L. C. Carlson

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.

Surface oxygen micropatterns on glow discharge polymer targets by photo irradiation

Recent simulations predict surface oxygen may be a significant source of disruptive perturbations in the implosion process of glow-discharge polymers (GDP) ablators at the National Ignition Facility. GDP material held in ambient atmospheric conditions showed an increase in mass when stored in light transparent containers, which suggests that photo exposure is a driving force for oxygen absorption. To investigate if surface oxygen is a contributing factor of disruptive perturbations during implosion, a method to imprint a periodic micropattern of oxygen on the surface of GDP was developed and used to fabricate a flat sample for empirical testing. Photo exposure using collimated blue light was used to generate micropatterns of surface oxygen on the GDP material. The periodic oxygen micropattern was confirmed by secondary ion mass spectrometry (SIMS) and energy dispersive spectroscopy. A SIMS depth profile showed the atomic percent of oxygen ranged from 8 at. % near the surface to 1 at. % at a depth of 2 μm ...

Update 2015 on Target Fabrication Requirements for NIF Layered Implosions, with Emphasis on Capsule Support and Oxygen Modulations in GDP

Abstract Experiments and analysis in the 3 years since the 2012 Target Fabrication Meeting have resulted in significant improvement in understanding of the requirements for high-performance layered implosions. Three issues have been identified that significantly degrade the performance of the implosions as they were originally configured for National Ignition Facility experiments: capsule support system, time-dependent radiation asymmetry, and transverse oxygen nonuniformity in the glow discharge polymer (GDP) ablator. Analyses suggest that the shortfalls in these three areas can explain the degraded performance of the National Ignition Campaign implosions. We present the status of work toward curing these three problems in the standard GDP ablator/gold hohlraum configuration as they affect target fabrication priorities. We also summarize the prospects for alternate ablators that might reduce these degradation mechanisms.

Automation of NIF Target Characterization and Laser Ablation of Domes Using the 4pi System

Abstract Capsules for inertial confinement fusion require precise measurement of isolated features and domes on the capsule’s outer surface. Features that are too large must be removed. A 4pi capsule mapping and characterization system has been developed to map, identify, and measure domes using a Leica confocal microscope. An ultraviolet wavelength laser was integrated to laser-ablate the offending domes that exceed the allowable mix mass. Current process methods to remove domes require three different stations in different locations. The 4pi system achieves automated capsule handling, metrology, and laser polishing/ablation of domes on one device without losing track of the capsule’s orientation. The measurement technique and metrology accuracy are compared to patch atomic force microscopy scans and phase-shifting diffraction interferometer measurements with good correlation. The laser polishing method has demonstrated analogous results to the current process methods, but in an automated fashion. Additionally, the 4pi capsule-handling capability of the system has been used to laser-ablate purposeful engineered designs into specialty capsules.

Surface Modification of ICF Target Capsules by Pulsed Laser Ablation

Abstract Topographical modifications of spherical surfaces are imprinted on National Ignition Facility target capsules by extending the capabilities of a recently developed full-surface (4π) laser ablation and mapping apparatus. The laser ablation method combines the precision, energy density, and long reach of a focused laser beam to preimpose sinusoidal modulations on the outside surface of high-density carbon capsules and the inside surface of glow discharge polymer capsules. Sinusoidal modulations described in this paper have submicron to tens of microns vertical scale and wavelengths as small as 30 μm and as large as 200 μm. The modulated patterns are created by rastering a focused laser fired at discrete capsule surface locations for a specified number of pulses. The computer program developed to create these raster patterns uses inputs such as the laser beam intensity profile, the material removal function, the starting surface figure, and the desired surface figure. The patterns are optimized to minimize surface roughness. In this paper, simulated surfaces are compared with actual ablated surfaces measured using confocal microscopy.

Automation of NIF Target Fabrication

Abstract The National Ignition Facility (NIF) is on schedule to increase its shot rate after a congressionally mandated efficiency study was enacted to develop strategies for increasing the number of experiments fielded on NIF. The study set an ambitious goal to double the number of shots over a short 2-year period. Through a variety of higher-efficiency means, NIF has geared up and is on track to meet this goal. General Atomics (GA), as a major target and component supplier for NIF, has pursued a number of higher-efficiency studies and enabled higher-throughput systems on its own in order to meet the target requests for the increased shot rate while maintaining the same high-precision level required for every target. Five automation processes have recently been implemented at GA, adding to a large suite of automated metrology, robotics, and laser-machining capabilities.

Quantitative Defect Analysis of Ablator Capsule Surfaces Using a Leica Confocal Microscope and a High-Density Atomic Force Microscope

Abstract High-density carbon (HDC) is being evaluated as an alternative to the current National Ignition Facility (NIF) point-design ablator material (glow discharge plasma, or GDP, plastic) due to its high density and optimal opacity, which leads to a higher implosion velocity. Chemical-vapor-deposition-coated HDC capsules have a near perfect surface figure but a microscopically rough surface. After polishing, the surface becomes smooth at nanometer scales but has numerous micron-sized surface pits, whose volumes, morphology, and distribution must be quantified to guide NIF target selection. Traditional metrology tools for GDP surface defects, such as the atomic force microscope (AFM) based Spheremapper and a phase-shifting differential interferometer, lack the resolution to characterize these localized features. In this paper, we describe how this metrology challenge is met by developing automated surface metrology solutions based on a high-density (HD) AFM and a Leica confocal microscope. These tools are complementary in nature. HD-AFM has a 0.1-μm spatial resolution and determines the overall shape distortion and pit statistics by tracing great circles on a capsule with high throughput. The Leica confocal microscope maps the two-dimensional (2-D) surface at low magnification to find all large defects that could be missed by HD-AFM. Then, a high magnification scan inspects at a <0.3-μm lateral resolution to characterize the defect volume. These 2-D maps provide an opportunity for modeling the shell performance at the peak implosion velocity, thereby aiding capsule selection. These new and improved metrology tools provide quantitative data for the continual refinement of the NIF specifications for HDC capsules. Finally, we report on the development of a laser ablation tool that, when combined with the Leica confocal microscope, can identify, quantify, and laser-ablate GDP domes that do not meet NIF specifications.

Update 2017 on Target Fabrication Requirements for High-Performance NIF Implosion Experiments

Abstract Experiments and analysis in the 2 years since the 2015 Target Fabrication Meeting have resulted in further evolution of the requirements for high-performance layered implosions. This paper is a status update on the experimental program and supporting modeling, with emphasis on the implications for fabrication requirements. Previous work on the capsule support has continued, with various other support options being explored in experiments and modeling. Work also continues on ablator composition nonuniformities, with important new results from CH experiments on Omega, and the first three-dimensional X-ray transmission measurements of Be capsules on the National Ignition Facility. Work on hohlraums continues to include near-vacuum hohlraums and U hohlraums without a gold lining. Overall, the understanding that has been achieved, along with the progress in fabrication technology, represents good continuing progress toward the goal of fusion in the laboratory.

Evolution of Magnetized Liner Inertial Fusion (MagLIF) Targets

Abstract The Magnetized Liner Inertial Fusion experimental campaign conducted at the University of Rochester’s Laboratory for Laser Energetics has evolved significantly since its start in 2014. Scientific requirements and OMEGA Extended Performance (EP) system technology both have progressed, resulting in necessary and available updates to the target design. These include, but are not limited to, optimizing target dimensions and aspect ratios to maximize survival at desired pressures; coating target components to improve physics diagnosis; precision-machining diagnostic windows along the axis of the target for enhanced diagnostic views; improving fiducial placement reproducibility and reducing subsequent assembly time by 50%; and implementing gas-pressure transducers on the targets. In addition, target fabrication techniques have changed and advanced, allowing for better target reproducibility and decreased assembly time. To date, 11 variations of targets have been fabricated, with successful target fielding ranging from 1- to 20 atm internal pressure and a maximum survivability of 33 atm.

Metrology Feasibility Study in Support of the National Direct-Drive Program

Abstract The 100-Gbar Laser Direct Drive program calls for ablator capsules with no defects larger than 0.5 μm in lateral dimension and fewer than ten defects with lateral dimensions between 0.1 and 0.5 μm. Compared to laser indirect drive capsules, this represents > 10× reduction of defect length scale and >500× reduction in defect number density. This presents major challenges to both fabrication and metrology. In this paper, we will discuss the proof-of-principle work conducted at General Atomics to identify metrology techniques suitable for 100-Gbar target characterization. We present a detailed study of dark-field imaging, laser scatterometry, and environmental scanning electron microscopey. We identify dark-field imaging as the best approach for meeting the 100-Gbar metrology needs.