Generating High Brightness Electron Beams Via High Transformer Ratio Plasma Wakefield Acceleration

Beam driven plasma wakefield accelerator?PWFA?has made great progress in the past decades,which produces high gradient accelerate field and has the potential to generate high quality beams.As a new kind of accelerator,it is promising for future compact light sources and linear colliders.One of the challenges is that how to produce high energy and high quality beams simultaneously.The thesis makes combination of high transformer ratio PWFA and density down-ramp injection to generate high energy and high quality beam.Through thereotical simulations,the feasibility of this method has been verified,and the physics between high transformer ratio and high quality injection has been studied.The possibility of implementing PWFA at SXFEL has been explored,which is important for future experiments.The work of this thesis is carried out as chapters below:In chapter 1,reviews to the important research and develpement of PWFA,high transformer ratio acceleration and high quality injection are given.As a main tool for the work of this thesis,PIC?particle in cell?simulation is introduced.In chapter 2,the nonlinear theory of high transformer ratio and the process of density down-ramp injection in blowout regime are summarized.In chapter 3,systematic simulations are performed to study the generation of high energy high brightness electron beams from a high transformer ratio PWFA with density down-ramp injection.Results show that an electron bunch with an average transformer ratio around 4,normalized emittance about 10nm and peak brightness near 10²⁰A/m²/rad²can be generated in this way.In addition,the effects of drive bunch shape,longitudinal length and transverse size on transformer raito and the quality of injected beam are discussed,which are useful for the parameter selection in future experiments.In chapter 4,the performance of high transformer ratio PWFA in hollow or near hollow channel is examined.Simulations show that hollow channel affects a little on transformer ratio,but it would increase the emittance of electron beams that injected from the density down-ramp,thus reducing the peak brightness.In chapter 5,the feasibility of implementing high transformer ratio PWFA at SXFEL is explored.Based on the shaped electron beam parameters obtained by start-to-end beam line simulations,we performed 3D PIC simulations.Transformer ratio is optimized by scanning the plasma density and the transverse size of the drive bunch.Finally an average transformer ratio of 3.6 is achieved.Meanwhile,combining with density down-ramp injection,it can generate electron bunch with a brightness more than 10¹⁸A/m²/rad².