Next-generation plasma-based accelerators can push electron bunches to gigaelectronvolt energies within centimetre distances^1,2. The plasma, excited by a driver pulse, generates large electric fields that can efficiently accelerate a trailing witness bunch^3,4,5, enabling the realization of laboratory-scale applications ranging from high-energy colliders⁶ to ultrabright light sources⁷. So far, several experiments have demonstrated large accelerations^8,9,10 but the resulting beam quality, particularly the energy spread, is still far from state-of-the-art conventional accelerators. Here we show the results of a beam-driven plasma acceleration experiment where we used an electron bunch as a driver followed by an ultrashort witness bunch. By setting a positive energy chirp on the witness bunch, its longitudinal phase space is rotated during acceleration, resulting in an ultralow energy spread that is even lower than the spread at the plasma entrance. This result will significantly impact the optimization of the plasma acceleration process and its implementation in forthcoming compact machines for user-oriented applications.

下一代基于等离子体的加速器可以在厘米距离内将电子束推到千兆电子伏特能量^1,2。由驱动脉冲激发的等离子体产生大电场，可以有效地加速尾随见证束^3,4,5，从而实现从高能对撞机⁶到超亮光源⁷的实验室规模应用。到目前为止，一些实验已经证明了大加速度^8,9,10但由此产生的光束质量，尤其是能量扩散，与最先进的传统加速器相比仍然相去甚远。在这里，我们展示了光束驱动等离子体加速实验的结果，其中我们使用电子束作为驱动器，然后使用超短见证束。通过在见证束上设置正能量啁啾，其纵向相空间在加速过程中发生旋转，从而产生甚至低于等离子体入口处的超低能量扩散。这一结果将显着影响等离子加速过程的优化及其在即将推出的面向用户应用的紧凑型机器中的实施。