Metre-scale plasma wakefield accelerators have imparted energy gain approaching 10 gigaelectronvolts to single nano-Coulomb electron bunches. To reach useful average currents, however, the enormous energy density that the driver deposits into the wake must be removed efficiently between shots. Yet mechanisms by which wakes dissipate their energy into surrounding plasma remain poorly understood. Here, we report picosecond-time-resolved, grazing-angle optical shadowgraphic measurements and large-scale particle-in-cell simulations of ion channels emerging from broken wakes that electron bunches from the SLAC linac generate in tenuous lithium plasma. Measurements show the channel boundary expands radially at 1 million metres-per-second for over a nanosecond. Simulations show that ions and electrons that the original wake propels outward, carrying 90 percent of its energy, drive this expansion by impact-ionizing surrounding neutral lithium. The results provide a basis for understanding global thermodynamics of multi-GeV plasma accelerators, which underlie their viability for applications demanding high average beam current.

米级等离子体尾场加速器已为单个纳库仑电子束提供了接近 10 吉电子伏的能量增益。然而，为了达到有用的平均电流，驱动器沉积在尾流中的巨大能量密度必须在两次发射之间有效地消除。然而，尾流将能量消散到周围等离子体的机制仍然知之甚少。在这里，我们报告了皮秒时间分辨的掠角光学阴影测量和大规模细胞内粒子模拟，这些离子通道是由 SLAC 直线加速器的电子束在稀薄的锂等离子体中产生的破碎尾流产生的。测量结果显示，通道边界以每秒 100 万米的速度径向扩展，持续时间超过一纳秒。模拟显示，原始尾流向外推动的离子和电子携带着 90% 的能量，通过碰撞电离周围的中性锂来驱动这种膨胀。研究结果为理解多 GeV 等离子体加速器的整体热力学奠定了基础，这也是其在需要高平均束流的应用中的可行性的基础。