Ultrarelativistic waves in overdense electron-positron plasmas

The interaction of singly periodic nonlinear waves of relativistic amplitude and electron-positron plasmas is investigated by means of fully self-consistent, relativistic, electromagnetic-particle computer simulation. When the frequency of a superluminous wave is slightly above the relativistic cutoff frequency, the initial waveform, be it sinusoidal or sawtooth (a self-consistent cold-fluid-theory solution), breaks up and creates a very hot plasma with a large particle flux $\Gamma \lesssim 2n_{0}c$ in the longitudinal direction. We find an important deviation from the cold-fluid-theory solution in the form of localized density spikes at the extrema of the wave magnetic field. For a wave frequency well above the cutoff, an initial sinusoidal waveform changes little but still creates a large longitudinal particle flow. Our observations of a large longitudinal flux of high-energy particles confirms a suggested mechanism for cosmic-ray acceleration.