Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 11774227) and the Science Challenge Project, China (Grant No.TZ2018005)
Received Date:02 September 2019
Accepted Date:15 November 2019
Published Online:05 February 2020
Abstract:Plasma wakefield acceleration driven by ultra short ultra intense laser pulse interacting with gas target has been studied for almost four decades. Monoenergetic electron beams with central energy of multi-giga electron-volt have been achieved in a centimeter-scale acceleration distance. Currently, the highest energy of electrons accelerated by laser wakefield is 8 GeV. In order to further improve the quality of such electrons, many kinds of electron injection schemes have been proposed such as density gradient injection, colliding pulse injection and ionization injection. Electrons under the suitable conditions can be trapped by the strong plasma wakefield. Those trapped electrons are then accelerated in the wakefield. In a nonlinear regime, the wakefield shows a “bubble” structure. Electrons with transverse momentum can oscillate in the wakefield and produce considerably betatron radiation in the ultraviolet and X-ray region. In this paper, we study the electron injection around the sharp plasma-vacuum boundary. The effects of the slant angle of the boundary on the final electron quality are investigated in detail. Our results show that with optimal slant density transition around the vacuum plasma boundary, both the beam quality and the injection charge in the second “bubble” can be improved. Two-dimensional particle-in-cell simulations show that the injection charge in the second wake bucket can be increased three times when an optimal slant angle is used compared with a vertical boundary. The driving pulse’s polarization also affects the injection charge. When the polarization is in the injection plane the injection charge in the first bucket can be triply increased compared with the case when the polarization is out of the plane. The reasons for the enhanced injection charge and transverse oscillation are found by tracing the initial injection positions and trajectories of the electrons. These studies would benefit the electron acceleration and its applications, such as compact betatron radiation source. Keywords:laser-plasma interactions/ laser plasma wakefield acceleration/ electron injection
首先研究真空等离子体边界面倾角对注入电量和最终电子能量的影响. 如图2(a)所示, 数值模拟研究表明, 在锐真空-等离子体边界注入下, 通常电子会被注入到第一和第二个尾波中. 为了研究倾斜角的影响, 分别对注入到不同空泡的电子进行统计, 如图2(b)所示, 对第二个空泡中的超粒子, 统计它们的电量、动量和能量, 并随机选取其中的100个计算它们的轨迹. 图 2 倾斜角为0°时激光传播500T0后等离子体密度与注入电子(γ ≥ 15)的位置分布 (a)注入电子的分布; (b)放大后第二个空泡中的电子分布 Figure2. Distributions of plasma density and injected electrons after 500T0 propagation when the boundary slant angle is 0°: (a) Injected electrons; (b) electrons in the second bubble
分别对0—500T0时间范围内不同倾斜角度情况下两个空泡中被加速电子进行统计, 在它们的相对论因子大于等于15时计入统计, 如图3所示. 从图3可以看出, 不同倾斜角度对应的激光电子加速到统计阈值(γ ≥ 15)所需的时长是不一样的. 简而言之, 倾斜角越大时, 激光后沿的尾波形成时间越晚, 电子越晚被捕获; 而低倾斜角情况下, 尾波形成较早, 电子被捕获时间也相对更早, 从而加速启动的更早. 图 3 激光传播过程中不同角度下的电子平均能量增长的情况 (a)第一个空泡内电子平均相对论因子γ的变化; (b) 第二个空泡内电子平均相对论因子γ的变化 Figure3. Average energy growth with time: (a) Average gamma factor of electrons in the first bubble; (b) average gamma factor of electrons in the second bubble
除了关注加速电量外, 被加速电子束的横向振荡对于电子束的Betatron辐射也是非常重要的, 直接关系到辐射的强度和辐射谱分布. 为了明确倾斜边界对电子横向运动产生的影响, 研究了被加速电子的运动轨迹. 对两个特定的倾斜角度(0°和45°), 在不同的空泡中各选取100个被加速电子, 根据它们的轨迹分析注入特征. 如图5(a)所示, 在锐的倾斜情形中(0°), 注入电子沿着激光传输方向是基本对称的; 在边界倾斜角为45°时, 如图5(b)所示, 产生的注入电子则具有上下非对称性. 注入电子的这种上下轨迹不对称性是斜入射与0°入射激光的主要不同之处. 在边界面与入射光非垂直的情况下, 可以预见到激光单侧将与等离子体优先接触, 另一侧的接触时间则相对滞后, 此时仅有最先接触的那一侧附近的电子能够发生注入. 这种机制是导致二者的加速电子在统计上轨迹不同的原因, 同时也会导致电子在加速过程中横向振荡的不同. 图 5 不同倾斜角度下的第一个空泡(红色)和第二个空泡(蓝色)中注入电子的轨迹(为了显示清晰, 对两种倾角情形, 各自只选取了10个典型的电子) (a) 0°; (b) 45° Figure5. Trajectories of electrons in the first bubble (red) and second bubble (blue): (a) 0°; (b) 45°. Ten electrons’ trajectories have been selected for clearer view
图6显示了所有高能电子的横向动量的统计结果, 对于0°和45°的两种边界下, 空泡中被加速电子的横向振荡过程存在差异. 0°边界倾角时, 第一个空泡内注入电子的横向动量较小, 而第二个空泡内注入电子的横向动量则沿入射光两侧对称振荡, 幅度与45°倾角边界注入时类似, 但该倾角下第一个空泡内电子的振荡幅度也得到了增强(这是由于第一个空泡内电子注入较早, 但注入时空泡形成具有不对称性, 电子获得初始横向动量较大), 有利于强的Betatron辐射. 通常大的横向振荡幅度有望提升小型化台面X射线辐射源的强度和辐射谱的中心频率. 图 6 (a) 0°倾斜边界角时注入空泡1 (红)和空泡2上下两侧注入电子(蓝)的平均动量; (b) 45°倾斜边界角时注入空泡1和空泡2电子的平均动量 Figure6. (a) Average transverse momentum of electrons in the first bubble (red) and second bubble (blue) when the boundary slant angle is 0°; (b) average transverse momentum of electrons in the first bubble (red) and second bubble (blue) when the boundary slant angle is 45°