Strapped bifilar helices for high-peak-power traveling-wave tubes

A method of increasing the peak-pulsed power output of broad-band traveling-wave tubes is described. The method involves the use of a modified bifilar helix for the slow-wave structure. The modification employs 1) special straps or 2) mode-selective attenuation to prevent backward-wave oscillation in the anti-symmetric mode. This results in the possibility of using helices in the symmetric mode at values of ka (circumference-to-free-space wavelength ratio) as large as 0.6 at the highest amplification frequency. This in turn makes possible an increase in peak-pulsed beam power of a factor of approximately sixteen times that possible with a single helix. Both analytical and experimental results regarding the behavior of the structures are presented showing the propagation characteristics. The experimental results include cold measurements to determine ω-β diagrams and measurements with an electron beam which yield experimental values of interaction impedance.     

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.     

毫米波行波管返波振荡的仿真研究

该文利用HFSS仿真了工作在Ka波段的螺旋线、反绕双螺旋线及耦合腔等慢波系统,通过傅里叶分析得到空间谐波,进而分析和比较了上述慢波系统的返波振荡特性.结果表明:螺旋线慢波系统中出现明显的角向谐波次数和轴向谐波次数不相等的窄问谐波分量,在π模附近,有产生返波振荡的危险:而反绕双螺旋线通过提高基波耦合阻抗及可工作的归一化频率来提高了抑制返波振荡的能力;为了避免返波振荡,耦合腔工作频带的选择应尽可能远离单腔相移为π,2π的频点.     

A Simple Closed-form Formula for Backward-Wave Start-Oscillation Condition for Millimeter-Wave Helix TWTs

An accurate and simple closed-form formula, for backward-wave start-oscillation condition for a millimeter-wave helix traveling-wave tube amplifier was developed, using an artificial neural network (ANN) algorithm. The analysis considers the effects of circuit loss and also the variation of electron beam diameter corresponding to beam filling. The formula is simple and amenable to easy computation, even using a scientific calculator, and without resorting to exhaustive numerical iterative search followed in conventional analyses and, at the same time, without sacrificing the accuracy in results. The formula was validated against published results, and excellent accuracy was observed. The analysis has been further used for inferring some physical interpretations on the effects of beam-filling factor and circuit loss on the start-oscillation condition of a typical millimeter-wave helix traveling-wave tube.     

Effect of Attenuator on BWO Start Oscillation Condition in a Helix Millimeter Wave TWT Under Magnetic Focusing

In a practical helix millimeter wave traveling-wave tube (TWT), there are always magnetic focusing system for constraining the electron beam as it passes through the interaction region and attenuator for suppressing the oscillations, including backward-wave oscillation (BWO) and improving the output power. In view of the attenuator and magnetic focusing system, a 2D linear theory is employed to analyze BWO start oscillation condition. Numerical results show that the start oscillation length of the millimeter wave TWT decreases when the start position of the attenuator is close to the input section of the slow wave structure (SWS), and that it increases with the decrease of the attenuation length or the increase of the attenuation quantity. Therefore, in order to predict the BWO accurately, we should take into the attenuator and magnetic focusing system account.     

A method for reducing the second harmonic level in wideband traveling wave tubes

To reduce the second harmonic level and enhance the efficiency and stability against backward-wave self-excitation of wideband helical traveling-wave tubes containing a screen with longitudinally conducting edges, it is suggested that the gap between edge ends and a helix in the output section be gradually increased toward the energy output, the helix pitch being constant or slightly varied for optimizing the efficiency. It is shown that the increase in the gap reduces the second harmonic level and enhances the efficiency and the start current of backward-wave self-excitation.     

Theoretical analysis of log-periodic circuits in microwave amplifiers

The feasibility of applying the log-periodic concept to microwave amplifiers has been investigated. This approach, successfully utilized in antenna design, offers the potential of greater bandwidth at a given power level than attainable from existing uniform devices. Log-periodic devices analogous to klystron, traveling-wave, and hybrid amplifiers have been studied. The dimensions of each section, including the beam diameter, in a log-periodic amplifier are a constant factor times those of the preceding section. Thus the general outline of such devices is conical. These devices have been simulated on a digital computer using a model in which the coupling between periodic sections is provided by one space-charge mode and one circuit mode. The accuracy of this model has been verified by simulation of existing traveling-wave tubes. Computer results on a 100kW log-periodic "klystorn," using a backward-wave circuit with a total taper ratio of 4.6 to 1, indicate that a useful bandwidth of approximately 3 to 1 can be obtained. Increasing the mechanical taper increases the bandwidth in direct proportion. Computer results on log-periodic versions of helix traveling-wave tubes are also given.     

Dispersion relations in time-space periodic media part II—Unstable interactions

The conditions sufficient to assert the existence of absolute (time-growing) instabilities in distributed parametric structures or media are shown. Treating the simple, linearized, time-space periodic transmission line model, it is shown that time-growing instabilities exist in an unbounded medium whenever the pump modulation velocity is greater then the characteristic velocity of the medium and is in the convergent ("nonsonic") range of operation. The existence of these instabilities is based upon the rigorous calculation of the dispersion relation for the model, accounting for all small-signal frequencies. Proof of the existence of the instability, using the dispersion relation of the active structure, is based upon the method of Briggs, previously used in electron-stream interactions. The waveform of these instabilities is that of the backward-wave oscillator and comparisons with backward-wave oscillator (electron-beam) tubes are made. The predictions of the theory hold only for the transient period, for which the linearized model is valid, before the fields reach the nonlinear regime. The analysis, however, forms the basis for certain simple calculations of oscillation frequency, growth rates, predictions of competing interactions, and identification of wave types--all useful for application. The method of analysis should be useful for application to various other types of parametric interactions involving elastic waves, spin waves, plasma waves, etc., in single or multimode coupling schemes. The frequency range of application of such traveling-wave parametric effects extends from the optical range down to the radio-frequency range.     

Properties of ring-plane slow-wave circuits

Ring-plane circuits consist of circular rings or slotted pipes supported by one or more radial planes. These circuits propagate slow waves(v_{p} < c)in one or more modes, with velocity and impedance governed primarily by the ring or slot resonances much as on planar ladders. The multimode behavior depends very much on the mutual coupling near the radial planes. Tests of many variations about a prototype configuration are presented, including methods for suppression of unwanted modes. The results are given in terms of ω -- β diagrams, field shapes and impedances. The study presents essentially all parameters necessary for design of circuits for traveling-wave amplifiers and backward-wave oscillators, as well as information useful in other applications, such as linear accelerators. Ring-plane circuits are of large diameter(ka ge 1), are quite fast(v_{p}/clarge) and have relatively large impedances. The radial support planes provide good heat conduction paths, allowing for high average power capability. These properties make these circuits well suited for use in high-power, high-frequency traveling-wave tubes.     

A model of a diaphragmatic-waveguide traveling-wave tube

On the basis of a physical notions of the specific features of the slow-wave field distribution in the helix structure of a traveling-wave tube (TWT), various methods for controlling the amount of wave slowing and the slope of the dispersion characteristic are analyzed. The effect of the dielectric supports and correcting elements placed outside the helix is considered. In addition to the known methods for correcting dispersion through the use of a dielectric shield and longitudinally conducting elements, which weaken the interaction with the electron flow, new methods that provide both an efficient interaction with the electrons and the possibility of operation without backward-wave self-excitation at voltages higher than the voltages used with conventional helices are proposed and analyzed. With an analysis based on the equivalent-line method, it is shown that the slope of the helix dispersion characteristic can be decreased by the appropriate use of ring-shaped supports. Design solutions using the new methods of correction are considered. The history of development of ultrabroadband TWTs, including the work done in the Soviet Union on tubes with centrifugal electrostatic focusing, whose actual amplification band can be as wide as three octaves, is partially presented.     

Correction of the dispersion characteristic of a helix slow-wave structure in traveling-wave tubes

On the basis of a physical notions of the specific features of the slow-wave field distribution in the helix structure of a traveling-wave tube (TWT), various methods for controlling the amount of wave slowing and the slope of the dispersion characteristic are analyzed. The effect of the dielectric supports and correcting elements placed outside the helix is considered. In addition to the known methods for correcting dispersion through the use of a dielectric shield and longitudinally conducting elements, which weaken the interaction with the electron flow, new methods that provide both an efficient interaction with the electrons and the possibility of operation without backward-wave self-excitation at voltages higher than the voltages used with conventional helices are proposed and analyzed. With an analysis based on the equivalent-line method, it is shown that the slope of the helix dispersion characteristic can be decreased by the appropriate use of ring-shaped supports. Design solutions using the new methods of correction are considered. The history of development of ultrabroadband TWTs, including the work done in the Soviet Union on tubes with centrifugal electrostatic focusing, whose actual amplification band can be as wide as three octaves, is partially presented.     

返波激励的级联折叠波导行波放大器

利用折叠波导返波振荡器(FW-BWO)作为激励源,用于激励工作频率为216 GHz的折叠波导行波放大器.利用3D-Magic进行仿真实验,通过仿真优化,最终得到96 W的输出功率,整个电路的长度被设计为只有1 cm左右.通过该方法,显著地缩短了高频结构长度,有利于实现小型化的真空电子学太赫兹源,对集成化的太赫兹源设计具有重要的参考价值.     

Formation and suppression of stationary and chaotic oscillations in a non-autonomous gyrotron backward-wave oscillator

Formation and suppression of chaotic oscillation modes in a non-autonomous gyrotron backward-wave oscillator is studied. It is shown that, in the considered system, developed chaotic oscillations and stationary single-frequency oscillations can be formed in a gyrotron tube upon variation in the frequency and power of an external signal. Domains in the plane of basic control parameters are found in which the considered oscillation modes of a non-autonomous gyrotron backward-wave oscillator are observed.     

Suppression of self-excitation of a backward-wave traveling-wave tube by decreasing the diameter and the pitch of the slow-wave structure toward the energy output

Mathematical expressions showing that the optimum way of suppressing the self-excitation of a backward-wave traveling-wave tube is to decrease the diameter and pitch of the slow-wave structure toward the energy output are obtained. The conditions for the values of the diameter and pitch before and after the decrease are found under which the phase velocities of the minus first spatial harmonic (backward wave) are maximally different, which ensures the maximum suppression of self-excitation, and phase velocities of the zero harmonic are optimal for obtaining the maximum efficiency.     

Wave propagation on multifilar helices

The modes of the sheath helix are shown to be related closely to the space harmonics of real helices. Multiwire helices propagate many modes, the principal space-harmonic component of each approaching more closely the appropriate sheath helix mode as the number of wires is increased. Each mode can be excited by a certain phase sequence of exciting currents for the wires in one transverse plane. Experimental verification of the theory is reported using a bifilar helix tube. Phase velocities and impedances are measured, using Kompfner's null method and the start of backward-wave oscillations for the principal forward and backward components respectively.     

A coupled mode description of the backward–wave oscillator and the Kompfner dip condition

The start oscillation condition for the backward-wave oscillator and the operation of the traveling-wave tube amplifier at the Kompfner dip point are described from the point of view of the coupling of two modes of propagation. Growing waves are not involved. Two waves are sufficient when the tube is more than a half plasma wavelength long. Operation in this "large space charge" domain is inherently simpler than in the "low space charge" domain. The start oscillation condition and the Kompfner dip condition are simply expressed in terms of the coupling constant between modes, HL = (2n ± 1) π/2, where n is an integer. In addition, the uncoupled modes must have the same velocity. The result is also expressed in terms of the more familiar parameters CN and hL. The effect of loss in the circuit mode is calculated. When the two waves carry energy in opposite directions, growing waves result. This case is discussed briefly.     

Traveling-Wave Tube Nanosecond Pulse Generator In 24-Gc Region

A 24-Gc traveling-wave tube of the type ECL-1180 /24W80 was used for generating nanosecond carrier pulses. The helix of the traveling-wave tube was modulated by a 40-Mc high voltage sine wave. Duration, power and carrier frequency shift of the output RF pulse were examined quantitatively with respect to the modulating voltage. It was found that the delay time of a traveling-wave tube has a large effect on the characteristics of the output pulse. The theory of transmission line modulators was developed to explain the pulse shaping mechanism in the traveling-wave tube. Observations of output pulses were made with three measuring devices, i.e., video-sampling oscilloscope with crystal detector, interferometer, and spectrum analyzer. The interferometer was found to be useful for measuring half-amplitude duration of carrier pulse. Output pulses with half-amplitude duration of as short as 1 nsec were observed.     

Fully balanced current-tunable sinusoidal quadrature oscillator

An integrable current-tunable sinusoidal quadrature oscillator is presented. A current-tunable all-pass filter using a signal-differencing technique is realized as the frequency-selective network. The implementation is fully balanced so as to enable accurate quadrature signals with symmetry. The oscillation frequency is current-tunable over a wide-frequency sweep range of approximately three orders of magnitude. The quadrature signals possess the amplitude matching and the quadrature phase matching of better than 0.004dB and 0.15°, respectively. The maximum useful frequency of oscillation is in excess of 8MHz. Total harmonic distortions are less than 0.5%.     

Theoretical Study for Folded Waveguide Traveling Wave Tube

A wideband folded waveguide traveling-wave tube (TWT) amplifier has advantages of simpler coupling structures and robust structure over the conventional helix TWT. The phase velocity of waves in folded waveguide is slowed down to the velocity of electron beam. Slow-wave interaction with the electron beam in folded waveguide is studied in a linear fashion. For a cold beam, the linear theory predicts a gain of 2 dB/cm and a bandwidth of 37% at the center frequency of 14 GHz. A closed algebraic dispersion relation for the frequency and the axial phase shift per period is obtained using an equivalent circuit model. Numerical solution calculated from the dispersion relation and three-dimensional electromagnetic code, HFSS simulations predict a mode coalescing in the folded waveguide. And a theoretical phase velocity prediction of the electromagnetic wave in this circuit is verified by HFSS simulations.     

An investigation and application of the contrawound helix

An investigation of the contrawound helix and the determination of its characteristics has been completed. By means of a hollow-beam traveling-wave tube and perturbation methods, the fundamental and minus-one space harmonics impedances have been measured and a number of space harmonics of the symmetric and antisymmetric modes identified. The fundamental impedance was measured to be equal or greater than the sheath-helix impedance depending, however, upon the thickness and width of the helix tape. The minus-one space harmonic impedance was measured to be larger than the fundamental impedance for ka > 0.65. The contrawound helix has been employed as a high-power X-band amplifier (8.5-9.5 kmc). The characteristics of the tube described are 25-db gain, power out > 1 kw, efficiency 7-10 per cent.