Beam-wave Interaction Simulation Research Articles

Performance Improvement of Helix Traveling- Wave Tubes Based on Multiobjective Optimization Technique

An optimization design tool for helix traveling-wave tubes (TWTs) is developed by combining the nondominated sorting genetic algorithm II (NSGA-II) and the beam–wave interaction simulator (BWIS), which can realize multiobjective optimization of various TWT figures of merit (FoMs). FoMs include the output power, gain, efficiency, and nonlinear distortion characteristics. This optimization design tool can automatically construct a Pareto-optimal solution set according to FoMs. Then, the final simulation design can be obtained by analyzing the output power variation over the operating band of the individuals in the optimized solution set. In order to ensure the comprehensiveness and accuracy of FoMs, a workflow is established in the optimization tool, which can simulate the physical process by transferring the data between different computing tasks automatically during the design of beam–wave interaction. Based on this design tool, the performance of an L-band helix TWT has been improved significantly. For the demonstrated interaction, the output power, phase distortion, interaction, and recovery efficiency are taken as objectives. After analyzing the output power variation over the operating band, the final parameter set is determined from the Pareto-optimal solution set.

Metamaterial-Inspired Interaction Structure for MW-Level Klystron at 714 MHz

A metamaterial (MTM)-inspired interaction structure for MW-level klystron at 714 MHz is proposed. The electromagnetic characteristics of MTM-inspired resonant cavities with single-bridge complementary electric split ring resonators (CeSRRs) are studied, and the cold-tested results confirm the feasibility of miniaturizing the resonant cavity. Furthermore, the beam–wave interaction simulation for the interaction structure constructed by MTM-inspired resonant cavities with single-bridge CeSRRs is carried out, demonstrating 2.28 MW of output power at 714 MHz for a 100 kV, 40 A electron beam. The radius and length of the MTM-inspired interaction structure are 1/2 and 2/3 of the conventional counterparts, respectively. The promising performance makes it attractive for future applications such as linacs.

A Sub-THz High-Order Mode Backward Wave Oscillator Driven by Pseudospark Sourced Multiple Sheet Electron Beams

A novel concept of combining the advantages of pseudospark (PS)-sourced electron beam (high beam current density), multiple-sheet-electron-beams (large total beam cross-sectional area), and high-order mode (HOM) slow wave structure (SWS) (high power capacity) to produce high-power terahertz signals is presented in this article. As an example, a sub-terahertz backward wave oscillator (BEO) driven by PS-sourced dual-sheet-electron-beams is designed. Measured results of the interaction circuit, including transmission/reflection coefficients and the dispersion relation, were in good agreement with simulation predictions. Beam-wave interaction simulations having considered the plasma effect and conductor loss predicted stable output signals with radiation power of 167.4–255.4 W over a tunable bandwidth of 234.4–241.0 GHz.

Design and Experiment of 1 THz Slow Wave Structure Fabricated by Nano-CNC Technology

This article reports a slow-wave structure (SWS) working at 1 THz which can be fabricated through nano-computer numerical control (nano-CNC) technology for the first time. First, a 1 THz deformed quasi sine waveguide (D-QSWG) SWS has been designed and simulated in this article. To reduce the metal loss of input and output waveguide, a coupler which transit the waveguide to over mode rectangular waveguide has been designed. The simulation result shows that input and output waveguides with this coupler have lower metal loss than that of WR1 rectangular waveguide. To fabricate the designed D-QSWG SWS, tungsten steel endmills have been used. The fabricated SWS has the surface roughness around 58&#x2013;74 nm, the corresponding effective conductivity is around <inline-formula> <tex-math notation="LaTeX">$2\times 10^{{7}}$ </tex-math></inline-formula>&#x2013;1.85 <inline-formula> <tex-math notation="LaTeX">$\times 10^{{7}}$ </tex-math></inline-formula> S/m. And the cold test results show that the insertion loss of the fabricated model is less than 30 dB while the reflection loss is less than &#x2212;15 dB in the frequency range of 0.998&#x2013;1.016 THz, which has a good consistency with the simulated results. The beam-wave interaction simulation results based on the cold test results show that the designed D-QSWG traveling wave tube (TWT) has the output power of 300 mW, and the 3 dB bandwidth is around 3 GHz.

Design and high-power test of 800-kW UHF klystron for CEPC

To reduce the energy demand and operation cost for circular electron positron collider (CEPC), the high efficiency klystrons are being developed at Institute of High Energy Physics, Chinese Academy of Sciences. A 800-kW continuous wave (CW) klystron operating at frequency of 650-MHz has been designed. The results of beam–wave interaction simulation with several different codes are presented. The efficiency is optimized to be 65% with a second harmonic cavity in three-dimensional (3D) particle-in-cell code CST. The effect of cavity frequency error and mismatch load on efficiency of klystron have been investigated. The design and cold test of reentrant cavities are described, which meet the requirements of RF section design. So far, the manufacturing and high-power test of the first klystron prototype have been completed. When the gun operated at DC voltage of 80 kV and current of 15.4 A, the klystron peak power reached 804 kW with output efficiency of about 65.3% at 40% duty cycle. The 1-dB bandwidth is ±0.8 MHZ. Due to the crack of ceramic window, the CW power achieved about 700 kW. The high-power test results are in good agreement with 3D simulation.

Particle-in-Cell Algorithm-Based Computation of Time-Dependent-Multimode Behavior of 42-GHz Gyrotron

The analysis of the multimode behavior of the interaction cavity for 42-GHz gyrotron is presented here. The in-house developed codes single-mode cold cavity solver (SMCCS) and GYCAD and commercial particle-in-cell (PIC) simulator MAGIC are used in the design and analysis of 42-GHz gyrotron cavity. SMCCS is used for the calculation of cold cavity parameters such as <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> factor, resonant frequency, and the axial electric field profile, which are further used in the beam–wave interaction computation. The PIC code is used here for the parametric optimization and time-dependent-multimode beam–wave interaction simulations of the interaction cavity. The mode purity in terms of the electromagnetic power in individual modes is also calculated using the data generated in the beam–wave interaction simulations. The single-mode analysis of the cavity is also performed using the analytical code GYCAD based on the generalized nonlinear theory. The results obtained by the PIC simulator and GYCAD are discussed and compared here. The analysis of the electron beam misalignment, which may occur due to the radial shifting or tilting of the different components during the fabrication of the device, is also performed and the effects on the efficiency, mode competition, and mode stability are discussed in detail.

Design and Microfabrication of an Interaction Circuit for a 0.3-THz Sheet Beam Extended Interaction Oscillator With Multiple-Mode Operation

Normally, the conventional sheet beam extended interaction oscillator (SB-EIO) operates in a single mode and has a narrow tunable bandwidth. To broaden the tunable bandwidth, the concept of multiple-mode operation is proposed and a 0.3-THz ladder-shaped interaction circuit is designed as described in this article. The interaction circuit was manufactured by using the computer numerical control milling technique and cold tested by using a vector network analyzer. Measured and simulated results were in good agreement with each other. Beam-wave interaction simulations with the ohmic loss being taken into account verified the feasibility of the multiple-mode operation and demonstrated that the SB-EIO produced an output power of >91 W in the frequency range of 293.7–294.9 GHz. Compared with the single-mode SB-EIO, the tunable bandwidth of the multiple-mode SB-EIO has increased from 0.3 to 1.2 GHz.

Design, Microfabrication, and Characterization of a Subterahertz-Band High-Order Overmoded Double-Staggered Grating Waveguide for Multiple-Sheet Electron Beam Devices

At present, there is a great interest in increasing the power level of terahertz (THz) radiation sources. Aimed to improve the output power of sheet electron-beam devices, a sub-THz-band rectangular metal column-loaded double-staggered grating waveguide (RMC-DSGW) is proposed in this article, which is suitable for the interaction between the high-order overmoded wave and multiple-sheet electron beams. The RMC is used to suppress adjacent competing modes. To verify the idea, a sub-THz-band TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sub> -like mode RMC-DSGW joined with two identical rectangular TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> -TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> mode converters was designed and manufactured by using nanocomputer numerical control milling. A reflection coefficient lower than ~ -12.6 dB and a transmission coefficient higher than ~ -8.9 dB were measured in the frequency range of 231.3-271.4 GHz. Beam-wave interaction simulations with the ohmic losses being taken into account for an RMC-DSGW-based dual-sheet beam traveling wave tube (TWT) predicted an output power of >332 W and a gain of >30.2 dB over a 3-dB bandwidth of 50 GHz (230-280 GHz). Its output power was approximately twice that of the conventional fundamental mode sheet beam TWT. It proves that the combination of the RMC-DSGW and multiple-sheet electron beams is an effective approach to improve the output power of sheet electron beam devices.

Design of Thin Wire Metamaterial-Based Interaction Structure for Backward Wave Generation

In this article, a thin wire (TW) metamaterial (MTM)-based interaction structure has been reported as a potential candidate for realizing a backward wave oscillator (BWO). The interaction structure has been designed by arranging an array of connected TWs periodically along the axis of a hollow rectangular waveguide. The TW array exhibits negative permittivity along the transverse dimensions and allows mode propagation below the transverse magnetic (TM) mode cutoff of the hollow waveguide. Two TW array variants, one with wires of square cross section (STW) and the other of circular cross section (CTW), have been considered. The frequency range (~12.3–13.3 GHz) of the fundamental backward TM mode has been verified using both the eigenmode and S-parameter simulations. For a stable operation, the operating point has been chosen at $\sim \pi $ /2 point on the fundamental backward mode at 12.8 GHz. The TW array has been designed in such a way so as to avoid the interception with the electron beam propagating down the interaction structure. A prototype of STW array inside a rectangular waveguide has also been fabricated. Preliminary beam–wave interaction simulation has also been carried out, and an output power of 12.98 kW has been observed with an electronic efficiency of 13.61%. This TW array is a simpler structure to realize and hence has the advantage of reduced fabrication complexity. This makes it a viable choice as a filling medium to realize an MTM-based BWO.

Study of a 0.35 THz Extended Interaction Oscillator Driven by a Pseudospark-Sourced Sheet Electron Beam

A compact high-power extended interaction oscillator (EIO) driven by a pseudospark-sourced (PS-sourced) sheet electron beam (SEB) is presented at 0.35 THz. It combines the advantages of a planar interaction circuit and a SEB generated from the PS discharge, including a large beam cross-section, high gain per unit length, and high current density with the additional benefit of not requiring an external focusing magnetic field. Staying within what is achievable with microfabrication techniques, the influence of tolerance on the Q value, resonance frequency, and characteristic impedance was investigated. The effect of surface roughness caused by the manufacturing method on Ohmic loss of the material surface was studied. The advanced microfabrication techniques of Ultra Violet Lithographie, Galvanik, and Abformung (UV-LIGA) and Nano-computer numerical control (Nano-CNC), which are capable of realizing high precision and a metal surface of sufficient smoothness, were proposed to manufacture the planar structures. The effect of plasma density in PS-sourced SEB on the resonance frequency of the EIO circuit was investigated. The simulation results showed that the output signal had a slight frequency upshift and a decrease of the output power as the plasma density increased at 0.35 THz, which is consistent with the theoretical analysis. Beam–wave interaction simulations for this planar EIO predicted a peak output power of 1.8 kW at ~0.35 THz using an effective value of conductivity of ${1.1} \times {10}^{{7}}$ S/m to take into account the skin depth and surface roughness.

Design and Measurement of a Terahertz Double Staggered Grating Waveguide With an Arc-Shaped Beam Tunnel

A millimeter-waveband double staggered grating waveguide (DSGW) with an arc-shaped beam tunnel was previously proposed for sheet beam traveling wave tubes. In this article, the design, fabrication, and cold test of a terahertz band DSGW with an arc-shaped beam tunnel are presented. Simulations demonstrated that the novel DSGW would have a higher coupling impedance and a better beam wave interaction in comparison to the conventional DSGW with a linear-shaped beam tunnel. Beam wave interaction simulations showed that the output power and the electronic efficiency, respectively, increased from ~420 W to ~580 W and from 7.0% to 9.3% at 240 GHz. These two kinds of DSGWs were fabricated by using the nanocomputer numerical control (CNC) milling and wire cutting techniques, and their performances were cold tested by using a vector network analyzer. The measurement results were in good agreement with the simulations and demonstrated that both the traditional and novel DSGWs achieved a wide frequency bandwidth of ~40 GHz (0.22–0.26 THz) with a transmission coefficient ${S}_{{21}}$ better than −2.7/−3.0 dB and a reflection coefficient ${S}_{{11}}$ lower than −13.0/−15.7 dB, respectively.

Triode Type Coaxial Inverse Magnetron Injection Gun for 2-MW, 240-GHz Gyrotron

A triode type coaxial inverse magnetron injection gun (CIMIG) is designed using the 2-D and 3-D beam trajectory simulation codes for 240-GHz, 2-MW coaxial gyrotron. In IMIG, the modulating anode is adjusted at the center of the emitter ring and it provides compact size and better control on the beam instability and beam halo. As per the beam-wave interaction simulations, the CIMIG is designed to deliver the 6.5-MW (100 kV, 65 A) helical electron beam of 8.72-mm guiding center radius at the cavity center. First, the synthesis of CIMIG is performed to estimate a range of electrical and geometrical parameters based on the tradeoff equations. The Gaussian type of magnetic field profile is optimized with the peak magnetic field value of 10 T at cavity center and the beam compression ratio of 35.11. The pitch factor of 1.306 and the transverse velocity spread of 2.74% are realized in 2-D trajectory code electron GUN. The design results obtained in 2-D simulations are validated in 3-D particle trajectory simulator CST-Particle Studio. In 3-D optimization, the gun geometry and the electrical parameters are kept similar to 2-D design. The pitch factor of 1.285 and transverse velocity spread of 3.01% are obtained in 3-D design which shows good agreement with the 2-D design.

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.

Study of Performance Improvement for a &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;${Q}$ &lt;/tex-math&gt; &lt;/inline-formula&gt;-Band Sheet-Beam Traveling-Wave Tube

The study of performance improvement for a ${Q}$ -band sheet-beam traveling-wave tube (SB-TWT) based on an improved double-staggered grating waveguide slow-wave structure (SWS) is presented. To enhance the tube performance, such as reducing the driving power requirement and increasing the electronic efficiency, three approaches were combined together in the tube design, including making the operating frequency be closer to the cutoff frequency of the SWS, improving the SWS with a curved beam tunnel, and introducing the phase velocity tapering technique. Beam–wave interaction simulations predicted that great improvements have been achieved. The output power, gain, and electronic efficiency at 42.5 GHz have, respectively, increased from 7.5 to 20.2 kW, from 23.7 to 60 dB, and from 13.9% to 30.3%, in comparison with a previously designed SB-TWT.

Numerical Simulation and Experimental Study of Sub-THz and THz CW Clinotron Oscillators

Results of computer modeling for the sub-THz and THz continuous-wave clinotrons are presented together with the results of their experimental investigations. Presented simulation results are obtained with both the computation methods and the statistical processing of real device operational parameters. This combination provides relevant output results. The effect of weakly inhomogeneous magnetic field on the electron beam adjustment in clinotron oscillator is clearly shown with the carried out 3-D particle tracking modeling. The effect of the surface roughness and thermal heating of the grating structure on a clinotron operation is also considered, and obtained results were considered during computer modeling. The 3-D thermal heating analysis in clinotron was performed, and its influence on ohmic losses was analyzed. Comparison of beam–wave interaction simulation and experimental results has been presented and discussed.

Theoretical analysis and Vsim simulation of a low-voltage high-efficiency 250 GHz gyrotron

Low-voltage, high-frequency gyrotrons with hundreds of watts of power are useful in radar, magnetic resonance spectroscopy and plasma diagnostic applications. In this paper, a 10 kV, 478 W, 250 GHz gyrotron with an efficiency of nearly 40% and a pitch ratio of 1.5 was designed through linear and nonlinear numerical analyses and Vsim particle-in-cell (PIC) simulation. Vsim is a highly efficient parallel PIC code, but it has seldom been used to carry out electron beam wave interaction simulations of gyro-devices. The setting up of the parameters required for the Vsim simulations of the gyrotron is presented. The results of Vsim simulations agree well with that of nonlinear numerical calculation. The commercial software Vsim7.2 completed the 3D gyrotron simulation in 80 h using a 20 core, 2.2 GHz personal computer with 256 GBytes of memory.

A dielectric-embedded microstrip meander line slow-wave structure for miniaturized traveling wave tube

A modified microstrip meander line (MML) slow-wave structure (SWS) embedded with dielectric rod for miniaturized traveling wave tubes (TWTs) is presented in this paper. High-frequency characteristics, fabrication processes, and beam-wave interaction of the modified MML SWS are analyzed. Compared with conventional MML SWSs supported by dielectric substrate, the modified MML SWS possesses a wider cold bandwidth (about 53.5% increased) and larger interaction impedances (about 21.4% increased). Moreover, this modified MML has better connection with the dielectric, which may improve the slow-wave structure’s stability and reliability under thermal shock, stress impact, and electron bombardment. The beam-wave interaction simulation results indicate that TWTs using the modified MML SWS is capable of delivering 75.85 W saturated output power, corresponding a gain of 28.8 dB and an electron efficiency of 6.32% at the frequency of 35 GHz, with the sheet electron beam of 6000 V and 200 mA.

Study of a 0.2-THz Extended Interaction Oscillator Driven by a Pseudospark-Sourced Sheet Electron Beam

The study of a planar G-band extended interaction oscillator (EIO) driven by a pseudospark-sourced (PS) sheet electron beam is presented. This enables the advantages of a planar interaction circuit combined with the merits of a PS sheet electron beam, including large beam cross section, high current density and the fact that a PS electron beam does not require the use of an external focusing magnetic field. Beam–wave interaction simulations for this planar EIO predicted a peak output power of 2.1 kW at ~0.2 THz. Investigations indicate that this planar EIO has a better tube performance with a higher radiation power compared with the PS pencil electron beam EIO.

Design and Analysis of RF Section and Beam Wave Interaction for C Band 250 kW CW High Power Klystron

Klystron is vacuum electron device operating in microwave range of frequencies. It is used as power amplifier in a variety of systems including radars, particle accelerators and thermonuclear reactors. A precise simulation of beam wave interaction in klystron is useful not only to understand its operation but also in development of tube with minimum iterations in fabrication. In the present case standard design codes like POISSON'S SUPERFISH, AJDISK and MAGIC 2D have been used to estimate and optimize different design parameters of klystron for desired tube performance. The paper presents the results of RF-section simulation of 250 kW CW C-band klystron with specifications as given in Table 1.

Study on phase velocity tapered microstrip angular log‐periodic meander line travelling wave tube

To improve the efficiency of beam-wave interaction and the gain of the microstrip angular log-periodic meander-line (MALPML) travelling wave tube (TWT), a modified phase velocity tapered MALPML (PVT-MALPML) with 45° chamfer is proposed in this study. Compared with MALPML slow wave structure (SWS), the PVT-MALPML SWS has better transmission characteristics. Meanwhile, the beam-wave interaction simulation results show that a higher gain and electron efficiency can be produced by the PVT-MALPML TWT. By using the novel PVT-MALPML SWS, the properties of the radial TWT is further improved to meet the requirements of electronic system to TWTs.