Dynamics of species separation induced by ultra-strong shock: A molecular dynamics investigation

Abstract

Species separation induced by ultra-strong shocks is a fundamental topic for both inertial confinement fusion and high-energy-density experiments. However, how this phenomenon unfolds during the propagation and release phases of shocks has not been fully understood. In this study, we conduct a systematic investigation of species separation dynamics induced by strong shocks covering both steady propagation stage and shock release stage. Our study employs the non-equilibrium molecular dynamics method to explore these processes in a medium composed of two representative isotopes. Simulation results unveil distinct physical scenarios and dynamic behaviors of species separation during the two stages: During the steady propagation stage, species separation is confined to a localized region near the shock front, where the dynamics of the binary isotopic mixture are well-captured by the single-fluid approximation. In contrast, at the shock release stage, the species expand independently into the vacuum with different speeds, leading to a separation region of several orders of the mean free path. Interactions between particles play a minimal role in this process, allowing the dynamics of the mixture to be effectively described by decoupled two-phase hydrodynamic equations. Specifically, a self-similar behavior in the degree of separation and the isentropic expansion laws governing the two highly rarefied species at the shock release stage are identified, offering quantitative insights for applications.