Coupled-cavity TWT Research Articles

Efficient linear analysis of coupled-cavity traveling-wave tubes using minimum number of dispersion frequency points

Coupled-cavity traveling-wave tubes (CCTWTs) are high-power amplifiers operating in the microwave and millimeter-wave regions of the spectrum, utilizing beam-wave coupling within an electromagnetic slow-wave structure (SWS). The linear (small-signal) analysis of these amplifiers is a crucial initial step in their electromagnetic design. This paper presents and evaluates a method for analyzing these structures in the linear regime using only three frequency points from the SWS. The results obtained from this method are compared with full-wave particle-in-cell simulations, demonstrating satisfactory agreement. This approach offers a rapid analysis method for examining CCTWTs in the linear state, eliminating the need for lumped circuit modeling and detailed knowledge of the SWS dispersion characteristics.

Fabrication and Cold Test of G-Band Symmetrical Double Slots Coupled-Cavity TWT

Fabrication and Cold Test of G-Band Symmetrical Double Slots Coupled-Cavity TWT

Analytic theory of coupled-cavity traveling wave tubes

Coupled-cavity traveling wave tube (CCTWT) is a high power microwave vacuum electronic device used to amplify radio frequency signals. CCTWTs have numerous applications, including radar, radio navigation, space communication, television, radio repeaters, and charged particle accelerators. Microwave-generating interactions in CCTWTs take place mostly in coupled resonant cavities positioned periodically along the electron beam axis. Operational features of a CCTWT, particularly the amplification mechanism, are similar to those of a multicavity klystron. We advance here a Lagrangian field theory of CCTWTs with the space being represented by one-dimensional continuum. The theory integrates into it the space-charge effects, including the so-called debunching (electron-to-electron repulsion). The corresponding Euler–Lagrange field equations are ordinary differential equations with coefficients varying periodically in the space. Utilizing the system periodicity, we develop instrumental features of the Floquet theory, including the monodromy matrix and its Floquet multipliers. We use them to derive closed form expressions for a number of physically significant quantities. Those include, in particular, dispersion relations and the frequency dependent gain foundational to the RF signal amplification. Serpentine (folded, corrugated) traveling wave tubes are very similar to CCTWTs, and our theory applies to them also.

Bandwidth enhancement for over-mode traveling-wave amplifiers

To increase the bandwidth of conventional over-mode traveling wave tubes (TWTs) with single-mode existing (SME) in an operation band, a theory on over-mode TWTs with multi-modes coexisting (MME) in the operation band is presented in this paper. To verify this theory, G-band dual-beam and shunted coupled Hughes-type coupled-cavity TWTs with MME and SME are investigated. The particle-in-cell simulation analysis confirms that, compared with SME TWT, the MME TWT can not only operate stably but also has 2.24 times bandwidth and a large increase in the output power.

Octree-based voxel model for representation of spatial conflicts detected using domain mapping matrix across multiple design domains

This paper discusses use of domain mapping matrix (DMM) to detect spatial conflicts across multiple design domains and use of octree-based voxel model for representation of these spatial conflicts. A framework has been developed to create octree-based voxel model linked with intended empty spaces in product, which are associated with design requirements, product lifecycle states and with connected design domains. Knowledge in system modelling language (SysML), is used to select criteria for building octree voxel model. Two case studies of coupled cavity travelling wave tube (CCTWT) slow wave structure (SWS) design and high power RF window design, have been taken to showcase the code capabilities. Octree voxels inside CAD platform, show and represent spatial conflicts, detected by associativity modelling of empty space blocks inside CAD along with the product knowledge in SysML model.

Octree-based voxel model for representation of spatial conflicts detected using domain mapping matrix across multiple design domains

This paper discusses use of domain mapping matrix (DMM) to detect spatial conflicts across multiple design domains and use of octree-based voxel model for representation of these spatial conflicts. A framework has been developed to create octree-based voxel model linked with intended empty spaces in product, which are associated with design requirements, product lifecycle states and with connected design domains. Knowledge in system modelling language (SysML), is used to select criteria for building octree voxel model. Two case studies of coupled cavity travelling wave tube (CCTWT) slow wave structure (SWS) design and high power RF window design, have been taken to showcase the code capabilities. Octree voxels inside CAD platform, show and represent spatial conflicts, detected by associativity modelling of empty space blocks inside CAD along with the product knowledge in SysML model.

Operating characteristics of a clamp klystron oscillator at E-band

We describe a klystron-like oscillator operating at a frequency of 77.2 GHz (E-band) using particle-in-cell (PIC) simulations with the code MAGIC. It is shown that the klystron oscillator with one “buncher” cavity and one “catcher cavity” shaped like a “clamp” driven with an applied voltage U=20 kV−100 kV can operate with an electronic efficiency of above 11% with MW output power. To increase the electronic efficiency, we combined two clamp klystrons along the z-axis in a hybrid configuration between a klystron and the slow wave structure of a coupled-cavity TWT. PIC simulations demonstrate that when this hybrid design operates in the TE30 mode, its electronic efficiency can be as high as 28% with an output power as high as 1.35 MW. This article describes in detail this approach to the high power millimeter-wave source design.

Development of a 1.5-kW Average Output Power Coupled-Cavity TWT With a 10% Bandwidth Operating in <inline-formula> <tex-math notation="LaTeX">${X}$ </tex-math> </inline-formula>-Band

A three-section coupled-cavity traveling-wave tube (CC-TWT) has been developed for avionic applications. The design parameters for a TWT operating in ${X}$ -band with a high average power, high power, high efficiency, high duty, wideband, low weight, and rugged structure have been obtained. Data presented show a good agreement between simulation results and prototype test results. The small-signal simulations were done with software developed in house for the evaluation of interaction impedance. The commercial code CST suite was used for the prediction of the dispersion diagram. The large-signal simulations were carried out with the “1-D code, computer program for analysis of CC-TWT,” by Cosmic. This paper explains the theory used for the design and shows the “cold test” and “hot test” results achieved with the prototype. The prototype has been able to deliver more than 1.5-kW average output power in a bandwidth larger than 1 GHz.

Design of two stage depressed collector for high-power traveling wave tube

This paper presents a method for design a suitable two stage depressed collector of X band large duty ratio coupled-cavity traveling wave tube (CC-TWT). The axial, radial velocities and radius of 96 electron trajectories in the output potion of this TWT were simulated and analyzed by 2.5 dimensional interaction package owned by 772nd factory. A refocusing segment was designed to improve the flow of electron injection layer with the help of re-simulation of these parameters. Then a two stage depressed collector was designed with optical design, structural design and thermal analysis. Finally, a TWT was assembled for testing. The measured proportion of the current distribution in every collector electrode and the collector efficiency are close to the analytical full-band average results, which means that this analytical method is suitable for design of two stage depressed collector for high-power CC-TWT.

Design and Large-Signal Modeling of a $W$ -Band Dielectric TWT

The performance capabilities of a dielectric traveling-wave tube (TWT), designed for operation near 94 GHz near the 100-W power level, are investigated using large-signal simulations. Key design elements include a dielectric liner with a moderately high dielectric constant of 13.5, consistent with magnesium orthotitanate ceramic, and the use of a solid round electron beam with properties (26-kV, 100-mA, and 0.185-mm beam radius) that are comparable to those employed in W -band coupled-cavity TWTs. The optimal simulated performance, computed assuming a 5-cm beam-wave interaction length, is 115 W of output power at 94 GHz, with 6-GHz half-power bandwidth, 10.3-dB large-signal gain, and 4.4% electronic efficiency. Technical issues, including the existence of a higher linearity Crestatron regime, the mitigation of beam charging, and thermal management, are discussed.

Study on Ka-band sheet-beam, three-slot-staggered-ladder coupled-cavity traveling-wave tube in a small tunable periodic cusped magnet

A Ka-band sheet electron beam (SEB) traveling-wave tube (TWT) based on three-slot staggered-ladder (TSTL) coupled-cavity slow-wave structure (CC-SWS) is proposed. The high-frequency characteristics of TSTL SWS were analyzed in detail. A well-matched input/output coupling system that solves the problem of energy coupling between the SWS and the external circuit was designed. A new method of loading the distributed attenuator for TSTL SWS was investigated for suppressing backward wave oscillation effectively. In addition, a SEB gun which can produce the SEB conveniently was optimized. A small tunable periodic cusped magnet (STPCM) system was designed to focus the low voltage, high-current SEB, which decreases the whole dimension of the TWT sufficiently. Moreover, the special STPCM system is predicted to exhibit 100% beam transmission efficiency. Finally, the beam-wave interaction was studied. The results obtained by CST simulation show that the SEB TWT can produce output power over 5 kW within the bandwidth ranging from 31.5 to 35.5 GHz, and the maximum output power is 10.15 kW at 33 GHz, corresponding electron efficiency of 10.36%. The fabrication of the device is under way.

Five-Wave Equation for Small-Signal Analysis of Traveling-Wave Tubes Considering the Effects of Axial Periodicity of the Interaction Structure

The conventional Pierce theory is used to analyze traveling-wave tube (TWT) amplifiers. This theory is a convenient method to design helix TWTs. However, in some cases such as coupled-cavity TWTs and folded-waveguide TWTs, this approach yields nonphysical results. In fact, backward-wave excitation may not be ignored for high-power wideband coupled cavities as well as for devices operating near the band edges. The Pierce-type equation is modified to include both backward and forward RF waves simultaneously. A fifth-order algebraic equation is derived that yields TWT small-signal gain in approximately the entire frequency band, including near cutoff frequencies. This approach is based on the coupling between two fast and slow space-charge waves and three RF space-harmonic waves, using independent space-harmonic approximation. The obtained equation may be used to analyze the coupled-cavity folded waveguide and ladder TWT structures.

Preliminary Design and Experiment of a Ridge-Loaded Staggered Single-Slot Rectangular Coupled-Cavity Structure for -Band Traveling-Wave Tube

A novel ridge-loaded staggered single-slot rectangular coupled-cavity traveling-wave tube (CC-TWT) operating at $X$ -band is presented. It features large interaction impedance and good thermal dissipation with a moderate bandwidth. Compared with the existing staggered single-slot rectangular CC-TWT and Hughes-type TWT, this structure has the advantage of large coupled impedance and high electron efficiency, which makes it potentially useful for miniaturization in CC-TWT. The electromagnetic characteristics and the beam–wave interaction based on this slow-wave structure are investigated. The calculation results predict that it potentially could provide saturated output power over 20 kW from 8.5 to 9.8 GHz when the cathode voltage is set from 25.4 to 26.4 kV and the beam current is 5 A, respectively. The corresponding saturated gain and the electron efficiency can reach over 34.4 dB and 15.2%. From 8.6 to 9.5 GHz, the gain is even over 38.6 dB, and the electron efficiency is more than 16.9%. The cold test results show that the voltage standing wave ratio of the high-frequency structure is below 2 at the range of 8.5–10 GHz.

Linear theory of beam–wave interaction in double-slot coupled cavity travelling wave tube**Project supported by the National Natural Science Foundation of China (Grant No. 11205162).

A three-dimensional model of the double-slot coupled cavity slow-wave structure (CCSWS) with a solid round electron beam for the beam–wave interaction is presented. Based on the “cold” dispersion, the “hot” dispersion equation is derived with the Maxwell equations by using the variable separation method and the field-matching method. Through numerical calculations, the effects of the electron beam parameters and the staggered angle between adjacent walls on the linear gain are analyzed.

Simulation of Drive-Induced Oscillation in Coupled-Cavity TWTs

Drive-induced oscillation (DIO) is an instability that limits the achievable output power in wideband coupled-cavity traveling wave tubes (CC-TWTs). It occurs under large-signal conditions, when the electron beam is slowed as a result of its interaction with the circuit at the drive frequency. DIO occurs at a frequency very near the $2\pi $ point (band edge) of the lower branch of the cold circuit dispersion diagram. It is commonly accompanied by a sudden increase in the body current and a flattening of the power transfer curve at the drive frequency. The accurate prediction of the conditions for onset of DIO would be useful to the designers of CC-TWTs. In this paper, we present an approach to the prediction of DIO thresholds using the multifrequency large-signal simulation code TESLA-CC.

A three-dimensional nonlinear beam–wave interaction theory for common traveling wave tubes

A three-dimensional (3-D) nonlinear theory model of beam–wave interaction for common traveling wave tubes (TWTs) is described. The equation governing the amplitude of electromagnetic wave is derived analogously to Poynting’s theorem. The electron dynamics are treated using the 3-D Lorentz force equations. RF space charge fields are obtained from solutions of the Helmholtz equations. In the model, the RF field profiles for cold structure are represented by the digitized RF field calculated by a finite-element software, HFSS. Because the digitized RF field can be obtained in the same way, the 3-D simulation code can be used to simulate common TWTs, such as helix TWTs, coupled-cavity TWTs, and even the folded waveguide TWTs. Results from the 3-D code are compared with those from 1-D code, experiment and the existing analytical theories for three types of slow wave structures.

An Overmoded W-Band Coupled-Cavity TWT

A 94-GHz overmoded traveling-wave tube (TWT) has been designed, fabricated, and successfully tested. The TWT operates in the rectangular TM31 mode of the cavity, while lower order modes are suppressed using selectively placed strips of lossy dielectric. The 87-cavity TWT circuit was directly machined from Glidcop, a dispersion-hardened copper. The TWT was tested in a 0.25 T solenoidal magnetic field in 3- $\mu \text{s}$ pulses. Operating at a voltage of 30.6 kV with 250 mA of collector current, the TWT was zero-drive stable and achieved 21 ± 2 dB linear device gain with 27-W peak output power. Taking into account 3 dB of loss in both the input and output coupling circuits, the gain of the TWT circuit itself is 27 ± 2 dB with 55 W of saturated circuit output power. Using the 3-D particle-in-cell code CST Particle Studio, the linear circuit gain was estimated to be 28 dB and the saturated output power 100 W, in good agreement with the experimental results. The measured bandwidth of 30 MHz was significantly smaller than the predicted value of 250 MHz. The overmoded TWT is a promising approach to high-power TWT operation at W-band and to the extension of the TWT to terahertz frequencies.

Large-Signal Investigation of Cold and Hot Matched Coupled-Cavity Traveling-Wave Tubes

Abstract The implications of different matching conditions for coupled-cavity traveling-wave tubes are considered. Using a small-signal approach the characteristic impedance for the coupled (hot) beam-wave system is derived. It is taken as reference impedance to model ideal transformers and severs. This matching condition is compared to the standard cold match and both cases are analyzed using a large-signal interaction tool. The implications on input reflection and output gain are discussed.

A 2.5-D frequency-domain nonlinear computer model of coupled-cavity traveling wave tubes

Purpose – The purpose of this paper is to present a 2.5-dimensional (2.5-D) frequency-domain nonlinear computer model for the beam-wave interaction of coupled-cavity traveling wave tubes (CC-TWTs). Design/methodology/approach – MKK (proposed by Malykhin, Konnov, and Komarov) equivalent circuit model is used to describe the coupled-cavity slow-wave structure. And the losses are taken into account in the MKK equivalent circuit. Instead of one-dimensional (1-D) disk model, the electron beam is divided into a set of discrete rays and the electron dynamics are treated using the three-dimensional (3-D) Lorentz force equations. Findings – The simulated result obtained show that the computer model can give a good predict for CC-TWTs in V-band. Practical implications – The computer model is capable of treating nonlinear problems. Compared with particle-in-cell simulation, the 2.5-D frequency-domain computer model spends less time. Besides, the 3-D electron trajectory can be used to design high-efficiency collectors for CC-TWTs. Originality/value – The computer model is able to simulate nonlinear problems of coupled-cavity TWT faster.

Recent Developments to the Microwave Tube Simulator Suite

Recent developments to microwave tube simulator suite (MTSS) are reported in this paper. The MTSS is a full-featured tightly integrated software package for microwave tube analysis and design. It includes a commercial grade user interface and three physics simulators. The electron optics simulator is a finite-element (FE) 2-D and 3-D electrostatic steady-state beam trajectory code that has been used to design beam optics, including electron gun and collector. High-frequency circuit simulator is a full 3-D FE computational electromagnetic code that aims to get the accurate electromagnetic wave character spreading in high-frequency structures, such as dispersion, coupling impedance, and attenuation. Beam wave interaction simulator is a large-signal simulation code for helix traveling wave tubes (TWTs), coupled-cavity TWTs, and klystrons. The first version of MTSS was released in 2007, and all of these three physics simulators and user interface are improved and updated over the past six years. This paper reports on some significant advances to MTSS; new Ribbon style user interface, a lot of component modeling templates, more abundant postprocessing features, parallel version, full 64-bit version, more physics, and computation methods improvement.