Richtmyer--Meshkov instability with ionization at extreme impact conditions

Abstract

Richtmyer–Meshkov instability (RMI) under extreme impacting conditions is studied via molecular dynamics (MD) simulation with an electron force field (eFF) model. It is revealed that the strong loading ionizes materials into heavy ions and free electrons, and subsequently, a quasi-steady electron/ion separation zone is established across the shock front because free electrons can move quickly to regions ahead of the shock wave. The electron/ion separation zone propagates at the same velocity as that of the shock wave, and its width and strength remain nearly constant. Based on this observation, a simple charge distribution profile is proposed for microscopic RMI with ionization, with which an analytical model for interface acceleration caused by electric field force can be derived. A nondimensional parameter ( η), which is defined as the ratio of the flow domain length to the length of the charge separation zone, is proposed. When η exceeds a certain value, the charge density distribution is similar to that of macroscopic RMI with ionization, and thus, an acceleration model for macroscopic RMI can be derived. Finally, a nonlinear model for the perturbation growth of macroscopic RMI with ionization is achieved by incorporating the acceleration model to the potential flow theory of Q. Zhang and W. Guo [“Universality of finger growth in two-dimensional Rayleigh–Taylor and Richtmyer–Meshkov instabilities with all density ratios,” J. Fluid Mech. 786, 47–61 (2016)]. The validity of the model is verified by the present large-scale eFF MD simulation and experimental results obtained with the Nova laser.