AI 中文总结
本文首次直接观测到激光等离子体加速器中高电荷电子束提取时的纵向膨胀与电子剥离现象，通过飞秒相对论电子显微镜和粒子模拟揭示其机制，并指出该效应降低束峰值电荷密度与加速器效率。
AI 中文摘要
激光等离子体加速器已展示出产生超短相对论电子束的能力，其峰值电流适用于紧凑型光源、超快衍射和强场研究。然而，其性能关键取决于电子束在离开等离子体加速器时保持纵向相空间密度。在此，我们报告了对一个先前未解析过程的首次直接观测：在从激光驱动尾场加速器提取过程中，高电荷电子束经历显著的纵向膨胀并逐渐丢失电子，我们将这一现象称为电子剥离。利用飞秒相对论电子显微镜，我们追踪了电子束在加速器出口之外很远处的演化，观察到电子束在传播过程中伸展超过多个等离子体波长后才发生电子剥离。粒子网格模拟再现了观测到的行为，并揭示其源于高电荷密度束离开加速器时多种效应的组合。这些动力学将束头能量重新分布到低能尾中，从而降低了有用的峰值电荷密度，最终降低了激光等离子体加速器的效率。我们的结果为激光等离子体加速器中的束提取和相空间演化提供了新见解，并强调了控制这些集体效应对未来应用的重要性。
英文摘要
Laser-plasma accelerators have demonstrated the ability to produce ultrashort relativistic electron bunches with peak currents suitable for compact light sources, ultrafast diffraction, and strong-field studies. However, their performance critically depends on preserving the longitudinal phase-space density of the beam as it exits the plasma accelerator. Here, we report the first direct observation of a previously unresolved process in which a highly charged electron bunch undergoes significant longitudinal expansion and progressively loses electrons during extraction from a laser-driven wakefield accelerator, a phenomenon we refer to as electron shedding. Using femtosecond relativistic electron microscopy, we tracked the evolution of the beam far beyond the accelerator exit and observed the bunch stretching over many plasma wavelengths before shedding electrons during propagation. Particle-in-cell simulations reproduce the observed behavior and reveal that it originates from a combination of effects when a high-charge-density beam exits the accelerator. These dynamics redistribute energy from the beam head into a low-energy tail, thereby reducing the useful peak charge density and ultimately decreasing the efficiency of the laser-plasma accelerator. Our results provide new insight into beam extraction and phase-space evolution in laser plasma accelerators and highlight the importance of controlling these collective effects for future applications.
