtained. With the help of this method an asymmetric mode converter TE01-TE11 was calculated and optimized for maximum efficiency. We have also calculated several multi-mode helical structures, which are planned to be used in gyro-TWT and pulse compressors, and analyzed their dispersion curves. Experiments with these components were carried out. The numerical results agree well with the measurements.

A time-domain self consistent theory of a gyrotron traveling wave tube with a helically corrugated operating waveguide has been developed. Based on this model, the process of short pulse amplification was studied in regimes of grazing and intersection of the dispersion curves of the electromagnetic wave and the electron beam. In the first case, the possibility of amplification without pulse form distortion was demonstrated for the pulse spectrum width of the order of the gain bandwidth. In the second case, when the electrons' axial velocity was smaller than the wave's group velocity, it was shown that the slippage of the incident signal with respect to the electron beam provides feeding of the signal by “fresh” electrons without initial modulation. As a result, the amplitude of the output pulse can exceed the amplitude of its saturated value for the case of the grazing regime, and, for optimal parameters, the peak output power can be even larger than the kinetic power of the electron beam.

The phenomenon of passive compression of frequency-modulated (FM) pulses in a dispersive media (DM) was used to increase the peak microwave power up to the multigigawatt level. A helically corrugated waveguide was used as the DM, while a relativistic X-band backward-wave oscillator (RBWO) with a descending-during-the-pulse accelerating voltage served as a source of FM pulses. Compression of pulses down to a halfwidth of 2.2 ns accompanied by a 4.5-fold power increase up to a value of about 3.2 GW has been demonstrated.

A novel scheme of a self-exciting single-cavity terahertz gyromultiplier is proposed and theoretically investigated. Simulations predict a possibility to obtain a power of 75 W at the frequency of 1.3 THz from the 80 kV/0.7 A electron beam when operating at the fourth cyclotron harmonic at the relatively low magnetic field of 14 T.

The paper presents new results on the development of a method to generate ultrahigh-power short-microwave pulses by using a known principle of compression (reduction in pulse duration accompanying with increase in pulse amplitude) of a frequency-swept wave packet propagating through a dispersive medium. An oversized circular waveguide with helical-corrugations of its inner surface ensures an eigenwave with strongly frequency dependent group velocity far from cutoff. These dispersive properties in conjunction with high rf breakdown strength and low Ohmic losses make a helically corrugated waveguide attractive for increasing microwave peak power. The experiments performed at kilowatt power levels, demonstrate that an X-band microwave pulse of 80 ns duration with a 5% frequency sweep can be compressed into a 1.5 ns pulse having 25 times higher peak power by optimizing the frequency modulation of the input wave packet.