Quantification of Changes in Fill Tube Curvature During Target Assembly

Inertial confinement fusion capsules fielded at the National Ignition Facility are filled with deuterium and tritium fuel by means of a fill tube. The fill tube introduces a low-density pathway into the fuel region of the capsule that allows high Z contaminant to invade the hot spot during the course of the implosion. A recent series of nominally identical high-yield implosions on the NIF has exhibited significant variability in performance. We evaluate the impact of the fill tube in these implosions computationally to determine whether variations in fill tube geometry could have contributed to this variability. The main contrast between the fill tube geometry in the six shots was the outer diameter of the capsule bore hole, a conical hole into which the fill tube is inserted. In our simulations, the geometry of the bore hole can play a significant role in the development of nonlinear flows seeded by the fill tube. We find that the amount of space between the bore hole and the fill tube is the primary factor that determines the amount of contaminant jetted into the hot spot by the fill tube and, in turn, the level of yield reduction due to the fill tube in our simulations. As a consequence, some capsules with 5 μm fill tubes are predicted to outperform capsules with 2 μm fill tubes. We also find that micrometer-scale changes to bore hole size can impact fusion yields by up to four times near the ignition threshold. Nevertheless, simulation trends do not reproduce experimental yield trends, suggesting that the fill tube geometry was not the primary factor contributing to the observed variability in performance and that the fill tube could be masking sensitivity to other asymmetries such as other micrometer-scale capsule defects like voids that were not included in our simulations.

Inertial confinement fusion (ICF) experiments at the National Ignition Facility (NIF) require cryogenic targets at the 1-cm scale to be fabricated, assembled, and metrologized to micron-level tolerances. During assembly of these ICF targets, there are physical dimension metrology steps to be made of the components, subassemblies, and completed targets. Metrology is primarily completed using optical coordinate measurement machines that provide repeatable measurements with micron precision, while also allowing in-process data collection for absolute accuracy in assembly. To date, 51 targets have been assembled and metrologized, and 34 targets have been successfully fielded on NIF relying on these metrology data. In the near future, ignition experiments on NIF will require tighter tolerances and more demanding target assembly and metrology capability.Metrology methods, calculations, and uncertainty estimates will be discussed. Target diagnostic port alignment, target position, and capsule location results will be reviewed for the 2009 Energetics Campaign. The information is presented via control charts showing the effect of process improvements that were made during target production. Certain parameters, including capsule position, met the 2009 campaign specifications but will have much tighter requirements in the future. To meet these new requirements assembly process changes and metrology capability upgrades will be necessary.

Steady progress is being made in inertial confinement fusion experiments at the National Ignition Facility (NIF). Nonetheless, substantial further progress is still needed to reach the ultimate goal of fusion ignition. Closing the remaining gap will require either improving the quality of current implosions, increasing the implosion scale (and correspondingly the energy delivered by NIF), or some combination of the two. But how much of an improvement in implosion quality or energy scale is required to reach ignition? To reliably answer this question, an accurate understanding of current and past experiments is first required. Previous modeling efforts of NIF implosions have shown the need to resolve a wide range of scales (from microns to millimeters) as well as a faithful representation of the genuinely three-dimensional (3D) character of the stagnation process. Modeling NIF implosions is further complicated by the many perturbation sources that have been found to influence integrated implosion performance: flux asymmetries from the surrounding hohlraum, engineering features such as support tents and fill tubes, surface defects and contaminants, and more recently the radiation shadow cast by the fill tube on the capsule. A model including all of these effects, and with adequate resolution, challenges current computing capabilities but has recently become feasible on the largest computers. This paper reviews the status of these multi-effect, 3D simulations of NIF implosions, their comparison to experimental data, and preliminary results on scaling these simulations to the threshold of ignition on NIF.

The National Ignition Facility (NIF) is a 192 beam Nd-glass laser facility presently under construction at Lawrence Livermore National Laboratory (LLNL) for performing inertial confinement fusion (ICF) and experiments studying high energy density (HED) science. When completed in 2009, NIF will be able to produce 1.8 MJ, 500 TW of ultraviolet light for target experiments that will create conditions of extreme temperatures (>108 K), pressures (10 GBar) and matter densities (>100 g/cm3). A detailed program called the National Ignition Campaign (NIC) has been developed to enable ignition experiments in 2010, with the goal of producing fusion ignition and burn of a deuterium-tritium (DT) fuel mixture in millimeter-scale target capsules. The first of the target experiments leading up to these ignition shots will begin in 2008. The targets for the NIC are both complex and precise, and are extraordinarily demanding in materials fabrication, machining, assembly, cryogenics and characterization.The DT fuel is contained in a 2-millimeter-diameter graded copper/beryllium or CH shell. The 75-μm-thick cryogenic ice DT fuel layer is formed to sub-micron uniformity at a temperature of approximately 18 Kelvin. The capsule and its fuel layer sit at the center of a gold/depleted uranium ‘cocktail’ hohlraum. Researchers at LLNL have teamed with colleagues at General Atomics to lead the development of the technologies, engineering design and manufacturing infrastructure necessary to produce these demanding targets. We are also collaborating with colleagues at the Laboratory for Laser Energetics (LLE) at the University of Rochester in DT layering, and at Fraunhofer in Germany in nano-crystalline diamond as an alternate ablator to Beryllium and CH.The Beryllium capsules and cocktail hohlraums are made by physical vapor deposition onto sacrificial mandrels. These coatings must have high density (low porosity), uniform microstructure, low oxygen content and low permeability. The ablator capsule has a 5-μm-diameter hole laser drilled to permit removal of the mandrel and introduction of the DT fuel. A 10-μm-diameter fill tube is bonded to the capsule to enable filling with the DT gas. These components must then be assembled to tolerances of approximately 5—10 microns, with comprehensive characterization and metrology. The DT ice is formed through controlled seeding, aided by beta decay of the tritium to help smooth the layer, and differential heating of the hohlraum to counteract the effects of natural convection.We present an overview of the technologies for target fabrication, assembly and metrology and advances in growth and imaging of DT ice layers. The sum of these efforts represents a quantum leap in target precision, characterization, manufacturing rate and flexibility over current state-of-the-art.This work performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-48.

Modifications to the National Ignition Facility (NIF) Cryogenic Target Positioner (Cryo-TarPos) are needed to provide polar-drive-ignition targets; ideally, these modifications will be completed and tested by 2017, the earliest date anticipated for polar-drive-ignition experiments. The extent of these modifications is defined by the mechanical and thermal requirements needed for the target to conform to the ignition design and the capabilities of the existing equipment. This paper describes the design of the polar-drive target assembly and the surrounding cryogenic environment that meets many of the specifications and requirements for the ignition target. Further work is necessary to optimize the design and provide more-detailed guidance for modifying the NIF Cryo-TarPos; however, there is sufficient information to begin the redesign effort at the conceptual level.A specialized facility has been constructed to test different target assembly and cryogenic hardware designs. The equipment provides the mechanical and cryogenic functionality available at the NIF, making it possible to test different target designs with deuterium in a configuration suitable for integration with the NIF Cryo-TarPos. The polar-drive target assembly has demonstrated a stable ice layer (170 to 350 μm thick) and the ability to control the thickness to ±3 μm of the desired value. The target is rotatable to fully characterize the D2 ice surface using shadowgraphy and X-ray phase contrast. Thermal models of the target and its environment indicate that (a) it should be possible to achieve the desired 1-μm root-mean-square smoothness using D-T, (b) the fill tube has little effect on the ice smoothness, and (c) it is possible to shape the isotherms surrounding the target sufficiently to form an oblate ice layer that may be more desirable for polar-drive implosions.

Fill tubes are used to inject deuterium and tritium fuel into inertial confinement fusion capsules fielded on the National Ignition Facility. These fill tubes have been shown to have a detrimental effect on capsule performance, primarily by introducing a low-density pathway into the central fuel region that enables the jetting of ablation material into the hot spot. Due to the complexity of the highly nonlinear flow associated with the fill tube and the challenge of diagnosing the evolution of the fill tube jet late in the implosion experiments, the uncertainty in how this perturbation source evolves is great. Here, we report on the results of a detailed code comparison performed to understand uncertainties in computational modeling of the impact of fill tubes on implosion performance. The study employed two radiation-hydrodynamics codes, HYDRA and xRAGE, which employ very different meshing strategies and hydrodynamics solvers, as well as two radiation transport methodologies, discrete ordinates and multi-group diffusion. Our results demonstrate generally good agreement between codes through most of the implosion although they indicate sensitivity to opacity averaging methods. Late in the implosion, differences arise in the distribution and amount of contaminant although these differences have a remarkably small impact on the amount of yield reduction due to the fill tube. While these results demonstrate sensitivity in fill tube modeling to algorithmic choices, the observed differences between codes are small relative to known sensitivities due to expected variations in the fill tube geometry. Finally, we have developed a methodology for performing multi-group diffusion simulations that show good agreement with the more accurate discrete ordinates method.

Fill tubes are being implemented to meet direct-drive National Ignition Facility (NIF) target designs and eliminate the need for permeation filling of targets. Significant improvements have been made to the fill tube designs for the NIF-scale CD and fast ignition targets to accommodate fuel-layering experiments at the University of Rochester Laboratory for Laser Energetics. The initial fill tube design had a number of issues that contributed to the nonuniformity of the deuterium (D2) ice layer and low fabrication yield of targets. Redesign of the entire target has significantly improved the D2 ice layering by reducing thermal perturbations. These design changes also made a more robust target that can survive the handling required in fabrication and testing. This paper will detail the target design aspects that were altered, including adjusting the fill tube aspect ratio, removing the thermally conductive support stalk, and adding a thermally conductive coating on the fill tube.

Hydrodynamic instability growth of the capsule support membranes (or “tents”) and fill tubes has been studied in spherical, glow discharge polymer plastic capsule implosions at the National Ignition Facility (NIF) [Campbell et al., AIP Conf. Proc. 429, 3 (1998)]. In NIF implosions, the capsules are supported by tents because the nominal 10-μm thick fill tubes are not strong enough to support capsules by themselves. After it was recognized that the tents had a significant impact of implosion stability, new support methods were investigated, including thicker, 30-μm diameter fill tubes and cantilevered fill tubes, as described in this article. A new “sub-scale” version of the existing x-ray radiography platform was developed for measuring growing capsule perturbations in the acceleration phase of implosions. It was calibrated using hydrodynamic growth measurements of pre-imposed capsule modulations with Legendre modes of 60, 90, 110, and 140 at convergence ratios up to ∼2.4. Subsequent experiments with 3-D perturbations have studied instability growth of 10-μm and 30-μm thick fill tubes to compare them with 30-nm thick tent perturbations at convergence ratios up to ∼3. In other experiments, the perturbations from cantilevered fill tubes were measured and compared to the tent perturbations. The cantilevered fill tubes were supported by 12-μm thick SiC rods, offset by 100 μm, 200 μm, and 300 μm from the capsule surfaces. Based on these experiments, 30-μm thick fill tubes and 300-μm offset cantilevered fill tubes were recommended for further tests using layered deuterium-tritium implosions. The effects of x-ray shadowing during the drive and oxygen-induced perturbations during target assembly produced additional seeds for instabilities and were also measured in these experiments.

Inertial Confinement Fusion (ICF) experiments at the National Ignition Facility (NIF) have unveiled a wealth of new insights into the physics of laser-plasma interaction (LPI) in the presence of many laser beams. Besides their crucial impact on ICF experiments, these findings have also revealed new ways in which LPI processes can alter the fundamental optical properties of plasmas (optical anisotropy, refractive index modifications, dispersion etc.). These findings have led to the proposal of new ideas to manipulate light using plasmas, including conceptual designs for new plasma-based optical systems such as plasma polarizers, wave plates and dielectric mirrors. Compared to traditional (crystal-based) optics systems, such plasma-based photonics devices essentially alleviate the constraints of optics damage, and can manipulate laser beams at fluences millions of times above the damage threshold of solid-state optics. In this presentation, some of the main recent findings on multi-beams LPI in NIF experiments will be reviewed, as well as how their analysis has led us to pursue new research in “plasma photonics”. Recent and on-going experiments at the Jupiter Laser Facility (LLNL) to test and validate these new schemes will also be presented. A tunable plasma waveplate was recently demonstrated (D. Turnbull et al., submitted), and was used to produce near-ideal circular polarization of an intense laser beam. Follow-up experiments aimed at demonstrating a plasma-based polarizer and dielectric mirror are currently in preparation.

Measurements of hydrodynamic instability growth for a high-density carbon ablator for indirectly driven inertial confinement fusion implosions on the National Ignition Facility are reported. We observe significant unexpected features on the capsule surface created by shadows of the capsule fill tube, as illuminated by laser-irradiated x-ray spots on the hohlraum wall. These shadows increase the spatial size and shape of the fill tube perturbation in a way that can significantly degrade performance in layered implosions compared to previous expectations. The measurements were performed at a convergence ratio of ∼2 using in-flight x-ray radiography. The initial seed due to shadow imprint is estimated to be equivalent to ∼50-100 nm of solid ablator material. This discovery has prompted the need for a mitigation strategy for future inertial confinement fusion designs as proposed here.

The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory is a stadium-sized facility that, when completed in 2008, will contain a 192-beam, 1.8-megajoule, 500-terawatt, ultraviolet laser system together with a 10-m-diam target chamber and room for 100 diagnostics. NIF is the world's largest and most energetic laser ex- perimental system and will provide a scientific center to study inertial confinement fusion and matter at extreme energy densities and pres- sures. NIF's energetic laser beams will compress fusion targets to con- ditions required for thermonuclear burn, liberating more energy than re- quired to initiate the fusion reactions. Other NIF experiments will study physical processes at temperatures approaching 10 8 K and 10 11 bar, conditions that exist naturally only in the interior of stars and planets. NIF has completed the first phases of its laser commissioning program. The first four beams of NIF have generated 106 kJ in 23-ns pulses of infrared light and over 16 kJ in 3.5-ns pulses at the third harmonic (351 nm). NIF's target experimental systems are being commissioned and experi- ments have begun. This work provides a detailed look at the NIF laser systems, laser and optical performance, and results from recent laser commissioning shots. We follow this with a discussion of NIF's high- energy-density and inertial fusion experimental capabilities, the first ex- periments on NIF, and plans for future capabilities of this unique facility. © 2004 Society of Photo-Optical Instrumentation Engineers. (DOI: 10.1117/1.1814767) Subject terms: high-energy-density physics; inertial confinement fusion; labora- tory astrophysics; solid-state lasers.

For the various tuning as well as ignition campaigns, targets on the National Ignition Facility (NIF) need to be filled with gases, typically with the different isotopes of H2 and He. Fill tubes that supply the two small chambers in the target, the capsule and the hohlraum, are microcapillaries that are only tens of microns in diameter and present significant impedance to flow. Knowledge of the exact pressures and gas compositions in the capsule and the hohlraum is critical for fielding targets on NIF. This requires modeling of the gas flow through the capillary tubes, at both room temperature and cryogenic temperatures. We present results from a comprehensive model and its experimental verification for a range of conditions such as temperature and pressure.

Target is one of the essential parts in inertial confinement fusion (ICF) experiments. To ensure the symmetry and hydrodynamic stability in the implosion, there are stringent specifications for the target. Driven by the need to fabricate the target required by ICF experiments, a series of target fabrication techniques, including capsule fabrication techniques and the techniques of target characterization and assembly, are developed by the Research Center of Laser Fusion (RCLF), China Academy of Engineering Physics (CAEP). The capsule fabrication techniques for preparing polymer shells, glow discharge polymer (GDP) shells and hollow glass micro-sphere (HGM) are studied, and the techniques of target characterization and assembly are also investigated in this paper. Fundamental research about the target fabrication is also done to improve the quality of the target. Based on the development of target fabrication techniques, some kinds of target have been prepared and applied in the ICF experiments.

The National Ignition Facility (NIF), the world's largest and most powerful laser system for inertial confinement fusion (ICF) and experiments studying high energy density (HED) science, is nearing completion at Lawrence Livermore National Laboratory (LLNL). NIF is a 192-beam Nd-glass laser facility that will produce 1.8 MJ, 500 TW of ultraviolet light, making it over fifty times more energetic than present ICF facilities. The NIF Project, begun in 1995, is over 90% complete and is scheduled for completion in 2009. The building and the entire beam path have been completed. The Project is presently installing the optics and electronics to build out the beams and is commissioning them in the laser bays. By September 2007, all of the lasers in one of the two laser bays will be commissioned with the capability of producing over 2 MJ of 1ω (1.05 μm) light, making NIF the world's first mega Joule laser system. A year later, the laser system will be essentially complete. Experiments using one beam in the Precision Diagnostic System (PDS) have shown that NIF can meet all of its 3ω light performance goals including energy, power, focusing, and shot rate and has the precision and accuracy required for ignition pulse performance. The plan is to have half of the beams commissioned to the target chamber in a symmetric geometry to begin 96-beam symmetric indirect-drive experiments. These first ICF experiments using more than 200 kJ of 3ω light will have an order of magnitude more energy than presently available and represent the beginning of experiments preparing for ignition. This national effort for ignition experiments is coordinated through a detailed plan called the National Ignition Campaign (NIC) that includes the science, technology, and equipment such as diagnostics, cryogenic target manipulator, and user optics required for ignition experiments. The goal is to have all of the equipment operational and integrated into the facility soon after Project completion to begin ignition experiments in 2010. In addition, experiments will begin to investigate HED science for defense and basic science applications. With over 50 times more energy than present facilities and the ability to produce ignition, NIF will explore new physics regimes. Following project completion in 2009, facility time at NIF will be allocated to the broad user community using the process outlined in a formal governance plan. A NIF User Office has been established to coordinate use of NIF by the national security and other user communities.

X-ray penumbral imaging has been successfully fielded on a variety of inertial confinement fusion (ICF) capsule implosion experiments on the National Ignition Facility (NIF). We have demonstrated sub-5 μm resolution imaging of stagnated plasma cores (hot spots) at x-ray energies from 6 to 30 keV. These measurements are crucial for improving our understanding of the hot deuterium-tritium fuel assembly, which can be affected by various mechanisms, including complex 3-D perturbations caused by the support tent, fill tube or capsule surface roughness. Here we present the progress on several approaches to improve x-ray penumbral imaging experiments on the NIF. We will discuss experimental setups that include penumbral imaging from multiple lines-of-sight, target mounted penumbral apertures and variably filtered penumbral images. Such setups will improve the signal-to-noise ratio and the spatial imaging resolution, with the goal of enabling spatially resolved measurements of the hot spot electron temperature and material mix in ICF implosions.