N. Acharya

Polymers are widely used in high energy density (HED) experiments, but rheological properties, such as viscosity and strength, are often assumed negligible in modeling despite limited experimental validation. We report initial measurements of rippled reflecting shocks in epoxy using the Sakharov shock perturbation decay technique supported by analytical formulation from Miller and Ahrens [Rev. Mod. Phys. 63(4), 919 (1991)]. Experiments at the Jupiter Laser Facility employed drive beams delivering a 10 ns, 800 J pulse across an ∼600 μm spot, launching a shock above 400 GPa into epoxy. The highly reflective shock in the epoxy sample was diagnosed using velocimetry and optical pyrometry. Velocimetry showed strong reflectivity and evidence of perturbation structure and streaked optical pyrometry clearly captured spatial evolution of the perturbed shock and showed amplitude growth, decay, and phase inversion, in agreement with hydrodynamic simulations using ARES. Our demonstration of technical viability and the observation of rippled reflecting shocks in epoxy establish a path toward future quantitative constraints on polymer rheology in HED experiments.

The evolution of non-uniform shocks produced by modulated laser irradiation or surface perturbations is relevant to studies of inertial confinement fusion and material properties at high-energy-density conditions. We present results from an experiment conducted at the OMEGA EP laser facility, where a 300 GPa shock was driven into a fused silica sample with pre-fabricated single-mode surface modulations. Using time-resolved optical velocimetry, we captured the continuous evolution of rippled shock motion, enabling a comprehensive mapping of the spatial amplitude history from formation to phase reversal in a single experiment. Initially, the ablation-driven shock inherits a fraction of the surface modulation amplitude from the sample, which subsequently grows before decaying, ultimately leading to the flattening of the rippled shock and a phase reversal. We find that two-dimensional inviscid hydrodynamic simulation of the experiment is able to qualitatively capture many aspects of the rippled shock evolution but over-predicts the initial amplitude growth. This experimental platform, capable of accommodating varying ripple wavelengths, lays the groundwork for a potential viscometry method at extreme pressures, where viscous effects manifest as differences in shock flattening times between rippled shocks of two distinct wavelengths propagating through the sample.

A laser-driven shock propagating through an isolated particle embedded in a plastic (CH) target was studied using the radiation-hydrodynamic code FLASH. Preliminary simulations using IONMIX equations of state (EOS) showed significant differences in the shock Hugoniot of aluminum compared to experimental data in the low-pressure regime [O(10) GPa], resulting in higher streamwise compression and deformation of an aluminum particle. Hence, a simple modification to the ideal gas EOS was developed and employed to describe the target materials and examine the particle dynamics. The evolution of the pressure field demonstrated a complex wave interaction, resulting in a highly unsteady particle drag which featured two drag minima due to shock focusing at the rear end of the particle and rarefaction stretching due to laser shut-off. Although ∼30% lateral expansion and ∼25% streamwise compression were observed, the aluminum particle maintained considerable integrity without significant distortion. Additional simulations examined the particle response for a range of particle densities, sizes, and acoustic impedances. The results revealed that lighter particles such as aluminum gained significant momentum, reaching up to ∼96% of the shocked CH's speed, compared to ∼29% for the heavier tungsten particles. Despite the differences seen in the early stage of shock interaction, particles with varying acoustic impedances ultimately reached the same peak velocity. This identified particle-to-host density ratio is an important factor in determining the inviscid terminal velocity of the particle. In addition, the modified EOS model presented in this study could be used to approximate solid materials in hydrocodes that lack material strength models.