Plasma wakefield accelerators use tabletop equipment to produce relativistic femtosecond electron bunches. Optical and X-ray diagnostics have established that their charge concentrates within a micrometre-sized volume, but its sub-micrometre internal distribution, which critically influences gain in free-electron lasers or particle yield in colliders, has proven elusive to characterize. Here, by simultaneously imaging different wavelengths of coherent optical transition radiation that a laser-wakefield-accelerated electron bunch generates when exiting a metal foil, we reveal the structure of the coherently radiating component of bunch charge. The key features of the images are shown to uniquely correlate with how plasma electrons injected into the wake: by a plasma-density discontinuity, by ionizing high-Z gas-target dopants or by uncontrolled laser–plasma dynamics. With additional input from the electron spectra, spatially averaged coherent optical transition radiation spectra and particle-in-cell simulations, we reconstruct coherent three-dimensional charge structures. The results demonstrate an essential metrology for next-generation compact X-ray free-electron lasers driven by plasma-based accelerators.

等离子体尾场加速器使用桌面设备产生相对论性飞秒电子束。光学和 X 射线诊断已证实它们的电荷集中在微米大小的体积内，但其亚微米内部分布已被证明难以表征，该分布对自由电子激光器的增益或对撞机中的粒子产量产生关键影响。在这里，通过同时对激光尾场加速电子束在离开金属箔时产生的不同波长的相干光学跃迁辐射进行成像，我们揭示了束电荷的相干辐射分量的结构。图像的关键特征显示出与等离子体电子注入尾流的方式独特相关：通过等离子体密度不连续性、通过电离高Z气体目标掺杂剂或通过不受控制的激光等离子体动力学。通过电子光谱、空间平均相干光跃迁辐射光谱和细胞内粒子模拟的额外输入，我们重建了相干三维电荷结构。结果证明了由等离子体加速器驱动的下一代紧凑型 X 射线自由电子激光器的基本计量学。