Controlling the plasma phase velocity for high energy acceleration in the PBWA

The authors have studied the propagation of diffraction limited laser beams in plasmas and find that under certain conditions the phase velocity of the ponderomotive wave can be made to increase with propagation distance into the plasma. This is achieved by adjusting the initial wave front curvatures and the minimum spot sizes of the two injected laser beams. In principle, this phase tuning can be maintained indefinitely, but will limited in practice by such effects as pump depletion and laser plasma instabilities, which will limit the maximum energy attainable. Design parameters of the lasers' optical characteristics for a PBWA will be presented