Electron Injection And Relay Acceleration In Laser Wakefield Accelerator

Laser wakefield acceleration(LWFA)was proposed by Tajima and Dawson in the1970 s.After 40 years of development,especially the invention of ultra-short ultraintense laser pulse based on chirped pulse amplification technology,laser wakefield acceleration has developed rapidly.Considerable progresses have been achieved in acceleration stability and electron beam quality,and LWFA is getting closer and closer to practical applications.At present,quasi-monoenergetic electron beams with several Ge V energy have been generated within centimeter scale acceleration distance,and the highest electron energy from LWFA has reached 8 Ge V.In recent years,besides free exploration,LWFA researches are also focused on specific problems for different applications.On one hand,in order to improve the stability and quality of the accelerated electron beam,a variety of electron injection methods have been proposed.On the other hand,in order to obtain higher energy electron beam,one needs to overcome the dephasing problem in single-stage acceleration.Schemes like phaselocked acceleration are being researched.In this dissertation,we propose our solutions to the electron injection problem and dephasing problem in laser wakefield acceleration,and present our experimental results of laser wakefield acceleration based on discharge capillary.We hope to provide new ways for future laser wakefield acceleration developments and applications.This dissertation mainly includes the following three parts:In the first part,we mainly studied the electron injection process in laser wakefield acceleration,and focused on an electron injection mechanism at the vacuum-plasmaboundary.We studied the electron injection mechanism in the case of tilted sharp boundaries,and evaluated the influence of different laser incident angles and polarization directions on the amount and energy of injected electrons.We found that the lateral momentum of injected electrons at different tilt angles will be quite different.The reason of this difference is analyzed in detail and the comparison with the electrons injected into the normal boundary is made.In the second part,we mainly studied the dephasing problem in laser wakefield acceleration.Electron dephasing is one of the key issues that affect the maximum energy in single-stage accelerator.We have proposed a solution--relay acceleration to solve this problem.In this scheme.In our scheme,electrons are injected into a bubble structure a little bit far from the drive laser.The relay acceleration of the electrons through multiple bubbles can be realized by proper density modulation in the longitudinal direction,which greatly increases the acceleration length and the maximum energy of the electrons compared to the normal single bubble acceleration.In our research,we found that when the electron beam passes through each bubble,it is scattered by the dense electron layer at the tail of the bubble.Electron beam scattering will cause the lateral momentum divergence and electron beam energy loss.This is different from the previous studies of one-dimensional wakefield field simulations.This problem only occurs in multi-dimensional simulations.The defocusing effects on the final beam energy spread and emittance,caused by electrons crossing the high-density electron layer locating between the neighboring buckets,can be suppressed by appropriately connecting the staged channels.In the end,our simulations succeeded in allowing the electron beam to jump from the fifth bubble to the second bubble continually,doubled the energy gain,proving the feasibility of multi-stage relay acceleration.In the third part,we carried out a laser wakefield acceleration experiment based on a discharge capillary on a 200 TW laser system in the laboratory of laser plasma,SJTU.The density of the capillary channel is scanned and calibrated.The capillary wave guiding of intense laser pulse and the final high-energy electron acceleration are studied.At the beginning of the experiment,the effective shots rate of this acceleration system is only about 20%～30%.After optimizing the horizontal and vertical density inside the capillary,we have determined the parameter range where plasma channel depth matches the laser width,which greatly increases the acceleration stability.The experimental results show that an electron beam of hundreds Me V can be stably accelerated in a 1cm capillary;and electron beam with Ge V energy can be obtained in a 3cm capillary.This result lays the foundation for our future staged laser wakefield acceleration based on a curved capillary.