The nonlinear group velocity of a laser pulse propagating in a cold underdense unmagnetized plasma is examined. Analytical expressions for the group velocity are derived for various pulse length regimes. These expressions reduce to the usual ∂ω/∂k form for small amplitude and are verified for aribtrary 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 velocity of the excited wake is less than the group velocity of the pulse for symmetrically shaped pulses. The techniques used can be applied to other waves in a plasma.

• Received 22 August 1994

DOI:https://doi.org/10.1103/PhysRevE.51.1364