C.A. Di Stefano

Experimental observations of turbulent mixing due to Kelvin–Helmholtz instability on the OMEGA Laser Facility

Shear-flow, Kelvin–Helmholtz (KH) turbulent mixing experiments were performed on the OMEGA Laser Facility [Boehly et al., Opt. Commun. 133, 495 (1997)] in which laser-driven shock waves propagated through a low-density plastic foam placed on top of a higher-density plastic foil. The plastic foil was comprised a thin iodine-doped plastic tracer layer bonded on each side to an undoped density-matched polyamide-imide plastic. Behind the shock front, lower-density foam plasma flowed over the higher-density plastic plasma, such that the interface between the foam and plastic was KH unstable. The initial perturbations consisted of pre-imposed, sinusoidal 2D perturbations, and broadband 3D perturbations due to surface roughness at the interface between the plastic and foam. KH instability growth was measured using side-on radiography with a point-projection 5-keV vanadium backlighter. Time-integrated images were captured on D-8 x-ray film. Spatial density profiles of iodine-doped plastic mixed with foam were inf...

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.

Target Fabrication at the University of Michigan

Abstract At the University of Michigan (U-M), we have successfully fabricated and characterized targets for our experimental campaigns since 2003. Because of the unique production environment, we iterate many models in the course of a single-shot plan and have the flexibility to test and alter target designs as needed throughout the build process. Over the past few years, many advances in target design and fabrication have allowed greater degrees of design complexity while retaining the high level of build precision necessary for microscale experiments on facilities such as the OMEGA laser. Extensive target metrology is carried out during and after the fabrication process to allow for full knowledge of experimental conditions and to ensure that all targets are within required specifications. Analysis of the variability in metrology measurements over the multiple-shot campaigns allows for the quantification of improvements in the target build quality and metrology measurements. We present a summary of the capabilities and recent developments of target fabrication at U-M, as well as progress and analysis of build repeatability.

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.

Measurement of high-energy (10-60 keV) x-ray spectral line widths with eV accuracy.

A high resolution crystal spectrometer utilizing a crystal in transmission geometry has been developed and experimentally optimized to measure the widths of emission lines in the 10-60 keV energy range with eV accuracy. The spectrometer achieves high spectral resolution by utilizing crystal planes with small lattice spacings (down to 2d = 0.099 nm), a large crystal bending radius and Rowland circle diameter (965 mm), and an image plate detector with high spatial resolution (60 μm in the case of the Fuji TR image plate). High resolution W L-shell and K-shell laboratory test spectra in the 10-60 keV range and Ho K-shell spectra near 47 keV recorded at the LLNL Titan laser facility are presented. The Ho K-shell spectra are the highest resolution hard x-ray spectra recorded from a solid target irradiated by a high-intensity laser.

Measurements of the energy spectrum of electrons emanating from solid materials irradiated by a picosecond laser

In this work, we present the results of experiments observing the properties of the electron stream generated laterally when a laser irradiates a metal. We find that the directionality of the electrons is dependent upon their energies, with the higher-energy tail of the spectrum (∼1 MeV and higher) being more narrowly focused. This behavior is likely due to the coupling of the electrons to the electric field of the laser. The experiments are performed by using the Titan laser to irradiate a metal wire, creating the electron stream of interest. These electrons propagate to nearby spectator wires of differing metals, causing them to fluoresce at their characteristic K-shell energies. This fluorescence is recorded by a crystal spectrometer. By varying the distances between the wires, we are able to probe the divergence of the electron stream, while by varying the medium through which the electrons propagate (and hence the energy-dependence of electron attenuation), we are able to probe the energy spectrum of the stream.

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.