D.C. Marion

Dual, orthogonal, backlit pinhole radiography in OMEGA experiments

Backlit pinhole radiography used with ungated film as a detector creates x-ray radiographs with increased resolution and contrast. Current hydrodynamics experiments on the OMEGA Laser use a three-dimensional sinusoidal pattern as a seed perturbation for the study of instabilities. The structure of this perturbation makes it highly desirable to obtain two simultaneous orthogonal backlighting views. We accomplished this using two backlit pinholes each mounted 12mm from the target. The pinholes, of varying size and shape, were centered on 5mm square foils of 50μm thick Ta. The backlighting is by K-alpha emission from a 500μm square Ti or Sc foil mounted 500μm from the Ta on a plastic substrate. Four laser beams overfill the metal foil, so that the expanding plastic provides radial tamping of the expanding metal plasma. The resulting x-rays pass through the target onto (ungated) direct exposure film (DEF). Interference between the two views is reduced by using a nose cone in front of the DEF, typically with a...

Three-dimensional blast-wave-driven Rayleigh-Taylor instability and the effects of long-wavelength modes

This paper describes experiments exploring the three-dimensional (3D) Rayleigh–Taylor instability at a blast-wave-driven interface. This experiment is well scaled to the He/H interface during the explosion phase of SN1987A. In the experiments,  ∼5 kJ of energy from the Omega laser was used to create a planar blast wave in a plastic disk, which is accelerated into a lower-density foam. These circumstances induce the Richtmyer–Meshkov instability and, after the shock passes the interface, the system quickly becomes dominated by the Rayleigh–Taylor instability. The plastic disk has an intentional pattern machined at the plastic/foam interface. This perturbation is 3D with a basic structure of two orthogonal sine waves with a wavelength of 71 μm and an amplitude of 2.5 μm. Additional long-wavelength modes with a wavelength of either 212 or 424 μm are added onto the single-mode pattern. The addition of the long-wavelength modes was motivated by the results of previous experiments where material penetrated unex...

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...

How high energy fluxes may affect Rayleigh–Taylor instability growth in young supernova remnants

Energy-transport effects can alter the structure that develops as a supernova evolves into a supernova remnant. The Rayleigh–Taylor instability is thought to produce structure at the interface between the stellar ejecta and the circumstellar matter, based on simple models and hydrodynamic simulations. Here we report experimental results from the National Ignition Facility to explore how large energy fluxes, which are present in supernovae, affect this structure. We observed a reduction in Rayleigh–Taylor growth. In analyzing the comparison with supernova SN1993J, a Type II supernova, we found that the energy fluxes produced by heat conduction appear to be larger than the radiative energy fluxes, and large enough to have dramatic consequences. No reported astrophysical simulations have included radiation and heat conduction self-consistently in modeling supernova remnants and these dynamics should be noted in the understanding of young supernova remnants.Radiation and conduction are generally considered as the main energy transport mechanisms for the evolution of early supernova remnants. Here the authors experimentally show the role of electron heat transfer on the growth of Rayleigh–Taylor instability in young supernova remnants.

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.

Initial conditions of radiative shock experimentsa)

We performed experiments at the Omega Laser Facility to characterize the initial, laser-driven state of a radiative shock experiment. These experiments aimed to measure the shock breakout time from a thin, laser-irradiated Be disk. The data are then used to inform a range of valid model parameters, such as electron flux limiter and polytropic γ, used when simulating radiative shock experiments using radiation hydrodynamics codes. The characterization experiment and the radiative shock experiment use a laser irradiance of ∼7 × 1014 W cm−2 to launch a shock in the Be disk. A velocity interferometer and a streaked optical pyrometer were used to infer the amount of time for the shock to move through the Be disk. The experimental results were compared with simulation results from the Hyades code, which can be used to model the initial conditions of a radiative shock system using the CRASH code.

Collapsing radiative shock measurements in xenon on the omega laser

Summary form only given. Radiative shocks occur in many high-energy density explosions, but prove difficult to create in laboratory experiments or to fully model with astrophysical codes. Here we describe an experiment significant to astrophysical shocks, which produces a driven, quasi-planar radiative shock in xenon gas at 6 mg/cc. A thin, low-Z disk is driven into a cylindrical volume of xenon gas via laser ablation pressure. This impact creates a shock in xenon, after which the disk travels behind the shock providing a continuing pressure source. With average shock speeds above 100 km/sec, this shock can radiate away energy just behind the shock front, creating a thin layer of dense xenon. Simulations suggest this material is compressed an order of magnitude more than strong shock relations would predict. X-ray backlighting techniques have yielded images of a collapsed shock compressed to <1/25 its initial thickness (45 mum) at a speed of ~100 km/s when the shock has traveled 1.3 mm. Optical depth before and behind the shock is important for comparison to astrophysical systems, where low densities combined with powerful explosions provide ideal conditions for producing radiative shocks

Design of radiative shock targets to enable improved diagnostic access

Summary form only given. Our group has been studying radiative shocks using laser targets that contain a gas, most often xenon. Our motivation includes the production of conditions in the laboratory relevant to astrophysical phenomena and astrophysical simulation codes. A specific phenomenon of interest is the production of a dense, collapsed layer in a given material. Past experiments have consisted of a drive disk made of a solid, dense, low-Z material, capping off one end of a polyimide tube, along with various components used in alignment and measurement. A fill tube for inserting xenon gas plugged the other end. A diagnostic viewed the target perpendicular to the tube and obtained an image of high density regions. Pending experiments will require different diagnostics to measure temperature and related quantities, and which therefore must view the shock along the axis of the tube instead of viewing it from the side. The new target design must provide access to the diagnostic which looks up the axis of the main polyimide cylinder. We plan to accomplish this by building a target with a thin film closing the end of the polyimide tube, in a geometry that allows a laser beam to push the thin film out of the way. This will open up the diagnostic view along the tube axis. The main variables in deciding upon the details of this design are the various types of polyimide arms, and thickness and material of the film at the end of the main cylinder. Determining the type of arm involves first investigating available materials and their cost, and second, evaluating the machining capabilities within the lab. Evaluating the proper film thickness and material will involve HYADES computer simulations, a one-dimensional, hydrodynamic code, which detail the behavior of polyimide (C22H10O5N2) and polyethylene (CH) films when varying the film thickness, and laser spot size. The shock must clear 500 mum within 10 ns in order to not interfere with the diagnostic and therefore ruin the data. We will show the final design of this target and examples of experimental prototypes

Three-dimenstional rayleigh-taylor instability in decelerating interface experiments

Summary form only given. Our goal is to experimentally confirm or disprove the hypothesis that the Rayleigh-Taylor instability could be responsible for the observed transport of heavy elements from the core of SN1897A, a core-collapse supernova, into its outer layers. Observational astrophysicists have been unable to explain the X-ray or luminosity data from SN1987A. Strong hydrodynamic instabilities could be one explanation of the data. Computer simulations of SN1987A have not been able to reproduce the high velocity of heavy elements in the supernova, however, no simulations to date have taken into account three-dimensional effects. Our experiments bridge the gap between simulations and observations by using intense lasers to create an extremely large amount of energy in a small volume. Experiments performed at the Omega laser facility use ~5 kJ of laser energy to create a blast wave similar to those in supernovae. The blast wave crosses a perturbed interface with a density drop and produces Rayleigh-Taylor growth