Theory of long term gain growth in traveling wave tubes

The properties of the attenuator materials used in traveling wave tubes (TWT) are a major factor in determining the gain of the tube. Carbon is a commonly used attenuator material which is typically deposited on the dielectric rods used to support the helix slow wave structure in TWTs. The deposited carbon layers are susceptible to ion bombardment induced damage during operation and outgassing of the tube, which increases the electrical resistivity of the layer and reduces the rf loss in the tube. The total gain in the tube grows during this process until the bombardment ceases or the layer damage saturates. A simple theory is developed that relates this gain change to the gas evolved in the tube that results in the particle bombardment of the layers during operation. The model accurately describes the observed time dependent behavior of the gain, and the time constant for outgassing the tube is found for several tubes ranging from S to Ka band. The time constant increases significantly for smaller, high frequency tubes with a low conductance and poor pumping speeds. The model shows that stabilization of the gain occurs after several time constants have passed, which may take hundreds to thousands of hours     

Reflexions and gain ripple in TWT's

The presence of the electron beam in a TWT introduces additional internal reflexions that are not present in the cold tube, at the output, at the input, and at the attenuator. These additional reflexions, that we denote "electronic reflexions,"increase the gain ripple. The electronic reflexions are hard to measure and therefore have to be calculated. In the present paper, equations are derived for them as functions of Pierce's TWT parameters. A conclusion is that the electronic reflexions often exceed the cold reflexions that can be obtained in practice. The electronic reflexions in TWT's with a backward fundamental coupled-cavity structure are almost an order of magnitude larger than those in helix TWT's.     

Use of a simple external nonreciprocal attenuator in coupled-cavity TWT's

It is well known that use of a nonreciprocal sever in traveling-wave tube (TWT) design offers several advantages. Previous attempts to obtain these advantages involved incorporation of ferrite material within the envelope of helix TWT's. These attempts have encountered serious difficulties, and have had no apparent effect on TWT design practice. A different approach to realization of a nonreciprocal sever for a coupled cavity TWT is described. This approach involves readily available wave guide components external to the tube, and does not appear to require the solution of significant research and development problems. A computer simulation performed on the CTS 200 W TWT design indicates that a substantial improvement in efficiency could be obtained through use of this technique.     

The effect of helix loss on noise figure in traveling-wave tubes

Helix loss increases the noise figure of a TWT by altering the amplifying properties of the tube, and also by acting as a source of thermal noise which is amplified by interaction with the beam. Noise power originating in the helix propagates as a wave and undergoes amplification in the same manner as the signal. This noise power adds to the noise figure of the tube by an amount proportional to the attenuation of the helix and to its temperature, and inversely proportional to the tube gain per unit length. The effect of refrigerating the helix has been demonstrated by several experiments in which low-noise TWT's were immersed in liquid nitrogen while monitoring the noise figure. The reduction in noise figure varied from a few tenths to nearly one db. One case of particular interest is that of a WJ-226 low-noise TWT, operating in the frequency band 1.4 to 2.3 Gc, which showed a minimum noise figure of 2.6 db when operating in a room temperature environment. On cooling the tube to liquid nitrogen temperature, a minimum noise figure of 1.7 db was obtained, at a frequency of 1.65 Gc.     

螺旋线行波管薄膜衰减器高频衰减特性的研究

利用HFSS仿真软件对螺旋线行波管中衰减器的衰减量分布情况进行了模拟分析。由于直接模拟螺旋线行波管中不同涂碳层厚度下衰减器的衰减分布情况难度大,计算量大,涂碳厚度不易模拟,因此,本文利用等效替换的方法进行了模拟计算,通过对单根夹持杆进行波导测试,获得S21,找到不同S21下的等效体电率或损耗角正切,并将他们对应的这些材料替换到行波管内,计算相应的衰减常数,得出不同位置下的衰减常数。     

A two wave theory of traveling-wave tubes and backward-wave oscillations

Starting from Pierce's small-signal TWT equations, a two-wave theory for complicated TWT's and BWO's is developed. Explicit gain expressions for TWT's with severs and velocity tapers are given. A detailed treatment is given of a BWA structure with a single-step velocity taper in the center. The BWO start oscillation current is calculated as a function of the taper strength. The effciency of a step velocity taper as a remedy for backward-wave oscillations in the higher passbands of a TWT is then analyzed. It is shown that a suitable taper can increase the start oscillation current by more than a factor of 10 without adverse effects on the TWT performance if the backward waye coupling impedance is low. A tube with a weak linear taper is analyzed by a perturbation technique and it is found that there is no significant difference between the start oscillation currents for tubes with weak linear or abrupt tapers of comparable strengths.     

倍频程螺旋线行波管2.5维大信号分析和设计

本文利用2.5维大信号分析程序和辅助分析程序,对一只7-18GHz带宽,2-2.5kW脉冲输出功率的螺旋线行波管进行计算机模拟,电子注感应电流基波分量在螺旋线慢波系统高频场中的相位分析和最佳工作点分析。在此基础上,探讨了色散曲线布局,衰减器配置,切断位置,再同步设计等倍频程螺旋线行波管的设计方法。     

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.     

毫米波折叠波导行波管的研究

采用等效电路方法和电磁场仿真软件Ansoft HFSS分析了折叠波导行波管的结构参数对其高频特性的影响,并在此基础上确定了Ka波段折叠波导行波管的尺寸.利用三维非线性粒子模拟软件MAGIC3D建立了两段式折叠波导行波管的模型,模拟研究了切断区长度和位置对折叠波导行波管的饱和输出功率及第2段电路单位长度增益的影响.最后设计了一个工作于33～36GHz的两段式折叠波导行波管,其输出功率的波动小于1dB,最大连续波输出功率达670W,对应电子效率高达7.55％.     

Traveling wave tubes for communication satellites

Traveling wave tubes (TWT's) have contributed markedly to the development of communications satellites. As the prime-power consuming and transmitting device, the major transponder gain element, and the largest contributor to transmission nonlinearities, the TWT has been the focal point for continuous but carefully measured evolutionary improvements. Efficiency improvements continue to be made without compromising desired communications characteristics or tube lifetimes. These improvements have been made primarily in the RF circuit through loss reduction and phase-velocity tapering techniques, and in the spent-beam region through better multielement collector designs. Traveling wave tubes developed for satellites at 4 and 12 GHz are used as examples. Since TWT's ate life-limited devices, emphasis has been placed on techniques ensuring long life in satellite applications. Both oxide- and dispenser-type cathodes are discussed and data on life characteristics are presented. During the past decade, while generally demonstrating excellent space lifetimes, operating TWT's continue to approach their potential cathode wear-out life, which is theoretically of the order of 10⁵h.     

Accurate Calculation of TWT Intermodulation in the Many-Carrier Case

The formulas derived by Stette [1] permit the calculation of TWT output carrier power and intermod power when the input to the TWT consists of an infinite number of infinitesimal carriers. The formulas for these quantities involve an integration of the TWT singlecarrier characteristic over a semi-infinite range, and for most of the TWT's one considers, a simple approximation of the integration by a summation gives satisfactory answers. However, when one considers special TWT's, e.g., linearized TWT's or the piecewise linear limiter [2], simple approximations are often not good enough. To get accurate results, a multipoint Gaussian quadrature must be used.     

Harmonic generation in octave bandwidth traveling-wave tubes

This paper describes the investigation of harmonic generation in traveling-wave tubes (TWT's) via large-signal analysis and digital computer techniques. Efficiency degradation and harmonic power content are shown to be importantly related to such TWT design considerations as circuit dispersion, harmonic coupling impedance ratio, and gain level. Also described is the phenomenon of second harmonic interference in the beam bunching process and how it leads to substantial efficiency reduction in TWT's employing relatively nondispersive structures.     

A study on helix pitch tolerance impact on TWT small-signal gain

Traveling-wave tubes (TWTs) consist of several sections with different helix pitch to provide the required small-signal gain. An extensive study aiming to improve the comprehension of the effect of the helix pitch manufacturing tolerances on the TWT small-signal gain has been completed. The obtained results demonstrate the relevant impact on the TWT small-signal gain when the helix pitch deviates from its nominal value and allow devising a tuning procedure for the realized tubes.     

螺旋线行波管注-波互作用三维粒子模拟

结合时域有限差分方法和粒子模拟方法,开发了螺旋线行波管的三维注-波互作用粒子模拟程序。从最基本的电磁场运动规律和力学规律出发,利用高速计算机直接求解完整的麦克斯韦方程组和洛伦兹方程,完成了对C 波段螺旋线行波管的注-波互作用的模拟计算,得到了电子的相位和空间分布以及管子的功率、增益等电性能参量,验证了程序的正确性,为后期优化和设计奠定了基础。     

Design of Tip Loss Profile on Support Rod for a Helix TWT

This paper describes the design of tip loss profile on support rod for a high gain, high efficiency helix TWT. The tip loss profile at sever ends for the input and the output section of the helix slow wave structure has been designed for return loss at severs ends better than -20 dB. The experimental results have been carried out in the input and the output section of the helix assemblies for the high gain helix TWT to find the return loss in the region of carbon coated tip loss on the support rods. The experimental result has been compared with the simulated performance for the return loss at the sever ends. The design of tip loss profile on the support rod for helix TWT has been carried out in real situation using Ansoft HFSS. A good agreement has been found in the simulated and experimental results.     

高增益行波管的振荡抑制

本文介绍我们在研制高增益小功率行波管过程中所遇到的振荡,分析了产生振荡的机理,并由此找到了抑制振荡的方法,管子最终所达到的增益在 70dB以上,最大的输出功率可达到3W。     

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.     

利用三维粒子软件计算螺旋线行波管的互作用和三阶互调

采用三维电磁软件CST粒子工作室模拟单信号输入时某Ku波段螺旋线行波管的互作用及双信号输入时该行波管的三阶互调。模拟计算时,采用理想电子注和均匀聚焦磁场,对衰减器采用分段处理,通过设置每段衰减器材料的损耗正切值来表征其衰减量。单信号输入互作用模拟计算结果与实测结果非常接近,通过模拟得到了行波管双信号输入时两个基波信号输出功率以及三阶互调（IM3）产物,并给出输出信号的快速傅立叶变化频谱。     

Traveling-wave tube amplifier performance evaluation and design optimization for applications in digital communications with multilevel modulations

In this paper, we demonstrate the use of a power margin as a figure-of-merit for evaluating the performance and optimizing the design of traveling-wave tube amplifiers (TWTAs) used in digital communication applications with multilevel modulations. The power margin is a system-level measure that balances both device efficiency and nonlinear distortion and provides a more direct prediction of the system-level performance of power amplifiers than device-level measures such as device efficiency or error-vector-magnitude. We calculate the power margin for M quadrature amplitude modulation for an existing TWTA to demonstrate the setting of an optimal amplifier operating drive level according to the criterion of the maximum power margin. The power margin can also be used to compare the performance of different traveling-wave tube (TWT) configurations. We compare the calculated power-margin performance for helix TWT circuits optimized with different optimization goal functions using the helix TWT design code CHRISTINE. The goal functions used in the optimization of the TWT circuits include AM/PM optimization, complex gain optimization, efficiency optimization, and a new digital goal function optimization. The digital goal function is shown to provide an enhanced power margin compared to the other three goal functions and demonstrates the potential of TWT device design optimization from a system perspective.     

Design of a tapered helix for suppressing backward wave oscillations in MMW TWT’s

Backward wave oscillations are easy occurred in high power helix millimeter wave traveling wave tubes (MMW TWT’s), owing to larger ka value and higher operating voltage. The start current of backward wave is calculated for helix TWT in Ka band. It is shown that start current will increase by used a tapered helix. Backward wave oscillations can be suppressed in tube. The design method of the tapered helix is given in this paper.