| Thermal cathode is widely used in accelerators,FELs and kinds of vacu?m electronics.For better performance and long stable run,low emittance with notable contribution from cathode is needed.Emittance of perfect thermal cathode only depends on radius and temperature.However,emission non-uniformity exist due to cathode surface profile,material distribution,temperature variation.etc.,which may cause intrinsic emittance growth,brightness decline,envelop size augment,device performance deterioration or even failure.In order to research effect of kinds of non-uniformity on intrinsic emittance,theoretical derivation and FDTD-PIC simulation are conbined in this paper.To understand how intrinsic emittance is manipulated by temperature variation and work function variation,a general intrinsic emittance model consisting of contributions from surface non-uniformity of smooth cathode is firstly developed based on general thermal emission.Numerical calculation for a cathode with temperature radical variation showed that RMS emittance declined about 15%when non-uniformity reaches 10%,which well conform to FDTD-PIC simulation results.Otherwise,non-uniformity effect will weaken by space charge field(SCF).For work non-uniformity,simulation is further verified by numerical calculation for work function radical variation and 1D cosine periodic variation.Furthermore,theoretical result and simulation for cathodes with work function of 2D cosine periodic variance and 2D random distribution showed that emittance increase coefficient(EIC)approached to 1 and variance of EIC decreased quickly when spatial frequency increased.This paper shows that work non-uniformity hardly affects intrinsic emittance if cathode diameter is much larger than the average size of micro work region;in contrast,intrinsic emittance may differ from uniform case notably if micro size is comparable to cathode diameter.Otherwise,space charge field will cause EIC augment for smooth cathode with work variation.In order to understand how emittance is manipulated by surface profile,Firstly,a theoretical form of intrinsic emittance-roughness characteristics of thermionic cathode is derived for the first time.The theoretical solution shows that the emissivity increases with roughness up,and is approximately proportional to the applied electric field.Then,based on the FDTD-PIC program YYPICMC,the influence of 2D cosine surface roughness on intrinsic emittance was analyzed.The simulation results were significantly different from the theoretical solutions.When the roughness amplitude increased to a certain value,the EIC tended to grow slowly.When the space period of profile is very small,the EIC tends to a lower value rather than rapidly increasing with the period down.Combined with the variation of current,electron trajectory and the influence of space charge effect,the mechanism of this difference is analyzed,and the results show that this difference is caused by the limitation of theoretical form.Finally,according to the limitations of the theoretical model,an idea to improve the emittance-roughness theory model by introducing "curve effect" and "particle loss effect" is proposed.Finally,EIC change along axis nearby rough surface and space charge effect are analyzed,results show that EIC decrease sharply at rough region due to parts of electrons are lost;meanwhile,intrinsic emittance caused by roughness and applied field will be partially eliminated by SCF.For real thermionic cathodes,intrinsic emittance is affected by kinds of non-uniformity mentioned above together.Simulation shows that total EIC is about the same as the product of EICs of all independent non-uniformitys.Furthermore,space charge is likely always weakening influence of non-uniformity to intrinsic emittance.The full text is divided into eight chapters.The contents of each chapter are briefly introduced as follows:Chapter 1 is the introduction,which introduces the background,research content and research scheme of this study.This paper introduces the previous research on thermionic cathode surface non-uniformity,puts forward the research contents and basic scheme of this study,as well as key problems to be solved,and introduces the characteristics and innovations of this study.Chapter 2 introduces the theory of thermionic emission,which supports theoretical research of cathode non-uniformity.Then,the basic parameters of beam current are introduced,and the concepts of emissivity increase coefficient and emission current increase coefficient are defined.Chapter 3 introduces in detail the FDTD-PIC simulation technology and the function of the self-developed program YYPICMC and key simulation technologies such as particle linked list,peristaltic camouflage generation,surface profile regeneration,etc.Chapter 4,Chapter 5 and chapter 6 are the main contents of this study,focusing on EIC of temperature heterogeneity,work function non-uniformity and surface roughness.The above non-uniformity characteristics and the influence of space charge field on the emittance are obtained based on theoretical derivation and simulation,which are briefly introduced in above paragraphs.Based on Chapter 4 to Chapter 6,a coupled non-uniformity inducing emittance growth model is established in Chapter 7,and the influence of coupled inhomogeneity on the intrinsic emittance is studied.In Chapter 8,we s?mmarize the research results of this paper,and put forward some suggestions for cathode development according to the demand of high quality beam. |
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