Non-monotonic growth and critical shock strength for Richtmyer–Meshkov instability in plasmas with multi-species transport

Abstract The Richtmyer–Meshkov instability (RMI) is a key mechanism triggering interfacial mixing in high-energy-density plasmas, with shock strength strongly influencing its evolution. In this work, we demonstrate that the growth of the RMI in plasmas does not increase monotonically with shock strength, as predicted by classical idealized theory, but instead exhibits a critical shock strength that maximizes the instability growth. This behavior is revealed through hydrodynamic simulations performed with the FLASH code (Fryxell et al 2000 Astrophys. J. Suppl. Ser. 131 273), which incorporates a multi-species transport module (Vold et al 2017 Phys. Plasmas 24 042702) using species-dependent coefficients computed from a unified transport theory (Simakov and Molvig 2016 Phys. Plasmas 23 032115); (Simakov and Molvig 2016 Phys. Plasmas 23 032116). Simulation results show that perturbation amplitude and vortex structures exhibit a non-monotonic dependence on shock strength, with a critical shock strength corresponding to maximal growth, whereas molecular mixing increases monotonically. Decoupled simulations reveal that increasing shock strength naturally introduces a competition between stronger shock compression and acceleration versus suppression by viscosity and mass diffusion, leading to the critical behavior of amplitude. A semi-empirical amplitude model, fitted from a limited number of simulations, is also proposed to rapidly estimate the critical shock strength. Similarly, vortex structures exhibit a critical shock strength due to the interplay between shock-induced compression and baroclinic vorticity deposition versus viscous damping. In contrast, molecular mixing is dominated by enhanced mass diffusion, leading to a monotonic increase of the mixing layer with Mach number. These findings reveal that careful adjustment of shock strength can control the relative contributions of instability-driven mixing and diffusion-driven mixing. This may provide guidance for optimizing laser drive conditions to improve implosion performance in inertial confinement fusion.