Folded Waveguide Slow Wave Structure Research Articles

Flatted Grating Folded Waveguide Slow Wave Structure for 0.2 THz Traveling Wave Tube

Flatted Grating Folded Waveguide Slow Wave Structure for 0.2 THz Traveling Wave Tube

Automated Design of Broadband Folded-Waveguide Slow-Wave Structures for Traveling-Wave Tubes via Deep Reinforcement Learning

In this article, deep reinforcement learning (DRL) is utilized to explore high-performance folded-waveguide (FWG) slow-wave structures (SWSs) for broadband application in traveling-wave tubes (TWTs). DRL combines deep learning (DL) for perception and reinforcement learning (RL) for decision-making to form a complete automated system from input data to output action control. The deep <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{Q}$</tex-math> </inline-formula> -network (DQN), a typical model of DRL is employed to learn how to design FWG SWSs like human beings, and to apply the learned design experience to meet different application requirements. To verify the effectiveness of this method concretely, a DQN is trained 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 to explore the general design policy of FWG SWSs for realizing broadband beam-wave interaction, and it will be exploited to search for well-structured FWG SWSs in the frequency band centered at 850 GHz. Using CST to predict the performance of the structure designed by the DQN, the simulation results show that the 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">$\textit{S}_{\text{11}}$</tex-math> </inline-formula> is less than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$-$</tex-math> </inline-formula> 20.3 dB and the transmission loss is less than 1.95 dB/mm from 750 to 1000 GHz. With a pencil electron beam of 20 kV and 10 mA, the hot simulation results show that the 3-dB bandwidth with the output power greater than 1.6 W and the gain greater than 26 dB can reach 110 GHz from 800 to 910 GHz. Such a design method will facilitate researchers to obtain a practical FWG structure at specified operating conditions, and help to get a deeper insight into the design process from the perspective of artificial intelligence (AI).

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.

An accuracy control strategy for micro-milling process of folded waveguide slow wave structure

The slow-wave structure (SWS) working in the terahertz frequency band features large aspect ratio and long span with characteristic dimensions of tens of microns. The development of micro-manufacturing technology for the high-quality fabrication of terahertz SWS is technically essential to promote the advancement of terahertz radiation source devices. In this work, micro-milling approaches were devised to process the 0.34 THz folded waveguide SWS with particle-reinforced metal matrix composite material. The causes of shape error and position error, especially within the arc-shape region, were analyzed in detail, considering the influence from the following error of machine tool and the unfavorable rigidity of milling tools. The optimization of regionalized cutting parameters was achieved, and two productive tool-path-planning schemes were conceived according to the structural features within the processing areas, attempting to minimize the external impact on the shape accuracy of SWS. A practical tool replacement scheme with the orthometric setting slots as a reference for resetting after tool replacement was determined, in order to avoid misalignment at the junction of adjacent units. In consideration of the structural complexity of SWS and the position specificity of burrs, the tool path in the horizontal plane was designed in the way of alternately milling of S-shape slot and straight slot, with cutting parameters adaptable to the depth of the processing subregion, which shows excellent suppression effect of burrs. The proposed micro-milling process strategy offers promises to improve the fabrication quality of high-aspect-ratio SWSs with the minimum structure size of ~50μm.

Design of a High-Efficiency Ka-Band TWT Power Amplifier for Radar Applications

The design of a high-efficiency traveling-wave tube (TWT) based on a nonconventional folded waveguide slow wave structure (FW-SWS) operating over 34–36 GHz for naval and ground Ka-band radars is presented. The ridge-vane-loaded FW structure allows a good compromise between high interaction impedance and wide bandwidth along with a low weight required for aircraft radar applications and a simple planar manufacturing process structure. Small-signal simulations are performed with a custom interaction impedance evaluation program. The CST suite 2021 3-D electromagnetic simulation code is used for the prediction of the dispersion diagram. Starting from a sensitivity analysis of the FW in terms of width, a high electronic efficiency value of 14.6% is obtained with a reduction in the width of 0.1 mm. To further improve electronic efficiency, a negative phase-velocity step tapering technique is applied and a value of 15.4% from 34 to 36 GHz is obtained. Finally, under an optimized beam voltage and a current of 21 kV and 550 mA, respectively, a peak value of the output power of approximately 1.7 kW and an output power of 800 W with a duty cycle of 12% over the frequency band are obtained. Measurement results of the interaction impedance of a ridge-vane-loaded FWG structure composed of a ten-period high-conductivity oxygen-free copper carried out according to the Langerstrom perturbation method confirm the goodness of the simulation results.

Beam–Wave Resynchronization Method of the Nonperiodic Meander Line and Folded-Waveguide SWS for TWTs

A beam-wave resynchronization (BWRS) method is proposed by changing the structural parameters of the nonperiodic (NP) slow wave structure (SWS) to reduce the phase velocity of the wave for traveling-wave tubes (TWT) application. The mathematical model and analysis for the BWRS method are introduced to investigate what condition the NP meander line SWS (ML-SWS) and the NP folded-waveguide SWS (FW-SWS) should satisfy so that the corresponding TWT can have a good output performance. As the applications, the NP zigzag ML-SWS and the concentric arc FW-SWS are designed according to the method, which is also compared with that designed according to the phase velocity synchronization (PVS). The simulated results show that, by using the BWRS method, the max output power (efficiency) of the zigzag ML-SWS TWT can be improved from 56.7 (18.9%) to 76.26 W (25.4%), and the bandwidth can be expanded from 6 to 10 GHz in the Ka-band. Furthermore, the max output power (efficiency) of the concentric arc FW-SWS TWT can be improved from 199 (27.6%) to 237 W (32.9%), and the bandwidth can be expanded more than twice in the W-band as before. These results indicate that the BWRS method is beneficial for the improvement of the output power and electron efficiency and may be beneficial for wider bandwidth.

Cold Characteristics of Overmoded Ridge- and Groove-Loaded Folded Waveguides for High-Power Traveling Wave Tubes

The TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> high-order mode operation characteristics of the ridge-loaded (RL) and groove-loaded (GL) folded waveguide slow wave structures (FWG SWSs) with two electron beam channels are studied for high-power traveling wave tubes (TWTs) at the terahertz (THz) frequency regime. Operating in a higher order mode means that the SWSs can have larger structure dimensions to reduce the fabrication difficulty at the THz frequency band and use higher current to increase the output power. For FWG SWSs operating in the high-order mode, it is difficult to suppress backward wave oscillation (BWO) of fundamental mode. In this article, we proposed the RL and GL FWG SWSs to suppress BWO and studied the dispersion, coupling impedance, and transmission loss of them. The results show that the RL structure can substantially increase the start oscillation length of the interaction circuit and avoid using many circuit sections for spurious mode depression which ease the assembly process of the tube. The GL structure can broaden the bandwidth with high-order mode operation but with the ease of BWO of fundamental mode. The straight RL and GL waveguides are used to explain the above results, and the effect of electron beam channel position on coupling impedance is also studied.

Design Method of the Non-Periodic Meander Line and Folded-Waveguide SWS for TWTs Based on the Effective Phase Velocity Synchronization

A design method of the non-periodic meander line slow-wave structure (ML-SWS) and folded-waveguide slow-wave structure (FW-SWS) for traveling-wave tubes (TWTs) is proposed based on effective phase velocity synchronization. For the non-periodic ML-SWS and FW-SWS, the unified effective axial phase velocity seen by electrons, the dispersion equation, and the ideal beam-wave synchronization voltage are derived. The theoretical results demonstrate that the non-periodic ML-SWS and FW-SWS reported to date are consistent with the proposed method. Furthermore, two examples are presented as applications for the proposed method to the non-periodic SWS design, including the concentric arc FW-SWS and the zigzag ML-SWS. In addition, the simulated results of the two examples are in good agreement with the theoretical ones.

Design and RF Characterization of High-Power Ferrule-Loaded Folded Waveguide SWS for mm-Wave TWT

Design and analysis of ferrule-loaded folded waveguide slow wave structure is carried out for a high-power millimeter-wave traveling-wave tube (TWT). The design of the structure is carried out by numerical simulation. A typical structure has been designed and fabricated at the Ka-band, and cold test measurement is carried out for the cold circuit parameters and the results are compared against numerical analysis. To compare the efficacy of the ferrule-loaded folded waveguide structure, a conventional folded waveguide structure is also designed for the same operating bandwidth, and cold circuit parameters are compared. The comparison results shows that the interaction impedance of the ferrule-loaded folded waveguide structure is double compared with the conventional structure at the cost of reduction in cold bandwidth. Furthermore, particle-in-cell (PIC) simulation is also carried out to estimate the output power and gain for both the structures for the same operating band, and the results are compared. The comparison results show that the cross section is reduced by 22% and the overall interaction length is reduced by 50% for the ferrule-loaded FW-SWS compared with the conventional FW-SWS for the same output power of about 500 W.

Design and Simulation of 10-GHz Bandwidth Phase-Velocity Tapers Folded Waveguide for 108-GHz Continuous-Wave Traveling-Wave Tube

Folded waveguide (FW) traveling-wave tubes (TWT) is a typical kind of vacuum electron device (VED) with some unique characteristics of high frequency, high power, and moderate bandwidth, which are widely used in radars, communication systems, electronic countermeasure, and so on. For the application to the demonstrations of ocean communication with the frequency of over 100 GHz and continuous-wave power with 15 W, an FWTWT operating at the frequency of 108 GHz and the CW power with 50 W is designed. The optimizations of dispersion characteristics and structural parameters of this FWTWT are optimized through a detailed theoretical analysis and a particle-in-cell (PIC) simulation, respectively. For the improvements of communication quality by the increase of output power, a novel method of phase-velocity tapered (PVT) FW slow wave structure (SWS) is used. Comparing with the conventional FWSWS, the output power with the proposed scheme is enhanced 1.5 times. Moreover, the effects of angle and kinetic spreads of electron beam velocity on output power are first investigated using PIC simulations. On the other hand, for obtaining the optimum signal transmission, a tapered and modified pill-box window is presented and used as the transmission structure, which is the characteristics of the broad bandwidth and low attenuation of transmission. With the help of optimized parameters, the CW FWTWT operates at the frequency of 108 GHz, and the maximum output power, gain, and bandwidth are obtained at 78.6 W, 31.9 dB, and 11 GHz, respectively. The optimum results are beneficial to the development of CW FWTWT.

Design and Fabrication of a Novel High Power THz Slow Wave Structure Based on MEMS Technology

With the development of TWT in terahertz band, the size of TWT is greatly reduced and the power gain is also reduced. In order to improve the performance of traveling wave tube (TWT), an E-plane and H-plane tapered folded waveguide slow wave structure operating at 0.34THz is designed. In order to reduce the RF loss, a gradual ridge is added to the E-plane ridge. Compared with other ridge loaded structures, the coupling impedance of this structure has been significantly improved, and it has better dispersion characteristics and higher maximum output power. According to the electromagnetic simulation results, under the condition of 16.8kV, 50mA banded electron beam, the maximum output power of the folded waveguide can reach 65.1w, the power gain can reach 31.8db, the 3dB working bandwidth can reach 12GHz, and the electronic efficiency can reach 7.75%. Finally, a complete DRIE process for processing slow wave structure is explored. The slow wave structure is fabricated by combining silicon with non-silicon.

High Linear Power E-Band Traveling-Wave Tube for Communication Applications

A low-voltage and high linear power E-band traveling-wave tube amplifier is designed to meet the demand of high-data-rate wireless communication. The integrated design, including the dispersion combination of folded waveguide slow wave structure, broadband diamond pillbox window, and AlN-SiC lumped attenuator, ensures the success of the prototype tube with satisfying performance. The test results show that the amplifier is capable of providing more than 45.2 W output power, 38.8 dB saturated gain, 14.3% total efficiency, and 45.6 dB small-signal gain in 71-76 GHz band with a low voltage of 12.88 kV and a current of 80 mA. The 1 dB compression point exceeding 20 W verifies the feasibility of dispersion combination technology for high linear power design.

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.

Design of high-power wide-band G-band third harmonic amplifier

To meet the demand of high-power and wide-band signal sources for G-band vacuum electronic devices, the research on G-band third harmonic amplifier is carried out. The amplifier utilizes the third harmonic current in the nonlinear beam-wave interaction of E-band TWT, and realizes G-band electromagnetic wave amplification by cascading harmonic interaction section. The design scheme of high performance and practical G-band wide-band high-power source adopts folded waveguide slow wave structure with modified circular bends, and the G-band third harmonic amplifier is simulated and optimized by using the microwave tube simulator package (MTSS) software. The result shows that the device can obtain harmonic output power greater than 3.6 W in the range of 15 GHz, with conversion gain＞33.3 dB and electronic efficiency＞0.36%. Compared with other miniaturized terahertz radiation sources in this band, it has superior performance in terms of output power and bandwidth, and thus provides a design basis for the subsequent research of G-band third harmonic amplifier.

Theory and Experiment of High-Gain Modified Angular Log-Periodic Folded Waveguide Slow Wave Structure

A novel modified angular log-periodic folded waveguide slow wave structure (MALPFW-SWS) is proposed for high-gain traveling wave tubes (TWTs). Each unit of the MALPFW-SWS has different dispersion characteristics and thus it has the ability to suppress oscillations in TWTs. The dispersion equation of the MALPFW-SWS is derived and a Ka-band MALPFW-SWS is designed according to the dispersion equation for verification. The particle-in-cell (PIC) simulation indicated that the output signal is steady even the gain of the designed MALPFW-TWT reaches 40 dB. The frequency spectrum of the output signal also proved that the MALPFW-SWS can suppress oscillations. The cold test experiment results show that the transmission characteristics of the fabricated MALPFW-SWS are good and are in good agreement with the simulation results.

A 0.34-THz High-Power, Slow-Wave Structure: Designing and Microfabricating an H-Plane Ridge-Loaded Folded Waveguide

High-power microvacuum electronic devices are one of the key technologies for the study of terahertz radiation sources. The traveling-wave tube (TWT) based on the folded waveguide slow-wave structure is the most promising terahertz device. This article designs the structural parameters of an H-plane ridge-loaded folded waveguide to improve the maximum output power of TWTs in the atmospheric window at 0.34 THz. The results predicted by a theoretical model and by electromagnetic (EM) simulation indicate the interaction impedance of the slow-wave structure is significantly increased. The simulation results show that the design is capable of raising the maximum output power to more than 50 W at 0.34 THz, with sheet electron beam parameters of 16.8 kV and 50 mA. The power gain, the 3-dB bandwidth, and the electronic efficiency of the folded waveguide can reach 30.5 dB, 10 GHz, and 6%, respectively. Finally, the designed slow-wave structure is microfabricated using multistep UVLIGA technology.

Experimental Verification of the Low Transmission Loss of a Flat-Roofed Sine Waveguide Slow-Wave Structure

The sine waveguide (SWG) is presented as a potential slow-wave structure (SWS) for the millimeter-wave (mm-wave) and terahertz (THz) traveling-wave tube (TWT) because of the advantages of wide bandwidth, natural electron beam tunnel, and easy fabrication. In particular, this theoretical study indicates that the transmission loss of the SWG is far less than that of the folded waveguide SWS, which is widely employed in the mm-wave and THz TWT. Here, the flat-roofed SWG slow-wave circuit working in the ${W}$ -band has been machined using nano-computer numerical control technology, which guarantees micron tolerances and surface roughness of tens of nanometers using carbide tooling with 100- ${\mu }\text{m}$ diameter. The cold test results show that the transmission coefficient ${S}_{{{21}}}$ of the entire high-frequency slow-wave circuit is more than −4.1 dB with a total length of 123.84 mm in the frequency range from 90 to 100 GHz, and the transmission loss is less than 0.36 dB/cm.

Design and cold test of period-tapered double-ridge-loaded folded waveguide slow wave structure for Ka band TWTs

A double-ridge-loaded folded waveguide (DRL-FW) travelling wave tube (TWT) based on period-tapered structure is proposed. Through analysing the dispersion characteristics of the DRL-FW slow wave structure (SWS), the physical mechanism of the band-edge oscillation is obtained. Period-tapered SWS is proposed and analysed for verifying the feasibility in suppressing upper-band-edge oscillation and increasing the output power. Then the electromagnetic characteristics and the beam-wave interaction of TWT based on the period-tapered DRL-FW SWS are investigated. The calculation results predict that it potentially could provide continuous wave power over 600W from 29 GHz to 32 GHz without upper-band-edge oscillation. The bandwidth expands from 29-31GHz to 29-32GHz and electron efficiency is increased from more than 8.3% to more than 11%, while the range of operating voltage expands from 22kV-22.5kV to 22kV-24kV. The corresponding saturated gain can reach over 36.8 dB. In addition, we have carried out experimental tests on the transmission characteristics of period-tapered DRL-FW SWS. The cold test results show that the voltage stand-wave ratio (VSWR) is below 1.8 in the range of 29-32GHz. Good transmission characteristics greatly reduce the risk of reflection wave oscillation, thus improving the stability of DRL-FW TWT.

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.

A Forward-Wave Oscillator Based on Folded-Waveguide Slow-Wave Structure

A new concept of oscillator based on a folded waveguide slow-wave structure (FWG SWS) is studied. The resonant cavity of the oscillator is formed by a section of FWG SWS, a length-adjustable waveguide, and a waveguide loaded by multilayer dielectric. The beam is synchronized with the forward wave on the SWS. If an FWG traveling-wave amplifier (TWA) is driven by this oscillator, there is a possibility that the amplifier and the oscillator share the same design and the same art of manufacturing and power supplier. A forward-wave oscillator based on a design of a TWA in 2-mm band is calculated by both a 1-D nonlinear code and particle-in-cell (PIC) simulation. From both the code and the simulation results, when the oscillator is 30-pitch long, the operation voltage 22000 V and the start current is 3 mA at 138 GHz. When the oscillator is 22-pitch long, operation current 15 mA, and operation voltage 22400 V, simulation result shows that the output power is 58.32 W, and the efficiency is 17.26% at 133.2GHz. When both the voltage and length of the length-adjustable waveguide are tuned, the oscillator can operate within all the first dispersion band of the SWS. If only voltage is tuned, the oscillator can oscillate at several discrete frequencies.