Kinetic enhancement of Raman backscatter, and electron acoustic Thomson scatter

Abstract

1-D Eulerian Vlasov-Maxwell simulations are presented which show kinetic enhancement of stimulated Raman backscatter (SRBS) due to electron trapping in regimes of heavy linear Landau damping. The conventional Raman Langmuir wave is transformed 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 of the pump laser off the EAW fluctuations is reported and referred to as electron acoustic Thomson scatter. This light is similar in wavelength to, although much lower in amplitude than, the reflected light between the pump and SRBS wavelengths observed in single hot spot experiments, and previously interpreted as stimulated electron acoustic scatter [D. S. Montgomery et al., Phys. Rev. Lett. 87, 155001 (2001)]. The EAW observed in our simulations is strongest well below the phase-matched frequency for electron acoustic scatter, and therefore the EAW is not produced by it. The beating of different beam acoustic modes is proposed as the EAW excitation mechanism, and is called beam acoustic decay. Supporting evidence for this process, including bispectral analysis, is presented. The linear electrostatic modes, found by projecting the numerical distribution function onto a GaussHermite basis, include beam acoustic modes (some of which are unstable even without parametric coupling to light waves) and a strongly-damped EAW similar to the observed one. This linear EAW results from non-Maxwellian features in the electron distribution, rather than nonlinearity due to electron trapping.

Persistent subplasma-frequency kinetic electrostatic electron nonlinear waves

“…This is what emerged from the simulation work of Strozzi et al 8 where they looked at the plasma as it evolved in the presence of significant SRS, which was in fact the condition in the actual experiment. 1,2 The reader is referred to the original paper 8 for the interesting details. Here it suffices to say that their simulation and analysis of that significantly non-Maxwellian plasma ͑see especially Figs.…”

“…The theory just cited [4][5][6][7] and work by many others [9][10][11][12][13][14][15][16] ͑previous to the work by Strozzi et al 8 ͒ addressed the problem of the possible persistence of such EAW-style modes but only when imposed as initial states ͑with necessarily complicated structure in the electron velocity distribution function near the phase velocity͒. No obvious means exist to impose such complicated initial states other than by a simple act of will by theorists in a computer model.…”

“…4,5 Other analyses on waves of this kind created by imposition of initial values on the electron distribution function ͑but not cited by Montgomery et al͒ had been published by Schamel 6 and by Krapchev and Ram. 7 Recently much more light has been shed on these scattering results 1,2 by Strozzi et al 8 who dealt specifically with the problem of the generation of EAW-style phenomenon in the presence of strong SRS. ͑Their work will be discussed briefly at the end of this paper.͒…”

Excitation of nonlinear ion acoustic waves in CH plasmas

“…Then on this basis, in 2003, Rose presented the estimates to the spatial gain rate coefficients of the backward stimulated Raman scatter (BSRS) and backward stimulated Brillouin scatter (BSBS) due to trapping effects. 22 And also, Strozzi et al 23 paid attention to the linear modes with distributions modified by electron trapping and researched the kinetic effects such as electron trapping in stimulated Raman backscatter by one-dimensional Eulerian VlasovMaxwell simulations. Then, they presented a framework for estimating when electron trapping nonlinearity was expected to be important in Langmuir waves.…”

Breakdown of electrostatic predictions for the nonlinear dispersion relation of a stimulated Raman scattering driven plasma wave

“…[5] and where In order to test the accuracy of our theoretical estimate, δω srs , we compare it with the frequency shift δω num measured from Vlasov-Maxwell simulations of SRS. The simulations are performed with the Eulerian Vlasov code elvis [6,7]. The space and time steps are ∆x/λ l = c ∆t/λ l = 0.03.…”