Plasma-based accelerators driven by either intense lasers¹ or charged particle beams² can accelerate electrons or positrons with extremely high gradients compared with conventional radio-frequency accelerators. For their use as next-generation light sources and in energy frontier colliders³, beams with good stability, high quality, controllable polarization and excellent reproducibility^4,5 are required. The accelerated electrons can be either internally injected directly from the background plasma or externally injected from conventional accelerators. Despite significant progress^{6,7,8,9,10,11,12,13,14}, the beam properties obtained with the internal injection scheme fall short of simultaneously reaching these requirements. In contrast, such high-property beams are routinely generated from conventional accelerators. Therefore, it is important to demonstrate the injection from a conventional accelerator into a plasma-based machine followed by further acceleration of the beam. Here we report the demonstration of external injection from a conventional linear accelerator into a laser wakefield accelerator and subsequent acceleration without any significant loss of charge, which is achieved by properly shaping and matching the beam into the plasma structure. The experimental results, combined with three-dimensional particle-in-cell simulations, indicate that this is possible with modest degradation in the beam quality. This work is an important step towards realizing a high-throughput, multistage, high-energy, hybrid conventional-plasma-based accelerator.

与传统的射频加速器相比，由强激光¹或带电粒子束^{2驱动的基于等离子体的加速器可以以极高的梯度加速电子或正电子。}为了将它们用作下一代光源和能量前沿对撞机³，需要具有良好稳定性、高质量、可控偏振和出色再现性^4,5的光束。加速的电子既可以直接从背景等离子体内部注入，也可以从传统加速器外部注入。尽管取得了重大进展^{6,7,8,9,10,11,12,13,14}，通过内部注入方案获得的光束特性无法同时达到这些要求。相比之下，这种高性能光束通常由传统的加速器产生。因此，重要的是要演示从传统加速器注入基于等离子体的机器，然后进一步加速光束。在这里，我们报告了从传统线性加速器到激光尾场加速器的外部注入以及随后的加速，而没有任何显着的电荷损失，这是通过将光束适当地整形和匹配到等离子体结构中来实现的。实验结果与 3D 细胞内粒子模拟相结合，表明在光束质量适度下降的情况下，这是可能的。