Advanced Direct Drive Shock Ignition Studies

The shock ignition approach to inertial confinement fusion offers a potential route to ignition and high gain. It proposes a low velocity fuel assembly on a low adiabat, and ignition through the launching of a late timed strong shock. Accurate descriptions of the coupling of laser energy into the capsule are required to model implosions, including driving the highly compressible fuel and the interaction of the shock launching spike with the coronal ablation plasma. Two well diagnosed experiments were performed on the Omega-60 laser facility that isolated key physics issues for the two steps of shock ignition.

The first used a novel conical target to access for the first time the laser-plasma conditions relevant for full-scale shock ignition, in order to characterise the laser-plasma interactions and subsequent supra-thermal hot electrons. The dominant instability was identified as convective stimulated Raman scattering, producing hot electrons of ~40 keV with a laser energy conversion efficiency of 1-3%. This is unique and an essential measurement, as inclusion of hot electron generation and propagation in shock ignition simulations is crucial for constructing implosion designs that might be capable of reaching ignition.

The second experiment investigated the implosion dynamics of warm deuterium filled capsules using shaped laser pulses that maintained a reduced shell adiabat and associated high fuel compressibility. A laser drive multiplier was tuned with trajectory measurements from a gated self-emission imager, a significant advancement in the ability to more accurately simulate reduced adiabat designs that are relevant for both shock ignition fuel and conventional central hot spot implosions. Despite significant low mode asymmetries that were identified during the in-flight fuel compression and within the late formed hot spot, the shell trajectory, hot spot morphology and peak neutron emission were well reproduced from one-dimensional simulations. More experiments coupled with predictive modelling are a necessity to determine whether inertial confinement fusion can be a future energy source.