High-power Traveling-wave Amplifier Research Articles

The interaction of symmetric and asymmetric modes in a high-power traveling-wave amplifier

A three-dimensional (3-D) model has been developed for the investigation of the coupling of the lowest symmetric and asymmetric modes in a high-power, high-efficiency traveling-wave amplifier. We show that in a uniform structure and for an initially nonbunched beam, the interaction efficiency of the asymmetric mode may be much higher than that of the symmetric mode. It is also shown that the coupling between these two modes is determined by a single parameter that depends on the beam characteristics; its value varies between zero when no coupling exists and unity in case of maximum coupling. For a beam that is uniform at the input end, this parameter varies linearly with the guiding magnetic field. In case of a bunched beam, it decreases linearly with the increasing phase-spread of the bunch. Because of the interaction, the radius of the beam increases linearly with the power associated with the asymmetric mode at the input end; it increases rapidly in the case of an initially uniform beam relative to the case of a prebunched beam. Selective damping to suppress the asymmetric mode is described and analyzed.

Symmetric and asymmetric mode interaction in high-power traveling wave amplifiers: experiments and theory

High-power microwave amplifier operation has been studied for use in a number of applications. The performance of the amplifiers has been marred, in some cases, by pulse shortening of the microwave signal. A possible source of the shortening is the loss of the beam due to hybrid HEM/sub 11/ mode interaction with the beam. In this paper, we describe experiments which investigate high-power operation and the effects of HEM modes on the amplifier performance. We report the high-output powers (>50 MW) with efficient (>54%) amplification of microwaves in an X-band traveling wave amplifier. In some experiments, peak power levels exceeding 120 MW were measured at an efficiency of 47%. The excitation of the asymmetric hybrid electromagnetic mode was monitored carefully, but does not seem to have a critical impact on the main interaction process in spite of the fact that its dispersion curve almost overlaps that of the symmetric interacting mode. Theoretical analysis of the interaction in a tapered traveling wave structure indicates that, even if the amount of power in the asymmetric modes at the input of the structure is comparable to that in the symmetric mode, the asymmetric modes cause no power reduction in the symmetric mode. For the case of off-axis beams the TM/sub 01/ output power may drop by about 30% and the power in the hybrid mode reach about one third of that in the symmetric mode. In order to avoid hybrid mode excitation it is necessary to suppress the reflections from both ends of the output structure several decibels below the gain level of the asymmetric mode.

Beam-quality and guiding-magnetic-field requirements for a high-power traveling-wave amplifier operating at 35 GHz

A two-dimensional quasianalytic model has been developed for the investigation of the performance of a high-efficiency traveling-wave amplifier operating at 35 GHz. Simulations indicate that a relative energy spread of less than 5{percent} is sufficient to reach high efficiency. It is also shown that there is an optimal guiding magnetic field for a given geometry of the slow-wave structure. {copyright} {ital 1998} {ital The American Physical Society}

The electron cyclotron maser as a high-power traveling wave amplifier of millimeter waves

The electron cyclotron maser instability has been exploited as the basis for a new type of traveling wave amplifier which operates at unusually high-power levels at millimeter wavelengths. The first experimental model of this amplifier has been operated at 35 GHz and has demonstrated a stable gain of 17 dB and an output power of 10 kW (unsaturated). The gain was linear over a dynamic range > 30 dB. The absolute value of the gain and its dependence on current and magnetic field were in excellent agreement with theoretical calculations. Bandwidth and saturated power have yet to be measured directly, but no fundamental problems were observed which will prevent successful achievement of the design predictions (viz., bandwidth ≃ 10 percent, power on the order of 105W, efficiency > 10 percent).

Measurement and evaluation of a high-power traveling-wave amplifier in multicarrier operation

The multicarrier communication measurements performed on a high power, broadband traveling wave amplifier are described in this paper. The technical requirements for fm multicarrier operation of satellite communication links are now being established and these requirements depend heavily on the components presently available. One of the important but previously unevaluated components is the high power, wideband amplifier for the ground stations. The measurements describe the performance of a new high power traveling wave amplifier in wideband multicarrier operation.

Behavior of magnetically focused large-signal traveling-wave amplifiers

Many high-power traveling-wave amplifiers operate under confined-flow conditions (high magnetic field) so that there is little radial motion of the electrons. However, the trend in recent years has been toward utilizing near-Brillouin magnetic fields (uniform and ppm) for beam focusing in order to achieve a small, lightweight device. In such a device operating under large-signal conditions, the radial excursions of the electrons can become quite large and may result in premature saturation, a reduction in gain and appreciable circuit current interception, seriously degrading its performance. It is thus of considerable interest to determine the minimum magnetic fields necessary to constrain the beam under large-signal conditions.

Phase focusing in linear-beam devices

The phenomenon of phase focusing in linear-beam devices is investigated theoretically by modifying the nonlinear interaction equations for devices such as the traveling-wave amplifier (TWA) to accommodate spatially-varying parameters and then investigating their use under a variety of operating conditions. Phase focusing can be accomplished either by tapering the RF circuit phase velocity or by applying a dc voltage gradient along the beam or structure. Both approximate analytical solutions and general computer solutions are presented for each of the phase-focusing techniques. The analytic solutions yield profiles which are explicit functions of tube length and these are compared with more general computer solutions where space-charge effects are included. An experimental S-band high-power TWA was constructed with a variable-pitch helix designed on the basis of the hard-kernel-bunch theory. Experimental data indicate an efficiency improvement from 18 per cent to 35 per cent and an improvement in the gain-frequency characteristics.

Waveguide-vane coupled slow-wave structure suitable for a high-power traveling-wave amplifier

A class of slow-wave structures, previously proposed for use in the magnetron amplifier, is examined in the linear version for application in a high-power, traveling-wave amplifier. Two versions are studied, both theoretically and experimentally, with particular emphasis on the frequency range suitable for wide-band, high-voltage application.

Why a circuit sever affects traveling-wave tube efficiency

The effect of a circuit sever on traveling-wave amplifier efficiency has been calculated from the large-signal traveling-wave tube equations. Measurements made on high-power traveling-wave amplifiers, which utilize a severed coupled-cavity interaction circuit, have confirmed the calculated results. The results show that a traveling-wave amplifier with C = 0.1 and QC = 0.25 must have 26 db or more growing-wave small-signal gain beyond the sever to obtain the maximum theoretical efficiency. If the growing-wave gain is less than this value, efficiency is reduced; for example, if the gain is 4 db less, the efficiency is reduced by one half. The mechanism which causes this reduction of saturated output power can be understood from the computed electron distribution in phase and velocity. From these phase-velocity diagrams, the degradation in efficiency is seen to be caused by the debunching of the beam by space-charge forces in the region between the sever and the point where the microwave signal builds up to the value it previously reached before the sever. Further study of the computer results shows that the loss in efficiency caused by insufficient gain beyond the sever cannot be made up by depressed collector operations, and that the use of over-voltage to obtain increased efficiency in a severed tube may not be effective because of the resulting reduction in gain that over-voltaging causes.

High-power traveling-wave amplifier for VHF

The electrical and mechanical design of a new-order high-power, CW, all metal ceramic, VHF traveling-wave amplifier will be discussed.

Large signal behavior of high power traveling-wave amplifiers

The results of an experimental investigation on the large signal behavior of a kilowatt power level helix type traveling-wave amplifier tube are presented. Operation with and without attenuators was investigated using a movable electromagnetic probe to measure power level along the tube. Quite different effects of drive power and beam voltage on the saturation level were found for operation with and without attenuators. The maximum power level is lower for attenuator operation. Also, power levels do not continue to increase with increasing beam voltage and drive power. In contrast, attenuator-less operation produces the highest efficiency, and the power levels continue to rise with increasing beam voltage and drive power. Conversion efficiencies as high as 25 per cent are obtained with an attenuator and as high as 40 per cent without an attenuator. Efficiency calculations based on small-signal theory can be made to agree reasonably well with the experimental attenuator-less operation efficiencies by assuming an appropriate ratio of the ac component of beam current to the dc component of beam current, i/I 0 .

A large-signal analysis of the traveling-wave amplifier: Theory and general results

Equations are derived describing the large-signal operation of the traveling-wave amplifier, including the effects of ac space charge and attenuation along the helical slow-wave structure. The equations constitute a system of nonlinear partial-differential-integral equations valid with reasonable approximations for all values of the parameters which are encountered in typical high-power traveling-wave amplifiers. The parameters which appear in the equations are the relative injection velocity b, the gain parameter C, the large-signal space-charge parameters K and B, the loss parameter d, and the input-signal level A <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> . The working equations were programmed for and solved on the Michigan Digital Automatic Computer, MIDAC, located at the University of Michigan's Willow Run Research Center. The rf voltage amplitude A(y), the phase lag of the rf wave relative to the electron stream θ(y), and the velocity deviation 2Cu(y, φ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> ) were computed and plotted for several values of C, K, b, and B at A <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> = 0.0025, and d = 0. Also, distance-phase plots are presented for each of the above cases. These flight-line diagrams for the traveling-wave amplifier are similar to the well-known "Applegate diagram" associated with klystrons. Zero-space-charge solutions are presented for C = 0.05, 0.1, and 0.2 with b as the parameter in order to determine the value of b which gives the maximum saturation gain and the optimum tube length. For C = 0.1 similar solutions are obtained for several values of the space-charge parameter K. Presented in graphical form, the results of these various solutions shed a considerable amount of light on the high-level operation of the traveling-wave amplifier.

A wide-band structure for high-power traveling-wave tubes

In an effort to overcome the low power handling capabilities of the conventional wire helix considerable effort has been devoted to the development of filter-type slow-wave structures for use in high-power traveling-wave amplifiers. However, the shortcoming of the various filter-type structures such as the disk-on-rod and the apertured disk, is their inherently narrow bandwidth. For a filter-type structure operating at a center frequency of 3000 mc, the maximum attainable bandwidth is approximately 10 per cent or 300 mc. With the type of structure considered herein it is hoped to achieve a bandwidth of approximately 30 per cent at 3000 mc. The helical waveguide structure is composed of a helical tape wound inside a cylinder. The hole for the passage of the beam would normally be located along the axis of the helical tape. However, the field analysis indicates that stronger coupling might be obtained by placing the beam hole somewhat off center. In order to gain maximum coupling to the beam, several off-center electron beams would be located at a radius corresponding to that of the maximum of the electric field. The cavities are coupled continuously by the helical tape. The bandwidth, i.e., dispersion curve, should approach that of the conventional wire helix except in the dispersive region because the waveguide has a cutoff. The power handling capability clearly is greater than that of the wire helix. However, the impedance will be lower, resulting In a lower gain parameter C. Other factors such as a higher permissible value of (I o /V o ) and a more nearly constant electric field along the spiral contribute to a higher gain parameter. In fact, the zero-order mode (β = O) will have no variation of electric field across the cavity. Because of the complexity of an exact mathematical analysis some approximations have been made. The assumption used in this analysis is that the pitch of the helical tape is considered small compared to its diameter. Each section of the waveguide is considered as a cylindrical cavity of height h, where h = 2 P and P = pitch of the tape. This is a valid assumption where the pitch is small, as the end faces of the cavities are essentially perpendicular to the cylindrical enclosure. The helical surface is considered to be a Riemann surface wherein φ is a periodic and single-valued coordinate; however, in the usual cylindrical coordinate system φ is a nonperiodic coordinate. Thus the fields are multi-valued functions of the coordinate φ. Under the above conditions and the generalization of the Bessel-function order n to its general parameter p, the familiar cylindrical wave-guide equations may be used. Under this transformation p is capable of assuming nonintegral values. On the basis of the theoretical analysis presented herein the helical waveguide structure seems best suited for high-frequency use, especially in the frequency range of 5–10 kmc. The bandwidth and gain approach those of the helix-type traveling-rave tube.