Folded Waveguide Traveling Wave Tube Research Articles

Design and Analysis of a 0.33-THz Sine-Shaped Folded Waveguide Traveling Wave Tube

Folded waveguide (FWG) traveling wave tube (TWT) is an attractive choice for a high-power amplifier. This article proposes a novel slow wave structure (SWS) called sine-shaped FWG (S-FWG), which is derived from conventional FWG. With the feature of transforming the arc-shaped segment of the traditional FWG into the sinusoidal waveguide, the proposed structure could be quickly optimized by modifying the amplitude of the two sine curves, and it has an ability to produce a greater interaction impedance compared to the traditional FWG SWS with the same frequency band, which, under the same operating conditions, is expected to result in a higher output power. Using CST Microwave Studio, the high-frequency properties and beam–wave interaction were simulated. Simulation results indicate that the S-FWG TWT reaches a maximum output power of 16.82 W and a gain of 32.25 dB at 330 GHz with an operating voltage of 19.4 kV, a beam current of 30 mA, and an input power of 10 mW. The S-FWG SWS should therefore be regarded as a promising SWS for the high-power terahertz (THz) traveling wave amplifier.

Design and Experimental Demonstration of a Circular Beam Electron Gun and a PPM System for D-Band TWTs

In this article, we present a design scheme of an electron-optical system (EOS) for terahertz vacuum electron amplifiers. The performance characteristics are studied by simulation, and the feasibility is verified by transmission experiment. Based on the Pierce design method, an electron gun with an isolated beam focus electrode (BFE) is designed, which has been simulated and confirmed by CST particle tracking solver and OPERA charged particle solver. A periodic permanent magnet (PPM) system including dual D-magnets of RF signal coupling port is utilized for focusing beam transportation in the 0.15-mm-radius tunnel. The experimental results of the magnetic field are in good agreement with the simulation results. The simulation results show that the beam-focusing system with the axial peak magnetic field of 5.2 kGs can confine the electron beam within the beam tunnel. Additionally, the EOS is successfully developed and used in the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</i> -band folded waveguide traveling-wave tube (TWT). The experimental results of electron transmission show that the thermionic cathode with a cathode load of 5 A/cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\text{2}}$</tex-math> </inline-formula> can provide a beam current of 56 mA, and 90% beam current is transmitted through the beam tunnel to the collector.

Ka-Band Low-Voltage Multiple-Beam Mini-TWT

We describe the design, development, and initial testing of a very compact, multiple-beam folded-waveguide traveling wave tube (TWT) operating in <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ka</i> -band at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 6 kV. A proof-of-principle four-beam, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 100 mA/beam device has been fabricated and tested, and a 16-beam version of the device has been designed and is predicted to produce an output power up to 1 kW. Here, we describe the design approach we devised and implemented to overcome two fundamental challenges: 1) generating and transporting the tightly spaced beams required to operate in the millimeter-wave regime and 2) developing folded waveguide circuits that can provide high interaction impedance for each of the beams as well as stable operation in a dense high-order-mode environment.

A Novel Design of G-Band Broadband Low-Gain Fluctuation Slow-Wave Structure With Improved Folded Waveguides

The design method for ultrawideband amplification with low-gain fluctuation is explored for the terahertz folded-waveguide traveling-wave tube (TWT). Although the folded-waveguide circuit has a large cold bandwidth ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 30%), effective amplification with a flat gain characteristic can only be achieved in a limited range due to sharply reduced interaction impedance. The situation further deteriorates in the design of a terahertz TWT where an oversized beam tunnel has to be used to facilitate the transport of the beam. Through careful analysis of the folded-waveguide (FWG) design characteristics, a modification to the geometry of the circuit has been proposed, leading to a significant enhancement in design capability. As a result, the interaction impedance is increased by 26% over a wide operation frequency band in comparison to the normal folded-waveguide circuit. This makes it possible to use the phase velocity tapering technology to increase efficiency and balance the gain at the same time. A design example in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{G}$</tex-math> </inline-formula> -band is presented. The interaction circuit is capable of producing 18-W output power over 30 GHz from 202 to 232 GHz with a gain fluctuation of less than 1.5 dB.

Development of G-Band Continuous-Wave Folded Waveguide Traveling-Wave Tube

A detailed development of a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$G$</tex-math> </inline-formula> -band continuous-wave (CW) traveling-wave tube (TWT) with a 160-GHz <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\cdot$</tex-math> </inline-formula> W product of bandwidth power is reported in the present work, in which the electron optics system (EOS), the tapered phase velocity folded waveguide (TPVFW) slow wave structure (SWS), the modified pill-box window (MPBW), and experimental results are described. The prototype of the proposed TWT is fabricated with optimized structural parameters obtained by particle-in-cell (PIC) simulations. Moreover, the experimental results show that TWT with 27-W output power at the frequency of 220 GHz at 100% duty cycle is obtained at the operation voltage of 17.1 kV and the current of 62 mA. The proposed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$G$</tex-math> </inline-formula> -band TWT will have broad applications in far-distance high-resolution imaging and large communications capacity.

Simulation Design of G-Band FWG TWT Amplifier Enhanced by π-Mode Extended Interaction

This article studies and summarizes the operating characteristics of a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${G}$ </tex-math></inline-formula> -band folded waveguide traveling wave tube (TWT) amplifier enhanced by the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula> -mode extended interaction cavities. Compared with conventional <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${G}$ </tex-math></inline-formula> -band TWTs, the proposed amplifier can at once obtain a higher gain in a shorter interaction circuit length and ensure a proper operating bandwidth. The key of the design is introducing the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula> -mode multigap resonant cavity with alternating wide and narrow slots, which will improve the working performance of the whole device by shortening the length of the interaction circuit, enhancing its interaction effect, and improving its gain of unit length. In this design, the operation bandwidth is expanded by stagger tuning technique. In addition, the influence of cavity loss is examined so that the optimal performance is achieved. The PIC simulation results show that when the operating voltage is 21 kV and the operating current 80 mA, the maximum average output power of the designed TWT amplifier is 65.78 W at 217.4 GHz, which is corresponding to the maximum gain and efficiency of 37.38 dB and 3.9%, respectively. The gain per unit length is 12.64 dB/cm and the 3-dB bandwidth is 3.5 GHz.

Using Phase Jumping Method to Enhance the Beam–Wave Interaction Efficiency in Terahertz Folded-Waveguide Traveling-Wave Tube

The folded-waveguide (FWG) traveling-wave tube (TWT) is a promising high-power terahertz (THz) amplifier. However, it is challenging to develop THz FWG-TWT with high beam–wave interaction efficiency. In this work, a phase jumping technology, which originates from free-electron lasers, is proposed to boost the beam–wave interaction efficiency of THz FWG-TWT. A modified slow wave structure acts as the phase shifter to adjust the phase gap between electron beam and electromagnetic wave. By jumping the phase gap to an appropriate value, the bunching center of electron beam keeps for a long time in the deceleration region. Thus, more energy is transferred to electromagnetic wave from bunching electron beam. To verify such proposition, a THz FWG-TWT example utilizing the phase jumping is presented in this article. Compared with conventional one, its efficiency within the operation bandwidth from 212 to 224 GHz is improved by 60%–140%.

Improved Gain Equalization Technique for Q-Band Folded-Waveguide TWT for Potential Application in High-Data-Rate Communication

With demanding applications in high-data-rate communication, a folded-waveguide (FWG) traveling-wave tube (TWT) faces the challenge of gain fluctuation and bandwidth limitation. To flatten gain and extend bandwidth, a novel 3-stage gain equalization technique (GET) is proposed and applied in FWG-TWT in this article. This technique shifts the operating point to a high-frequency range in the middle section. Gain equalization can be implemented by improving gain at high frequency and decreasing gain at low frequency. Furthermore, the GET also has a significant advantage in extending bandwidth. Accordingly, a 3-stage FWG-TWT is realized. Compared with a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> -band FWG-TWT with a uniform slow-wave structure (SWS), the 3-stage FWG-TWT extends the 3-dB bandwidth by more than ~45% and the gain variation less than 0.3 dB in the operating bandwidth, which demonstrates great improvement.

Design of high frequency system of 0.34 THz high order mode two-beam folded waveguide traveling wave tube

High frequency system is the key part of folded-waveguide (FW) traveling-wave tube (TWT), it will directly affect the operating frequency, bandwidth, gain and other indicators of TWT. In order to obtain larger output power and higher gain than a conventional single-beam FWTWT, the basic characteristics of the 0.34 THz high order mode two-beam FWTWT are studied. Firstly, the dispersion characteristics and interaction impedance of two-beam FW are calculated and compared with the results of simulation. The results show that the theory of dispersion characteristics is consistent with the simulation results and the interaction impedance matches well in high frequency band. CST studio suite is used to simulate the beam-wave interaction of the two-beam FW, and the output power is 41.68 W. In order to obtain high output, the height of the straight is increased. And the 63.12 W output is obtained with an increase of 52.7%. High frequency system is constituted by mode converter and output window structure, and good transmission characteristics are obtained within 25 GHz bandwidth. In the operating bandwidth, |S11| is greater than 15 dB, |S21| is less than 4.5 dB.

Thermal Management Techniques for Novel Single-Stage Collector of THz Folded Waveguide TWT

Folded waveguide traveling wave tubes (FW-TWTs) exhibit maximum potential in sub-THz to THz regime than its counterparts with respect to power and bandwidth, thereby facilitating its application in high data rate 6G communication systems. However, at high frequency, for synchronous and efficient interaction between electron beam and RF wave, FW-TWTs have to be operated at considerably high beam voltages. Furthermore, it has been reported that the interaction efficiency of FW-TWT is very poor; thus, most of the beam energy remains unspent. The efficiency of such devices may be increased by recovering the unspent beam energy in the collector. A corrugated single-stage collector has been designed with multiple periodic corrugations for an FW-TWT considering the size constraints. This geometry modification has resulted in self-trapping of secondary electrons and increased surface area yielding in >84% collector efficiency (~6% improvement in collector efficiency as compared with conventional collector). Detailed thermal and structural analyses of this collector have been carried out for different electrode materials, namely, copper and graphite. Various thermal management techniques have been employed to improve the thermal dissipation which includes forced air cooling, axial grooves on the outer envelope, and radiator fins. Simulation results reveal that a percentage reduction in the maximum temperature of 32.68%, 22.63%, and 72.59% has been achieved using forced air cooling, axial grooves, and radiator fins respectively. Further simulations also show a percentage decrease of 57.14% and 81.41% in the maximum collector temperature by using forced air cooling in combination with axial grooves and radiator fins, respectively. Based on the thermal and structural results, recommendations for the optimum thermal management technique have also been provided.

Demonstration of W-Band 30-W Continuous-Wave Folded Waveguide Traveling Wave Tube with Lowered Operation Voltage and Improved Gain Flatness

We have demonstrated and developed a W-band 30 W continuous-wave folded waveguide traveling wave tube (CW FWTWT) with 13.5-kV operation voltage and 4.8 dB gain flatness. In this paper, we quantitatively study the relationship between operating voltage and the key geometric dimensions of a folded waveguide, which is the key to the design of the low-voltage TWT. In addition, problems of improving beam transmission and suppressing oscillations are also studied, and the related design methodologies of the long-range electron gun, periodic permanent magnet (PPM) focusing system, low-voltage standing wave ratio (VSWR) coupling system, and horseshoe attenuator are discussed. On the basis of the above analysis, we have designed and fabricated two types of W-band CW TWTs. The test results show that beam transmissions are over 98.5% at DC and better than 96.5% at the highest peak RF power. Measured saturated output powers are greater than 33.6 W with saturated gain more than 42.2 dB within frequency range of 91–99 GHz, and an improved gain flatness of 4.8 dB is achieved.

Development of W-Band Folded Waveguide TWT With Lowered Operating Voltage and Improved Gain Flatness

A W-band folded waveguide traveling wave tube (FWTWT) with lowered 19.4-kV operation voltage and improved gain flatness was investigated and developed. In this article, design methodology, fabrication, and test results were presented and discussed. Two kinds of slow-wave circuits with the positive and negative period stepping were chosen to carry out the contrastive study. A folded waveguide automatic measurement code was independently developed, and the FWTWT was fabricated by LIGA method. The test results showed that a saturated output power was greater than 87 W in the frequency range of 93-100 GHz and greater than 102 W in the frequency range of 95-98.3 GHz with the maximum output power of 123 W at 96.9 GHz. Within the frequency range of 93-100 GHz, saturated gains were higher than 30.05 dB, and gain fluctuation was 3.47 dB with the maximum gain of 33.52 dB at 96.9 GHz. The maximum electronic efficiency of 7.04% and the maximum total efficiency of 13.52% were obtained.

Technology, Assembly, and Test of a W-Band Traveling Wave Tube for New 5G High-Capacity Networks

The folded waveguide (FWG) traveling wave tube (TWT) developed in the frame of the Horizon 2020 TWEETHER project for enabling a novel ${W}$ -band (92–95 GHz) high capacity wireless network for 5G is presented. The FWG TWT was designed by particle-in-cell (PIC) simulations. The technology and the results in terms of measured RF losses and beam transmission from the realized beam tester are presented. Amplification on the first TWT breadboard has been observed but with a poor multireflection pattern resulting from spurious burrs inside the FWG. It indicates, however, that the FWG technology offers great manufacturing simplification compared to conventional helix TWTs, thus enabling a low-cost device with large series production suitable for the wide market of wireless communications.

A $Ka$ -Band Folded Waveguide Traveling Wave Tube With Lumped Resistance Metamaterial Absorber

A Ka -band, folded waveguide traveling wave tube (FWG-TWT) composed of a folded waveguide (FWG) slow wave structure (SWS) and a compact lumped resistance metamaterial absorber (LR-MMA) as a sever load is proposed in this article. The LR-MMA consists of five resonant rings with a metallic ground plane and a separating silica layer. Lumped resistances are used and concentrated resistances are added at the opening of the resonant ring to match the surface impedance of the LR-MMA with that of the metallic waveguide. Simulation results show that the proposed LR-MMA has an absorption bandwidth of 3 GHz at 34-GHz center frequency with absorption exceeding 90%. A FWG-TWT using this LR-MMA is simulated; the output power of this traveling wave tube (TWT) is 112.5 W at 34 GHz, corresponding to a gain of 51.5 dB. The performance of the FWG-SWS using the LR-MMA is similar to that of the FWG-SWS using a wedge sever (WS). For this FWG-SWS, the sever load length of the proposed metamaterial attenuator (MMA) is 0.5 mm, which is much shorter than 9.4 mm of the traditional WS. The LR-MMA is suitable for microfabrication technology and it can be easily assembled with a FWG-SWS, which is helpful for achieving an integrated SWS for the TWT.

X-ray LIGA Microfabricated Circuits for a Sub-THz Wave Folded Waveguide Traveling-Wave-Tube Amplifier

We report experimental results on the microfabrication of three types of folded waveguide circuits with a high aspect ratio by using a high-energy X-ray LIGA technique. A W-band folded waveguide with a total of 80 periods was designed for a traveling-wave-tube (TWT) amplifier operating at 90 GHz–100 GHz. The measured return loss and insertion loss of the W-band fabricated circuit were–10 dB and 0.42 dB/cm, respectively. A G-band circuit showed a higher insertion loss of 1.57 dB/cm. An additional circuit fabrication for a 850-GHz folded waveguide TWT was also successfully carried out. The broad-wall angle measured with respect to the narrow wall of the rectangular waveguide was very close to 90◦, indicating an excellent resolution compared to that of other microfabrication techniques, such as deep reactive ion etching (DRIE) and ultraviolet (UV) lithography, electroplating, and polymer replication (LIGA).

DIMOHA: A Time-Domain Algorithm for Traveling-Wave Tube Simulations

To simulate traveling-wave tubes (TWTs) in time domain and more generally the wave-particle interaction in vacuum devices, we developed the DIscrete MOdel with HAmiltonian approach (dimoha) as an alternative to current particle-in-cell (PIC) and frequency approaches. Indeed, it is based on a longitudinal N-body Hamiltonian approach satisfying Maxwell's equations. Advantages of dimoha comprise: (i) it allows arbitrary waveform (not just field envelope), including continuous waveform (CW), multiple carriers or digital modulations (shift keying); (ii) the algorithm is much faster than PIC codes thanks to a field discretization allowing a drastic degree-of-freedom reduction, along with a robust symplectic integrator; (iii) it supports any periodic slow-wave structure design such as helix or folded waveguides; (iv) it reproduces harmonic generation, reflection, oscillation and distortion phenomena; (v) it handles nonlinear dynamics, including intermodulations, trapping and chaos. dimoha accuracy is assessed by comparing it against measurements from a commercial Ku-band tapered helix TWT and against simulations from a sub-THz folded waveguide TWT with a staggered double-grating slow-wave structure. The algorithm is also tested for multiple-carriers simulations with success.

High power folded waveguide traveling wave tube based on variable-width technology

Variable-width technology applied to the folded waveguide traveling wave tube (FW-TWT) is proposed in this paper to suppress the lower band-edge oscillation and to expand the operating bandwidth when the operating frequency shifts toward the lower band for obtaining larger output power. Changes in the width of the slow wave structure (SWS) determine the variation of the lower-cutoff-frequency (LCF), which makes the LCF's distribution a frequency range rather than a specific frequency-point. The lower band-edge oscillation will not be focused on a special position but distributed in a frequency-band range in which multiple oscillation modes compete with each other near the LCF. The lower band-edge oscillation can be suppressed by mode competition to some extent. In addition, the width variation of SWS also results in the phase-velocity variation, and thus, the operating bandwidth of FW-TWT can be further improved by the optimized design of variable-width-SWS parameters. Beam-wave interaction simulations predicted that power and bandwidth improvements have been achieved without the lower band-edge oscillation. Meanwhile, four solutions are given for further certifying the effectiveness of the variable-width method in lower band-edge oscillation suppression and bandwidth widening. An output power level of over 350 W in a frequency range of 85–90 GHz (5 GHz) is obtained, with a maximum power of ∼648 W and an electron efficiency of ∼12.1% at 88 GHz with a beam current of 260 mA and a beam voltage of 20.5 kV.

Self-Driving Energy-Recirculating Micromachined &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;${G}$ &lt;/tex-math&gt; &lt;/inline-formula&gt;-Band Folded-Waveguide Traveling-Wave Tube Oscillator

We propose a self-driving folded-waveguide traveling-wave tube (TWT) oscillator internally driven by a backward-wave oscillator (BWO). As a driving source, the initial beam energy obtained from the electrostatic field is converted to electromagnetic energy in the BWO. The TWT is designed to amplify the radio frequency signal from the spent electron beam condition as an energy-recirculating method. This paper presents a detailed design study and analysis of the individual circuits, as well as those of the combined circuit, which are conducted using the CST particle-in-cell commercial software and MAGIC code. A maximum average output power of 82.63 W at a frequency of 272 GHz is predicted under the lossless condition. Although the proposed self-driving circuit exhibits a relatively low output power, the key concept behind the novel circuit, which uses a spent electron beam as an amplification source, is validated via simulation. Furthermore, the tunable frequency of the proposed device is tested by changing the initial electron energy using CST Particle Studio, and the expected tunable frequency is 268–272GHz.

Design of a high gain and high efficiency W-band folded waveguide TWT using phase-velocity taper

The folded-waveguide (FW) slow-wave structure (SWS) is a very promising beam-wave interaction circuit for high-frequency traveling wave tubes (TWTs). This paper presents the design methodology for enhancing the gain and efficiency of a practical W-band FWTWT using phase-velocity taper. Initially, phase-velocity, interaction impedance, and gain-growth of a single-section FWTWT are predicted theoretically. Further, these parameters are optimized using the eigenmode solver and transient solver of 3-D CST Studio. The simulated results agreed with theoretically predicted results within ~1% discrepancy. Then the single-section SWS along with the input and output couplers consisting of quarter-wave impedance transformers and pillbox windows are simulated using transient solver and a return-loss of around −20 dB is obtained over the desired band. Finally, the output power, gain and electronic efficiency for a two-section TWT with the phase-velocity taper are obtained using particle-in-cell simulation of CST Studio. At an applied beam voltage of 20 kV and a current of 0.1 A, an output power of 130 W, corresponding to a saturated gain of 39.4 dB, an instantaneous 3-dB bandwidth of around 5.3%, and electronic efficiency of around 6.5% at 94 GHz has been obtained. It was found that at 94 GHz the ‘positively and negatively phase-velocity tapered’ sections enhanced the gain by almost 3 dB and doubles the electronic efficiency of the two-section FWTWT design as compared to the two-section FWSWS with the same interaction circuit length with no phase-velocity taper.

Nonlinear Investigation of Beam-wave Interaction in E Band Folded Waveguide TWT

A E-band folded waveguide traveling-wave tube (FWTWT) is modelled, and the nonlinear interaction between the electron beam and electromagnetic field is investigated by utilizing a 3-D particle-in-cell (PIC) code, MAGIC3D. The process of the interaction for millimeter wave TWT are presented, including the longitudinal momentum of the electron, the averaged radiation power, and the influence of input power on output power. The radiation power and gain of the FWTWT at 85GHz are 88.5W and 27.5dB.The output power of the FWTWT changes only 1 dB across the 6-GHz bandwidth ranging from 82GHz to 88GHz, which is attractive for many applications, e.g. 5G communication networks , satellite communication, electronic counter measures.