Hydrodynamics Experiments on the Shock Ignition Approach to Direct-Drive Inertial Fusion

The recent NIF breakthrough showed the credibility of the inertial fusion approach. However, it does not seem possible to extrapolate the indirect drive scheme followed at NIF to future energy production due to complicated targets, intrinsic low efficiency, and finally the implication for defense programs.

Alternatively, direct laser drive offers higher efficiency and simpler scheme, but it is more prone to laser non-uniformities and the impact of Rayleigh-Taylor instability. Decoupling compression and ignition, the basis of the shock ignition (SI) approach, could mitigate such instability. Here compression is followed by high intensity irradiation (≈1016 W/cm2) creating a strong shock converging at the center of the compressed target and increasing the temperature thus triggering nuclear reactions.

Presently there are many unknowns in SI, in particular on Hot electrons (HE) from laser–plasma interaction. Here, the results of two experiments conducted at the Omega facility will be presented.

The first one, in planar geometry, aimed at characterizing strong shock propagation and HE generation. Time-resolved radiographies were performed to study the hydrodynamic evolution. The HE source was characterized using x-rays spectrometers. Obtained values were used in simulations to reproduce radiography results, further constraining HE parameters. We found ∼10% energy conversion into HE with T∼27 keV. HE produced an increase of the pressure around the shock front. The low temperature found in this experiment could be advantageous for SI.

The second one, in spherical geometry in the so-called “40+20 configuration” used a laser temporal shaping typical of SI including a final spike. Neutron yield and areal density were measured for implosion performed changing the launching time of the final spike. The detrimental effect of HE on areal density and neutron yield was shown for an early spike launch. For a later spike launch, this effect is minimized because of higher target compression.