On the role of electron acoustic waves and beam acoustic modes in laser backscatter from plasmas

Abstract

Non-Maxwellian particle distributions can allow the plasma to support a variety of modes often omitted from studies of laser-plasma interactions. Such modes, which require a kinetic description of the plasma, can significantly affect the scattering of incident laser light. Both the electron acoustic wave (EAW), which can be supported by electrons trapped in a finite-amplitude wave, and the beam acoustic mode (BAM), which can be supported by drifting beam electrons, have been identified as possible longitudinal daughter waves for stimulated scattering in plasmas. These modes permit undamped oscillation at frequencies significantly lower than the electron plasma frequency ωpe, and so provide an additional mechanism for stimulated Raman-like scattering. Single hot-spot experiments using the Trident laser facility have indeed observed backscatter which resembles stimulated Raman scattering, and can occur in combination with it, but arises from a lower-frequency mode with ω ≈ 0.41ωpe identified with the EAW. Here we report fully nonlinear kinetic simulations using a one dimensional Vlasov-Maxwell system of electrons and immobile protons, used previously to model other kinetic phenomena relevant to laser-plasma interactions. These simulations demonstrate stimulated scattering both from Langmuir waves and from modes with frequencies significantly below the plasma frequency, in the range 0.6ωpe to 0.8ωpe. The importance of kinetic effects in saturating the conventional SRS component is also highlighted. For example, in some of our simulations, an initial burst of stimulated Raman scattering saturates via trapping of electrons, thereby providing an environment in which EAWs can grow; stimulated scattering from these EAWs is then seen later in the simulation. We also outline work currently underway to extend the treatment to two spatial dimensions, where lateral transport and scattering losses may effect the formation and evolution of trapped electron structures, such as the EAW and BAM.