Ultrarelativistic electromagnetic pulses in plasmas

We study through computer simulation the physical processes of a linearly polarized electromagnetic pulse of highly relativistic amplitude ($\frac{\mathrm{eE}}{m\omega c}\gg 1$) in an underdense plasma accelerating particles to very high energies. We consider first an electron-positron plasma. The maximum momenta achieved scale as the square of the wave amplitude. This acceleration stops when the bulk of the wave energy is converted to particle energy. The pulse leaves behind as a wake a vacuum region whose length scales as the amplitude of the wave. The results can be explained in terms of a snow plow or pistonlike action of the radiation on the plasma. When a mass ratio other than unity is chosen and electrostatic effects begin to play a role, first the ion energy increases faster than the electron energy and then the electron energy catches up later, eventually reaching the same value.