电子束尾波对质子束自调制尾波相速度影响的理论和数值模拟研究

Theoretical and numerical studies of the phase velocity of wakefields in plasma driven by self-modulated proton beams with electron beam seeding

高能质子束在等离子体中通过自调制不稳定激发尾波的研究在过去的十年里有了长足的发展,在欧洲核子研究中心(CERN)人们已经在相关AWAKE实验中利用这种尾波加速电子,并获得了最高能量约2 GeV的电子束.针对高能粒子加速应用需求,近几年人们又进一步提出了利用电子束种子尾波控制质子束自调制过程的方案,用于提升尾波的强度与稳定性.本文研究了电子束种子尾波对质子束自调制尾波相速度的影响,着重讨论了导致尾波相速度改变的多种物理机理及电子束所起到的作用.通过理论分析和二维粒子模拟研究发现,电子束的引入可以提升质子束自调制尾波的增长率和尾波的相速度,且电子束的电荷密度越高其效果愈明显.本文还探讨了电子束能量和质子束的纵向密度分布对相速度变化的影响.

Significant progress has been made in the studies of wakefield excitation in plasma by a self-modulated high energy proton beam in the past decade.The electron beams accelerated up to 2 GeV by using such a wakefield were demonstrated in the AWAKE experiment at CERN in 2018.Aiming at the application of high energy particle accelerators,new ideas have been investigated in recent years,such as seeding the proton beam self-modulation with an electron beam in order to enhance the strength and stability of the wakefield or adding a density transition in the plasma distribution to enhance the phase velocity and the strength of the wakefield.Here in this work,we investigate the effects of electron beam seeding on the phase velocity of the wakefield generated by the modulated proton beam in plasma.The physical mechanisms responsible for the phase velocity change and the roles played by the electron beam seeding are discussed.The theoretical analysis and two-dimensional particle-in-cell simulations show that both the growth rate and the phase velocity of the wakefield generated by the modulated proton beam can be enhanced by the electron beam seeding.The higher the charge density of the electron beam,the more significant the enhancement effects.The effects of electron beam energy and proton beam longitudinal profiles on the increase of phase velocity are also studied.It is shown that the evolution of the electron beam distribution has a significant effect on the seeding self-modulation process,and thus affecting the phase velocity.A self-focusing electron seeding beam can increase the phase velocity of the wakefield even to superluminal while an expanding seeding beam can reduce the phase velocity and destroy the stability of the whole process.This work may benefit the proton beam seeding self-modulation acceleration and its applications.