Experimental investigation of the density-scale-length effects on laser–plasma instabilities toward the shock-ignition scheme in direct-drive inertial-confinement fusion

C Nakatsuji, K Kawasaki, T Idesaka, S Matsuura, D Tanaka, T Sato, H Nagatomo, Y Sentoku, Y Takagi, G Cristoforetti, D Batani, Ph Nicolaï, N Ozaki, A Yogo, S Fujioka, K Shigemori

Phys. Plasmas 32, 052710 (2025)

Abstract

In direct-drive inertial-confinement fusion, understanding and controlling laser–plasma instabilities (LPIs) is crucial to optimizing energy coupling and achieving high-gain fusion. Herein, we report the experimental investigation of the effects of density scale-length on LPIs. The experiment was performed at the GEKKO-XII Laser facility [C. Yamanaka et al., IEEE J. Quantum Electron. 17, 1639 (1981)], specifically to characterize stimulated Raman scattering (SRS) and two-plasmon decay (TPD), and the effects of density scale-lengths on the relationship between these LPIs and hotelectron generation. The experimental results consistently indicate that the reduction in hot-electron generation with increasing density scale-length is strongly correlated with decreases in both SRS and TPD in high-density regions. Rosenbluth gain analysis implies that pump depletion by stimulated Brillouin scattering is not responsible for the observed reduction in SRS and TPD. Instead, spatial and temporal incoherence of propagating laser beams, driven by filamentation, could have suppressed SRS and TPD, as evaluated by filamentation figure of merit (FFOM). On the other hand, the thermally corrected FFOM suggests that hot spots by random phase plates are the origin of the early growth of SRS in low-density regions.