相对论皮秒激光在低密度等离子体中直接加速的电子束的横向分布特征研究  

Transverse distribution of electron beam produced by relativistic picosecond laser in underdense plasma

作　　者：张晓辉 董克攻[2] 华剑飞[1] 朱斌[2] 谭放[2] 吴玉迟[2] 鲁巍[1] 谷渝秋[2,3] Zhang Xiao -Hui;Dong Ke -Gong;Hua Jian -Fei;Zhu Bin;Tan Fang;Wu Yu -Chi;Lu Wei;Gu Yu -Qiu(Department of Engineering Physics, Tsinghua University, Beijing 100084, China;Science and Technology on Plasma Physics Laboratory, Laser Fusion Research Center, China Academy ofEngineering Physics, Mianyang 621000, China;Academy of Engineering Physics, Shenzhen Technology University, Shenzhen 518118, China)

机构地区：[1]清华大学工程物理系,北京100084 [2]中国工程物理研究院激光聚变研究中心,等离子体物理重点实验室,绵阳621000 [3]深圳技术大学工程物理学院,深圳518118

出　　处：《物理学报》2019年第19期165-171,共7页Acta Physica Sinica

基　　金：国家重点研发计划(批准号:2016YFA0401100);科学挑战计划(批准号:TZ2018005)资助的课题~~

摘　　要：相对论皮秒激光与低密度等离子体作用可以通过“激光直接加速”机制获得超有质动力定标率的高能电子,且电荷量可以达到百 nC 级,在伽马射线产生、正电子产生等方面具有重要应用.然而激光直接加速电子束相比激光尾场加速电子束具有更大的发散角,同时实验观测的横向束分布也不均匀,但是其中的物理机制研究较少.本文通过二维粒子模拟证明,相对论皮秒激光在低密度等离子体中驱动的激光直接加速中,高能电子束会在激光偏振方向分叉,而且电子能量越高这种现象越明显.文章通过细致的理论分析解释了这种高能电子横向分布产生“分叉”结构的内在原因.在激光直接加速的过程中,电子在纵向获得加速的时候,它在激光偏振方向(横向) betatron 振荡的动能也会随之增加,当电子的能量足够高时,二者呈线性关系,因此高能电子的横向速度的振幅近似相等,这种相等的振幅最终导致了高能电子束在激光偏振方向的分叉.Energetic electron beam can be generated through the directlaser acceleration (DLA) mechanism when high power picosecond laser propagates in underdense plasma, and the electron yield can reach several hundred nC, which has a great application in driving secondary radiations, such as bremsstrahlung radiation and betatron radiation. When a linearly polarized laser is used, the beam divergence is always larger in the laser polarization direction. What is more, the forked spectral-spatial distribution is observed in the experiments driven by femtosecond laser where DLA is combined with the laser wakefield acceleration (LWFA). The forked distribution is regarded as an important feature of DLA. However, an analytical explanation for both the bigger divergence and the forked spectral-spatial distribution is still lacking. Two-dimensional (2D) particle-in-cell simulations of picosecond laser propagating in underdense plasma are conducted in this paper to show how the fork is formed in DLA. The fork structure is a reflection of the distribution of electron transverse velocity. We find that when electrons are accelerated longitudinally, the transverse oscillation energy in the laser polarization direction increases correspondingly. If the electron energy is high enough, the transverse oscillation energy will increase linearly with the electron energy. As a result, the most energetic electrons will have an equal amplitude of vy, where vy denotes the velocity in the laser polarization direction. For a single electron, the distribution of its transverse velocity over a long period dP/dvy,will peak at ±vm (vm denotes the amplitude of vy). If all the electrons have the same vm, the distribution of vy at a given time will be the same as dP/dvy.That means they will split transversely, leading to a forked spectral-spatial distribution. By using a simplified model, the analytical expression of vm is derived, showing good agreement with vm in the PIC simulation. However, the oscillation energy in the direction perpendicular to pola

关 键 词：激光直接加速 电子横向分布 

分 类 号：O53[理学—等离子体物理]