Time evolution of stimulated Raman scattering and two-plasmon decay at laser intensities relevant for shock ignition in a hot plasma

Abstract

Laser–plasma interaction (LPI) at intensities $10^{15}{-}10^{16}~\text{W}\cdot \text{cm}^{-2}$ is dominated by parametric instabilities which can be responsible for a significant amount of non-collisional absorption and generate large fluxes of high-energy nonthermal electrons. Such a regime is of paramount importance for inertial confinement fusion (ICF) and in particular for the shock ignition scheme. In this paper we report on an experiment carried out at the Prague Asterix Laser System (PALS) facility to investigate the extent and time history of stimulated Raman scattering (SRS) and two-plasmon decay (TPD) instabilities, driven by the interaction of an infrared laser pulse at an intensity ${\sim}1.2\times 10^{16}~\text{W}\cdot \text{cm}^{-2}$ with a ${\sim}100~\unicode[STIX]{x03BC}\text{m}$ scalelength plasma produced from irradiation of a flat plastic target. The laser pulse duration (300 ps) and the high value of plasma temperature ( ${\sim}4~\text{keV}$ ) expected from hydrodynamic simulations make these results interesting for a deeper understanding of LPI in shock ignition conditions. Experimental results show that absolute TPD/SRS, driven at a quarter of the critical density, and convective SRS, driven at lower plasma densities, are well separated in time, with absolute instabilities driven at early times of interaction and convective backward SRS emerging at the laser peak and persisting all over the tail of the pulse. Side-scattering SRS, driven at low plasma densities, is also clearly observed. Experimental results are compared to fully kinetic large-scale, two-dimensional simulations. Particle-in-cell results, beyond reproducing the framework delineated by the experimental measurements, reveal the importance of filamentation instability in ruling the onset of SRS and stimulated Brillouin scattering instabilities and confirm the crucial role of collisionless absorption in the LPI energy balance.

Observation and modelling of stimulated Raman scattering driven by an optically smoothed laser beam in experimental conditions relevant for shock ignition

“…Several recent experiments aimed at the investigation of LPI and HE generation in conditions relevant for Shock Ignition -i.e. laser intensities in the range 10 15 − 10 16 W/cm 2 , with wavelenght in the UV range, impinging on plasmas of a few keV temperature and of a few hundreds of microns density scalelength -have been reported [9][10][11][12][13][14][15][16][17]. None of them, however, could meet all of these conditions simultaneously, because of laser energy limitations in the available laser facilities.…”

“…Here, absolute SRS could prevail on TPD at densities close to the quarter critical density because of the higher growth rate, due to the dependence on the plasma temperature, while convective SRS at lower densities could also damp the TPD growth by pump depletion mechanisms. Recent experiments at PALS carried out with 1ω irradiation at ≈ 10 16 W/cm 2 suggest that TPD is driven at early times, during the interaction of the leading part of the laser pulse, while it is successively damped, probably due to pump depletion caused by the onset of convective SRS at lower plasma densities [11]. In typical exploding-foil experiments, in fact, SRS is driven at later times of interaction, when the plasma scalelength has become sufficiently large, and is convectively amplified at densities well below the quarter critical density, close to the Landau damping cutoff determined by the plasma temperature (k epw λ D ≈ 0.3) [11,12].…”

“…Recent experiments at PALS carried out with 1ω irradiation at ≈ 10 16 W/cm 2 suggest that TPD is driven at early times, during the interaction of the leading part of the laser pulse, while it is successively damped, probably due to pump depletion caused by the onset of convective SRS at lower plasma densities [11]. In typical exploding-foil experiments, in fact, SRS is driven at later times of interaction, when the plasma scalelength has become sufficiently large, and is convectively amplified at densities well below the quarter critical density, close to the Landau damping cutoff determined by the plasma temperature (k epw λ D ≈ 0.3) [11,12]. Very few experiments [13,22,23], however, explored LPI at laser intensities close to 10 16 W/cm 2 together with plasma density scalelength higher than 200 µm, as envisaged in the SI scheme, where the non-linear character of SRS is expected to be strong.…”

Bremsstrahlung cannon design for shock ignition relevant regime

“…These instabilities result in reflected laser light (SRS, SBS) and the generation of nonthermal electron populations (SRS,TPD) 10 , the so-called fast electrons. One of the major open issues within the SI approach is the effect of the fast electrons on the ability to drive a strong shock 11 .…”

Stimulated Raman scattering in a non-eigenmode regime

“…Generally, laser plasma instabilities [10,11] , especially stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS) and two-plasmon decay (TPD) instability, have been mainly considered in ICF with the incident laser intensity less than 10 15 W/cm 2 [12][13][14] . However, the laser intensity may be of the order of 10 16 or even 10 17 W/cm 2 in shock ignition [15][16][17][18][19] , Brillouin amplification [20,21] and the interactions of high-power laser with matter [22][23][24] . Therefore, the parametric instabilities close to the regime of subrelativistic intensity need to be explored in depth.…”