Laser wakefield acceleration, as an advanced accelerator concept, has attracted great attentions for its ultrahigh acceleration gradient and the capability to produce high brightness electron bunches. The three-dimensional (3D) density serves as an evaluation metric for the particle bunch quality and is intrinsically related to the applications of an accelerator. Despite its significance, this parameter has not been experimentally measured in the investigation of laser wakefield acceleration. We report on an electro-optic 3D snapshot of a laser wakefield electron bunch at a position outside the plasma. The 3D shape of the electron bunch was detected by simultaneously performing optical transition radiation imaging and electro-optic sampling. Detailed 3D structures to a few micrometer levels were reconstructed using a genetic algorithm. The electron bunch possessed a transverse size of less than 30 micrometers. The current profile shows a multi-peak structure. The main peak had a duration of < 10 fs and a peak current > 1 kA. The maximum electron 3D number density was ~ 9 × 10²¹m ^-3. This research demonstrates a feasible way of 3D density monitoring on femtosecond kilo-ampere electron bunches, at any position of a beam transport line for relevant applications.

激光尾场加速作为一种先进的加速器概念，以其超高的加速梯度和产生高亮度电子束的能力而备受关注。三维（3D）密度作为粒子束质量的评价指标，与加速器的应用有着内在的联系。尽管该参数很重要，但在激光尾场加速的研究中尚未通过实验测量。我们报告了等离子体外部位置的激光尾场电子束的电光 3D 快照。通过同时进行光学跃迁辐射成像和电光采样来检测电子束团的 3D 形状。使用遗传算法重建了几微米级别的详细 3D 结构。电子束的横向尺寸小于30微米。目前的剖面显示出多峰结构。主峰的持续时间< 10 fs，峰值电流> 1 kA。最大电子3D数密度约为9×10 ²¹ m ^-3。这项研究展示了一种在相关应用的束流传输线的任何位置对飞秒千安电子束进行 3D 密度监测的可行方法。