Study On The Period-varying Slow-wave Structure

Slow-wave structures are the major components of the travelling wave tube(TWT).To utilize the interaction between the microwave and the electron beam in aTWT,the microwave phase velocity should be made approximately equal to theelectron velocity.And this velocity is far slower than the light velocity.Sequentially,microwave circuits in which the microwave propagates are defined as slow-wavestructures (or slow-wave circuits).If the structures are periodic,it is called periodicslow-wave structures.However,when electrons transfer the kinetic energy to the microwave,theelectron velocity is decrease.In the process of the beam-wave interaction,electronscontinuously lose their kinetic energy.Therefore,their velocities continuously decrease.Sequentially,the decrease of the electron velocity would cause the electron beamnon-synchronizes the microwave.This is an important reason to the limitation of thetravelling wave efficiency.According to reason above,it is assumed that if the electron beam synchronizesthe microwave during the whole interaction process,the interaction efficiency can beimproved.Currently,two ways are presented to improve efficiency.The one is to applya dc voltage gradient along the beam or structure,the other is to taper the RF circuitphase velocity.And the theory adopted in this dissertation belongs to the velocitytapering.In this dissertation,the space harmonic selectivity of period-varying foldedwaveguides are analyzed,the nonuniform folded waveguides are studied,thesynchronization in period- varying folded waveguides is analyzed,and the conclusionsobtained are proved by the software simulation.The structure of this dissertation is presented as follow:Chapter 1 is introduction.The background of research and the obstacles ofpresent TWTs will be firstly explained.Then,the significance and the researching scopeof this dissertation will be presented.In chapter 2,the space harmonics in period-varying folded waveguides and folded waveguides with jumping-period are analyzed,then conditions to select spaceharmonic in these structures are obtained.In chapter 3,the space harmonics are discussed in nonuniform folded waveguides,conditions to maintain one space harmonic synchronizes the electron beam during thewhole interaction process are presented,and the coupling impedance in each period isanalyzed.Finally,output power and interaction efficiency in this structure are analyzedby the particle simulation software MAGIC,and these conditions are proved.In chapter 4,the space harmonics in period-varying slow-wave structures areanalyzed,conditions to maintain synchronization between the electron beam and themicrowave in period-varying slow-wave structures are discussed.And the physicalmeaning of these structures is presented.In chapter 5,the space harmonics are analyzed in period-varying foldedwaveguides,the space harmonics amplitude are deduced,principles of one spaceharmonic synchronizing the electron beam during the whole interaction process areobtained,and these principles are proved by the software simulation.Finally,improvements of output power and interaction efficiency in period-varying foldedwaveguides are presented.The chapter 6 is conclusion.