Beam Tunnel Research Articles

Wide band high gain modified-folded-wave-guide slow wave structure for millimeter-wave traveling wave tube amplifier in 6G and beyond back-haul communications

Abstract Millimeter-wave technology is crucial in telecommunications as it enables higher bandwidths and faster data transfer. V-band represents frequencies ranging from 50 to 75 GHz and is used for short-range back-haul communication, defense radar, inter-satellite communications, and space debris detection. Folded waveguide slow-wave structures (FWG-SWS) are an excellent choice for V-band amplifiers due to their wide bandwidths, large gain, and ease of fabrication using modern lithographic processes. This article presents a modified folded waveguide slow-wave structure (MFW-SWS) for V-band traveling wave tube amplifiers (TWTAs) desirable for multi-gigabit 6G and beyond back-haul communications. The MFW slow-wave structure is an FWG-SWS variant that supports an increased RF phase velocity, accommodating additional SWS length for much-improved amplification. The dispersion analysis effectively provides a 15% operational bandwidth ranging from 64–74 GHz with a reduced phase velocity of three-tenths the speed of light apt for enhanced beam-wave interactions. The two-section SWS has independent attenuators designed to provide over 20 dB attenuation throughout the functional bandwidth effortlessly. Additional wave-guide transitions are introduced to achieve the best attenuation characteristics. Along the axis of the SWS, a square beam tunnel is employed to support a 26 kV round electron beam with a beam current of 70 mA. To contain the electron beam within the beam tunnel, this study used a focusing mechanism with a constant magnetic field of 0.25T, which is highly appropriate for miniaturization and large-scale implementation in 6G back-haul towers. Particle-in-cell (PIC) simulations were performed using the CST (Computer Simulation Technology) Studio Suite, and a gain of over 26 dB with an output power of 50 W was successfully attained across the operational bandwidth.

Present and new operational quantities evaluated from photon spectrum measurements at workplaces in the research reactor and accelerator facility at the JAEA

The International Commission on Radiation Units and Measurements (ICRU) has proposed to change the definitions of the operational quantities used for the area and individual monitoring of external exposure in ICRU Report 95. Since introducing new operational quantities into the radiation monitoring may affect the dose assessment using the present personal dosimeters, precise assessments of the influence on monitoring are necessary by characterizing the energy spectrum in the workplace and the energy dependency of the dosimeters to be used. This study measured photon spectra using a NaI(Tl) scintillation detector or a LaBr3(Ce) scintillation detector at the Japan Research Reactor No. 3 (JRR-3) and Japan Proton Accelerator Research Complex (J-PARC) workplaces at the Japan Atomic Energy Agency (JAEA). The photon energy spectra were obtained by unfolding the pulse height distributions measured at each workplace. The present and new operational quantities were then evaluated and compared using the spectra measured at the workplaces.In JRR-3, the photon energy spectra were measured at the workplaces in the reactor room and thermal neutron beam hall while the reactor was operated. The ambient dose equivalent rates H˙*(10) at the workplaces were 0.2 μSv·h−1 – 13 μSv·h−1. High energy gamma-rays of 6–7 MeV from 16N were observed near the fuel failure detection system in the reactor room. Those of 7–8 MeV produced by the thermal neutron capture of iron were observed in the reactor room and the beam hall. In J-PARC, the photon energy spectra were measured at the workplaces in the beam tunnel of the 3 GeV synchrotron beam ring five days after the operation. The ambient dose equivalent rates H˙*(10) at the workplaces were 35 μSv·h−1 –430 μSv·h−1.It was found that the new personal doses, Hp, were 10%–20% smaller than the present personal dose equivalents, Hp(10), at the workplaces.

An E-plane loaded sine waveguide for G-band traveling wave tube

A new slow wave structure (SWS), named E-plane loaded sine waveguide (EPLSW) SWS is proposed for the development of G-band traveling wave tubes (TWTs). The structure, derived from the sine waveguide (SW) SWS, features staggered rectangular gratings on the two E-planes, intersecting with the sine-shaped gratings. The natural electron beam tunnel lies between these gratings. The EPLSW-SWS offers improved characteristics compared to the conventional SW-SWS. High-frequency simulation studies reveal that the passband of the EPLSW-SWS extends by more than 33 GHz over the SW-SWS under identical dispersion conditions. Additionally, the interaction impedance of EPLSW-SWS surpasses that of the SW-SWS in the frequency range of 195–255 GHz. Further analysis through particle-in-cell (PIC) results presents that when operating at 19.5 kV and 145 mA for the sheet beam current, it attains a maximum saturated power of 132.7W and a 3 dB bandwidth exceeding 50 GHz.

Suppression of Parasitic Modes in a Cylindrical Gyrotron Beam Tunnel by a Wall Corrugation With Schroeder Diffuser Structures

Suppression of Parasitic Modes in a Cylindrical Gyrotron Beam Tunnel by a Wall Corrugation With Schroeder Diffuser Structures

Design of an ultrashort H‐plane coupler without transition structure for sheet beam traveling‐wave tube amplifier at W‐band

AbstractAn ultrashort H‐plane sheet beam traveling‐wave tube (SB‐TWT) coupler without transition structure is proposed. The coupler adopts a new idea of directly coupling the microwave from H‐plane of the slow‐wave circuit (SWC) and optimizes the matching by introducing an asymmetric circular ridge on the E‐plane of the coupled waveguide. As a result, the length of the SB‐TWT coupler is reduced from more than 3λ to 1λ. At the same time, the beam tunnel area is enlarged to twice that of the SWC beam tunnel. In the working band (90–100 GHz) of the SWC, the simulation results show that the reflection is less than −25 dB, the isolation is greater than −22 dB. And the influence of the dimensions and locations of ridge on the performance of the coupler is studied. Particle‐in‐cell simulation results also show that the new coupler can not only complete the microwave coupling well but also reduce the number of SWC periods.

A Novel Folded Rectangular-Grating Slow Wave Structure for Dual-Beam Traveling Wave Tube

Increasing the input current is an important method to achieve the high-power output for the traveling wave tubes (TWTs) operating in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{W}$</tex-math> </inline-formula> -band and higher terahertz (THz) band. For the folded waveguide (FW) TWT, increasing the height of the straight waveguide section of slow wave structure (SWS) can realize a double tunnel to increase the input current, but its electromagnetic characteristics, such as the operating passband, interaction impedance, and ohmic loss, become worse. Thus, a novel SWS, called folded rectangular-grating (FRG), with two tunnels for more input current, was proposed. Its two beam tunnels, where the electromagnetic wave in the operating band is cutoff, are located on the upper and lower edges of the grating, respectively, which makes the FRG-SWS have a wider operating passband, higher interaction impedance, and lower ohmic loss by freely adjusting its grating height. More importantly, its structural complexity is the same as that of FW-SWS. First, the electromagnetic characteristics, including dispersion, interaction impedance, and ohmic loss, are analyzed. Then, the particle-in-cell (PIC) simulation was carried out to verify that the saturated power, gain, and electron efficiency of TWT using FRG-SWS have been significantly improved compared with the TWT using FW-SWS. In the end, the cold test of transmission characteristics for FRG-SWS was done, and the experimental results are basically in good agreement with the simulation ones when the conductivity is set as 2.25 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times$</tex-math> </inline-formula> 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\text{7}}$</tex-math> </inline-formula> S/m.

A Modified Fold Waveguide Slow Wave Structure for W-Band Dual-Beam TWT

Increasing the interaction current capacity and the interaction impedance is crucial to achieving high output power for traveling wave tubes (TWTs) in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{W}$</tex-math> </inline-formula> -band and terahertz (THz)-band. With the increased distance between the upper and lower metal walls of folded waveguide (FW), the slow wave structure (SWS) named dual-tunnel FW (DTFW) is formed. Benefiting from such a rational design, the electric field in the noninteracting region decreased and naturally formed dual electron beam tunnels. Meanwhile, the operating mode is cutoff at the beam tunnel, the electromagnetic wave in DTFW can be propagated along the path similar to FW and operate in the fundamental mode. Furthermore, compared with FW, due to its larger size, the DTFW has the same structural complexity and less manufacturing difficulty, which is significant for <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{W}$</tex-math> </inline-formula> -band and THz-band. In this article, through the analysis of the high-frequency characteristics of the SWS using FW and DTFW, the physical mechanism of bandwidth and interaction impedance improvement of DTFW-SWS are revealed. Moreover, the particle-in-cell (PIC) simulation results reveal that, compared with a TWT using unprecedentedly excellent FW-SWS, the performance of TWT using DTFW-SWS has been significantly further improved in the aspects of saturated power, gain, and electron efficiency. Therefore, DTFW-SWS is a promising SWS for high-power and wide-bandwidth TWT in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{W}$</tex-math> </inline-formula> -band and THz-band.

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.

Theoretical Study of a Novel G-Band Sheet Beam Slots-Loaded Coupled-Cavity Traveling Wave Tube

In this article, the design of a novel symmetrical double slots coupled-cavity (SDSCC) traveling wave tube (TWT) operating 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 novel slow wave structure (SWS) suits sheet electron beam and has a large sheet beam channel and high coupling impedance. The novel structure shows great potential in high-power terahertz amplifier field. The dispersion characteristics are studied by both theory and simulation. An equivalent circuit of SDSCC SWS is built. The coupling impedance and electric field of operating mode are simulated. A suitable input–output structure is designed for this novel SWS. The transition characteristics are exhibited. The beam-wave interaction process is also simulated. The peak output power arrives at 384 W at 218 GHz by the usage of a sheet beam with 26.2 kV and 300 mA. The average dc current density in the sheet beam tunnel is around 160 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> . The spectrum of the output signal is pure, and the risk of mode competition is very low.

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.

Compact E-Band Sheet Beam Folded Waveguide Traveling Wave Tube for High Data Rate Communication

The design of a compact <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{E}$</tex-math> </inline-formula> -band (71–76 GHz) sheet beam (SB) folded waveguide (FWG) traveling wave tube (TWT) for high data rate communications is presented in this article. The round beam tunnel for a pencil FWG slow wave structure (SWS) has been redesigned with a rectangular beam tunnel to be suitable for the SB with increased beam current. A novel periodic permanent magnet (PPM)–periodic quadrupolar magnet (PQM) focusing system has been developed to reduce the beam ripple and improve the transmission ratio. A multistep side-out coupler is designed to import and extract the RF power from the SWSs and be compatible with the planar periodic magnetic focusing system. Particle-in-cell (PIC) simulations are employed to evaluate the performance of the high-frequency circuit. Driven by a 1 W signal, the 30-period circuit is estimated to produce more than 100 W power and achieve a 21 dB gain with a less than 0.4 dB variation over 71–76 GHz. Moreover, phase and linearity analysis show a less than 6 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> in-band phase distortion and <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 dB third-order intermodulation distortion (C/3IM) at 30 W of output power, meeting the requirements of a communications-focused TWT. Over 5% circuit efficiency and 40% overall efficiency when equipped with a single-stage collector can be achieved by the compact TWT over the target band.

Metamaterial-Inspired 0.22 THz Traveling-Wave Tubes With Double Sheet Beams

To develop high power and compact terahertz (THz) sheet beam traveling-wave tube (TWT), an all-metal metamaterial (MTM)-inspired slow wave structure (SWS) is proposed. This MTM-inspired SWS exhibits obvious advantages such as high interaction impedance, double beam tunnels, and compactness. The MTM-inspired SWS is devised and then simulated. The calculated average interaction impedance of the SWS for the first spatial harmonic of the fundamental mode is over <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5 \Omega $ </tex-math></inline-formula> in the frequency range of 0.21–0.226 THz, which is bigger than that of the counterparts. Further, the beam-wave interaction of the MTM-inspired 0.22 THz TWT with double sheet beams is simulated. The maximum output power is predicted to be 400 W with 3-dB bandwidth of 5.4 GHz when the input power is 100 mW. Most importantly, the total length of the interaction structure of the 0.22 THz TWT is only 29.2 mm. The cross section of the proposed SWS and beam tunnels are <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.5\times0.42$ </tex-math></inline-formula> mm and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.5\times0.37$ </tex-math></inline-formula> mm, respectively. The simulation results show that the compact THz radiation source presented here is promising for future applications such as in imaging and material analysis.

Exploratory Study on High-Efficiency High-Power W-Band Klystron Based on Kladistron Technology

To achieve tens of kilowatt (kW) output power, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${W}$ </tex-math></inline-formula> -band klystron usually employs a large beam tunnel to accommodate high current. The deteriorated effective impedance of the cavity could be reenhanced by adopting extended interaction (EI) cavities. In this article, a new approach utilizing simple fundamental modes in cascaded single-gap cavities for <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${W}$ </tex-math></inline-formula> -band klystron is proposed and discussed. Although the effective impedance of each cavity is far lower than its EI klystron (EIK) counterpart, this disadvantage could be efficiently compensated by densely populating those single-gap cavities using the kladistron technology previously conceived for the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${C}$ </tex-math></inline-formula> - or <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${X}$ </tex-math></inline-formula> -band high-efficiency (HE) klystrons. However, kladistron is rarely reported and normally regarded as less a promising HE bunching technique overwhelmed by the self-excitation issues caused by adjacent mode coupling. Fortunately, such instability could be naturally suppressed in a higher frequency regime, so further exploration of kladistron technology is intriguing. In this article, an exemplary 30-kW <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${W}$ </tex-math></inline-formula> -band klystron equipped with big circular beam tunnel is designed using the kladistron technology, which finally shows no instability issues in 3-D particle-in-cell (PIC) simulation. This novel device delivers up to 37% electronic efficiency and 24% RF efficiency, validating that the kladistron concept is well suited for this millimeter-wave amplifier, which indicates possible HE substitutes for existing EIK technology.

A Modified Staggered Double Grating Slow Wave Structure for W-Band Dual-Beam TWT

To improve the performance of staggered double grating traveling wave tube (SDG-TWT), such as the saturated power, electron efficiency, and gain, a modified SDG (MSDG) slow wave structure (SWS) is proposed for a wide-band and high-power TWT operating in W-band or higher terahertz band. The MSDG-SWS provides not only high interactive impedance over a relatively wide frequency range but also the double beam tunnels. The physical mechanism of bandwidth broadening, interaction impedance improvement of MSDG-SWS is revealed by analyzing its electromagnetic guided wave characteristics, and the results of high-frequency characteristic indicates that, under the condition that the dispersions of the two SWSs maintain basically consistent, the passband of MSDG-SWS increases by 14 GHz over SDG-SWS, and the average interaction impedance of MSDG-SWS is larger than SDG-SWS in the frequency band below 103 GHz. Moreover, the simulation results indicate that at 17.6 kV of beam voltage and 400 mA of beam current, the MSDG-TWT can provide more than 260 W of saturated output power in the frequency range of 78–122 GHz and can reach 551 W of maximum power at 86 GHz with 7.61% of maximum electron efficiency. Thus, it can be regarded that MSDG-SWS is a promising SWS for TWTs operating in W-band or higher terahertz band.

Three-Way Serpentine Slow Wave Structures With Stationary Inflection Point and Enhanced Interaction Impedance

We introduce two novel variants of the serpentine waveguide slow-wave structure (SWS), often utilized in millimeter-wave traveling-wave tubes (TWTs), with an enhanced interaction impedance. Using dispersion engineering in conjunction with transfer matrix methods, we tune the guided wavenumber dispersion relation to exhibit stationary inflection points (SIPs), and also non-stationary, or tilted inflection points (TIPs), within the dominant TE10 mode of a rectangular waveguide. The degeneracy is found below the first upper band-edge associated with the bandgap where neighboring spatial harmonics meet in the dispersion of the serpentine waveguide (SWG) which is threaded by a beam tunnel. The structure geometries are optimized to be able to achieve an SIP which allows for three-mode synchronism with an electron beam over a specified wavenumber interval in the desired Brillouin zone. Full-wave simulations are used to obtain and verify the existence of the SIP in the three-way coupled waveguide and fine-tune the geometry such that a beam would be in synchronism at or near the SIP. The three-way waveguide SWS exhibits a moderately high Pierce impedance in the vicinity of a nearly-stationary inflection point, making the SWS geometry potentially useful for improving the power gain and basic extraction efficiency of millimeter-wave TWTs. Additionally, the introduced SWS geometries have directional coupler-like behavior, which enables distributed power extraction at frequencies near the SIP frequency.

Study on a Novel Dual-Beam Flat-Roofed Sine Waveguide Traveling-Wave Tube

The present study develops a dual-beam flat-roofed sine waveguide slow wave structure (SWG SWS) by complying with the conventional flat-roofed SWG SWS in this article. Through the placement of a metal plate in the electron beam tunnel center of conventional flat-roofed SWG SWS, the electron beam tunnel is divided into two tunnels. The SWS exhibits a wide working bandwidth, low loss, and small reflection by optimizing the structural size of the dual-beam flat-roofed SWG. Moreover, a comparison is drawn of the slow wave characteristics and beam–wave interaction between dual-beam flat-roofed SWG and conventional flat-roofed SWG. As revealed from the results, the output power level of the dual-beam flat-roofed SWG traveling-wave tube (TWT) is significantly higher than that of the flat-roofed SWG TWT exhibiting the identical current density or the input power. Furthermore, this study develops a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$W$ </tex-math></inline-formula> -band high-gain dual-beam flat-roofed SWG TWT with an attenuator. The maximum output power is 756.6 W at 93 GHz, and the corresponding gain is 38.73 dB. In the frequency range of 89–100 GHz, the output power reaches over 500 W, and the 3-dB bandwidth exceeds 10 GHz.

Design and Simulation for 100-Watt-Class 340-GHz Extended Interaction Klystron

An extended interaction klystron (EIK) working at the frequency of 340 GHz is designed in this present article. The characteristic impedance, coupling coefficient, and normalized electron conductance of the cold cavities are analyzed and optimized. To improve the output power, the EIK adopts a prebunching (PB)-cavity high-frequency structure (HFS). Compared with the conventional terahertz (THz) EIK, the coherence of PB-cavity EIK is enhanced, and the output power is improved. The 3-D particle-in-cell (PIC) simulation predicts the EIK, driven by a 0.20-A, 22.4-kV electron beam confined in a 0.2-mm-diameter beam tunnel, and generates an output power of 138.3 W at the frequency of 339.7 GHz. The gain, electronic efficiency, and 3-dB bandwidth are 39.6 dB, 3.1%, and 500 MHz, respectively. The proposed EIK can be applied to the THz high-resolution radar.

Design and Analysis of Truncated Velocity Tapered Sine-Waveguide Structure for Traveling-Wave Tube in THz Band

A truncated sine-waveguide traveling-wave tube (SWGTWT) is studied to obtain wideband amplification at 600 GHz. The cold test parameters are obtained and finalized through simulation to achieve minimum dispersion coupled with a high interaction impedance. The interaction impedance at various points of the beam tunnel cross section area is evaluated to choose the right shape of the electron beam for the TWT. The effect of velocity tapering on the SWG structure is studied. Particle-in-cell (PIC) simulation is carried out using cylindrical as well as sheet beams. The velocity-tapered structure exhibits higher power and efficiency. The maximum output power and 3-dB bandwidth are estimated to be 7 W and 40 GHz, respectively. The corresponding gain and electronic efficiency of the truncated SWGTWT are 25.4 dB, and 1.88%, respectively.

Reduced Plasma Frequency Calculation Based on Particle-in-Cell Simulations

We propose a scheme to calculate the reduced plasma frequency of a cylindrical-shaped electron beam flowing inside of a cylindrical tunnel, based on results obtained from Particle-in-cell (PIC) simulations. In PIC simulations, we modulate the electron beam using two parallel, non-intercepting, closely-spaced grids which are electrically connected together by a single-tone sinusoidal voltage source. The electron energy and the beam current distributions along the length of the tunnel are monitored after the system is operating at steady-state. We build a system matrix describing the beam's dynamics, estimated by fitting a 2x2 matrix that best agrees with the first order differential equations that govern the physics-based system. Results are compared with the theoretical Branch and Mihran model, which is typically used to compute the plasma frequency reduction factor in such systems. Our method shows excellent agreement with the theoretical model, however, it is also general. Our method can be potentially utilized to determine the reduced plasma frequencies of electron beams propagating in differently-shaped beam tunnels, where no theoretical model yet exists, such as the case of a cylindrical or elliptical electron beam propagating inside of a metallic beam tunnel of cylindrical, square, or elliptical cross-section. It can be applied also to electron beams composed of multiple streams.

G-Band Broad Bandwidth Extended Interaction Klystron with Traveling-Wave Output Structure

In this paper, we investigate a large-sized beam tunnel, G-band extended interaction klystron (EIK) with a traveling wave output structure for the development of broad bandwidth EIKs. The high-quality factor F was introduced to estimate the bandwidth characteristics of the cluster cavities, and the optimal cluster cavity structure parameters were obtained based on this factor. The simulation mode of the device was designed by the 3D particle-in-cell (PIC) commercial simulation software. Four cluster cavities with a staggered distribution of frequencies were employed to expand the bunching bandwidth, and two traveling wave modes, 2π−π/10 and 2π−2π/10, were used as the operating modes in the output structure, effectively increasing the output bandwidth. The simulation findings show that the maximum output power is 170 W, the corresponding gain is 37.5 dB, and the 3-dB bandwidth is up to 1.25 GHz. The three-hole coupling structure with a large-sized beam tunnel provides convenience for the fabrication of devices in the G-band, and our study shows a potential method for the realization of a G-band broadband EIK.