Norman E. Elliott

Observation of mix in a compressible plasma in a convergent cylindrical geometry

Laser beams that directly drive a cylindrical implosion are used to create a measurable region of mixed material in a compressible plasma state, for the first time in a convergent geometry. The turbulence driven by the Richtmyer–Meshkov instability by shock passage across a density discontinuity mixes marker material that is radiographically opaque. The width of the mix layer is compared between a system with large surface roughness and an initially smooth system. The experiment is described and results are compared to multi-dimensional numerical simulation, including three-dimensional turbulence calculations. The calculations adequately match the observations provided the measured initial conditions are used.

Off-axis multiple scattering of a laser beam in turbid media: comparison of theory and experiment

The off-axis forward-scattered radiation from a low energy He-Ne laser, scattered by high density hydrosols, is studied experimentally and theoretically. The validity range of theoretical calculations is determined using four measurements with two optical depths and two detector fields of view. For narrow field of view detectors, experimental and theoretical agreement is good close to the beam axis and decreases with transverse distance; agreement is good at greater distances from the beam axis using an open detector. The small-angle approximation adequately describes multiple scattering close to the beam; calculations are significantly improved by combining the small-angle approximation with diffusion theory.

The characterization of asymmetric alumina membranes by Rutherford backscattering spectrometry

Abstract Effective membrane structures often rely on the presence of thin discriminating layers that exert selectivity control while maintaining high fluxes. Characterizing these separating layers is often difficult given the scarcity of analytical tools capable of measuring structural detail below sample surfaces. We have shown that Rutherford backscattering spectrometry (RBS) is an effective tool for characterizing asymmetric inorganic membranes. The ability to accurately measure the thickness and composition of thin metal layers using RBS offered a means of analyzing composite structures in which metals are deposited to influence separation or provide strength to the membrane. This analysis of commercially available alumina membranes by RBS quantified the interpenetration depth of deposited gold layers. RBS has also been applied to the measurement of sample porosity in Al 2 O 3 membranes using specialized 16 O resonance experiments. The measured porosity of these asymmetric membranes resulting from the resonance measurements was compared to a graphical analysis of scanning electron microscopy images. The results of this study suggest that RBS may offer advantages in a wide range of membrane materials analysis applications.

Characterization of surface roughness and initial conditions for cylindrical hydrodynamic and mix experiments

Abstract Hydrodynamic experiments in cylindrical geometry are used to study both mix (compressible, in convergent geometry) and mode coupling (impact of short wavelengths on long). For both types of experiments, knowledge of the initial conditions (the surface roughness spectrum, amplitude versus wavelength, as well as all target metrology) is very important. This paper is a discussion of the techniques and efforts to document and understand our initial conditions and their uncertainties and how well we can control them.

Characterization of density and metal content in low density foam targets for inertial confinement fusion

Targets for Inertial Confinement Fusion (ICF) experiments to measure isoelectronic temperatures achieved in open configurations have been manufactured from low density polymethylpentene. The temperature measurements are made by ratioing the intensity of two nearby X-ray lines, in this case from titanium and chromium. This required these metals to be added to the foams during manufacture. Individual targets consisted of machined cylinders of foam 2 mm in diameter by 2 mm long. Because of small scale variability in the foam, it was necessary to individually characterize each target for metal content and density to determine the total mass of plastic and metal present to allow a correct calculation of the temperature to be made. The requested density of the foam was less than 5 mg/cm3 and the metal loading was to be 0.5 wt.% each of titanium and chromium. This means the mass of plastic was roughly 10 μg and the metal mass was roughly 50 ng. This low level of material to be detected required the application and modification of X-ray fluorescence and beta particle transmission methods to characterize the foams.

Pockels readout optical modulator: An x‐ray imaging detector that maintains good efficiency over a broad energy range

We present measurements of the performance of a Pockels readout optical modulator (PROM) x‐ray imaging detector using a 30‐kV constant potential bremsstrahlung source. A nickel step wedge was used to measure the spatial resolution and noise of the PROM image. PROMs are unique high‐efficiency x‐ray detectors that image with good spatial resolution over a wide range of x‐ray energy. The PROM is constructed from an optically transparent, active crystal that is sensitive to x‐ray dose. This unique construction allows the use of a thick crystal to increase efficiency for x‐ray detection while avoiding the spatial resolution degradation that usually accompanies thick detectors.

Chemical Compatibility of Silica Aerogel Processes with ICF Hohlraums

Abstract Inertial Confinement Fusion (ICF) energy hohlraums are composed of a high-Z material filled with foam. Because of the small pore size and transparency, silica aerogels are used in some ICF targets. The traditional synthesis of silica aerogels require sol-gel polymerization of silicon alkoxide followed by supercritical drying. Some constituents in sol-gel polymerization have been found to contribute to leaching of certain metals at the silica/metal interface. Since the hohlraums are composed of metals, possible chemical reactivity at the silica aerogel and metal hohlraum interface was investigated. The hohlraums studied are aluminum lined with either copper or copper/chromium. Upon initial inspection, the aerogel appeared transparent and uniform, however, closer inspection of the copper wall suggested possible leaching. Alternatively the quality of the aerogel in the copper-chromium hohlraum was very poor with the chromium layer of the hohlraum and some copper completely etched. Control experiments were used to determine the cause of the leaching. When copper is in the presence of sol-gel constituents, Cu2+ ion formed, thus leaching copper from the hohlraum walls. In the presence of chromium, Cr2O72- or CrO42- was identified in solution with the Cu2+, these anions are believed to form copper chromite under the aerogel synthesis procedures utilized.

BEST PRACTICE PROCEDURES FOR MAKING DIRECT DRIVE CYLINDRICAL TARGETS FOR STUDIES OF CONVERGENT HYDRODYNAMICS

Abstract The production of cylindrical targets involves numerous steps. These steps are shared in common with many other types of Inertial Confinement Fusion (ICF) targets but no other single target encompasses such a wide range of fabrication techniques. These targets consist of a large number of individual parts, virtually all fabricated from commercially purchased raw material. As an example, the polystyrene used is synthesized in house from purchased monomer material. This material must be polymerized, purified, characterized and put into solution before it is even first used in the making of a target. Because virtually every manufacturing and assembly process we currently use is involved in the production of these targets, this paper is written as a way documenting the methods used.

Recent Developments in Fabrication of Direct Drive Cylinder Targets for Hydrodynamics Experiments at the OMEGA Laser

Abstract Experimental campaigns are being conducted at the 60 beam OMEGA laser at the University of Rochester’s Laboratory for Laser Energetics to acquire data to validate hydrodynamic models in the high energy-density regime. This paper describes targets that have been developed and constructed for these experimental campaigns. Targets are 860 μm inner diameter by 2.2 mm length cylinders with 70 μm thick polymer ablator. On the ablator inner surface and located halfway along the axis of the cylinder is a 500 μm wide Al marker band. Band thicknesses in the range 8-16 microns are used. CH foam with densities in the range 30-90 mg/cc fills the inside of the cylinder. While these targets have been fabricated for years, several new improvements and features have recently been developed. Improvements include the use of epoxy instead of polystyrene for the ablator, and the use of electrodeposited Al for the marker band. A critical feature of the target is the surface feature that is placed on the marker band. Experiments are aimed at understanding the hydrodynamic behavior of imploding cylinders as a function of this surface feature. Recent development work has focused on production of engineered surface features on the target marker band. Using a fast tool servo on a diamond turning lathe, a wide range of specified surface features have been produced. This paper will address improvements to the cylinder targets as well as current development efforts.

Surface Treatment of PMMA with Polyvinylpyrrolidone and a Titanium Compound and Its Effect on Blood Compatibility

The surfaces of poly(methyl methacrylate) (PMMA) have been modified through a chemical infusion process by treatment of the sample with a solution containing varying amounts of titanium (IV) isopropoxide and poly (vinylpyrrolidone) (PVP). The resulting sample is modified in the outermost 150–325 micron region while the underlying polymeric material is unchanged. The chemical infusion process, a solvent based surface modification technique, is described in detail along with a study correlating treatment conditions with penetration depth. The treated samples were characterized by scanning electron microscopy, optical microscopy, and neutron activation analysis. These samples were evaluated for blood compatibility using two biological assays: (1) an adherence assay in which the adherence of human polymorphonuclear leukocytes to the samples was determined, and (2) a dynamic hemolysis assay using rat blood erythrocytes to determine the hemolytic activity of the samples. Based on the results of these assays, the PMMA samples treated with PVP alone resulted in an improvement in reactivity with the blood cells in a manner comparable to TecoflexR, a commercially available polymer with relatively benign characteristics.