Design of high-efficiency wide-bandwidth coupled-cavitytraveling-wave tube phase velocity tapers with simulated annealingalgorithms

The output circuit section of a traveling-wave tube (TWT) routinely contains an RF phase velocity taper for the purpose of increasing RF output power and efficiency. By slowing the RF phase velocity in approximate synchronism with the decelerating electron beam bunches, the taper increases power transfer from the beam to the RF wave. Recently, the computational optimization technique of simulated annealing was shown to be very effective in the design of an RF phase velocity taper that significantly increased computed RF power and efficiency of a coupled-cavity TWT. In this paper, two new broadband simulated annealing algorithms are presented that optimize (1) minimum saturated efficiency over a frequency bandwidth and (2) simultaneous bandwidth and minimum efficiency over the frequency band with constant input power. The algorithms were incorporated into the NASA 2.5-dimensional (2.5-D) coupled-cavity TWT computer model and used to design optimal phase velocity tapers using a 59-64 GHz coupled-cavity TWT as a baseline model. Compared to the baseline taper design, the computational results of the first broadband algorithm showed an improvement of 73.9% in minimum saturated efficiency. The second broadband algorithm indicates an improvement of 272.7% in minimum RF efficiency with constant input power drive and an increase in simultaneous bandwidth of 0.5 GHz over that calculated for the baseline TWT     

Start-oscillation conditions in nonuniform backward-wave oscillators

The effects of nonuniformities in the circuit phase velocity and the beam potential on the start-oscillation conditions of an O-type backward-wave oscillator (BWO) are investigated. The nonuniformities include linear, quadratic, and exponential circuit phase velocity tapers and beam potential gradients. It is shown that for weak circuit phase velocity tapers or voltage gradients the start-oscillation current may be lower than that of a uniform tube; however, when the taper or gradient is made stronger than a particular value which depends on the operating parameters, a no-oscillation region results. The latter result indicates that such nonuniformities may be utilized to suppress backward-wave oscillations in forward-wave amplifiers.     

Design of single-mode step-tapered waveguide sections

Single-mode step-tapered waveguide sections are analyzed in an effort to determine how the various device parameters, specifically, taper function, mask spot size, and taper length, affect power loss. The results indicate that 1) for short to medium length tapers, linear tapers exhibit the lowest loss, whereas for long tapers, all of the taper functions exhibit essentially the same loss; 2) for single-mode transitions with waveguide widths in the range2-15 mum, the loss in a step-tapered waveguide section can be nearly minimized as long as the mask spot size is chosen such that there are at least 20-25 steps in the taper; and 3) the length of a given taper for minimum power loss is strongly influenced by the asymmetry of the waveguide structure.     

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.     

Engineering Computation and Analysis of Enhancing Efficiency for Folded Waveguide TWT in MMW

Large signal operating equations in TWT have been solved by numerical solve method. The relations between interaction efficiency and design parameters in TWT are analyzed. A fast engineering design method of enhancing efficiency for folded waveguide TWT by velocity resynchronization is described. The computation is shown that the efficiency of Ka-band folded waveguide TWT with double taper can be enhanced over twice as original tube. This method of approach is very useful for tube designer.     

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.     

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.     

The shape of fiber tapers

A model for the shape of optical fiber tapers, formed by stretching a fiber in a heat source of varying length, is presented. Simple assumptions avoid any need for the techniques of fluid mechanics. It is found that any decreasing shape of taper can be produced. The procedure for calculating the hot-zone length variation required to produce a given shape of taper is described, and is used to indicate how an optical adiabatic taper can be made. A traveling burner tapering system is capable of realizing the model's prediction, and a complete practical procedure for the formation of fiber tapers with any reasonable shape is thus presented     

Gain stability of traveling wave tubes

The long-term gain stability of traveling wave tubes (TWT's) with helix slow-wave structures is examined. A major variable in the gain of TWT's is the stability of the attenuator material that is placed in the tube to damp oscillations and improve input-to-output isolation. Thin carbon layers are often used for this purpose in TWT's and are deposited onto the helix support rods by several different techniques that produce a variability in the material structure and properties. The carbon layers are also susceptible to physical damage due to the environment in the tube during conditioning and long-term operation. Modification of the electrical conductivity of the layer by energetic particle bombardment and chemical erosion decreases the net RF loss in the tube and causes the gain to increase with time. The presence of impurity gases and rapid conditioning procedures produce gain increases due to the lattice damage of the attenuator material of up to 10 dB in a TWT in the first several hundred hours of operation. Properly designed attenuator loss-patterns and minimization of the gas evolution in the TWT causes these effects to saturate and the gain to stabilize quickly. Techniques to ensure long-term stability of these layers are discussed     

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％.     

Design of Coupling Structure for MMW Folded Waveguide TWTs

A significant advantage of the folded wave traveling wave tube (TWT) is simple coupling structure. In millimeter wave band, owing to higher frequency, linear double taper can be used as impedance transformer. The design method and calculation results for the folded waveguide TWTs in Kα-band are given.     

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.     

E波段连续波空间行波管研制进展

针对高波段空间行波管的卫星通信应用需求,介绍了E波段连续波空间行波管的研制情况。该行波管通过进一步优化折叠波导慢波结构参数和调整周期跳变方案,实现改善带内增益波动性、提高效率的目的。研制出的样管在14.7 kV、74 mA条件下,实现电子注动态流通率高于98%,在71~76 GHz范围内,输出功率大于85 W,总效率大于37%,增益大于40 dB。     

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.     

Synthesis of Optimum Finline Tapers Using Dispersion Formulas for Arbitrary Slot Widths and Locations

The theory of TEM matching sections has been modified so that it can be applied to finline tapers. A step-by-step procedure is given to calculate the taper contour for a given maximum VSWR. The taper is optimum in the sense that its length is the shortest possible for the required VSWR. To achieve fast convergence, a transversal resonance method was developed to calculate finline dispersion, which is valid for arbitrary slot widths and slot locations. The finline can be unilateral as well as bilateral, and the slot may be off-centered. The dispersion data are compared with values found in the literature, and the calculated taper performance with the authors' own measurements, both showing good agreement.     

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.     

改善效率的0.14 THz折叠波导慢波结构设计

在研究0.14 THz折叠波导行波管中,提出一种三段相速跳变的设计,使得电子能够在输出段与行波场发生速度再同步,从而提高了电子工作效率。根据色散公式,找到一种影响相速变化的结构因素。通过优化设计进行大信号程序计算,在电压14.95 kV、工作电流30 mA时,与未采用相速变化的结构相比,140 GHz时功率提高了0.84 W,效率提高了9.13%;在142 GHz时功率提高了0.88 W,效率提高了10.4%;-1 dB带宽由原来的5 GHz提高到7 GHz,扩展了行波管的带宽,提高了电子与波的互作用效率。     

Efficient nonadiabatic planar waveguide tapers

We report taper designs with high transmission efficiencies and with lengths shorter than those needed for adiabatic operation. The tapering occurs between rectangular optical waveguides with the same vertical silicon-on-insulator layer structure, but with different horizontal widths, namely 0.5 and 2.0 /spl mu/m, and for taper lengths between 0.5 and 3.0 /spl mu/m. After a comparison between two different optimization methods in a two-dimensional calculation scheme, one of these is repeated using three-dimensional calculations. The results show that, also in the length region where conventional linear and parabolic tapers are not yet adiabatic, tapers with a high efficiency can be designed by applying complex taper structures with more degrees of freedom.     

Reflection Coefficient of E-Plane Tapered Waveguides

This paper treats the reflection of linearly and sinusoidally tapered waveguides. In the first part, reflection coefficients of linearly tapered waveguides for dominant modes are calculated. Graphs of the vswr of tapers for different impedance ratios are plotted showing that the vswr does not go to unity at multiples of a half wavelength. In the second part, reflection coefficients of sinusoidally tapered waveguides are calculated. Experimental data verify the theory for both kinds of tapers of various lengths at 4 kmc band. Linear tapers perform almost as well as exponential tapers, and better than shorter hyperbolic tapers. The reflection coefficients of sinusoidal tapers can be about half as small as that of the linear tapers, and these tapers compare favorably with the Dolph-Tchebycheff and the Willis taper of improved design.     

Reflection of a Pyramidally Tapered Rectangular Waveguide

The reflection coefficient Gamma of a pyramidally tapered rectangular waveguide is derived by assuming that the taper impedance is proportional to the height and guide wavelength and inversely proportional to the width of the taper cross section. It is a shown that the loci of Gamma, plotted in the K plane as a function of taper length for some conventional tapers, do not pass through the center of the chart at multiples of a half-guide wavelength as for an exponential line, but instead they converge almost concentrically. The frequency characteristic of the pyramidally tapered waveguide is compared with other types of tapers. Typical 7-kmc experimental results for several tapers differing in length are presented.