Early-Time “Shine-Through” in Laser Irradiated Targets

Abstract

In direct-drive laser-fusion experiments, a deuterium or deuterium-tritium filled microballoon is imploded by direct illumination with a number of high-power laser beams. The implosion is driven by the ablation of material from the outer surface of the target. The ablation and compression occurs only after a plasma has been established at the surface of the target. This plasma, once established, dominates the interaction since the laser can no longer propagate past the critical density surface where the plasma frequency equals the laser frequency. Energy is absorbed at densities below critical and transported through the overdense region to the ablation front. Another important function of the plasma is to smooth, by thermal conduction, some of the the spatial non-uniformities in the laser energy deposition on the way to the ablation layer.