1.Department of Engineering Physics, Tsinghua University, Beijing 100084, China 2.Science and Technology on Plasma Physics Laboratory, Laser Fusion Research Center, China Academy ofEngineering Physics, Mianyang 621000, China 3.Academy of Engineering Physics, Shenzhen Technology University, Shenzhen 518118, China
Fund Project:Project supported by the National Key Program for S&T Research and Development, China (Grant No. 2016YFA0401100), and Science Challenge Project, China (Grant No.TZ2018005)
Received Date:18 July 2019
Accepted Date:23 August 2019
Available Online:01 October 2019
Published Online:05 October 2019
Abstract: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 $\dfrac{{{\rm d}P}}{{{\rm d}{v_y}}}$, 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 $\dfrac{{{\rm d}P}}{{{\rm d}{v_y}}}$. 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 polarization will decrease when electrons are accelerated longitudinally (acceleration damping). As a consequence, the divergence perpendicular to the polarization direction will be smaller. Our research gives a quantitative explanation for the transverse distribution of electrons generated by DLA. With some modification, it can also be used in DLA combined LWFA to better control the dephasing length. Keywords:direct laser acceleration/ transverse distribution
图2(a)展示了$t = 5965\omega _0^{ - 1}$时刻电子在能量-vy相空间的分布(vy代表电子在y方向的速度), 对应的是实验中测量到的电子能谱. 从图中我们可以看到电子能量基本为连续分布, 最高能量在我们的模拟条件下可以达到100 MeV左右, 并且电子的横向分布出现了明显的分叉结构, 且越到高能端越明显. 这种分布与之前fs激光驱动的LWFA-DLA混合加速实验中的观察到的电子能谱[18,19]非常相似. 为了更好地展示, 我们在图片的右侧用黑色实线画出了能量高于60 MeV的电子在横向的分布, 其计数值做了归一化处理. 从这个图可以更清晰地看到电子在激光偏振方向会分裂为两团, 电子大量聚集在$ \pm 0.15\;c$的位置, 在$ \pm 0.15\;c$之间则相对较少, 而在$ \pm 0.15\;c$之外则几乎很少. 图2(b)展示了能量在60—70 MeV之间的电子在$y - {p_y}$相空间的分布, 其中py代表电子在y方向的动量. 我们可以看到, 这些电子组成了一个空心的圆环, 这暗示我们这些电子可能在y方向具有相同的振幅. 在第3节中我们将结合简化解析模型细致分析电子束产生这样结构的内在原因. 图 2$t = 5965\omega _0^{ - 1}$时刻电子在相空间的分布 (a)电子在能量-vy相空间的分布, 白色虚线是电子横向速度振幅的理论值, 右侧的黑色实线代表着能量大于60 MeV的电子的vy的分布, 为了更好地展示, 其计数值做了归一化处理; (b)能量在60—70 MeV之间的电子在y-py相空间的分布 Figure2. Electron phase space at $t = 5965\omega _0^{ - 1}$: (a) Energy -vy phase space, the white dashed lines denote the amplitude of vy from analytical solution,the black solid line denotes the vy distribution of electrons above 60 MeV, the counts are normalized to achieve a better illustration; (b) the y-py phase space of electrons within energy range from 60 MeV to 70 MeV.