Kinetic enhancement of Raman backscatter, and electron acoustic Thomson scatter

1-D Eulerian Vlasov-Maxwell simulations are presented which show kineticenhancement of stimulated Raman backscatter (SRBS) due to electron trapping inregimes of heavy linear Landau damping. The conventional Raman Langmuir wave istransformed into a set of beam acoustic modes [L. Yin et al., Phys. Rev. E 73,025401 (2006)]. For the first time, a low phase velocity electron acoustic wave(EAW) is seen developing from the self-consistent Raman physics. Backscatter ofthe pump laser off the EAW fluctuations is reported and referred to as electronacoustic Thomson scatter. This light is similar in wavelength to, although muchlower in amplitude than, the reflected light between the pump and SRBSwavelengths observed in single hot spot experiments, and previously interpretedas stimulated electron acoustic scatter [D. S. Montgomery et al., Phys. Rev.Lett. 87, 155001 (2001)]. The EAW is strongest well below the phase-matchedfrequency for electron acoustic scatter, and therefore the EAW is not producedby it. The beating of different beam acoustic modes is proposed as the EAWexcitation mechanism, and is called beam acoustic decay. Supporting evidencefor this process, including bispectral analysis, is presented. The linearelectrostatic modes, found by projecting the numerical distribution functiononto a Gauss-Hermite basis, include beam acoustic modes (some of which areunstable even without parametric coupling to light waves) and a strongly-dampedEAW similar to the observed one. This linear EAW results from non-Maxwellianfeatures in the electron distribution, rather than nonlinearity due to electrontrapping.Comment: 15 pages, 16 figures, accepted in Physics of Plasmas (2006