Helix Circuit Research Articles

Development of a K-band traveling wave tube based on carbon nanotube cold cathode

In this work, a K-band traveling wave tube (TWT) based on carbon nanotube (CNT) cold-cathode is developed, which achieves a large output power as well as a high power gain. Compared to traditional thermionic cathode TWT, the CNT-based cold-cathode TWT shows significant advantages in fast response speed, miniaturization, integration, and high efficiency. The CNT-cathode of the TWT can emit a current of 9.26 mA, and a high current of 7.80 mA is delivered to the TWT's electron beam tunnel. The maximum current of the collector is 5.90 mA, indicating that the current density of the electron beam in the helix circuit tunnel reaches 750 mA/cm2 and higher, and the maximum transmittance rate in the experiment can reach 92%. The demonstrated CNT-based TWT can output a maximum power of 10.90 W with a power gain of 15.60 dB, and the maximum power gain achieved is 18.97 dB. The above experiment results imply that the CNT-cathode has great potential for application in TWT devices.

Design and development of high linearity millimeter wave traveling-wave tube for satellite communications**Project supported by the National Natural Science Foundation of China (Grant No. 61401430).

The linearity of the traveling-wave tube is a very important characteristic for a modern communication system. To improve the linearity of the traveling-wave tube at no expense of the saturated output power and overall efficiency, a modified pitch profile combined with a small adjustment of operating parameters is proposed. The optimal design of the helix circuit is evaluated theoretically by a large signal analysis, and the experimental test is also carried out to make a comparison of performance between the novel and original designed traveling-wave tubes. The experiments show that the saturated output powers and efficiencies of these two tubes are close to each other, while the linearity of the traveling-wave tube is obviously improved. The total phase shift and AM/PM conversion at saturation of the novel tube, averaged over the operating band, are only 30.6°/dB and 2.5°/dB, respectively, which are 20.1°/dB and 1.6°/dB lower than those of the original tube, respectively. Moreover, the third-order intermodulation of the novel tube is up to 2.2 dBc lower than that of the original tube.

Integrated Design of a Compact and Lightweight S-Band Traveling-Wave Tube Amplifier for a New Class of Microwave Power Module

A new class of compact and lightweight S-band 1 kW traveling-wave tube (TWT) is being developed for a microwave power module (MPM) that will be used for phased antenna array radar applications. The proposed S-band MPM provides a tenfold peak power increase compared to state-of-the-art S-band MPMs. In this paper, the design of the vacuum power booster TWT part of the MPM is presented. The compact and lightweight S-band TWT is driven by a 6 kV, 0.9 A electron beam. The amplifier is predicted by large-signal simulations to generate over 1 kW at S-band with 25 dB saturated gain and over 40% efficiency. The stability from unwanted oscillations has been investigated. To suppress the oscillations, the helix circuit has been coated with carbon composite material. The coaxial input and output antennas have been fabricated. For efficiency enhancement, a multi-stage depressed collector (MDC) has been designed using a 3D particle-in-cell (PIC) simulator, VORPAL. The collector design makes use of a current loop based on a feedback mechanism for effective design process. The integrated designs of a helix circuit, an electron gun, a periodic permanent magnet (PPM), antennas, and a collector are presented.

A 3-D Analysis of a Microfabricated Ladder Slow-Wave Structure for a Millimeter-Wave Traveling-Wave Tube

The design and the analysis of a ladder slow-wave structure (SWS) operating at 50 GHz is presented. A 3-D particle-in-cell (PIC) modeling of a ladder SWS is presented for the first time. Compared with a conventional helix circuit, the ladder SWS provides wide bandwidth, high interaction impedance, and ease of manufacturing at millimeter wavelengths. A small-signal code based on Pierce's theory was developed to determine the dimensions and the small-signal characteristics of the device. Cold- and hot-test simulations were performed using High Frequency Structure Simulator and VORPAL codes. VORPAL, a 3-D PIC simulator that makes use of the conformal finite-difference time-domain method, predicts a small-signal gain of 15 dB and an instantaneous bandwidth of 6% for a 68-period ladder SWS that is driven by a 22-kV and 196-mA electron beam. Dispersion characteristics, large-signal characteristics, and nonlinear-beam dynamics are investigated.

Small-signal analysis of a rectangular helix structure traveling-wave-tube

This paper investigates the properties of traveling wave-beam interaction in a rectangular helix traveling-wave-tube (TWT) for a solid sheet electron beam. The ‘hot’ dispersion equation is obtained by means of the self-consistent field theory. The small signal analysis, which includes the effects of the beam parameters and slow-wave structure (SWS) parameters, is carried out by theoretical computation. The numerical results show that the bandwidth and the small-signal gain of the rectangular helix TWT increase as the beam current increases; and the beam voltage not obviously influences the small signal gain. Among different rectangular helix structures, the small-signal gain increases as the width of the rectangular helix SWS increases, however, the bandwidth decreases whether structure parameters a and L or Ψ and L are fixed or not. In addition, a comparison of the small-signal gain of this structure with a conventional round helix is made. The presented analysis will be useful for the design of the TWT with a rectangular helix circuit.

Radio-Frequency Characteristics of a Printed Rectangular Helix Slow-Wave Structure

A new type of printed rectangular helix slow-wave structure (SWS) is investigated using the field-matching method and the electromagnetic integral equations at the boundaries. The radio-frequency characteristics including the dispersion equation and the coupling impedance for transverse antisymmetric (odd) modes of this structure are analysed. The numerical results agree well with the results obtained by the EM simulation software HFSS. It is shown that the dispersion of the rectangular helix circuit is weakened, the phase velocity is reduced after filling the dielectric materials in the rectangular helix SWS. As a planar slow-wave structure, this structure has potential applications in compact TWTs.

Characterization of Ring-Bar and Contrawound Helix Circuits for High-Power Traveling-Wave Tubes

In this paper, we present results from an extensive study concerning the phase velocity and the interaction impedance characteristics of slow-wave structures (SWSs) for high-power traveling-wave tubes (TWTs). The SWS under analysis is the ring-bar one, which has been very suitable when high-power levels are needed in TWTs. We develop an analytical model based on the contrawound helix theory, which allows us to characterize the electromagnetic behavior of an equivalent ring-bar SWS. The theoretical model presented in this paper is able to reproduce all previously published results for the contrawound helix, and we also present new results. Additionally, we compare analytical results from the model to experimental results and to 3-D simulation results obtained using CST Microwave Studio.

CAD of Tape Helix Circuits for Traveling Wave Tubes by Convolution Methods

This work describes a new general approach to the analysis of periodic tape helix circuits and its application in practical computer programs for numerical design. The procedure is based on transforming the stationary periodic tape helix itself to Fourier space, in the same manner as the RF fields. The interaction between the RF field harmonics and the stationary circuit harmonics is conceptually a convolution process. The mathematical description of this process is done through the intermediary of a convoluting circuit matrix created from the geometrical harmonics of the stationary metallic circuit, representing its Fourier space transform. Similar representations are made of the external dielectric rod and metallic vane system so that the entire configuration is analyzed by the same approach. The procedure separates the overall fields into two orthogonal distributions belonging to the nullspace and the range of the convolution matrices of the relevant circuit regions. All relevant data such as the dispersion relation and fundamental and space harmonic impedances are obtained as part of the nullspace distribution alone. The distributions belonging to the range have the character of independent idler fields playing no part in the solution process. The presented theory is exact for all configurations of tape helix structures and of anisotropic dielectric rods and metallic vanes. In order to benefit from reduced programming efforts, the exact theory is approximated by disregarding certain second-order effects, and implemented into a comprehensive computer program for design of tape helix circuits for traveling wave tubes. A set of computer-generated numerical data compared with experimental results are presented at the end.

Development of linear traveling wave tubes for telecommunications applications

Traveling wave tubes (TWTs) designed for telecommunications applications in multichannel power amplifiers (MCPAs) are required to have high linearity, low intermodulation distortion, high efficiency and high power outputs. The linearity of the TWT can be greatly improved by operation of the tube significantly backed off from saturation and by optimization of the design of the helix circuit. This results in two tone intermodulation products that are 25 to 40 dB below the carrier level, depending on the amount of back-off selected. However, operation backed off from saturation results in a greatly reduced efficiency of the TWT, which must be compensated for by optimal circuit and collector design. Several L-band and S-band Hughes TWTs have been developed for telecommunications applications and feature saturated power levels of up to 2 kW and average power of over 600 W with overall efficiencies of over 20% at 10 dB back-off and 40% at 6 dB back-off. These tubes provide high average power, high efficiency amplification with modest size and reduced cooling requirements compared to solid state amplifiers.

Application of field emitter arrays to microwave power amplifiers

This paper describes the operation of a field emitter array (FEA) as the electron source of a traveling-wave tube (TWT) amplifier. Issues of beam control and focus at high current density and low magnetic field are addressed as well as issues relating to the inherent high emittance of the FEA beam and cathode protection from ion bombardment. Large signal, nonlinear RF-modulated FEA-TWT interaction simulations show circuit efficiencies that approach 50%, even for minimal bunching of average-to-peak current ratios of 0.7-0.9. Direct radio frequency (RF) modulation at the cathode is predicted to significantly improve linearity in the high-efficiency regime as well as reduce harmonic power levels. An unmodulated C-Band FEA-TWT was built to test the focusing approach as well as the robustness of the emitters in an operating vacuum device. The device uses a 1-mm diameter Spindt emitter with a custom-designed electron gun and helix circuit. The FEA-TWT has operated to date with a maximum current of 91.3 mA and shows 99.5% transmission under both drive and no-drive conditions. Output power of the device is 55.0 W at 1.5 GHz with a saturated gain of 23.4 dB and efficiency of 17%, and performs as predicted by simulation. During all operation, the FEA emission appears extremely stable, with no temporal variations observed at any time.

Characteristics of Bifilar Helix Circuit

The characteristics of two kinds of bifilar helix circuit are discussed in this paper. One is alike wound bifilar helix and another is contrawound bifilar helix. Alike wound bifilar helix is often used in O-type backward wave tube (BWO). Owing to wider operating bandwidth, higher interaction impedance and large dimensions, the contrawound helix is suitable for broadband millimeter wave traveling wave tube (MMW TWT).

Three-dimensional simulation of helix traveling-wave tube cold-test characteristics using MAFIA

The cold-test parameters including dispersion, impedance, and attenuation have been calculated for a helix traveling-wave tube (TWT) slow-wave circuit using MAFIA, the three-dimensional (3-D) electromagnetic finite-integration computer code. The helix model includes tape thickness, rectangular dielectric supports, and material properties consistent with the actual circuit. Measured cold-test data and computer-derived results for the helix circuit are presented with excellent agreement.

Design of traveling wave tubes based on field theory

A method is described for the design of helix traveling wave tubes (TWT) which is based on the linear field analysis of the coupled beam-wave system. The dispersion relations are obtained by matching of radial admittances at boundaries instead of the individual field components. This approach provides flexibility in modeling various beam and circuit configurations with relative ease by choosing the appropriate admittance functions for each case. The method is illustrated for the case of a solid beam inside a sheath helix which is loaded externally by lossy dielectric material, a conducting cylinder, and axial vanes. Extension of the analysis to include a thin tape helix model is anticipated in the near future. The TWT model may be divided into axial regions to include velocity tapers, lossy materials and severs, with the helix geometry in each region varied arbitrarily. The relations between the AC velocities, current densities, and axial electric fields are used to derive a general expression for the new amplitudes of the three forward waves at each axial boundary. The sum of the fields for the three forward waves (two waves in a drift region) is followed to the circuit output. Numerical results of the field analysis are compared with the coupled-mode Pierce theory. A method is suggested for applying the field analysis to accurate design of practical TWT's that have a more complex circuit geometry, which starts with a simple measurement of the dispersion of the helix circuit. The field analysis may then be used to generate a circuit having properties very nearly equivalent to those of the actual circuit.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

SELF-CONSISTENT FIELD THEORY OF GENERAL PERIODIC CIRCUITS FOR TRAVELING WAVE TUBES

A general method for exact analysis of cylindrical periodic circuits for traveling wave tubes without initial assumptions of field or current distributions is described. The procedure is based on transforming the stationary periodic circuit to Fourier space, in the same manner as one does the ordinary RF fields. By this approach the circuit is represented in Fourier space by an infinite number of harmonic circuit components. The interaction, or coupling, between the field harmonics and the stationary circuit harmonics is conceptually a convolution process. The mathematical description of this process involves the intermediary of a convoluting circuit matrix formed from the harmonics of the stationary metallic circuit. For any circuit this matrix is Hermitian and therefore guaranteed a full set of orthogonal eigenvectors and corresponding real eigenvalues. This complete set of vectors serves as a basis for expansions of the RF electric and magnetic harmonic fields. Due to the general nature of the metallic circuit, with surface areas that are partly metallic and partly open, the eigenvectors separate into two distinct sets, one set having eigenvalues equal or close to unity, and the other set eigenvalues equal or close to zero. The first set is associated directly with the metallic parts of the circuit and describes localized surface modes, including surface currents on the two circuit boundaries. The second set, which essentially is a nullset, does not "see" the metallic parts of the circuit, and describes the fields in the open areas of the circuit. Similar expansions are made in the dielectric rod support system exterior to the circuit itself. The paper provides a systematic and general analysis of the entire periodic configuration, based on a conversion of the field equations to a set of first order differential equations in the tangential components of the RF electric and magnetic field harmonics. In all regions the solutions are expressed in terms of transmission matrices which combine through simple boundary conditions to form an overall homogeneous set of equations. Its numerical solution provides all relevant data for the entire periodic configuration, comprising the finite thickness circuit itself, the dielectric support rods, and the enclosing metallic shield. The paper provides explicit formulae for numerical calculations of circuit characteristics, such as omega-beta diagram, interaction impedance, loss due to finite circuit conductivity, and detailed field and current distributions. Throughout the paper emphasis is placed on generality, so that the results are applicable to any kind of periodic cylindrical circuits. Application of the theory to a tape helix circuit is discussed briefly.

The effects of dielectric and metal loading on the dispersion characteristics for contrawound helix circuits used in high-power traveling-wave tubes

The theory developed by M. Chodorow and E.L. Chu (Stanford Univ. Tech. Rep. TR249, 1954) is expanded to include the problem of a contrawound helix interposed between two dielectric regions, symmetrically oriented inside a conducting cylinder. The single helix oriented in an identical fashion is simultaneously analyzed. Numerical results are presented in the form of dispersion diagrams over a wide range of parameters. Interesting behavior found within these diagrams is discussed and, whenever possible, comparable to experimental results. >

Phase velocity dispersion of a generalized metal-segment-loaded helix as used in broad-band traveling-wave tubes

An analytical model for predicting phase velocity dispersion of tape helix circuits loaded with solid-type or vane-type metal segments is described. An accurate design formula for dispersion is derived from the concept of the developed tape helix, taking account of the thickness effects of the tape and the edge effects of the metal segments. Comparisons with measurements of four solid-type and one vane-type helices show about +or-1% accuracy, which is seen to be sufficient for practical applications, The model can also be used to calculate a wide variety of asymmetry effects in loading elements around a tape helix. >

High average power dissipation in helix tubes

By precisely shrinking the glass envelope around the helix to a controlled depth, thus providing an exceptionally low thermal impedance to the outside of the envelope, and passing a suitable coolant over the envelope, it has been found that the power dissipation capability of helix circuits can be extended by more than one order of magnitude. A qualitative analysis indicates that the heat transfer characteristics should be as good as the experimental results indicate.

Electrostatic periodically focused backward-wave oscillator

A low voltage electrostatic periodically focused BWO has been developed which operates over most of X-band and delivers a maximum output power of 20 m.w. This device employs a hollow beam and a bifilar helix circuit. This is a remarkable result as the performance of such a tube is known to be extremely critical to the quality of the focusing. It experimentally demonstrates the rigid focusing predicted theoretically for the bifilar arrangement. Theoretical and experimental investigations have been conducted to determine the feasibility of using a modified focusing scheme which employs a uniform radial electric field in addition to the periodic fields of the bifilar helix. An improved S-band model under construction is expected to give about 50 m.w. output over a 2 to 1 bandwidth with maximum beam voltage less than 1 KV. The test results from this tube will be presented.

Use of multiple-helix circuits in 100-watt traveling-wave amplifiers

The problem of increasing the power level of a tw amplifier can be partially solved by using slow-wave circuits in parallel. It has been found that up to four helix circuits can be paralleled without adversely affecting the phase velocity characteristics, and that some increase in effective impedance may be realized. Paralleling is accomplished in a way which eliminates most of the unwanted propagation modes usually associated with coupled systems. 100-watt cw amplifiers for S and X band are described. These tubes use 4 helix slow-wave circuits, and are of metal ceramic construction. A 4-beam gun is used to provide a beam in each helix. In constructing the multiple-helix circuits, all helices are brazed together and are brazed in turn to sapphire rods to provide good cooling. Some loss in impedance is experienced due to dielectric loading. These tubes have saturation gains of the order of 20 per cent. The operating voltage is less than 3 kv, resulting in a relatively short tube. The X-band tube is short enough to permit permanent magnet focusing.

Traveling-wave amplifiers and backward-wave oscillators for VHF

An important aspect of the design of traveling-wave amplifiers and backward-wave oscillators for frequencies below 500 mc is the problem of obtaining a tube of reasonably small physical dimensions. Hollow beams of greater perveance than is obtainable with solid beams offer one method of reducing the size of such tubes by permitting operation at a lower voltage and greater gain per wavelength, for a specified beam power, than is possible in a solid beam tube. Some aspects of the design of minimum size hollow-beam forward-wave amplifiers using single helix circuits and backward-wave oscillators using bifilar helix circuits are presented. Several tubes of these types for operation below 500 mc have been built. Amplifier bandwidths and oscillator tuning ranges in excess of four to one in frequency have been obtained experimentally. Amplifier efficiencies in excess of 20 per cent and oscillator efficiencies in excess of 10 per cent have been achieved.