Coupled-cavity TWT Research Articles

Ion Instabilities Initiated in the Gun of a Linear Beam Microwave Tube

A theory is presented which offers an explanation for many of the causes of ion-induced instabilities of the phase and amplitude of the RF output of a solenoid-focused 10-kW continuous-wave coupled cavity traveling-wave tube (TWT). This theory is based on the effects of ions in the electron gun. Some of these ions enter the gun from the interaction region of the TWT. Ions generated in the gun can also have a significant effect on the instabilities. The characteristics (in particular, the timing and shapes) of the waveforms of the instabilities are discussed in detail. The sawtooth waveform often observed is explained by the rates of ion generation in the beam and then the rapid discharge of ions into the gun. When gas pressure in the gun is elevated because of relatively high pressure in the TWT, cathode evaporation, or sputtering, the waveforms can take on a complex double exponential shape that sometimes has a double period. Some of the reasons for these effects are given. The effects that are observed of solenoid current, cathode voltage, RF drive level, and collector voltage on the instabilities are explained by the effects of these parameters on an ion barrier at the collector end of the TWT. When these quantities are such that the barrier is small, the drainage of beam ions to the collector is increased and to the gun is decreased. With reduced ion flow to the gun, gas generation in the gun is reduced, and so the causes of the instabilities are suppressed.

Novel coupled-cavity TWT structure using two-step LIGA fabrication

In order to make millimeter-wave sources practical, improved fabrication methods are required. The use of Lithographie, Galvanoformung, Abformung; German acronym (LIGA) to fabricate a coupled-cavity traveling-wave tube (TWT) is presented. The proposed circuit design will focus on two-step LIGA fabrication and dual in-line ladder structure with a rectangular beam tunnel. The fabrication sequence of two-step LIGA is illustrated and the advantages and tradeoff of this approach are described. Three electromagnetic codes-HFSS, GdfidL, and MAGIC3D-are employed to analyze the circuit characteristics in cold and hot test simulations. Finally, the results from small and large signal analyses are presented to verify the validity of the circuit design.

2.5-Dimensional Multi-Signal Large-Signal Analysis of Helix TWTs

A computer model (SUNRAY-3D) for the 2.5-dimensional multi-signal large-signal analysis of helix TWTs has been developed based on the approach developed earlier by the author for the one-dimensional large-signal modelling of helix and coupled-cavity TWTs. Some of the features of O'Malley's work for three-dimensional modelling of coupled-cavity TWTs were incorporated successfully for the modelling of helix TWTs. The present model is capable for the analysis of multiple input signals together with their higher order harmonics and inter-modulation products. The expressions for the circuit field and the induced current at a plane were derived from the electric field in a helical slow-wave structure instead of using the conventional equivalent circuit approach. The program analyses TWTs with arbitrary variations in the helix pitch and rf loss (including sever and tip loss) along the slow-wave structure. Induced backward voltage components and reflected voltage components due to mismatches at the terminations of the helix were included. The program has been shown to be accurate and self-consistent for drive powers from 60dB below saturation to well above saturation. The energy balance factor is satisfied to better than ±0.1% even at and above saturation. The CPU time to compute the performance of a typical TWT for a single drive power with 96 rings and 16 steps per wavelength is less than one minute on a DEC-ALPHA computer and is much less than 20 seconds on an IBM-P-IV.

Nonlinear time-domain analysis of coupled-cavity traveling-wave tubes

A time-dependent nonlinear analysis of a coupled-cavity traveling-wave tube (CCTWT) is presented. The coupled-cavity structure is modeled by a set of equivalent circuit equations where the equations for currents and voltages are coupled to the nearest neighbor cavities. Input and output coupler models as well as sever cavities are included in the formulation. The electron dynamics are treated using the three-dimensional Lorentz force equations although the RF field representation is an analytic model based on cylindrically symmetric geometry. The magnetic focusing fields are also cylindrically symmetric and can be either a solenoid or a periodic permanent magnet stack. The space-charge fields are found by mapping charge to a two-dimensional grid (r, z) and solving Poisson's equation by a finite difference grid formulation. The circuit and Lorentz force equations are integrated in time in a self-consistent manner. The formulation is capable of treating multiple drive frequencies and the associated intermodulation products as well as oscillations and backward wave instabilities. Hence, the model can be used to perform stability analyses. Furthermore, the cavity parameters can be varied to model dynamic velocity tapering for efficiency enhancement. The simulation is applied to the analysis of a sample C-Band CCTWT, and comparisons with measured performance of a Ka-Band CCTWT at Communications and Power Industries, Palo Alto, CA, are made.

The Key Technology of Efficiency Enhancement for Traveling Wave Tubes in Millimeter Wave

The efficiency enhancement methods for helix and coupled cavity traveling wave tube (TWT) in millimeter wave (MMW) are described. The interaction efficiency has been reached 14.56% by dynamic velocity taper in Ka-band TWT. Efficiency improvement has been carried out by used copper helix for broadband MMW TWT. Owing to a ferruleless coupled cavity TWT used phase-adjusted taper, the interaction efficiency has been increased from 9.6 to 22.6%. Finally, enhancing collector efficiency used multistage depressed collectors is discussed in the paper. These concept and technology of the approach are very useful for tube designer.

The Beam-Wave Interaction in a Ka-Band Relativistic Coupled-Cavity TWT

By the method of the eigen mode expansion, a full three-dimensional (3-D) model has been developed that can be used to investigate the beam-wave interaction in a high-power, Ka-band relativistic coupled-cavity traveling-wave tube (TWT). In the tube studied by us, a sever in the interaction section is used to restrain the oscillation of RF electromagnetic fields between the input and output end. In this case, the asymmetric hybrid HTM11 mode has little impact on the main interaction process [1,2], so the RF electromagnetic fields mainly interacting with the modulated electron beam belong to the symmetric mode TM01. The presented model includes three-dimensional RF fields, three-dimensional electron motions, and improved three-dimensional space-charge fields including dc and ac space-charge fields. Moreover, this model can also calculate backward radiation excited by modulated electron beam and the direct effect of the transverse electron motion on the energy exchange. Our calculation results show that the space-charge field has evident effect on the interaction process, the transverse electron motion has some, and the backward radiation has little.

Software Packages for Design of TWTs with Large-signal Analysis

TWTs are widely used as high power microwave amplifiers for numerous defence, communication, and scientific applications. These devices are extremely complex in geometry and very expensive in manufacturing. Numerical modeling and simulation techniques are being used to improve the performance of these devices for higher output power, gain, efficiency, bandwidth and reliability even at very high frequencies. This paper presents details of the in-house developed software packages for the design and analysis of both helix and coupled-cavity TWTs. One software package, SUNRAY code, is described for large signal analysis of TWTs. Accuracies of these codes are ascertained by comparing simulated results with the experimental results and these codes are found highly accurate, self consistent and fast for interactive use. Recently, a space TWT of high efficiency, high gain, high linearity, small size and low weight is designed for a communication satellite system using in-house developed software packages.

Expanding to the Bandwidth of the Coupled Cavity TWTs in MMW

The power capability of millimeter wave coupled cavity traveling wave tube (CCTWT) is larger than other type TWTs, but the operating bandwidth is narrow. Two methods (staggered bandwidth and coalescent mode design) of expanding to the CCTWT in NMW are described. The simple calculation method for realizing coalescent mode is given.

Design of high-efficiency wide-bandwidth coupled-cavity traveling-wave tube phase velocity tapers with simulated annealing algorithms

The output circuit section of a traveling-wave tube (TWT) routinely contains an RF phase velocity taper for the purpose of increasing RF output power and efficiency. By slowing the RF phase velocity in approximate synchronism with the decelerating electron beam bunches, the taper increases power transfer from the beam to the RF wave. Recently, the computational optimization technique of simulated annealing was shown to be very effective in the design of an RF phase velocity taper that significantly increased computed RF power and efficiency of a coupled-cavity TWT. In this paper, two new broadband simulated annealing algorithms are presented that optimize (1) minimum saturated efficiency over a frequency bandwidth and (2) simultaneous bandwidth and minimum efficiency over the frequency band with constant input power. The algorithms were incorporated into the NASA 2.5-dimensional (2.5-D) coupled-cavity TWT computer model and used to design optimal phase velocity tapers using a 59-64 GHz coupled-cavity TWT as a baseline model. Compared to the baseline taper design, the computational results of the first broadband algorithm showed an improvement of 73.9% in minimum saturated efficiency. The second broadband algorithm indicates an improvement of 272.7% in minimum RF efficiency with constant input power drive and an increase in simultaneous bandwidth of 0.5 GHz over that calculated for the baseline TWT.

Theory and simulation of ion noise in microwave tubes

Since there is always some ambient gas in electron beam devices, background ionization is ubiquitous. For long pulse times, the electrostatic potentials associated with this ionization can reach significant levels and give rise to such observed phenomena as phase noise in microwave tubes. This noise is usually associated with the motion of ions in the device; therefore, it is called ion noise. It often manifests itself as a slow phase fluctuation on the output signal. Observations of noise in microwave tubes such as coupled-cavity traveling wave tubes (CC-TWTs) and klystrons have been discussed in the literature. In this paper, a hybrid model is discussed in which the electron beam is described by the beam envelope equation, and the ions generated by beam ionization are treated as discrete particles using the one-dimensional equations of motion. The theoretical model provides good qualitative as well as reasonable quantitative insight into the origin of ion noise phenomena. The numerical results indicate that the model reproduces the salient features of the phase oscillations observed experimentally. That is, the scaling of the frequency of the phase oscillations with gas pressure in the device and the sensitive dependence of the phase oscillations on the focusing magnetic field. Two distinct time scales are observed in simulation. The fastest time scale oscillation is related to the bounce motion of ions in the axial potential wells formed by the scalloping of the electron beam. Slower sawtooth oscillations are observed to correlate with the well-to-well interactions induced by the ion coupling to the electron equilibrium. These oscillations are also correlated with ion dumping to the cathode or collector. As a practical matter, simulations indicate that the low frequency oscillations can be reduced significantly by using a well-matched electron beam propagating from the electron gun into the interaction circuit.

A time-domain circuit simulator for coupled-cavity traveling-wave tubes

The Curnow equivalent circuit was used to predict the dispersion of cold coupled-cavity traveling-wave tubes, as wen as the voltage and current characteristics for lossless and lossy multicavity circuits. The equivalent circuit is extended to have three ports. The added beam port allows the future modeling of the interaction between beam and cavity. Losses are introduced into the circuit as resistors in series with the corresponding inductors. The time-domain solution to the multicavity circuit is developed. It can be applied to the full-spectrum signal. It is also useful for the transient analysis for both single frequency and full-spectrum signals, including the turn-on transients. Numerical methods to solve the time-domain equations are discussed; a second-order leap-frog method and a fourth-order Runge-Kutta method are implemented and analyzed. Simulation results from both codes are compared, and match well with the theory.

Free-electron lasers: vacuum electronic generators of coherent radiation

Vacuum electronic sources have shown marked improvement since the invention of the magnetron before World War II, and dramatic increases in both average powers and frequencies have been achieved. Of course, many of these gains have been achieved by the development of different devices. The typical development pattern for a given device exhibits and initial period of rapid improvement followed by a plateau determined by technological or physical limitations on the concept. Slow wave devices such as magnetrons and/or klystrons operate efficiently at frequencies up through X-band or somewhat higher. Helix or coupled cavity traveling-wave tubes are used for various applications at frequencies ranging up through W-band. At still higher frequencies, fast wave devices such as gyrotrons and free-electron lasers are required for high-power operation. The free-electron laser concept is unique in that the mechanism is applicable across the entire electromagnetic spectrum; free-electron lasers have been built at wavelengths from microwaves through the ultraviolet, and plans are under development for X-ray systems. Our purpose in this paper is to describe the principal directions of free-electron laser research at the present time. To this end, we first give a brief tutorial of the physics underlying the concept and then describe the principal development paths under way.

Resonant Loss Technique Applied in Coupled Cavity Traveling Wave Tubes

The reasons of band-edge oscillation in coupled cavity traveling wave tubes (CCTWT's) have been analyzed . A band-edge oscillation can be effectively suppressed by resonant loss method. Common structure type of resonant loss technique is loss button. The computation of loss button is discussed. Finally, the test method and results of the loss button are described.

Space charge effects in plasma-filled traveling-wave tubes

A theory describing the ac space charge effects in plasma-filled traveling-wave tubes (TWT) is presented. It is shown that in the case of operation at frequencies between the plasma frequency and the upper-hybrid frequency, the space charge forces can significantly enhance the efficiency. The analysis of a concrete, plasma-filled, coupled-cavity TWT shows that over a 20% bandwidth the calculated efficiency can exceed 40%.

3-Dimensional Large Signal Modelling of High Power CCTWTs

An improved 3-dimensional large-signal model has been developed for design and interactive simulation of high power Coupled Cavity Travelling-Wave Tubes (CCTWTs) based on Lagrangian particle dynamics. It has been shown that both the accuracy and computational speed can be improved by suitably modelling the rf beam incorporating a few modifications in the existing theory. In order to validate the efficiency of the model, two tubes have been analysed, one a low power low space-charge NASA CTS 200W tube and the other a high power high space-charge 10KW tube for which experimental results are available in the literature. The analytical results show good agreement with the experimental results with an increase in the computational speed by 20-22%.

A simulated annealing algorithm for optimizing RF power efficiency in coupled-cavity traveling-wave tubes

Decreasing the radio frequency (RF) phase velocity in the output section of a traveling-wave tube (TWT) is a technique commonly used to increase RF power efficiency. In order to optimize the profile of the phase velocity, a simulated annealing algorithm has been developed and implemented into the NASA multidimensional large-signal coupled-cavity TWT computer model. This algorithm allows the determination of the lengths of the individual cavities at the end of the output section necessary to provide the optimized phase velocity profile. The resulting nonlinear computer-generated phase velocity profile provides a design with optimized RF efficiency. In this paper, the optimization algorithm is described and computational results are shown. These results indicate an increase in center-frequency RF efficiency from 7.1 to 13.5% for a V-band coupled-cavity TWT.

Instabilities and spurious noise in coupled-cavity traveling-wave tubes

The main concepts of design and operation of a traveling-wave tube are outlined, with a discussion of rf instabilities and the causes of spurious noise. Methods and principles for avoiding rf instabilities in coupled-cavity TWTs are described. Suggestions are included for applying similar principles to minimize unwanted spurious effects in EBIS devices.

Simulation of cold-test parameters and RF output power for a coupled-cavity traveling-wave tube

Procedures have been developed which enable the accurate computation of the cold-test (absence of an electron beam) parameters and RF output power for the slow-wave circuits of coupled-cavity traveling-wave tubes (TWT's). The cold-test parameters, which consist of RF phase shift per cavity, impedance, and attenuation, are computed with the three-dimensional electromagnetic simulation code MAFIA and compared to experimental data for an existing V-band (59-64 GHz) coupled-cavity TWT. When simulated in cylindrical coordinates, the absolute average differences from experiment are only 0.3% for phase shift and 2.4% for impedance. Using the cold-test parameters calculated with MAFIA as input, the NASA Coupled-Cavity TWT Code is used to simulate the saturated RF output power of the TWT across the V-band frequency range. Taking into account the output window and coupler loss, the agreement with experiment is very good from 60-64 GHz, with the average absolute percentage difference between simulated and measured power only 3.8%. This demonstrates that the saturated RF output power of a coupled cavity TWT can be accurately simulated using cold-test parameters determined with a three dimensional electromagnetic simulation code. >

Novel high-gain, improved-bandwidth, finned-ladder V-band traveling-wave tube slow-wave circuit design

In this paper, the three-dimensional electrodynamic simulation code MAFIA (Solution of MAxwell's Equations by the Finite-Integration-Algorithm) is used to investigate methods of increasing the bandwidth and lowering the operating voltage of the ring-plane circuit. Calculations of frequency-phase dispersion, beam on-axis interaction impedance, attenuation, and small-signal gain per wavelength were performed for various geometric variations and loading distributions of the ring-plane TWT slow-wave circuit. Based on the results of the variations, a circuit termed the finned-ladder TWT slow-wave circuit was designed and is compared here to the scaled prototype ring-plane and a conventional ferruled coupled-cavity TWT circuit over the V-band frequency range. The simulation results indicate that this circuit has a much higher gain, significantly wider bandwidth, and a much lower voltage requirement than the scaled ring-plane prototype circuit, while retaining its excellent thermal dissipation properties. The finned-ladder circuit has a much larger small-signal gain per wavelength than the ferruled coupled-cavity circuit, but with a moderate sacrifice in bandwidth.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Generated Output of Large Signal Program for the Analysis of TWTs

One-dimensional large signal (nonlinear) models for coupled-cavity and helix travelling wave tubes (TWTs) were developed. These models are at least 5 to 6 times faster than the other known large-signal models with no loss in accuracy and present detailed description of the rf circuit field and modulated beam characteristics along a tube and at the output, in both tabulated and graphical forms. This paper describes the details of the generated output from the large signal models for helix and coupled-cavity TWTs and how such outputs can be interpreted for analysing the performances of different types of TWTs.