Theory of laser-induced adiabat shaping in inertial fusion implosions: The decaying shock

Abstract

The evolution of a decaying shock driven by a laser prepulse in inertial confinement fusion (ICF) capsules is investigated including the effects of mass ablation and residual ablation pressure. The adiabat (entropy) profile left behind by a decaying shock is calculated and compared with the results of one-dimensional simulations. It is found that the adiabat shape follows a simple power law of the areal density α=αinn(mshell/m)δa, where αinn is the shell inner surface adiabat, mshell is the initial total shell areal density, and m is the local shell areal density. The power law index δa varies from 1.12 for prepulses in the 100 ps range to 1.20 for prepulses in the 300 ps range. This adiabat profile accurately reproduces the simulation results to within a few percent. A simple procedure aimed at designing a laser pulse inducing the maximum adiabat ratio between the outer- and inner-shell surface is also developed. ICF capsules with a shaped adiabat are expected to benefit from improved hydrodynamic stability while maintaining the same one-dimensional performance as flat adiabat shells.