GROUP-VELOCITY OF LARGE-AMPLITUDE ELECTROMAGNETIC-WAVES IN A PLASMA

The nonlinear group velocity of a short intense laser pulse propagatin g in a cold underdense unmagnetized plasma is examined. Analytical exp ressions for the group velocity are derived. These expressions reduce to the usual partial derivative omega/partial derivative k form for sm all amplitude and are verified for arbitrary amplitude using particle in cell simulations on a cyclic mesh. We find that the leading edge of a pulse moves at the linear group velocity and that the phase velocit y of the excited wake is found to be less than the group velocity of t he pulse. The techniques used can be applied to other waves in a plasm a.