J. D. Sater

Solid deuterium-tritium surface roughness in a beryllium inertial confinement fusion shell

Solid deuterium-tritium (D-T) fuel layers for inertial confinement fusion experiments were formed inside of a 2 mm diameter beryllium shell and were characterized using phase-contrast enhanced x-ray imaging. The solid D-T surface roughness is found to be 0.4 {micro}m for modes 7-128 at 1.5 K below the melting temperature. The layer roughness is found to increase with decreasing temperature, in agreement with previous visible light characterization studies. However, phase-contrast enhanced x-ray imaging provides a more robust surface roughness measurement than visible light methods. The new x-ray imaging results demonstrate clearly that the surface roughness decreases with time for solid D-T layers held at 1.5 K below the melting temperature.

X-Ray Imaging Of Cryogenic Deuterium-Tritium Layers In A Beryllium Shell

Solid deuterium-tritium (D-T) fuel layers inside copper-doped beryllium shells are robust inertial confinement fusion fuel pellets. This paper describes the first characterization of such layers using phase-contrast x-ray imaging. Good agreement is found between calculation and experimental contrast at the layer interfaces. Uniform solid D-T layers and their response to thermal asymmetries were measured in the Be(Cu) shell. The solid D-T redistribution time constant was measured to be 28 min in the Be(Cu) shell.

Deuterium-Tritium Fuel Layer Formation for the National Ignition Facility

Abstract Inertial confinement fusion requires very smooth and uniform solid deuterium-tritium (D-T) fuel layers. The National Ignition Facility (NIF) point design calls for a 65- to 75-μm-thick D-T fuel layer inside of a 2-mm-diam spherical ablator shell to be 1.5 K below the D-T melting temperature (Tm) of 19.79 K. We find that the layer quality depends on the initial crystal seeding, with the best layers grown from a single seed. The low modes of the layer are controlled by thermal shimming of the hohlraum and meet the NIF requirement with beryllium shells and nearly meet the requirement with plastic shells. The remaining roughness is localized in grain-boundary grooves and is minimal for a single crystal layer. Once formed, the layers need to be cooled to Tm - 1.5 K. We have studied dependence of the roughness on the cooling rate and found that cooling at rates of 0.03 to 0.5 K/s is able to preserve the layer structure for a few seconds after reaching the desired temperature. The entire fuel layer remains in contact with the shell during this rapid cooling. Thus, rapid cooling of the layers is able to satisfy the NIF ignition requirements.

Numerical raytrace verification of optical diagnostics of ice surface roughness for inertial confinement fusion experiments

Targets for future laser-fusion ignition experiments will consist of a frozen deuterium-tritium ice layer adhering to the inner surface of a spherical shell, and the specifications for the inner surface quality of this ice layer are extremely demanding. We have developed a numerical raytrace model in order to validate backlit optical shadowgraphy as an ice-surface diagnostic, and we have used the code to simulate shadowgraph data obtained from mathematical ice layers having known modal imperfections. We find that backlit optical shadowgraphy is a valid diagnostic of the mode spectrum of ice-surface imperfections for mode numbers as high as 80 provided the experimental data are analyzed appropriately. We also describe alternative measurement techniques, which may be more sensitive than conventional backlit shadowgraphy.

Single crystal growth and formation of defects in deuterium-tritium layers for inertial confinement nuclear fusion

We identify vapor-etched grain boundary grooves on the solid-vapor interface as the main source of surface roughness in the deuterium-tritium (D–T) fuel layers, which are solidified and then cooled. Current inertial confinement fusion target designs impose stringent limits to the cross-sectional area and total volume of these grooves. Formation of these grain boundaries occurs over time scales of hours as the dislocation network anneals and is inevitable in a plastically deformed material. Therefore, either cooling on a much shorter time scale or a technique that requires no cooling after solidification should be used to minimize the roughness.

First Measurements of Fuel-Ablator Interface Instability Growth in Inertial Confinement Fusion Implosions on the National Ignition Facility.

Direct measurements of hydrodynamic instability growth at the fuel-ablator interface in inertial confinement fusion (ICF) implosions are reported for the first time. These experiments investigate one of the degradation mechanisms behind the lower-than-expected performance of early ICF implosions on the National Ignition Facility. Face-on x-ray radiography is used to measure instability growth occurring between the deuterium-tritium fuel and the plastic ablator from well-characterized perturbations. This growth starts in two ways through separate experiments-either from a preimposed interface modulation or from ablation front feedthrough. These experiments are consistent with analytic modeling and radiation-hydrodynamic simulations, which say that a moderately unstable Atwood number and convergence effects are causing in-flight perturbation growth at the interface. The analysis suggests that feedthrough from outersurface perturbations dominates the interface perturbation growth at mode 60.

Generating Low-Temperature Layers with Infrared Heating

Abstract Cryogenic targets for the National Ignition Facility require uniform solid layers inside spherical capsules at temperatures ~1.5 K below the triple point of hydrogen. Uniform layers have been successfully formed near the triple point. However, upon subsequent cooling the layers degrade. We report here recent attempts to form uniform deuterium hydride (HD) layers 1.5 K below the triple point using infrared (IR) radiation. Pumping the IR collisionally induced vibration-rotation band of solid HD contained inside a transparent plastic shell generates a volumetric heat source in the HD lattice. This in turn allows the formation of a spherical crystalline shell of HD inside the transparent plastic shell. HD layers ~50 μm thick have been formed near the triple point and slowly cooled 1.5 K under high IR power without layer degradation.

Optical and X-Ray Characterization of Groove Profiles in D-T Ice Layers

Abstract Deuterium-tritium (D-T) single-crystal ice layers in spherical shells often form with localized defects that we believe are vapor-etched grain boundary grooves built from dislocations and accommodating slight misorientations between contacting lattice regions. Ignition implosion target requirements limit the cross-sectional areas and total lengths of these grooves, and since they are often the dominant factor in determining layer surface quality, it is important that we be able to characterize their depths, widths, and lengths. We present a variety of ray-tracing and diffraction image modeling results that support our understanding of the profiles of the grooves, which is grounded in X-ray and optical imaging data. We also describe why these data are nevertheless insufficient to adequately determine whether or not a particular layer meets the groove requirements for ignition. We present accumulated data showing the distribution of groove depths, widths, and lengths from a number of layers, and we discuss how these data motivate the adoption of layer rejection criteria in order to ensure that layers that pass these criteria will almost certainly meet the groove requirements. We also describe future improvements that will provide more quantitative information about grooves in D-T ice layers.

A High-Pressure Filling and Layering Apparatus for Cyrogenic Hohlraums

Abstract A newly operational facility known as the Deuterium Test System (D2TS) has become available at Lawrence Livermore National Laboratory (LLNL). The D2TS provides the capability to perform integrated tests with many of the technologies necessary to deliver and shoot a cryogenic target on the National Ignition Facility (NIF). Procedures used to successfully fill and cool NIF ignition scale targets to cryogenic temperatures are reported. The first attempts at making cryogenic layers in these targets will also be discussed. These experiments are the first without fill tubes at LLNL. The primary technique used to create symmetrical layers of deuterium ice is infrared enhancement.

Benchmarking X-Ray Phase Contrast Imaging for ICF D-T Ice Characterization Using Roughened Surrogates

Abstract We use X-ray phase contrast imaging to characterize the inner surface roughness of deuterium-tritium (D-T) ice layers in capsules for future ignition experiments. It is therefore important to quantify how well the X-ray data correlate with the actual ice roughness. We benchmarked the accuracy of our system using surrogates with fabricated roughness characterized with high precision standard techniques. Cylindrical surrogates with azimuthally uniform sinusoidal perturbations with 100-μm period and 1-μm amplitude demonstrated 0.02-μm accuracy limited by the resolution of the imager and the source size of our phase contrast system. Spherical surrogates with random roughness close to that required for the D-T ice for a successful ignition experiment were used to correlate the actual surface roughness to that obtained from the X-ray measurements. We compare first the average power spectra of individual measurements. The accuracy mode number limits of the X-ray phase contrast system benchmarked against surface characterization performed by atomic force microscopy are 60 and 90 for surrogates smoother and rougher than the required roughness for the ice. These agreement mode number limits are about 100 when comparing matching individual measurements. We will discuss the implications for interpreting D-T ice roughness data derived from phase contrast X-ray imaging.