Richtmyer-Meshkov instability with ionization at extreme impact conditions

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 (eta), which is defined as the ratio of the flow domain length to the length of the charge separation zone, is proposed. When eta 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. Published under an exclusive license by AIP Publishing.

Automated summary

In this study, the Richtmyer-Meshkov instability (RMI) under extreme impacting conditions is investigated using molecular dynamics (MD) simulation. The results show that ionization occurs when the material is strongly loaded, resulting in the formation of a quasi-steady electron/ion separation zone across the shock front. This separation zone propagates at the same velocity as the shock wave and remains approximately constant in width and strength. Based on these findings, a simple charge distribution profile is proposed for microscopic RMI with ionization, and an analytical model for interface acceleration can be derived. When a nondimensional parameter (eta) exceeds a certain value, the charge density distribution is similar to that of macroscopic RMI with ionization, and an acceleration model for macroscopic RMI can be developed. The validity of the model is confirmed through large-scale MD simulations and experimental results.