Study of Beam Wave Interaction Nonlinear Theory and CAD Technique for Coupled Cavity Traveling Wave Tubes 

Author  BaiChunJiang 
Tutor  LiJianQing 
School  University of Electronic Science and Technology 
Course  Physical Electronics 
Keywords  Coupled cavity traveling wave tubes (CCTWTs) beamwave interaction equivelate circuit Drive induced oscillation (DIO) computer aided design(CAD) 
CLC  TN124 
Type  PhD thesis 
Year  2013 
Downloads  22 
Quotes  0 
Coupledcavity travelingwave tubes (CCTWTs) are vacuum electronic amplifierswith high gain and high output power. CCTWTs have been developed for uniquelydemanding applications in communications, radar and electronic counter measures. It isvery difficult to promote performance of CCTWTs in power, bandwidth, gain,efficiency and nonlinearity comprehensively by traditional design method based onexperience. The technique of modeling and simulation is the key method to improve thecapability of CCTWTs. The process of beamwave interaction for CCTWTs is themost important part of the mechanism and kernel theory for CCTWTs. And thebeamwave interaction for CCTWTs is the foundation of CCTWTs CAD software.Therefore, it is very helpful for optimizing the structure, improving output power,widening bandwidth and enhancing stability to analyse the beamwave interactiontheory.In this doctoral dissertation, the work is mainly focused on the study of beam waveinteraction basic theory and the CAD technology of CCTWTs. And several importantand valuable results are listed as below：1. Two equivalent circuit models are introduced to represent the coupled cavityhigh frequency structure. The two circuits can both give a good representation of thedispersion and total impedance. The formulas about dispersion, total impedance andcharacteristic impedance of the two circuits are also deduced. According to thecharacteristic of the circuits, two approaches to calculate the lumped elements of thecircuits are proposed with the formulas for each circuit. In order to validate theapproaches, the work to compare the approaches with different high frequencystructures which can work in different frequency bands is done. The content ofcomparasion mainly includes the dispersion and total impedance.2. To study the dispersion and total impedance of the coupled cavity highfrequency structure, the simulation software HFSS is used. Combining the analyticalformula of high frequency field and HFSS, the distribution of high frequency field,including axial field and radial field, is discussed in the coupled cavity high frequency structure. The calculation about total impedance and the relationship between theinteraction impedance and total impedance are also discussed. And it is of great use toresearch the beam wave interaction of CCTWTs.3. Through the equivalent circuits, Curnow and MKK, the matrixes includinglumped elements are construced. The matrixes are divided into two types：singlecavitymatrix and multipcavity cascaded matrix. The singlecavity matrix is used onlyconsidering the forward wave. While the backward wave is taken into account, themultipcavity cascaded matrix is used. Comparing with the multipcavity cascadedmatrix, the singlecavity matrix can calculate faster due to with few matrix elements.Worthing notice, the effect of attenuations is also taken into account in these matrixes.Generaly speaking, in a equivalent circuit, the loss resistance is used to represent theattenuation of the coupled cavity structure. And the unit of loss resistance andattenuation are different. In the multipcavity cascaded matrix, the NewtonRaphsonmethod is used to obtain the accurate equivalent loss resistance from transform theattenuation. In general, it is difficult and unaccurate to transform the attenuation intoequivalent loss resistance using analytical fornulars. In the singlecavity matrix, thevoltages of the equivalent circuit are attenuated instead of solving the equivalent lossresistance.4. The onedimensional (1D) and threedimensional (3D) beam wave interactiontheores of CCTWTs are constructed. And the formulas including phase equation,motion equation, field equation and source current equation are all deduced. TheHughes Aircraft Company’s961HA TWT which is a CCTWT for intersatellitecommunications is taken as an example to validate the1D and3D beam waveinteraction theoties. The simulated results are compared with the experimental results.And the simulated results agree with the experimental results. Since the backward waveis taken into account in the theory, the theory can also be used to predict the driveinduced oscillation (DIO). Also using the parameters of961HA TWT, the DIO issimulated. And it is proved that the approach to controlling DIO and improving themaximum output power by using attenuation is validated.5. The Hughes high frequency structure is analyzed in detail with simplifiedCurnow model and anaystical formulars solving dispersion and total impedance. Amethod which is used to optimize the high frequency structure and improve the output power of CCTWTs is developed. To validate the method, the961HA TWT is as anexample and the1D code based on1D beam wave interaction theory is used. It isproved that the method is useful for improving the output power. Discussing the changeof the dispersion, total impedance and wideband with the dimension of Hughes structure,a preliminary topdown design method which can be used to do fast design forhighefficiency coupledcavity high frequency structure according to the designspecifications of CCTWTs is developed. And the method is used to design a Kabandcoupledcavity high frequency structure. And the tested results show that thenewlydeveloped design method works pretty well.