Shock ignition laser-plasma interactions in ignition-scale plasmas

We use a sub-ignition scale laser, the 30 kJ Omega, and a novel shallow-cone target to study laser-plasma interactions at the ablation-plasma density scale-lengths and laser intensities anticipated for direct drive shock-ignition implosions at NIF-scale. Our results show that, under these conditions, the dominant instability is convective Stimulated Raman Scatter with experimental evidence of Two Plasmon Decay (TPD) only when the density scale-length is reduced. Particle-in-cell simulations indicate this is due to TPD being shifted to lower densities, removing the experimental back-scatter signature and reducing the hot-electron temperature. The experimental laser energy-coupling to hot-electrons was found to be 1 – 2.5%, with electron temperatures between 35 and 45 keV. Radiation-hydrodynamics simulations employing these hot-electron characteristics indicate that they should not pre-heat the fuel in MJ-scale shock ignition experiments.