Raman–Brillouin interplay for inertial confinement fusion relevant laser–plasma interaction

Abstract

The co-existence of the Raman and Brillouin backscattering instability is an important issue for inertial confinement fusion. The present paper presents extensive one-dimensional (1D) particle-in-cell (PIC) simulations for a wide range of parameters extending and complementing previous findings. PIC simulations show that the scenario of reflectivity evolution and saturation is very sensitive to the temperatures, intensities, size of plasma and boundary conditions employed. The Langmuir decay instability is observed for rather small k epw λ D but has no influence on the saturation of Brillouin backscattering, although there is a clear correlation of Langmuir decay instability modes and ion-fractional decay for certain parameter ranges. Raman backscattering appears at any intensity and temperature but is only a transient phenomenon. In several configurations forward as well as backward Raman scattering is observed. For the intensities considered, I λ 2 o above 10 15 W µm 2 /cm 2 , Raman is always of bursty nature. A particular setup allows the simulation of multi-speckle aspects in which case it is found that Raman is self-limiting due to strong modifications of the distribution function. Kinetic effects are of prime importance for Raman backscattering at high temperatures. No unique scenario for the saturation of Raman scattering or Raman-Brillouin competition does exist. The main effect in the considered parameter range is pump depletion because of large Brillouin backscattering. However, in the low k epw λ D regime the presence of ion-acoustic waves due to the Langmuir decay instability from the Raman created electron plasma waves can seed the ion-fractional decay and affect the Brillouin saturation.

Progress in understanding the role of hot electrons for the shock ignition approach to inertial confinement fusion

“…The reflectivity saturates at that time at the level of 40% (figure 12(a)) and the average electron temperature increases from 2.3 to 4 keV. A high SBS reflectivity is explained by a low ion temperature and consequently a very low damping of ion acoustic waves [37,[51][52][53][54]. Consequently, SBS saturated on a rather high level.…”

Collective absorption of laser radiation in plasma at sub-relativistic intensities

“…Laser penetration into the dense plasma terminates with the formation of cavities seen at x ∼ 317λ 0 in Figure 7(c). Formation of such cavities or "bubbles" was reported already in early publications on the laser absorption and parametric processes in near-critical plasmas [6,[16][17][18] , but because of much shorter spatial scales the role of laser filamentation and the parametric processes near the quarter critical plasma was overlooked.…”

Measurements of parametric instabilities at laser intensities relevant to strong shock generation

“…A similar growth followed by a rapid saturation has also been found in experiments carried out at the Trident laser facility 17 aimed at investigating SRS occurring in single hot spots and in PIC simulations in the kinetic regime reported in the literature. 19,[66][67][68] In these works, the saturation is due to a nonlinear frequency detuning occurring in large EPWs excited in the BRS process. 69,70 The frequency shift can be due to ponderomotive and electron trapping effects as, for example, in bowing and filamentation of plasma waves in speckles.…”

“…69,70 The frequency shift can be due to ponderomotive and electron trapping effects as, for example, in bowing and filamentation of plasma waves in speckles. 19,66 It was found 23,67,68,71,72 that the nonlinear phase detuning results also in a non-stationary Raman saturation, in the form of a transition from a strongly modulated quasi-periodic to intermittent chaotic regime, with an increasing laser intensity. 23,73 Since the SRS saturation is not mainly determined by Landau damping but rather by convective and side loss effects, we do not expect inflationary SRS or autoresonance 74 effects due to the ponderomotive trapping of electrons to be relevant for SRS growth, as discussed by Vu et al 64 The occurrence of kinetics mechanisms in BRS saturation in the present experiment is suggested by the chaotic character of spectra in Fig.…”