Multibeam absolute stimulated Raman scattering and two-plasmon decay

Area of Science:

• Plasma Physics
• Fusion Energy
• Laser-Plasma Interactions

Background:

• Inertial confinement fusion (ICF) relies on understanding laser-plasma instabilities.
• Stimulated Raman scattering (SRS) and two-plasmon decay (TPD) are key instabilities affecting ICF performance.
• Previous studies often focused on single-beam interactions, neglecting multibeam effects.

Purpose of the Study:

• To calculate three-dimensional absolute instability thresholds for SRS and TPD.
• To compare multibeam coupling efficiencies for SRS and TPD.
• To explain observed experimental regimes on OMEGA and NIF based on instability behavior.

Main Methods:

• Three-dimensional numerical calculations of instability thresholds.
• Analysis of plasma conditions relevant to OMEGA and NIF direct-drive ICF experiments.
• Comparison of single-beam and multibeam instability thresholds.

Main Results:

• Multibeam effects are significant for both SRS and TPD.
• SRS exhibits less efficient multibeam coupling compared to TPD.
• The minimum instability threshold for NIF parameters occurs for SRS near quarter-critical density.

Conclusions:

• The findings align with experimental observations of TPD-dominated regimes on OMEGA and SRS-dominated regimes on NIF.
• Despite lower single-beam SRS thresholds, multibeam coupling influences the dominant instability.
• Understanding these multibeam effects is crucial for optimizing ICF performance.