Direct observation of relativistic broken plasma waves

Plasma waves contribute to many fundamental phenomena, including astrophysics¹, thermonuclear fusion² and particle acceleration³. Such waves can develop in numerous ways, from classic Langmuir oscillations carried by electron thermal motion⁴, to the waves excited by an external force and travelling with a driver⁵. In plasma-based particle accelerators^3,6, a strong laser or relativistic particle beam launches plasma waves with field amplitude that follows the driver strength up to the wavebreaking limit^5,7, which is the maximum wave amplitude that a plasma can sustain. In this limit, plasma electrons gain sufficient energy from the wave to outrun it and to get trapped inside the wave bucket⁸. Theory and numerical simulations predict multi-dimensional wavebreaking, which is crucial in the electron self-injection process that determines the accelerator performances^9,10. Here we present a real-time experimental visualization of the laser-driven nonlinear relativistic plasma waves by probing them with a femtosecond high-energy electron bunch from another laser-plasma accelerator coupled to the same laser system. This single-shot electron deflectometry allows us to characterize nonlinear plasma wakefield with femtosecond temporal and micrometre spatial resolutions revealing features of the plasma waves at the breaking point.