Kinetic inflation of stimulated Raman backscatter in regimes of high linear Landau damping

Abstract

Kinetic simulations and analysis show that backward stimulated Raman scattering (BSRS), in regimes of large linear Landau damping of the primary Langmuir wave, attains levels greatly exceeding the predictions of models based on fixed damping. These regimes are encountered in plasma conditions expected for target designs to be fielded at the National Ignition Facility [J. D. Lindl, Inertial Confinement Fusion (Springer-Verlag, New York, 1998)]. Trapped electrons in the Langmuir wave have the dual effect of reducing its damping, thereby enhancing the BSRS response, and saturating this response by phase detuning, a consequence of the trapping-induced, time-dependent, frequency shift. BSRS, then, occurs as a train of sub-picosecond pulses, arising from the competition between phase detuning and parametric regeneration. A simple three wave parametric model, including the effect of the nonlinear frequency shift and residual nonlinear damping, reproduces these essential features. A similar scenario applies to backward stimulated Brillouin scattering (BSBS). BSRS activity many orders of magnitude above noise level is found for intense laser speckles even when the primary Langmuir wave number times the Debye length is as high as 0.55. The simulation model consistently accounts for the competition of other instabilities, including BSBS, forward stimulated Raman scattering, and the Langmuir decay instability with cavitation.

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

“…Therefore, in agreement with the results of Ref. [1], we do find that the frequency shift induces a detuning, δϕ, which slows down the growth of SRS.…”

“…For example, Vu et al [1] invoked phase detuning between the plasma wave and the laser drive, a consequence of the nonlinear frequency downshift of the EPW, as a mechanism for Raman saturation and as an explanation for the chaotic behavior of the Raman reflectivity. Brunner and Valeo [2] proposed the alternative view that the growth of electrostatic sidebands, produced by the trapped particle instability, is at the origin of Raman saturation and burstiness.…”

Trapped particle bounds on stimulated scatter in the large kλD regime

“…The dashed curve is the locus of nonlinear resonance, Re(ε) = 0, constrained by (17). As the density is varied, for fixed value of the other parameters, the maximum normalized gain rate 28 is shown in figure 9 for T e =2.5 and 5 keV.…”

“…The transition from (14) to (21) For each value of ω +δω, the wavenumber argument of ε is chosen according to (17).…”

The dependence of spatial autoresonance in SRS onkLλD

“…Even the inversion of the electron distribution function around the phase velocity has been observed. The regime associated with this process, called "kinetic inflation" [7] is known to destabilise SRS, and has been investigated in homogeneous plasmas, leading to reflectivities far higher than those predicted by linear theory in the kinetic regime (the regime in which the dominant nonlinear processes are of kinetic origin), typically defined as corresponding to k L D 0.29 [8].…”