Pump-Depletion Dynamics and Saturation of Stimulated Brillouin Scattering in Shock Ignition Relevant Experiments

TL;DR

This study investigates the pump-depletion and saturation behavior of stimulated Brillouin scattering in shock ignition experiments, revealing significant laser energy depletion that impacts energy coupling and electron generation in inertial confinement fusion.

Contribution

First experimental demonstration of near 100% pump-depletion in shock ignition conditions and analysis of its dynamics related to ion-acoustic wave breaking in stimulated Brillouin scattering.

Findings

Pump-depletion reaches nearly 100% within 0.5 ns of high-intensity laser pulse.

Depletion starts at 0.01-0.02 critical density and progresses to 0.1-0.2 critical density.

Pump-depletion dynamics are explained by ion-acoustic wave breaking in SBS.

Abstract

As an alternative inertial confinement fusion scheme with predicted high energy gain and more robust designs, shock ignition requires a strong converging shock driven by a shaped pulse with a high-intensity spike at the end to ignite a pre-compressed fusion capsule. Understanding nonlinear laser-plasma instabilities in shock ignition conditions is crucial to assess and improve the laser-shock energy coupling. Recent experiments conducted on the OMEGA-EP laser facility have for the first time demonstrated that such instabilities can $\sim$100\% deplete the first 0.5 ns of the high-intensity laser pump. Analysis of the observed laser-generated blast wave suggests that this pump-depletion starts at 0.01--0.02 critical density and progresses to 0.1--0.2 critical density. This pump-depletion is also confirmed by the time-resolved stimulated Raman backscattering spectra. The dynamics of the pump-depletion can be explained by the breaking of ion-acoustic waves in stimulated Brillouin scattering. Such strong pump-depletion would inhibit the collisional laser energy absorption but may benefit the generation of hot electrons with moderate temperatures for electron shock ignition [Shang et al. Phys. Rev. Lett. 119 195001 (2017)].