High-Energy-Density Physics Experiments of Rayleigh-Taylor Instability Growth at Low-Density-Contrast

Abstract

This dissertation describes experiments performed at the Omega-60 laser facility to investigate the nonlinear growth stage of the Rayleigh-Taylor instability (RTI) at a low-density-contrast embedded interface initialized with 2D and 3D single-mode sinusoidal perturbations. RTI occurs at the interface between two fluids of different densities when the lower-density fluid pushes the higher-density fluid. Such a system is energetically unstable: a small-amplitude perturbation at the interface evolves into features described as "bubbles" (parcels of light-fluid rising upward) and "spikes" (regions of heavy-fluid falling downward). Bubbles and spikes interpenetrate across the interface, forming a mixed-fluid region which continues to grow, thereby lowering the potential energy of system. This fundamental hydrodynamic instability is encountered throughout nature and engineered systems. In the realm of high-energy-density (HED) physics, RTI occurs in astrophysical phenomena such as supernovae explosions and in the laboratory during implosion of inertial confinement fusion (ICF) capsules. The ability to model and predict the evolution of RTI has important consequences for fundamental scientific understanding and engineering applications.

Theoretical approaches to RTI consider three separate cases: single-mode, multi-mode, and turbulent mixing, which evolve differently. Analytical models successfully predict macroscopic growth rates of the mixed-fluid layer for these three cases under certain conditions, but neglect the small-scale mixing dynamics, which are essential to describing transitional states. To develop reliable predictive capabilities, we must understand how the seed spectrum, density contrast of the two fluids, miscibility, acceleration history, and Reynolds number affect the evolution of RTI. Experiments with well-controlled initial conditions enable us to isolate and study particular aspects of the problem. Recent classical fluids experiments and numerical simulations investigate the late-nonlinear growth stage of single-mode RTI and dependence on density contrast. At low-density-contrast, single-mode RTI growth appears to reaccelerate, beyond the terminal velocity predicted by potential-flow models. In the late-nonlinear stage, secondary instabilities arise which modify the internal mixing dynamics and growth rate. No existing models describe this stage of RTI growth, where the mixed-fluid region is partially coherent, partially chaotic, but not fully turbulent. The work presented here investigates this regime in a high-energy-density system relevant to astrophysics and ICF.

In this dissertation, I describe experiments performed at Omega-60, a 10-kJ-class laser facility. In this experimental platform, a blast wave drives RTI growth at an embedded interface inside a shock tube. Using dual-axis X-ray radiography, we observed the evolution of the mixed-fluid region from 17-47 ns. Experimentally measured spike- and bubble-front positions are compared with buoyancy-drag models and radiation-hydrodynamics simulations. The experiments at Omega-60 did not sustain acceleration for long enough to drive RTI into the desired regime, but provided valuable information to inform the design of future experiments at the National Ignition Facility, a MJ-class laser facility.