Ignition Capsule Research Articles

Status of the development of ignition capsules in the U.S. effort to achieve thermonuclear ignition on the national ignition facility

An important component of the U.S. effort to achieve thermonuclear ignition in 2010 on the National Ignition Facility is the development of high quality 2 mm diameter spherical capsules to function as the ablator and contain the cryogenic DT fuel. Three ignition capsule designs have been developed, and detailed fabrication specifications for each design have been established and placed under change control. A research program with activities coordinated mainly between Lawrence Livermore, General Atomics and Los Alamos is underway to demonstrate fabrication of capsules meeting specifications. The point design for ignition campaigns beginning in 2010 is a Cu-doped Be capsule that has a radial gradient in Cu dopant level in the capsule wall. This capsule is being produced by sputter deposition of Be and Cu onto either a hollow glow discharge polymer (GDP) spherical mandrel or a solid spherical mandrel, followed by removal of the mandrel and polishing of the capsule. A key goal in the U.S. is to demonstrate fabrication of this capsule by the end of 2006. Two other ignition capsule designs are also being developed as contingencies to the point design. One contingency design is a GDP capsule that has a radial Ge dopant level in its wall. This capsule is produced by co-deposition of Ge and GDP onto a PAMS mandrel followed by thermal removal of the mandrel. The second contingency design is a uniform Cu-doped Be capsule that is fabricated from high purity fine grain Be0.3at.%Cu alloy using a precision machining route followed by polishing. Ignition targets to be fielded in 2010 will be filled with DT fuel through a small fill hole. Laser drilling capability has been developed and used to drill approximately 5 μm diameter holes through capsule walls for DT filling. Characterization methods necessary for characterizing capsules are being developed.

Laser-based platform for studying material hydrodynamics under heated and shocked conditions

Understanding how target defects and surface finish perturbations affect ignition capsule mixing is a critical goal of the inertial confinement fusion (ICF) community. While initial characterization of these features is essential to understanding the physics of ICF implosions, it is the condition of the features at the time of shock passage that ultimately dictates their impact on capsule performance. The Off-Hugoniot experiment was designed to quantify the evolution of material interfaces under heated and subsequently shocked conditions. The platform uses tin L-shell radiation to uniformly heat an epoxy/foam-layered package. As the epoxy expands into the foam, an independently controlled shock impacts the evolving interface. The resulting hydrodynamics are imaged via x-ray radiography. Beyond the flexibility of independent heating and shock control, the epoxy can be configured with any desired features, such as gaps, chamfers, and single and multimode perturbations. An overview of the experimental platform, data from the ongoing OMEGA campaign, and future plans are presented.

Calculation of re-emission diagnostic in NIF ignition Hohlraum at 1MJ laser energy

Preliminary results of a two-dimensional design study are discussed for a SiO2 foam filled Hohlraum containing a Bi-coated re-emission capsule. The Hohlraum wall consists of a Au–U “cocktail” designed to maximize the amount of x-ray energy produced for the capsule to absorb, given the 1MJ laser energy into the Hohlraum. The foam fill acts to minimize wall expansion while maintaining symmetric drive on the capsule. Various foam densities and laser pointings for most efficient drive are considered. Sensitivities to drive asymmetries during the long “foot” portion of the laser drive are calculated and shown for the re-emission capsule. The foam fill was found to affect the re-emit symmetry much more than a H∕He gas fill. Compensating effects in beam balance or pointing are required to maintain best symmetry. The effect of the diagnostic on the Hohlraum environment with a Cu-doped Be ignition capsule is discussed. Effects of optimal filtering for maximum signal detectability are considered.

Reduced reshock growth in a convergent/divergent system: Effect of reshock strength

The Richtmyer-Meshkov instability creates or seeds hydrodynamic instabilities that will mix cold shell material into the hot DT fuel in an ignition capsule and may prevent ignition. Characteristic of this process is multiple shocks crossing converging interfaces. To mimic this situation, strong converging shocks were created, passed over an unstable interface, reflected by an inner cylinder, and then reshocked the interface. Analysis of the mix width at the unstable surface shows no additional growth, within experimental uncertainty, due to an initially perturbed surface and no dependence on reshock strength.

Calculations of a Fill Hole/Tube in an Ignition Capsule

Filling an ignition capsule through a drilled hole in the ablator is the current approach to fielding an ignition capsule. But it adds an initial defect to the capsule which might grow large enough to affect or even prevent ignition. We present here calculations of the effects of fill tubes and holes for the 1.4 MJ 300 eV BeCu NIF capsule. The code used is the 3D AMR code written by Los Alamos and SAIC, “RAGE”. Several fill tube/hole sizes were tried. Most calculations were made in a planar 2D geometry, providing reliable information on the first part of the implosion before convergence effects become important. A 5 μm diameter hole generates a 25 by 30 μm jet when the main shock breaks out into the DT gas. The mass involved in the jet is insignificant, less than 1/1000 of the hot spot mass. There is no large difference between the jets formed by a plug and a fill tube, before they break out into DT gas. High resolution spherical calculations are still in progress to understand the end of the implosion. Experiments are planned as a support to this study.

Using laser entrance hole shields to increase coupling efficiency in indirect drive ignition targets for the National Ignition Facility

Coupling efficiency, the ratio of the capsule absorbed energy to the driver energy, is a key parameter in ignition target designs. The hohlraum originally proposed for the National Ignition Facility (NIF) [G. H. Miller, E. I. Moses, and C. R. Wuest, Nucl. Fusion 44, S228 (2004)] coupled ∼11% of the absorbed laser energy to the capsule as x rays. Described here is a second generation of the hohlraum target which has a higher coupling efficiency, ∼16%. Because the ignition capsule’s ability to withstand three-dimensional effects increases rapidly with absorbed energy, the additional energy can significantly increase the likelihood of ignition. The new target includes laser entrance hole (LEH) shields as a principal method for increasing coupling efficiency while controlling symmetry in indirect-drive inertial confinement fusion. The LEH shields are high Z disks placed inside the hohlraum on the symmetry axis to block the capsule’s view of the relatively cold LEHs. The LEH shields can reduce the amount of laser energy required to drive a target to a given temperature via two mechanisms: (1) keeping the temperature high near the capsule pole by putting a barrier between the capsule and the pole; (2) because the capsule pole does not have a view of the cold LEHs, good symmetry requires a shorter hohlraum with less wall area. Current integrated simulations of this class of target couple 140kJ of x rays to a capsule out of 865kJ of absorbed laser energy and produce ∼10MJ of yield. In the current designs, which continue to be optimized, the addition of the LEH shields saves ∼95kJ of energy (about 10%) over hohlraums without LEH shields.

Exposure of NIF Relevant Polymeric Samples to Deuterium-Tritium Gas at Elevated Temperature and Pressure

The purpose of the experiments described in this paper was to expose samples of polymeric materials to a mixture of deuterium-tritium (DT) gas at elevated temperature and pressure to investigate the effects (i.e., damage) on the materials. The materials and exposure parameters were chosen to be relevant to proposed uses of similar materials in inertial fusion ignition experiments at the National Ignition Facility. Two types of samples were exposed and tested. The first type consisted of 10 4-lead ribbon cables of fine manganin wire insulated with polyimide. Wires of this type are proposed for use in thermal shimming of hohlraums and the goal of this experiment was to measure the change in electrical resistance of the insulation due to tritium exposure. The second type of sample consisted of 20 planar polymer samples that may be used as ignition capsule materials. The exposure was at 34.5 GPa (5010 psia) and 70°C for 48 h. The change in electrical resistance of the wire insulation will be presented. The results for capsule materials will be presented in a separate paper in this issue.

Progress in the Production of Materials and Fabrication of NIF Beryllium-Copper Ignition Capsules at Los Alamos National Laboratory

Work is underway at Los Alamos National Laboratory to fabricate machined-and-bonded target capsules of Be-6 wt% Cu for the National Ignition Facility. Significant progress has been made in producing material with the desired composition, purity, and homogeneity of composition, by arc melting. This material is thermomechanically processed by equal channel angular extrusion, to break down the coarse ascast structure and refine the grain size, to about 20 μm. Machining with diamond tooling results in a significant improvement of the as-machined roughness, that also results in improved bond strengths. Bonding with a sputtered layer of Al can achieve high strengths with a bond 1.2 μm thick, and thinner bonds are being investigated. Laser-drilled holes and fill-tube counterbores produced by electrodischarge machining appear to be feasible, but will require improvements in specimen positioning.

Liquid Cryogenic Targets for Fast Ignition Fusion

Fast ignition fusion targets require a uniform cryogenic D-T fuel layer for efficient fuel assembly. Uniform beta layering of solid D-T fuel within a fast ignition capsule will be complicated by the presence of a reentrant cone for short-pulse laser access. We discuss an alternative approach to cryogenic fast ignition targets currently being developed at Sandia National Laboratories in which a liquid cryogenic fuel layer is condensed from a low-pressure external gas supply and confined between concentric plastic shells. This concentric-shell cryogenic liquid fuel target concept is particularly well adapted to a hemispherical capsule configuration for single-sided X-ray drive. Liquid cryogenic D-T targets have a number of potential advantages, including greatly reduced system cost, temperature control, fill time, and cryogenic handling requirements, compared to beta-layered D-T targets. The shape and surface quality of the liquid fuel layer is determined entirely by the bounding shells, opening the possibility for simplified fast ignition fusion energy targets. Technology issues for target fabrication are discussed, and radiation-hydrodynamics simulations of liquid fuel capsule performance are presented.

The effects of fill tubes on the hydrodynamics of ignition targets and prospects for ignition

The notion of using a narrow bore fill tube to charge an ignition capsule in situ with deuterium-tritium (DT) fuel is very attractive because it eliminates the need for cryogenic transport of the target from the filling station to the target chamber, and in principle is one way of allowing any material to be considered as an ablator. We are using the radiation hydrocode HYDRA [M. M. Marinak et al., Phys. Plasmas 8, 2275 (2001)] in two dimensions to study the effect of fill tubes on graded copper-doped Be ignition capsule implosions. The capsule is ∼1.1-mm radius and driven at ∼300eV. Fill tubes are made of glass and range in diameter from 10–20μm. These are inserted between 5 and 40μm into the ablator surface, and a glue layer around the capsule ∼2-μm thick is included. The calculations are unusually demanding in that the flow is highly nonlinear from the outset, and very high angular resolution is necessary to capture the initial evolution of the tube, which is complex. Despite this complexity, the net result is that by the time the capsule implosion takes off, a preferred, simple Bessel-like mode is set up that is almost independent of, and much larger than, the initial tube size, and close to the fastest growing mode for the capsule. The perturbation continues to grow during the unstable acceleration phase, and inverts as the capsule begins to stagnate, sending a spike of cold DT into the forming hot spot. In all cases studied the capsule ignites and gives close to clean one-dimensional yield. The principal seed of the perturbation appears to be shielding of the ablator in the close vicinity of the fill tube, and the growth is found to vary linearly with the diameter of the tube. The simulations and results are discussed.

Progress on neutron pinhole imaging for inertial confinement fusion experiments

Neutron imaging provides a powerful diagnostic for understanding the performance of inertial confinement fusion ignition capsules and the drive mechanism imploding them. To achieve the spatial resolution and fielding capability needed at the National Ignition Facility requires a staged approach that simultaneously pushes the limits of extant capabilities while developing new techniques that will extend to the National Ignition Facility regime. To this end, new pinhole assemblies have been designed and fabricated using very high-precision machining equipment. These assemblies have been fielded successfully at Laboratory for Laser Energetics, University of Rochester and have provided impetus for new aperture designs and new ideas for detectors, which are now the limiting element in the system resolution.

Yield and hydrodynamic instability versus absorbed energy for a uniformly doped beryllium 250 eV ignition capsule

A copper doped beryllium ablator capsule design is geometrically scaled from 190 kJ to 600 kJ absorbed energy for use as an ignition capsule driven at 250 eV on the National Ignition Facility [J. A. Paisner, J. D. Boyes, S. A. Kumpan, W. H. Lowdermilk, and M. S. Sorem, Laser Focus World 30, 75 (1994)]. The capsule design was previously optimized for 190 kJ fixed capsule absorbed energy. The optimization is confirmed at 377 kJ. Two-dimensional simulations are reported that determine surface roughness requirements and tolerance to radiative drive asymmetry over this absorbed energy range.

Design of a 250 eV cryogenic ignition capsule for the National Ignition Facility

Optimized performance of a capsule intended to produce ignition on the National Ignition Facility [J. A. Paisner, J. D. Boyes, S. A. Kumpan, W. H. Lowdermilk, and M. S. Sorem, Laser Focus World 30, 75 (1994)] is presented. Performance is optimized, for a 250 eV isotropic drive on a beryllium(copper) ablator, by varying the ablator outside radius, ablator thickness, the concentration of copper dopant in the ablator, and the fuel thickness, while keeping the absorbed energy fixed. Dopant concentration is constrained to be uniform in the ablator. The drive shock timing is adjusted to produce a low entropy implosion for each set of dimensions. The absorbed energy is kept fixed at 190 kJ, which results in the ablator outside radius remaining practically constant, about 0.137 cm. For capsule geometry near that resulting in optimal implosion yield, the absorbed energy depends only slightly on the ablator or fuel thickness. The parameter space of capsule dimensions was searched for central vapor densities of 0.3 and 0.5 mg/cc. Despite the detailed optimization, it is found that the capsule is notably more unstable than comparable capsules with a graded dopant in the ablator, as reported in previous literature.

Direct-drive cryogenic target implosion performance on OMEGA

Layered and characterized cryogenic D2 capsules have been imploded using high-contrast pulse shapes on the 60-beam OMEGA laser at the Laboratory for Laser Energetics [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. These experiments measure the sensitivity of the direct-drive implosion performance to parameters such as the inner-ice-surface roughness, the adiabat of the fuel during the implosion, and the laser power balance. The goal is to demonstrate a high neutron-averaged fuel ρR with low angular variance using a scaled, α∼3 ignition pulse shape driving a scaled all-DT ignition capsule. Results are reported with improvements in target layering and characterization and in laser pointing and target positioning on the OMEGA laser over previous experiments [T. C. Sangster et al., Phys. Plasmas 10, 1937 (2003)]. These capsules have been imploded using up to 23 kJ of 351-nm laser light with an on-target energy imbalance of less than 2% rms, full beam smoothing (1-THz bandwidth, two-dimensional smoothing by spectral dispersion, and polarization smoothing), and new, optimized, distributed phase plates. Pulse shapes include high-adiabat (∼25) square pulses and low-adiabat (<5) shaped pulses. The data from neutron and charged-particle diagnostics, as well as static and time-resolved x-ray images of the imploding core, are compared with one- and two-dimensional numerical simulations. Scaling of target performance to a weighted quadrature of inner-ice roughness at the end of the acceleration phase is investigated.

Low mode surface perturbation tolerance of ignition capsule implosions for the National Ignition Facility

The stability of a capsule intended to produce ignition on the National Ignition Facility (NIF) [J. A. Paisner, J. D. Boyes, S. A. Kumpan, W. H. Lowdermilk, and M. S. Sorem, Laser Focus World 30, 75 (1994)] is examined. Sensitivity to target fabrication defects in spherical harmonics modes L⩽12 is quantified in terms of a mode dependent tolerance to surface perturbations. Simulations of NIF capsule implosions with single mode perturbations on a single surface allow the determination of modal growth factors. Simulations with large initial perturbations were also done to determine how large a final perturbation could be tolerated. Combining the growth factors and tolerances determines specifications on initial perturbations. This allows an estimate of modal tolerance for each capsule surface and thickness variation for a polyimide ablator capsule design with central gas fills of 0.3 or 0.6 mg/cm3.

Production of Fine-Grained Beryllium-6 WT% Copper for Fusion Ignition Capsules by Arc Melting and Equal Channel Angular Extrusion

Beryllium doped with 6 weight % copper is the material of choice for fabrication of target capsules for the National Ignition Facility because of its combination of attractive neutronic, electronic, physical, and mechanical properties. The target capsules are 2 mm in diameter and thin-walled (150 microns) and must meet demanding dimensional specifications. The material must be fine-grained and of low inclusion content. Arc-melted Be-Cu is being produced to eliminate the oxide content that is inevitably present in conventional powdermetallurgy materials. Equal channel angular extrusion (ECAE) is being used to refine the as-cast grain structure. Be-Cu rods produced by the arc-melting process (5 mm in diameter by 30 mm in length) are enclosed in nickel cans with electron-beam welded plugs. The Be-in-Ni billets (9.5 mm in diameter by 45 mm in length) have been processed by ECAE at temperatures from 500 to 750°C in tooling with a 120° angle. Selected samples have been annealed for 1 hour at temperatures from 700 to 775°C. The ECAE processing creates a heavily deformed and finely subdivided structure, and the annealing can produce an equiaxed microstructure with a grain size of approximately 20 μm.

The physics basis for ignition using indirect-drive targets on the National Ignition Facility

The 1990 National Academy of Science final report of its review of the Inertial Confinement Fusion Program recommended completion of a series of target physics objectives on the 10-beam Nova laser at the Lawrence Livermore National Laboratory as the highest-priority prerequisite for proceeding with construction of an ignition-scale laser facility, now called the National Ignition Facility (NIF). These objectives were chosen to demonstrate that there was sufficient understanding of the physics of ignition targets that the laser requirements for laboratory ignition could be accurately specified. This research on Nova, as well as additional research on the Omega laser at the University of Rochester, is the subject of this review. The objectives of the U.S. indirect-drive target physics program have been to experimentally demonstrate and predictively model hohlraum characteristics, as well as capsule performance in targets that have been scaled in key physics variables from NIF targets. To address the hohlraum and hydrodynamic constraints on indirect-drive ignition, the target physics program was divided into the Hohlraum and Laser–Plasma Physics (HLP) program and the Hydrodynamically Equivalent Physics (HEP) program. The HLP program addresses laser–plasma coupling, x-ray generation and transport, and the development of energy-efficient hohlraums that provide the appropriate spectral, temporal, and spatial x-ray drive. The HEP experiments address the issues of hydrodynamic instability and mix, as well as the effects of flux asymmetry on capsules that are scaled as closely as possible to ignition capsules (hydrodynamic equivalence). The HEP program also addresses other capsule physics issues associated with ignition, such as energy gain and energy loss to the fuel during implosion in the absence of alpha-particle deposition. The results from the Nova and Omega experiments approach the NIF requirements for most of the important ignition capsule parameters, including drive temperature, drive symmetry, and hydrodynamic instability. This paper starts with a review of the NIF target designs that have formed the motivation for the goals of the target physics program. Following that are theoretical and experimental results from Nova and Omega relevant to the requirements of those targets. Some elements of this work were covered in a 1995 review of indirect-drive [J. D. Lindl, “Development of the indirect-drive approach to inertial confinement fusion and the target physics basis for ignition and gain,” Phys. Plasmas 2, 3933 (1995)]. In order to present as complete a picture as possible of the research that has been carried out on indirect drive, key elements of that earlier review are also covered here, along with a review of work carried out since 1995.

Preheat effects on shock propagation in indirect-drive inertial confinement fusion ablator materials.

The velocities and temperatures of shock waves generated by laser-driven hohlraum radiation fields have been measured for several indirect-drive inertial confinement fusion capsule ablator materials. For the first time, a time-resolved measurement of the preheat temperature ahead of the shock front has been performed and included in the analysis. It is found that preheat ahead of the shock front can cause significant shock propagation variations in the ignition capsule ablator materials being considered for the National Ignition Facility (NIF). If unaccounted for, these preheat effects could potentially preclude ignition at the NIF.

Degradation of radiatively driven inertial confinement fusion capsule implosions by multifluid interpenetration mixing

The observed degradation of radiatively driven single shell capsule yield with increasing convergence ratio can be quantitatively understood by mixing between the gaseous deuterium-tritium (DT) or deuterium (DD) and the pusher. Calculations using a multifluid interpenetration mix model can replicate the performance of experiments at the NOVA [C. Bibeau et al., Appl. Opt. 31, 5799 (1992)] and OMEGA [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] lasers, using a single fitting parameter, a mixing length ratio. When mix is similarly treated at the ablator–solid DT interface, cryogenic single shell ignition capsules for the National Ignition Facility [J. A. Paisner, E. M. Campbell, and W. J. Hogan, Fusion Technol. 26, 755 (1994)] and the Laser Mega Joule [ P. A. Holstein et al., Laser Part. Beams 17, 403 (1999)] calculate to ignite and burn with gains of several due to isolation of the ignition hot spot from the pusher material.

Direct-drive cryogenic target implosion performance on OMEGA

Layered and characterized cryogenic D2 capsules have been imploded using both low- and high-adiabat (α, the ratio of the electron pressure to the Fermi-degenerate pressure) pulse shapes on the 60-beam OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] at the Laboratory for Laser Energetics (LLE). These experiments measure the sensitivity of the direct-drive implosion performance to parameters such as the inner-ice-surface roughness, the adiabat of the cryogenic fuel during the implosion, the laser power balance, and the single-beam nonuniformity. The goal of the direct-drive program at LLE is to demonstrate a high neutron-averaged fuel ρR at a significant fraction of the predicted one-dimensional (1-D) neutron yield using an energy-scaled, low-adiabat (α∼3) ignition pulse shape driving a hydrodynamically scaled deuterium–tritium ignition capsule. New results are reported from implosions of ∼920-μm-diam, thin (∼5 μm) polymer shells containing 100 μm D2-ice layers with characterized inner-surface ice roughness of 3–12 μm rms. These capsules have been imploded using ∼17–23 kJ of 351 nm laser light with a beam-to-beam rms energy imbalance of less than 5% and full beam smoothing [1 THz bandwidth, two-dimensional (2-D) smoothing by spectral dispersion and polarization smoothing]. Near-1-D performance has been measured for a high-adiabat (α∼25) drive pulse, and the implosion performance with a low-adiabat (α∼4) pulse is in agreement with 2-D hydrocode predictions.