Helix TWTs Research Articles

LARGE-SIGNAL FIELD ANALYSIS OF A LINEAR BEAM TRAVELING WAVE AMPLIFIER FOR A SHEATH-HELIX MODEL OF THE SLOW-WAVE STRUCTURE SUPPORTED BY DIELECTRIC RODS. PART 1: THEORY

A rigorous fleld-theoretic method of analyzing the large-signal behavior of a linear beam traveling wave tube amplifler (TWTA) with slow-wave structure modeled to be a dielectric-loaded sheath helix is presented. The key step in the analysis is a representation of the fleld components as nonlinear functionals of the electron arrival time through a Green's function sequence for the slow-wave circuit. Substitution of this functional representation for the axial electric fleld component into the electron ballistic equation casts the latter into a flxed point format for a nonlinear operator in an appropriate function space. The flxed point, and therefore the solution for the electron-arrival time and hence the solution for the electromagnetic fleld components, can be obtained by standard successive approximation techniques. The calculations of the gain, the e-ciency and the other amplifler parameters, comparison of the results of the present theory with experimental results etc., on the basis of such a successive approximation solution for the fleld components, will be presented in the second part of this paper. Ever since the invention by Rudolf Kompfner in 1943, the linear beam traveling wave tubes making use of an helix for the slow-wave structure (popularly known as helix TWTs), with their wide bandwidths and large power outputs, continue to be unsurpassed as broadband ampliflers of microwave power except possibly by gyro-TWTs. The phenomenal growth of the satellite communication industry and the proliferation of radar applications have fueled an unprecedented demand for TWTs meeting stringent design speciflcations. In high power applications, the traveling wave tube amplifler (TWTA) will invariably be operated on the verge of saturation, and hence an analytical study of its large-signal behavior will be of immense interest. The analysis of TWT as an amplifler has been carried out by Pierce and Kompfner (1{3). This theory was developed some six and half decades ago, and it was based on a coupled-wave analysis utilizing the vacuum modes of the helix and the positive and the negative energy space | charge waves of the beam. An improved theory based on an eigen-vector analysis of Maxwell's equations for the helix has been developed by Reydbeck (4). Chu and Jackson (5) and Collin (6) considered the fleld approach for a small-signal analysis of the helix-TWTA. The linearized 'solution' (6) for the axial electric fleld was obtained as a linear combination of three space charge waves, a growing wave and a decaying wave, both having phase speeds slightly smaller than the beam speed and a constant-amplitude wave with a phase speed greater than the beam speed. However, the input boundary conditions (that the total a.c. beam current density and the r.f. perturbation in the electron speed vanish at the input plane) could exactly be satisfled for a flnite beam only by the trivial solution as the (complex) phase-shift constants associated with the three waves were not equal. Moreover, in the nonlinear regime this approach is completely intractable without the introduction of multitudinous limiting assumptions (7). Freund etal. (8,9) presented a linear fleld analysis for a TWTA using radial admittances at the boundaries. The analyses

Design of Coaxial Couplers for High Efficiency Helix TWT

The main objective of the paper is to make an efficient design of the input and output coaxial coupler for a helix TWTs. An approach has been developed for the efficient design and analysis of the coaxial couplers in the practical situation. Normally multi-section impedance transformer approach is used for any wide band coupler. For a space helix TWT, coupler should be wide bandwidth and small size. In this case coupler is matched with helix slow wave structure and the standard 50-ohm connectors. The simulated return loss (dB) profile for different type of couplers is obtained by using Ansoft HFSS, CST microwave studio and compares those with experimental results. The tip loss design at sever ends for the input and the output section has been also optimized.

Analysis of dielectric rods with arbitrary shape for low-dispersion slow-wave structures in helix TWTs

The introduction of rods with optimized cross-sectional shape in slow-wave structures (SWSs) for wideband helix traveling-wave tubes can sensibly improve the dispersion maintaining an adequate interaction-impedance level. In this brief, a procedure for analyzing helix SWSs loaded with rods of arbitrary shape, by using an analysis method adopting the inhomogeneous dielectric loading approach, is proposed. The advantages, in terms of SWS-dispersion improvement, of adopting optimized rod shapes in comparison to the typical rod shapes (i.e., rectangular, circular, and T-shaped) utilized up to now, are also demonstrated.

Study of space charge field based on sheath helix model of TWTS

The electromagnetic field of helix TWTs in the sheath model is analyzed. The dispersion and coupling impedance equations are derived and the one-dimensional space charge reduction factor is discussed in detail. The study shows that the difference of the space charge reduction factor between the sheath model and the perfect conducting tube is small at high frequency. The difference can be interpreted as the effect of the sheath helix. The higher frequency electromagnetic field has greater ability to affect the e-beam. Therefore, the space charge reduction factor equation based on the sheath helix model is inappropriate at high frequency.

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.

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.

Control of IM3 distortion in helix TWTs by harmonic injection-an Eulerian hydrodynamical study

Helix traveling-wave tube equations were formulated under Eulerian-fluid approximations, extending the nonlinearity up to the third-order. Closed-form solutions for the nonlinear equations were obtained by the method of Laplace transforms for the circuit electric field, which was then interpreted, for RF output power. The RF fundamental output power estimated by the present Eulerian theory was validated against that done by the Lagrangian analysis and the regime of agreement of the two theories was ascertained. Then within this range of validity were found the third-order intermodulation products at the frequencies 2f/sub 1/-f/sub 2/ and 2f/sub 2/-f/sub 1/, for the case of two-balanced signal inputs, one at the frequency f/sub 1/ and the other at f/sub 2/. The multisignal output power spectrum predicted by the two theories also showed a fairly close agreement. The role of the two additional second harmonic signals injected at the input, one at the frequency 2f/sub 1/ and the other at 2f/sub 2/, in controlling the third-order intermodulation products (distortion) was next studied by the simple Eulerian hydrodynamic analysis. By harmonic injection at the signal frequency 2f/sub 1/ (2f/sub 2/) alone, the intermodulation product at 2f/sub 1/-f/sub 2/ (2f/sub 2/-f/sub 1/) could be reduced but not at 2f/sub 2/-f/sub 1/ (2f/sub 1/-f/sub 2/). For the reduction of both the intermodulation products, the harmonic inputs at the frequencies 2f/sub 1/ and 2f/sub 2/, respectively, need to be simultaneously injected. In particular, the optimum values of the RF input power levels and phases of the second harmonic inputs relative to the corresponding fundamental inputs, which would result in a minimum in the third-order intermodulation distortion in the device, were found.

A Fast Computation Technique for RF Performances in Broadband Millimeter Wave Helix TWTs

A computation technique for gain, efficiency and output power in broadband millimeter wave helix TWTs from their dimensions is described. The computed results by modeling are good agreement with experimental measurements. It is shown that the method is fast and useful accuracy. It can be used as engineering design for MMW TWTs.

Improved nonlinear model for multisignal analysis of helix TWTs

A simplified and improved approach for the one-dimensional nonlinear multisignal analysis of a complete helix TWT has been developed. The model takes into account both the loss and circuit variation along a tube and the inherent backward wave and reflections due to mismatch at the input and output couplers. The model is generalised to consider any number of simultaneous signals, including generation of higher-order harmonics and intermodulation components. It is fast, for interactive use, and takes less than two minutes on a Micro-Vax computer to determine the detailed characteristics of a typical tube for four-signal operation with frequencies separated by nearly 10%. The energy balance factor is satisfied within ±1%, even at saturation. The accuracy of the model was checked by comparing computed results with published and experimental results for different types of helix TWT.

A new technology for slow wave structure welding

The capability of heat conduction of the slow wave structure is one of the main obstacles to the development of the high-frequency high-power helix TWTs. A simple but reliable technological method for obtaining high heat conduction capability and low rf loss is suggested. The method of measurement of the heat conduction capability and the results of this measurement are also given.