Effects of hydrogen concentration in ablator material on stimulated Raman scattering, two-plasmon decay, and hot electrons for direct-drive inertial confinement fusion

Laser plasma instabilities, such as stimulated Raman scattering (SRS) and two-plasmon decay (TPD), are basic phenomena in intense laser science and applications. In direct-drive inertial confinement fusion (ICF) where a fuel capsule is imploded by high-power lasers, SRS and TPD are generally problematic because hot electrons (HEs) generated by SRS and TPD cause fuel preheating, whereas HEs with acceptable energy are expected to contribute to ablation pressure enhancement. In all cases, it is necessary to clarify the occurrence of SRS, TPD, and subsequent HE generation. The ablator of a fuel capsule in direct-drive ICF typically consists of carbon with a variable amount of hydrogen (H). We investigated the H effects in the ablator on SRS, TPD, and HEs under direct-drive ICF conditions at the GEKKO laser facility in planer geometry. The experimental results showed an increase in SRS, TPD, and HEs when H was present in the ablator. The analysis indicated that the variations in plasma inhomogeneity and plasma temperature obtained by H addition were insufficient to explain the observed results. Thus, the enhancement is mainly attributed to the high ion acoustic wave damping driven by the H ions into the plasmas, suggesting that Langmuir decay instability caused SRS saturation, whereas other mechanisms, such as cavitation, could overwhelm the TPD saturation. These results suggest that a suitable choice of H concentration in the ablator is critical for mitigating and controlling the extent of SRS, TPD, and HEs to achieve robust and efficient implosion in direct-drive ICF.