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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.

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.

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.

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.

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.

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.

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.