Observation of Electron Energies Beyond the Linear Dephasing Limit from a Laser-Excited Relativistic Plasma Wave

The spatial extent of the plasma wave and the spectrum of the accelerated electrons are simultaneously measured when the relativistic plasma wave associated with Raman forward scattering of an intense laser beam reaches the wave breaking limit. The maximum observed energy of 94 MeV is greater than that expected from the phase slippage between the electrons and the accelerating electric field as given by the linear theory for preinjected electrons. The results are in good agreement with 2D particle-in-cell code simulations of the experiment.