Lei Bifeng, Liu Bin, Shi Mingyuan, Seidel Andreas, Seipt Daniel, Zepf Matt, Qiao Bin
Center for Applied Physics and Technology, HEDPS, and SKLNPT, School of Physics, Peking University, Beijing 100871, China.
Guangdong Institute of Laser Plasma Accelerator Technology, Guangzhou 510415, China.
Phys Rev E. 2024 Jan;109(1-2):015204. doi: 10.1103/PhysRevE.109.015204.
Shot-to-shot electron beam pointing instability in the plasma bubble, defined here as electron beam pointing jitter (EBJ), is a long-standing problem that limits the potential of the laser wakefield accelerator (LWFA) in a range of demanding applications. In general, EBJ is caused by variations in laser and plasma parameters from shot to shot, although the exact physical mechanism by which EBJ grows in the plasma wave remains unclear. In this work we theoretically investigate the fundamental physics of EBJ inside the plasma bubble and show how the intrinsic betatron oscillation can act as an amplifier to enhance EBJ growth. The analytical formulas for electron trajectory, pointing angle, and EBJ are derived from the basic momentum equation of an electron and verified numerically. It is shown that the shot-to-shot fluctuations of the laser and plasma parameters, such as laser strength, focus, and carrier-envelope phase, as well as the ambient plasma density and profile, lead to EBJ. The evolution of EBJ is dictated by the dynamics of the plasma bubble. Two amplification processes of the betatron oscillation are found in the rapidly evolving bubbles and play important roles in EBJ growth. The first is driven by a linear resonance in the wobbling bubble due to the coupling of the betatron oscillation and the bubble centroid oscillation. The second is a parametric resonance seen in the breathing bubble, where EBJ grows exponentially due to the strong frequency modulation of the betatron oscillation. Their characteristic functions, growth rates, and resonance conditions are deduced analytically and validated numerically. Finally, we also studied how radiation reaction affects EBJ. Our research provides a clear understanding of the basics of EBJ dynamics in LWFA and will help improve the use of LWFA in demanding applications.
等离子体泡中逐发电子束指向不稳定性，在此定义为电子束指向抖动（EBJ），是一个长期存在的问题，限制了激光尾场加速器（LWFA）在一系列高要求应用中的潜力。一般来说，EBJ是由激光和等离子体参数的逐发变化引起的，尽管EBJ在等离子体波中增长的确切物理机制仍不清楚。在这项工作中，我们从理论上研究了等离子体泡内EBJ的基本物理过程，并展示了本征贝塔振荡如何作为放大器增强EBJ的增长。电子轨迹、指向角和EBJ的解析公式由电子的基本动量方程推导得出，并通过数值验证。结果表明，激光和等离子体参数的逐发波动，如激光强度、聚焦和载波包络相位，以及周围等离子体密度和分布，会导致EBJ。EBJ的演化由等离子体泡的动力学决定。在快速演化的泡中发现了贝塔振荡的两种放大过程，它们在EBJ增长中起重要作用。第一种是由于贝塔振荡与泡质心振荡的耦合，在摆动泡中由线性共振驱动。第二种是在呼吸泡中出现的参量共振，其中由于贝塔振荡的强频率调制，EBJ呈指数增长。对它们的特征函数、增长率和共振条件进行了解析推导，并通过数值验证。最后，我们还研究了辐射反应如何影响EBJ。我们的研究为LWFA中EBJ动力学的基本原理提供了清晰的理解，并将有助于改进LWFA在高要求应用中的使用。
