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 accelerators3,6, a strong laser or relativistic particle beam launches plasma waves with field amplitude that follows the driver strength up to the wavebreaking limit5,7, 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 performances9,10. 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.