Ultrahigh intensity laser–plasma interaction: A Lagrangian approach*

A Lagrangian analysis simplifies considerably the description of the ultrahigh intensity regime of laser–plasma interaction, and facilitates the identification of several new and important effects. First, the vacuum figure ‘‘8’’ orbit is shown to be unstable, with respect to a stochastic instability leading to collisionless heating. Second, in the generation of plasma wakes using ultrahigh intensity laser pulses, it can be shown that, for long-duration laser pulses, the plasma wake is insignificant, but, through proper phasing of a set of short-duration pulses, a dramatic amplification of the wake amplitude occurs. Third, in the generation of third-harmonic waves using ultrahigh intensity, long-duration laser pulses, it can be seen that a mismatch in the phase velocity limits severely the power conversion, but a conversion efficiency free from saturation might indeed be possible by employing either a density-modulated plasma or an ionized buffer gas.