T. Bunn

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.

Planarization of Isolated Defects on ICF Target Capsule Surfaces by Pulsed Laser Ablation

Abstract Demanding surface-quality requirements for inertial confinement fusion (ICF) capsules motivated the development of a pulsed laser ablation method to reduce or eliminate undesirable surface defects. The pulsed laser ablation technique takes advantage of a full surface (4π) capsule manipulation system working in combination with an optical profiling (confocal) microscope. Based on the defect topography, the material removal rate, and the laser pulse energy and its beam profile, a customized laser raster pattern is derived to remove the defect. The pattern is a table of coordinates and number of pulses that dictate how the defect will be vaporized until its height is level with the capsule surface. This paper explains how the raster patterns are optimized to minimize surface roughness and how surface roughness after laser ablation is simulated. The simulated surfaces are compared with actual ablated surfaces. Large defects are reduced to a size regime where a tumble-finishing process produces very high-quality surfaces devoid of high mode defects. The combined polishing processes of laser ablation and tumble finishing have become routine fabrication steps for National Ignition Facility capsule production.

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.

Hydrodynamic instabilities seeded by the X-ray shadow of ICF capsule fill-tubes

During the first few hundred picoseconds of indirect drive for inertial confinement fusion on the National Ignition Facility, x-ray spots formed on the hohlraum wall when the drive beams cast shadows of the fuel fill-tube on the capsule surface. Differential ablation at the shadow boundaries seeds perturbations which are hydrodynamically unstable under subsequent acceleration and can grow to impact capsule performance. We have characterized this shadow imprint mechanism and demonstrated two techniques to mitigate against it using (i) a reduced diameter fuel fill-tube, and (ii) a pre-pulse to blow down the fill-tube before the shadow forming x-ray spots from the main outer drive beams develop.

Platinum Electrodeposition for Supported ALD Templated Foam Hohlraum Liners

Abstract Two commercially available platinum plating solutions (Platanex III and Platanex Luna) were evaluated for the electrodeposition of platinum layers on gold hohlraums and cylindrically shaped silver-gold ingots. The successful deposition of thin Pt layers on gold hohlraums as well as thick Pt layers on silver-gold alloys will allow for the integration of atomic layer deposition templated foam inside a hohlraum. It was found that when the manufacturer’s recommendations for the Pt plating solutions were used, the coatings obtained were unacceptable because of cracking, poor adhesion, or thin and powdery Pt deposits. Therefore, alternative plating parameters were investigated, and the conditions resulting in acceptable coatings are reported here.

Metal Alloy ICF Capsules Created by Electrodeposition

Abstract Electrochemical deposition is an attractive alternative to physical vapor deposition and micromachining to produce metal capsules for inertial confinement fusion (ICF). Electrochemical deposition (also referred to as electrodeposition or plating) is expected to produce full-density metal capsules without seams or inclusions of unwanted atomic constituents, the current shortcomings of micromachine and physical vapor deposition, respectively. Here, we discuss new cathode designs that allow for the rapid electrodeposition of gold and copper alloys on spherical mandrels by making transient contact with the constantly moving spheres. Electrodeposition of pure gold, copper, platinum, and alloys of gold-copper and gold-silver are demonstrated, with nonporous coatings of >40 µm achieved in only a few hours of plating. The surface roughness of the spheres after electrodeposition is comparable to the starting mandrel, and the coatings appear to be fully dense with no inclusions. A detailed understanding of the electrodeposition conditions that result in different alloy compositions and plating rates will allow for the electrodeposition of graded alloys on spheres in the near future. This report on the electrodeposition of metals on spherical mandrels is an important first step toward the fabrication of graded-density metal capsules for ICF experiments at the National Ignition Facility.

Hydrodynamic Instability Measurements in DT-Layered ICF Capsules Using the Layered-HGR Platform

The first measurements of hydrodynamic instability growth at the fuel-ablator interface in an ICF implosion are reported. Previous instability measurements on the National Ignition Facility have used plastic capsules to measure ablation front Rayleigh-Taylor growth with the Hydro.-Growth Radiography (HGR) platform. These capsules substituted an additional thickness of plastic ablator material in place of the cryogenic layer of Deuterium- Tritium (DT) fuel. The present experiments are the first to include a DT ice layer, which enables measurements of the instability growth occurring at the fuel-ablator interface. Instability growth at the fuel-ablator interface is seeded differently in two independent NIF experiments. In the first case, a perturbation on the outside of the capsule feeds through and grows on the interface. Comparisons to an implosion without a fuel layer produce a measure of the fuels modulation. In the second case, a modulation was directly machined on the inner ablator before the fuel layer was added. The measurement of growth in these two scenarios are compared to 2D rad-hydro modeling.