In order to achieve the several hundred Gbar stagnation pressures necessary for inertial confinement fusion ignition, implosion experiments on the National Ignition Facility (NIF) [E. I. Moses et al., Phys. Plasmas 16, 041006 (2009)] require the compression of deuterium-tritium fuel layers by a convergence ratio as high as forty. Such high convergence implosions are subject to degradation by a range of perturbations, including the growth of small-scale defects due to hydrodynamic instabilities, as well as longer scale modulations due to radiation flux asymmetries in the enclosing hohlraum. Due to the broad range of scales involved, and also the genuinely three-dimensional (3D) character of the flow, accurately modeling NIF implosions remains at the edge of current simulation capabilities. This paper describes the current state of progress of 3D capsule-only simulations of NIF implosions aimed at accurately describing the performance of specific NIF experiments. Current simulations include the effects of hohlraum radiation asymmetries, capsule surface defects, the capsule support tent and fill tube, and use a grid resolution shown to be converged in companion two-dimensional simulations. The results of detailed simulations of low foot implosions from the National Ignition Campaign are contrasted against results for more recent high foot implosions. While the simulations suggest that low foot performance was dominated by ablation front instability growth, especially the defect seeded by the capsule support tent, high foot implosions appear to be dominated by hohlraum flux asymmetries, although the support tent still plays a significant role. For both implosion types, the simulations show reasonable, though not perfect, agreement with the data and suggest that a reliable predictive capability is developing to guide future implosions toward ignition.

Three-dimensional simulations of low foot and high foot implosion experiments on the National Ignition Facility

More than two dozen inertial confinement fusion ignition experiments with cryogenic deuterium-tritium layers have now been performed on the National Ignition Facility (NIF) [G. H. Miller et al., Opt. Eng. 443, 2841 (2004)]. Each of these yields a wealth of data including neutron yield, neutron down-scatter fraction, burn-averaged ion temperature, x-ray image shape and size, primary and down-scattered neutron image shape and size, etc. Compared to 2-D radiation-hydrodynamics simulations modeling both the hohlraum and the capsule implosion, however, the measured capsule yield is usually lower by a factor of 5 to 10, and the ion temperature varies from simulations, while most other observables are well matched between experiment and simulation. In an effort to understand this discrepancy, we perform detailed post-shot simulations of a subset of NIF implosion experiments. Using two-dimensional HYDRA simulations [M. M. Marinak, et al., Phys. Plasmas 8, 2275 (2001).] of the capsule only, these simulations represent as accurately as possible the conditions of a given experiment, including the as-shot capsule metrology, capsule surface roughness, and ice layer defects as seeds for the growth of hydrodynamic instabilities. The radiation drive used in these capsule-only simulations can be tuned to reproduce quite well the measured implosion timing, kinematics, and low-mode asymmetry. In order to simulate the experiments as accurately as possible, a limited number of fully three-dimensional implosion simulations are also being performed. Despite detailed efforts to incorporate all of the effects known and believed to be important in determining implosion performance, substantial yield discrepancies remain between experiment and simulation. Some possible alternate scenarios and effects that could resolve this discrepancy are discussed.

Detailed implosion modeling of deuterium-tritium layered experiments on the National Ignition Facilitya)

The recently completed National Ignition Campaign (NIC) on the National Ignition Facility (NIF) showed significant discrepancies between post-shot simulations of implosion performance and experimentally measured performance, particularly in thermonuclear yield. This discrepancy between simulation and observation persisted despite concerted efforts to include all of the known sources of performance degradation within a reasonable two-dimensional (2-D), and even three-dimensional (3-D), simulation model, e.g., using measured surface imperfections and radiation drives adjusted to reproduce observed implosion trajectories [Clark et al., Phys. Plasmas 20, 056318 (2013)]. Since the completion of the NIC, several effects have been identified that could explain these discrepancies and that were omitted in previous simulations. In particular, there is now clear evidence for larger than anticipated long-wavelength radiation drive asymmetries and a larger than expected perturbation seeded by the capsule support tent. This paper describes an updated suite of one-dimensional (1-D), 2-D, and 3-D simulations that include the current best understanding of these effects identified since the NIC, as applied to a specific NIC shot. The relative importance of each effect on the experimental observables is compared. In combination, these effects reduce the simulated-to-measured yield ratio from 125:1 in 1-D to 1.5:1 in 3-D, as compared to 15:1 in the best 2-D simulations published previously. While the agreement with the experimental data remains imperfect, the comparison to the data is significantly improved and suggests that the largest sources for the previous discrepancies between simulation and experiment are now being included.

Radiation hydrodynamics modeling of the highest compression inertial confinement fusion ignition experiment from the National Ignition Campaign

The goal of an inertially confined, igniting plasma on the National Ignition Facility (NIF) [M. L. Spaeth, Fusion Sci. Technol. 69, 25 (2016)] remains elusive. However, there is a growing understanding of the factors that appear to be limiting current implosion performance. And with this understanding, the question naturally arises: What conditions will ultimately be required to achieve ignition, either by continuing to improve the quality of current implosions, or by hydrodynamically scaling those implosions to larger driver energies on some future facility? Given the complexity of NIF implosions, answering this question must rely heavily on sophisticated numerical simulations. In particular, those simulations must respect the three-dimensionality of real NIF implosions and also resolve the wide range of scales for the many perturbation sources that degrade them. This prospectus article reviews the current state of detailed modeling of NIF implosions, the scaling to ignition from recent experiments that that modeling implies, and areas for future improvements in modeling technique that could increase understanding and further enhance predictive capabilities. Given the uncertainties inherent in any extrapolation, particularly for a process as nonlinear as ignition, there will be no definitive answer on the requirements for ignition until it is actually demonstrated experimentally. However, with continuing improvements in modeling technique and a growing experience base from NIF, the requirements for ignition are becoming clearer.

Three-dimensional modeling and hydrodynamic scaling of National Ignition Facility implosions

Two sets of identical tungsten (W) targets are irradiated at 300 K with 12.3 MeV W4+ ions to peak damage levels ranging from 0.5 to 10 displacements per atom (dpa). This results in a damage profile that is peaked at ~0.8 µm and extends to a depth of ~1.5 µm. Both sets of targets are exposed to high-density (ne,center = 3 × 1020 m−3), low-temperature (Te,center = 1.6 eV) deuterium (D) plasma in Pilot-PSI. One set of irradiated targets is exposed at high surface temperatures (TW = 950–680 K) and the other at low surface temperatures (TW = 480–340 K). The surface temperature is determined by the local plasma conditions. Nuclear reaction analysis (NRA) is used to determine the D depth profiles at specific radial locations, thus giving a surface temperature scan of the D retention in the damaged W. Global retention is determined by thermal desorption spectroscopy, which yields total D retained in the target and also gives information of the different types of lattice defects that are trapping the D in the W lattice. The main results are that there is no measurable difference between the different dpa levels, implying a saturation of the retention enhancement at a level ≤0.5 dpa. For both irradiated and unirradiated tungsten, a peak in the retention is seen at TW = 480 K; however, the W4+ irradiation clearly enhances the retention. This enhancement is also temperature dependent and increases with increasing surface temperature up to an enhancement by a factor of 15–23 at TW = 950 K. At the lowest surface temperatures, a fluence dependence appears since the implanted deuterium is diffusion limited to only a small fraction of the irradiated zone. TDS spectra show an enhancement of both low-energy trap sites and high-energy trap sites. For these conditions, diffusion-limited, low fill fraction trapping determines the hydrogenic retention of the W.

/pdf/hydrogenic-retention-in-irradiated-tungsten-exposed-to-high-pqlnz1hyp1.pdf

Hydrogenic retention in irradiated tungsten exposed to high-flux plasma

Recently, we developed a new method for making spherical poly(α-methylstyrene) (PαMS) mandrels. The process utilizes a small amount (<0.1wt%) of high-molecular-weight poly(acrylic acid) (PAA) in th...

Development of High Quality Poly(α-Methylstyrene) Mandrels for NIF

Inertial confinement fusion shells have previously been evaluated on the basis of microscopic examination for local defects and limited surface profiling to represent their average fluctuation powe...

/pdf/complete-surface-mapping-of-icf-shells-23p4mj19io.pdf

Complete Surface Mapping of ICF Shells

We have developed a mathematical algorithm to reconstruct the entire ICF shell surface to 10 nm accuracy. Mode 0 and 1 information is inherently missing in the Spheremapper measurements and must be retrieved systematically by minimizing the radius discrepancy at trace intersections. The R(θ,ϕ) map with 1 deg. angular resolution can be constructed by this method to give the target designer a way to model implosions, to give the process engineer feedback and to give the customer a tool for product visualization

3D Surface Reconstruction of ICF Shells after Full Surface Spheremapping

Abstract Currently, poly(α-methylstyrene) mandrel precursors are suspended in a salt-containing water bath during curing. Matching the density of the precursors to their bath almost, but not quite, eliminates shell sagging caused by gravity. This sagging is opposed by the interfacial tension between the plastic containing oil solution and the water bath, but the tension is barely adequate to give satisfactory sphericity. We found that adding a small amount (<0.1 wt%) of high-molecular-weight poly(acrylic acid) to the water bath substantially increased the interfacial tension. Combining that change with a gentler shell curing process enabled consistent production of more spherical mandrels.

Decreasing Out-of-Round in Poly(α-Methylstyrene) Mandrels by Increasing Interfacial Tension

Irregularities in ICF shells need to be characterized in detail. Outside and inside surface, and wall thickness fluctuations are all Raleigh-Taylor unstable and can cause a shell to fail during com...

Jane Gibson

Papers

Development of High Quality Poly(α-Methylstyrene) Mandrels for NIF

Complete Surface Mapping of ICF Shells

3D Surface Reconstruction of ICF Shells after Full Surface Spheremapping

Decreasing Out-of-Round in Poly(α-Methylstyrene) Mandrels by Increasing Interfacial Tension

Seeing Shell Wall Fluctuations