Beam Quality Improvement Of Energetic Electrons In Laser Wakefield Acceleration

Laser wakefield acceleration has the advantages of ultrahigh acceleration gradient,small size,and low cost.Ionization-induced injection is an electron injection scheme with simple operation and high repetition rate.However,an excessively long injection distance results in a high energy spread of electron beam.This paper uses a combination of theoretical analysis and numerical simulation.Based on ionization injection,the aim is to improve beam quality and reduce energy spread.The main content is divided into the following three parts:The first part(chapters 1 and 2)first introduces the development history of laser technology and the basis of plasma physics,and points out that laser wakefield acceleration is a hot research area of laser-plasma interaction physics.Secondly,we introduce the theory of electron injection and common injection schemes,including self-injection,optical injection and ionization-induced injection.Finally,based on high energy spread of ionization injection,several proposed beam quality optimization schemes are introduced.The second part(Chapter Three)proposes a scheme to reduce energy spread of ionized injection electron by using positive chirped pulse,making the experimental operation easy from the perspective of laser shaping.Results show that there is compression effect when the positive chirp pulse propagates through underdense plasma,resulting in laser duration shortened and peak electric field increased.These effects lift the minimum value of wakefield potential and break injection condition in advance.The effective injection distance is shorten to hundreds of micrometers.Simulation results show that we finally obtain a bunch of electrons with energy spread 5.03%.The third part(chapter 4)proposes a scheme to reduce energy spread by using two-staged mixed gas target,improving beam quality from the perspective of plasma density configuration.Preset mixed gas with high-to-low density distribution is set.Ionization injection occurs only in high-density region and stops in low-density region due to a decrease of potential difference.In addition,this scheme can modulate electron density profile into bimodal shape and smooth the electric field.The energy spread will not increase due to the extension of the acceleration distance.Simulation results show that we obtain monoenergetic electron beam with energy spread 5.11%.