Dynamics of particles near the surface of a medium under ultra-strong shocks

Zixiang Yan,Xinyu Zhang,Hao Liu,Weiyan Zhang,Jie Liu,Xiantu He,Guoli Ren,Wei Kang

doi:10.1063/5.0030906

Abstract

Through nonequilibrium molecular dynamics simulations, we provide an atomic-scale picture of the dynamics of particles near the surface of a medium under ultra-strong shocks. This shows that the measured surface velocity vf under ultra-strong shocks is actually the velocity of the critical surface at which the incident probe light is reflected, and vf has a single-peaked structure. The doubling rule commonly used in the case of relatively weak shocks to determine particle velocity behind the shock front is generally not valid under ultra-strong shocks. After a short period of acceleration, vf exhibits a long slowly decaying tail, which is not sensitive to the atomic mass of the medium. A scaling law for vf is also proposed, and this may be used to improve the measurement of particle velocity u in future experiments.

Highlights

In recent years, with the advent of high-power lasers, laser-driven shocks have become widely used to determine equations of state (EOS) for various materials1–13 of fundamental interest to inertial confinement fusion (ICF).14–19 According to the Rankine–Hugoniot relations, the determination of all the flow variables of a shock front in such experiments can be reduced to the determination of two kinematic parameters, namely, the propagation speed vs of the shock wave and the average particle velocity u behind the shock front.20 vs can be measured with relative ease, determining u is more difficult experimentally
Through nonequilibrium molecular dynamics simulations, we provide an atomic-scale picture of the dynamics of particles near the surface of a medium under ultra-strong shocks
A scaling law for vf is proposed, and this may be used to improve the measurement of particle velocity u in future experiments

Summary

Introduction

With the advent of high-power lasers, laser-driven shocks have become widely used to determine equations of state (EOS) for various materials of fundamental interest to inertial confinement fusion (ICF). According to the Rankine–Hugoniot relations, the determination of all the flow variables of a shock front in such experiments can be reduced to the determination of two kinematic parameters, namely, the propagation speed vs of the shock wave and the average particle velocity u behind the shock front. vs can be measured with relative ease, determining u is more difficult experimentally. One commonly used indirect method is the so-called freesurface approximation, which assumes that the velocity of the free surface is twice the average particle velocity, i.e., vf 2u, which is known as the doubling rule. It has been shown both theoretically and experimentally that this approximation works well at relatively low Mach number. The way in which vf departs from this rule under strong shocks remains an open question Better understanding of this question is of great interest to laser-driven EOS experiments, in which ultra-strong shocks are often generated

Methods

Results

Conclusion