Revealing the nonlinear transition of beam-driven plasma wakefield

Plasma wakefield accelerators, powered by high-current particle beams, are rapidly emerging as a revolutionary platform for both compact light sources and the next generation of high-energy colliders. A critical aspect that remains unresolved in advancing these accelerators is the precise characterization of the field structure and its intricate correlation with the drive particle bunch. In this article, the drive electron bunch is produced by a laser-plasma accelerator, while the plasma wakefield it generates is probed by an ultrashort electron bunch from a separate laser-plasma accelerator. This configuration allows for the simultaneous extraction of both the induced wakefield and the space-charge field of the driving bunch in a single shot. As the charge of the driver is increased, two fundamental phenomena emerge: first, the electric wakefield structure transitions from a linear, sinusoidal pattern to a nonlinear, sawtooth pattern; second, the shape of the field is modified due to the influence of the trailing tail of the driver. Additionally, these observations reveal a strong correlation between the strength of the driver and the wakefield, which aligns closely with quasi-three-dimensional simulations and classical wakefield theory. These findings not only deepen our comprehension of beam-driven plasma wave dynamics but also provide crucial quantitative insights for high-quality plasma wakefield acceleration optimization in real time.