Group velocity of large amplitude electromagnetic waves in a plasma

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Abstract
The nonlinear group velocity of a short intense laser pulse propagating in a cold underdense unmagnetized plasma is examined. Analytical expressions for the group velocity are derived. These expressions reduce to the usual ∂ω/∂k form for small amplitude and are verified for arbitrary amplitude using particle in cell simualations 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 found to be less than the group velocity of the pulse. The techniques used can be applied to other waves in a plasma.

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