Rayleigh-Taylor Instability and Resulting Failure Modes of Ablatively Imploded Inertial Fusion Targets

Abstract

The ablation process is thought to be useful for driving the spherical implosion of inertial confinement fusion (ICF) targets¹. By “ablation” is meant the process of applying pressure continuously in time to a surface by heating the surface. The pressure applied may be thought of as the reaction to acceleration of heated material away from the surface. In fact, an implosion is only “ablative” if the time scale required for establishment of the outward flow from the surface is short compared to the implosion time; i.e., if the ablative flow is quasistationary. If the spherical target pellet is a shell of initial thickness Δr_p and radius r_p which is hollow or contains a much lower density fuel, then this shell, whose initial aspect ratio is defined as A_p = r_p/Δr_p, will accelerate inwardly. This acceleration causes the outside surface of the shell to experience instability of the Rayleigh-Taylor type, which is thought to be potentially troublesome for A_p ≳ 5². Subsequently, as the implosion of the shell is decelerated by compression of lower density fuel material inside, the inside surface becomes unstable . This article presents a theory of these instabilities and potential modes of failure caused by them.