Traveling Wave Tube Research Articles

Study on 1-THz Sine Waveguide Traveling-Wave Tube

In this article, a 1-THz traveling-wave tube (TWT) based on flat-roofed sine waveguide (SWG) slow wave structure (SWS) has been studied theoretically and experimentally, and parts of the component have been fabricated. First, the flat-roofed SWG SWS has been optimized for working at 1 THz, the normalized phase velocity is around 0.287, and the interaction impedance is over 3.5 Ω in the frequency range of 1.02-1.04 THz. Meanwhile, the performance of the designed TWT has been simulated, whose saturated output power is 720 mW and the corresponding gain is 28.57 dB. What is more, to decrease the transmission loss of the high-frequency structure, the input and output structures have been designed and a transition structure that transforms the waveguide to standard rectangular waveguide has been proposed. The simulation results show that the reflection coefficient of the novel proposed transmission system is less than -20 dB, and the insertion losses of the transition structure with the input and output waveguides are 2.45 and 1.68 dB, respectively. Finally, the designed SWS has been fabricated by deep reactive-ion etching (DRIE) and tested; S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> is generally less than -10 dB in the frequency range of 1020-1040 GHz.

A Substantially Lowered-Voltage High-Power W-Band Pulsed Traveling Wave Tube

Using a new kind of folded waveguide (FWG) with modified circular bends and two-stage depressed collector, we have designed a W-band-pulsed traveling wave tube (TWT) operating at substantially lowered-voltage with high power and efficiency. A prototype tube has been tested at voltage of 15.66 kV and 30% duty cycle, showing output power over 180 W and gain over 36 dB in 91-97-GHz frequency range. The electronic and overall efficiency are above 7.8% and 29%, respectively, within the bandwidth of 6 GHz. The packaged TWT has outer dimensions of 270×49×46 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> and weighs less than 1.5 kg.

Demonstration of a W-Band Traveling-Wave Tube Power Amplifier With 10-GHz Bandwidth

We present the experimental demonstration of a traveling-wave tube (TWT) power amplifier operating in the W-band (75-110 GHz) frequency range. The device is based on a serpentine waveguide (SWG) amplification circuit, a slow wave circuit type capable of high power and broad instantaneous bandwidth in the upper millimeter-wave range. A 20 kV, 140-mA round solenoid-focused electron beam powers the device. At 20 kV, we measure 215 ± 2 W peak RF output power at 93 GHz with 20.1 ± 0.15 dB saturated gain, pulsed at 0.1% duty. We observe 10-GHz instantaneous amplification bandwidth at 100-W minimum output power, covering the range 88-98 GHz. Operating at 20.8 kV, the TWT produces 285 ± 3 W at 91 GHz with 22.4 ± 0.15 dB gain, and 7 GHz, 3-dB bandwidth. The peak electronic efficiency measured is approximately 10%.

Design of an Asymmetric Multistage Depressed Collector for K-Band 100W Traveling Wave Tube

Efficiency and back-streaming current are important indicators in a traveling wave tube (TWT). A new asymmetric multistage depressed collector (MDC) is needed when designing a K-band 100W space TWT for high collector efficiency and low back-streaming current. In the new collector, the second, third, and fourth electrodes are asymmetrically designed to form an asymmetric electric field, and the potential and geometry of the electrode are appropriately optimized. The simulation shows that when the electronic efficiency is 27.28%, the collector efficiency is 85.34% with zero back-streaming current.

Research on GaN-Based RF Devices: High-Frequency Gate Structure Design, Submicrometer-Length Gate Fabrication, Suppressed SCE, Low Parasitic Resistance, Minimized Current Collapse, and Lower Gate Leakage

As an important part of wireless communication systems, the technology of RF devices and circuits has been progressing rapidly. For high-frequency and high-power applications, vacuum electronic devices (e.g., traveling wave tubes) have been widely used, although the vacuum tube is large in size and hard to integrate.

Novel double-band input coupler for gyroklystron and gyro-TWT

A novel input coupler that can operate in two different frequency bands separately is proposed in this paper. The dual-band (X/Ku) signals are input by two rectangular waveguides, respectively, in particular, a Bragg reflector is inserted in one of the rectangular waveguides to avoid the electromagnetic backflow, and then the circular TE01/TE02 modes are excited in the cylindrical cavity by field diffraction through the coupling holes with the help of a coaxial cavity. The simulation results show that dual-band input couplers for both dual-band gyroklystron and gyrotron traveling wave tube obtain excellent performance, laying the foundation for the development of the dual-band gyro-amplifier.

Power Enhancement in W-Band Pulsed Folded Waveguide TWT

To obtain higher power in ${W}$ -band traveling-wave tubes (TWTs), a modified folded waveguide (FWG) with non-concentric bends is introduced and demonstrated, which has higher coupling impedance compared with the normal FWG structure and is utilized to eliminate the band-edge instability of TWT. Phase velocity tapering (PVT) is also used to improve the interaction efficiency between the beam and the circuit. Furthermore, a higher beam current is used and the corresponding magnetic focusing system is designed to confine the beam. A prototype tube was then fabricated and tested at a beam current of 220 mA and beam voltage of 21.9 kV. The testing data show that this ${W}$ -band TWT is capable of delivering an output power of over 450 W with a 4.5-GHz bandwidth at a duty cycle of 10%, with a maximum power of 647 W and electronic efficiency of 13.4%. That is the highest output power and electronic efficiency test results of ${W}$ -band TWTs reported so far.

Numerical Simulation and Theoretical Analysis of Terahertz Sheet Beam TWTs Based on a Ridge-Loaded Folded Waveguide SWS

Generation of terahertz (THz) radiation from an interaction between sheet electron beam with the electromagnetic wave in a smooth transition ridge-loaded folded waveguide (RFWG) structure is proposed in this article. It was found that in the structure the electric field density can be increased so that the average interaction impedance of the proposed circuit is higher than that of conventional sheet beam (SB) folded waveguide (FWG) circuit. According to the beam–wave simulation results, at a designed voltage of 13.1 kV and an electron beam current of 100 mA, a maximum output power of 91.13 W can be achieved, which corresponds to a power gain of 32.88 dB and an efficiency factor of 6.96%. Therefore, the smooth transition RFWG traveling wave tube (TWT) should be considered as a promising THz radiation source. Moreover, based on a recently published 3-D nonlinear theory for a SB FWG-TWT, a version of the 3-D large-signal frequency-domain nonlinear theory model representation for the proposed smooth-transition SB RFWG-TWT was developed. The code is orders of magnitude faster than, and nearly as accurate as, full 3-D particle-in cell (PIC) simulations.

A Wideband High-Voltage Longitudinal Output Structure for Ka-Band Sheet Beam Traveling-Wave Tubes

This article proposes a longitudinal output structure in Ka-band sheet beam traveling-wave tube (SBTWT) with an ultrawideband transmission and high breakdown collector voltage. A novel electromagnetic bandgap (EBG) cutoff structure is designed to separate the slow-wave structure (SWS) and the depressed collector. The large stopband property of the EBG structure is applied to achieve not only the high transmission and the low gap leakage but also the long gap distance to provide a high breakdown voltage of the depressed collector. The 3-D simulations of a single longitudinal structure show a full Ka-band (26.5–40 GHz) reflection of less than −20 dB and gap leakage less than −0.02 dB. The entire longitudinal output collector, including two large gaps cutoff structures and the over mode electron collecting parts, is designed and manufactured. The simulations show a bandwidth of 10.4 GHz with a reflection coefficient <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S} _{\mathbf {11}}$ </tex-math></inline-formula> of lower than −20 dB. The cold test demonstrates the wild bandwidth of ~11.1 GHz with a reflection coefficient <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S} _{\mathbf {11}}$ </tex-math></inline-formula> of lower than −15 dB. The hot test reveals a stable collector dc voltage of −14.2 kV.

A High-Power Sheet Beam Slow-Wave Structure of Traveling Wave Tubes

In future electronic devices, the trade-off between power capacity and thermal management will be a severe challenge. In this letter, a sheet beam slow-wave structure (SB-SWS) has been proposed to realize high-power output while taking into account cooling design inside the traveling-wave tubes (TWT). The design of the modified structure ensures a sufficient axial electric field component to realize efficient interaction, meanwhile expands the width of the metal vanes by 327% to design micro-channel cooling. The power load capacity for electronic interception increased by 25%. Utilizing all-period optimization technology, the maximum output power can reach 4 kW, corresponding 18.8% electronic extraction efficiency. Also presented further is a proof-of-concept design model of Ka-band for practical test. The consistency between experiment and theory shows that this SWS is not only suitable for microwave frequency band, but more suitable for future miniaturized devices in sub-millimeter wave and terahertz bands.

Electron-optical system for dual radial sheet beams for Ka-band cascaded angular log-periodic strip-line traveling wave tube

A novel electron-optical system is proposed for dual azimuthally stacked radial sheet electron beams (ARSEBs). The two sheet beams are emitted from curved surfaces of two cathodes and are focused by a focusing electrode, which is carefully designed to produce a symmetric field distribution for the two beams and to avoid interference between the beams. A common tunable periodic cusped magnetic focusing system is also designed to focus the two ARSEBs. The dual sheet beams are used in a traveling wave tube (TWT) consisting of a cascade of two azimuthally supported angular log-periodic strip-line (ASALS) slow-wave structures (SWSs). The transmission efficiency in the presence of beam–wave interaction is 93%. The particle-in-cell simulation results show that for a beam voltage of 4400 V and a current of 0.39 A, the proposed cascaded TWT has a maximum gain of 32.5 dB at 35 GHz and a maximum output power of 180 W; this is a significant gain improvement over a TWT using a single ASALS SWS.

Experimental study on acoustic condensation fog elimination in traveling wave tube

Acoustic condensation artificial fog elimination has the advantages of wide application range, good economy, flexible and controllable operation area, and does not affect traffic operation. The characteristics of the fog, the frequency and intensity of the acting sound waves have an important influence on the effect of condensation and fog elimination. This paper studies the influence of the frequency of fog on the microscopic particle size distribution and the change of light transmission in the process of fog elimination from both theoretical and experimental aspects. The research results show that for natural fog conditions with low concentration, the time of pure sound wave action takes several minutes to tens of minutes. Under the condition of high-concentration artificial fog, the anti-fog time can be shortened to tens of seconds. Simulations and experiments have both confirmed that sound waves can significantly promote fog elimination, and there is an optimal frequency between 200Hz-700Hz.

Accurate and Efficient Thermal Analysis of Slow Wave Structures for Helix Traveling-Wave Tubes by Finite-Element Method

In slow wave structures (SWSs) for helix traveling-wave tubes, both thermal conductivities (TCs) of materials and thermal contact resistances (TCRs) are temperature-dependent, which have a great influence on the heat dissipation of helix SWSs. This article presents an accurate and efficient thermal analysis method of helix SWSs by finite-element method (FEM). In this method, a novel contact boundary condition (CBC) approach is proposed. This CBC approach can easily integrate various TCR models to simulate temperature-dependent TCRs accurately and conveniently. Specially for double nonlinearities arising from temperature-dependent TCs and TCRs within helix SWSs, a fast nonlinear solver named FMONS is also proposed to significantly improve the efficiency of the thermal simulation for helix SWSs. Based on this method, we further developed a 3-D thermal design tool for helix SWSs called TS2. Different from all the present FEM codes, for example, ANSYS, our TS2 can directly model temperature-dependent TCRs. By simulating various helix SWSs including a global model of the entire helix SWS, it is found that our TS2 has very high accuracy and efficiency in the thermal analysis of helix SWSs, which would be very useful for the design of high-power helix traveling-wave tubes.

A Reduction of Peak-to-Average Power Ratio Based Faster-Than-Nyquist Quadrature Signals for Satellite Communication

The increase in the throughput of digital television and radio broadcasting (DVB) channels can be achieved due to application of signals with a compact spectrum and a relatively small peak-to-average power ratio (PAPR). The reason is the usage of traveling wave tubes (TWT) for amplifying and transmitting signals from a satellite repeater in DVB-S2X systems. At the same time, given that the bandwidth allocated for transmission should be used as efficiently as possible, a high reduction rate of out-of-band emissions level is required. The most effective solution in this direction is the transition to spectrum-economic signals, such as optimal Faster-Than-Nyquist (FTN) signals, which can provide a certain reduction rate of the out-of-band emissions level and minimum acceptable PAPR. This article proposes a method for obtaining optimal FTN pulses, which have symmetry in time domain, with specified PAPR and reduction rate of out-of-band emissions for the quadrature phase shift keying (QPSK) and offset quadrature phase shift keying (OQPSK). The possibility of synthesizing signals with OQPSK modulation is presented theoretically for the first time. Optimal FTN signals can provide PAPR reduction by at most 3 dB and outperform known root raised cosine (RRC) pulses. The simulation model adopts an architecture for quadrature generation of optimal FTN signals with OQPSK modulation with blocks for adjustable pre-amplification, clipping, and power amplification. The proposed signals can be used to increase the spectral and energy efficiencies of satellite broadcasting systems, such as DVB-S2/S2X, as well as low-rate return channels of interactive broadcasting systems with a frequency resource shortage.

Experimental Validation of Phase Velocity and Interaction Impedance of Meander-Line Slow-Wave Structures for Space Traveling-Wave Tubes

Meander lines are promising slow-wave structures (SWSs) for millimeter-wave traveling-wave tubes (TWTs) due to low-cost manufacture, low-voltage operation, and high interaction impedance. However, experimental results on meander lines are rare in the literature. Phase velocity and interaction impedance are the most important parameters for the design and characterization of TWT SWSs. Their experimental determination in meander lines is crucial for validating simulations and developing new topologies. Based on a new theoretical model, this article presents an experimental procedure to determine the phase velocity and the interaction impedance. The method is validated on four different Ka -band (33–37 GHz) meander-line SWSs, including two of a novel topology.

Nonlinear dynamics of radiation of high-current beams of charged particles in spatially periodic structures

Provided a review of physical processes underlying the functioning of vacuum electronic devices, namely, nonlinear processes of radiation of high-current beams of charged particles during their passage through spatially periodic structures (resonators) and their interaction with an electromagnetic field. Principles of functioning of travelling wave tubes, backward wave tubes, multi-wave Cherenkov generators, free electron lasers and free electron masers, volume free electron lasers are considered. These devices operate in a wide spectrum range from microwave to X-ray, have high efficiency, and allow obtaining high radiation powers in narrow spectral ranges. Increase the performance and reliability of electronic vacuum devices is based on improving the complex electromagnetic structures in resonators. The article demonstrates a variety of such devices with an obvious generality of physical principles used in them. Nonlinear chaotic dynamics of their functioning is considered. Particular attention is paid to consideration of principles of operation of volume free electron lasers as well as their difference from other devices.

Demonstration of a High-Power and Wide-Bandwidth G-Band Traveling Wave Tube With Cascade Amplification

We propose and experimentally demonstrate a high-power and wide-bandwidth terahertz travelling wave tube (TWT), which is with two cascade amplification sections: the output wave of the first TWT is used as the input of the second one. Using a 17 kV, 71 mA electron beam, the measured peak power is over 60 watt and the average output power is over 12 watts with 20% duty cycle at the frequency around 0.22 THz. The gain is about 30 dB and the bandwidth is over 6 GHz. The average power-bandwidth-gain product reaches record high in the designed spectral range. We envisage that this new developed terahertz TWT will have broad application prospects, such as in long-range radar and communication systems.

Design of coaxial and waveguide couplers for helix TWT

Abstract In this paper, two types of coaxial coupler and waveguide coupler for different frequency helix traveling wave tubes (TWTs) are designed, fabricated and cold tested. The coaxial coupler includes of window ceramic and RF transformer section. At present multi-section impedance transformer design approach is used for wideband helix TWTs. In any helix TWT, impedance of the source is transformed to the characteristic impedance of helix. This is done by the quarter-wavelength (λ/4) impedance transformation approach. The simulated results of different types of couplers are carried out by HFSS and CST microwave studio software and compare with experimental results. Three-dimensional electromagnetic field simulators allowing the any geometry with port excitations it is possible to model the complex coaxial and waveguide type couplers with helix SWS assembly and predict its desired return loss performances.

A Sheet Beam Electron Gun With High Compression Ratio and Long Transmission Distance Performance for W-Band TWT Applications

A sheet beam electron gun with high compression ratio and long transmission distance performance for $W$ -band traveling-wave tube (TWT) is presented. A beam focusing electrode (BFE) with two pairs of unequal length outstretched structures is designed to provide equal positions of beam waist in both horizontal and vertical planes, which is beneficial to obtain large compression ratio and keep the beam cross section shape over a long distance. The operational voltage is set as 30 kV. The beam current is 0.505 A, and the corresponding cathode current density is about 5.59 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The simulation results show that the electron gun can provide a stable sheet beam under a 0.85-T magnet field. The compression ratio of the obtained sheet beam can be over 35, and the sheet beam can be transported through a 126-mm beam tunnel without interception with a 0.6 fill factor. The parameter sensitivities of the gun are analyzed and discussed as well.

The High-Order Coalesced TM11-Like Mode Operation for 220 GHz Sheet Beam Traveling-Wave Tube

Sheet beam vacuum electron tubes are an attractive solution for high-power sources or amplifiers at millimeter-wave and terahertz range. In this article, a high-order coalesced TM11-like mode operation for 220 GHz sheet beam traveling-wave tube is proposed. The feasibility of coalesced TM11-like mode operation is investigated. The utilization of high-order mode expands the RF circuit power capacity, increases the beam current and reduces the cathode emission density. The self-consistent nonlinear code and three-dimensional particle-in-cell simulations are used to verify the 6.7% beam-wave interaction at 220 GHz without phase matching optimization. The maximum output power reaches 370 W. The results demonstrate this methodology is a good solution to build a high-power device for a wide region of terahertz.