Shock Ignition Laser-Plasma Interactions in Ignition-Scale Plasmas.

Area of Science:

• Laser-plasma interactions
• Inertial confinement fusion physics
• High-energy-density physics

Background:

• Direct-drive shock ignition is a promising inertial confinement fusion approach.
• Understanding laser-plasma instabilities is critical for achieving ignition.
• Previous studies have not fully characterized interactions at relevant density scale lengths.

Purpose of the Study:

• To investigate laser-plasma interactions under conditions relevant to direct-drive shock ignition.
• To identify dominant instabilities and their impact on energy coupling and hot-electron generation.
• To assess the potential for hot-electron preheating in megajoule-scale experiments.

Main Methods:

• Utilized the 30 kJ Omega laser facility with a novel shallow-cone target.
• Studied plasma conditions at ablation-plasma density scale lengths and laser intensities relevant to NIF shock ignition.
• Employed particle-in-cell and radiation-hydrodynamics simulations to interpret experimental results.

Main Results:

• Convective stimulated Raman scatter was the dominant instability.
• Experimental evidence of two plasmon decay (TPD) was observed only when density scale length was reduced.
• Laser energy coupling to hot electrons was 1%-2.5%, with temperatures of 35-45 keV.
• Simulations indicated reduced TPD backscatter and lower hot-electron temperatures due to density shifts.

Conclusions:

• Under simulated shock ignition conditions, convective SRS is the primary instability.
• TPD is sensitive to density scale length, shifting to lower densities and reducing hot-electron impact.
• The characterized hot-electron generation is unlikely to cause significant fuel preheating in MJ-scale shock ignition experiments.