G. Malamud

Richtmyer-Meshkov evolution under steady shock conditions in the high-energy-density regime

This work presents direct experimental evidence of long-predicted nonlinear aspects of the Richtmyer-Meshkov process, in which new modes first arise from the coupling of initially-present modes, and in which shorter-wavelength modes are eventually overtaken by longer-wavelength modes. This is accomplished using a technique we developed employing a long driving laser pulse to create a strong (Mach ∼ 8) shock across a well-characterized material interface seeded by a two-mode sinusoidal perturbation. This technique further permits the shock to be sustained, without decay of the high-energy-density flow conditions, long enough for the system to evolve into the nonlinear phase.

Observation and modeling of mixing-layer development in high-energy-density, blast-wave-driven shear flow

In this work, we examine the hydrodynamics of high-energy-density (HED) shear flows. Experiments, consisting of two materials of differing density, use the OMEGA-60 laser to drive a blast wave at a pressure of ∼50 Mbar into one of the media, creating a shear flow in the resulting shocked system. The interface between the two materials is Kelvin-Helmholtz unstable, and a mixing layer of growing width develops due to the shear. To theoretically analyze the instabilitys behavior, we rely on two sources of information. First, the interface spectrum is well-characterized, which allows us to identify how the shock front and the subsequent shear in the post-shock flow interact with the interface. These observations provide direct evidence that vortex merger dominates the evolution of the interface structure. Second, simulations calibrated to the experiment allow us to estimate the time-dependent evolution of the deposition of vorticity at the interface. The overall result is that we are able to choose a hydrodynamic model for the system, and consequently examine how well the flow in this HED system corresponds to a classical hydrodynamic description.

Observation of single-mode, Kelvin-Helmholtz instability in a supersonic flow

We report the first observation, in a supersonic flow, of the evolution of the Kelvin-Helmholtz instability from a single-mode initial condition. To obtain these data, we used a novel experimental system to produce a steady shock wave of unprecedented duration in a laser-driven experiment. The shocked, flowing material creates a shear layer between two plasmas at high energy density. We measured the resulting interface structure using radiography. Hydrodynamic simulations reproduce the large-scale structures very well and the medium-scale structures fairly well, and imply that we observed the expected reduction in growth rate for supersonic shear flow.

Observation of dual-mode, Kelvin-Helmholtz instability vortex merger in a compressible flow

We report the first observations of Kelvin-Helmholtz vortices evolving from well-characterized, dual-mode initial conditions in a steady, supersonic flow. The results provide the first measurements of the instabilitys vortex merger rate and supplement data on the inhibition of the instabilitys growth rate in a compressible flow. These experimental data were obtained by sustaining a shockwave over a foam-plastic interface with a precision-machined seed perturbation. This technique produced a strong shear layer between two plasmas at high-energy-density conditions. The system was diagnosed using x-ray radiography and was well-reproduced using hydrodynamic simulations. Experimental measurements imply that we observed the anticipated vortex merger rate and growth inhibition for supersonic shear flow.

The Effect of a Dominant Initial Single Mode on the Kelvin–Helmholtz Instability Evolution: New Insights on Previous Experimental Results

This paper brings new insights on an experiment, measuring the Kelvin–Helmholtz (KH) instability evolution, performed on the OMEGA-60 laser facility. Experimental radiographs show that the initial seed perturbations in the experiment are of multimode spectrum with a dominant single-mode of 16 μm wavelength. In single-mode-dominated KH instability flows, the mixing zone (MZ) width saturates to a constant value comparable to the wavelength. However, the experimental MZ width at late times has exceeded 100 μm, an order of magnitude larger. In this work, we use numerical simulations and a statistical model in order to investigate the vortex dynamics of the KH instability for the experimental initial spectrum. We conclude that the KH instability evolution in the experiment is dominated by multimode, vortex-merger dynamics, overcoming the dominant initial mode.

Ablative stabilization of Rayleigh-Taylor instabilities resulting from a laser-driven radiative shock

The Rayleigh-Taylor (RT) instability is a common occurrence in nature, notably in astrophysical systems like supernovae, where it serves to mix the dense layers of the interior of an exploding star with the low-density stellar wind surrounding it, and in inertial confinement fusion experiments, where it mixes cooler materials with the central hot spot in an imploding capsule and stifles the desired nuclear reactions. In both of these examples, the radiative flux generated by strong shocks in the system may play a role in partially stabilizing RT instabilities. Here, we present experiments performed on the National Ignition Facility, designed to isolate and study the role of radiation and heat conduction from a shock front in the stabilization of hydrodynamic instabilities. By varying the laser power delivered to a shock-tube target with an embedded, unstable interface, the radiative fluxes generated at the shock front could be controlled. We observe decreased RT growth when the shock significantly heats t...

Study of shock waves and related phenomena motivated by astrophysics

This paper discusses the recent research in High-Energy-Density Physics at our Center. Our work in complex hydrodynamics is now focused on mode coupling in the Richtmyer- Meshkov process and on the supersonic Kelvin-Helmholtz instability. These processes are believed to occur in a wide range of astrophysical circumstances. In radiation hydrodynamics, we are studying radiative reverse shocks relevant to cataclysmic variable stars. Our work on magnetized flows seeks to produce magnetized jets and study their interactions. We build the targets for all these experiments, and simulate them using our CRASH code. We also conduct diagnostic research, focused primarily on imaging x-ray spectroscopy and its applications to scattering and fluorescence.

Innovations in Target Fabrication Techniques at the University of Michigan

Abstract The University of Michigan has been fabricating targets for OMEGA campaigns since 2003. These experiments explore supernova-relevant high-energy-density physics. The complexity of recent target designs has made it necessary to explore new methods of producing components that satisfy experimental needs. Interest in the dynamics of nonaxisymmetric shocks has led to the development of polyimide tubes with noncircular cross sections. For our latest Thomson scattering target, shielding was a very important component to the target design. We employed techniques to bend gold foils, enabling complex geometries without any of the seams inherent when two separate foils are pieced together. Machined acrylic bases are used to support all the components on our targets, contributing further to their repeatability and providing us with a method that eases our build. Here, we present improvements in our techniques, along with our basic tried-and-true methods of producing repeatable targets.