Numerical study on the feedthrough mechanism of double-interface Rayleigh-Taylor instability growth

These data offer the bubble and spike growth rates at the left and right interfaces. Abstract: Multi-interface Rayleigh-Taylor (RT) instability is one of the critical factors limiting the performance of double-shell inertial confinement fusion (ICF) targets. This work presents a numerical study of the double-interface RT instability in a three-fluid system with a dense central layer. The right interface is subject to RT instability. At the right interface, spike growth remains decoupled from the left interface. In contrast, bubble growth is strongly suppressed by feedback from the other interface, especially for thinner shells. This result contradicts the finding of Baker for a vacuum-bounded shell [J. Fluid Mech. 178, 161 (1987)]. The discrepancy stems from the absence of drag in vacuum versus the strong drag present in our finite-density configuration, which reduces the bubble growth rate. The left interface is mainly driven by feedthrough. At the left interface, bubble growth converges to the evolution pattern of the right side, while the spike region exhibits concurrent in-phase growth and anti-spike formation. The anti-spike forms due to the baroclinic effect associated with a local pressure maximum. We then quantify the roles of shell thickness, wavelength, Atwood number, and acceleration. Parametric analysis shows that bubble feedthrough increases with thinner shells and larger values of the other parameters. In contrast, effective spike feedthrough shifts negatively when the shell thickness and left Atwood number increase, but positively when the other parameters increase. Based on these insights, we extend the classical buoyancy-drag model by incorporating feedthrough mechanisms. For a typical thin-shell case, the extended model reduces the bubble-growth error to one-seventh of the original one and more reliably captures the shell-thinning process. This work investigates the feedthrough mechanism in the double-interface RT instability and provides a useful model for instability growth in double-shell ICF targets.