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

Callahan, D. A.; Amendt, Peter; Dewald, E. L.; Haan, S. W.; Hinkel, D. E.; Izurni, N.; Jones, O. S.; Landen, O. L.; Lindl, J. D.; Pollaine, S. M.; Suter, L. J.; Tabak, M.; Turner, R. E.

doi:10.1063/1.2196287

Cited by 26 publications

(14 citation statements)

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“…This drive corresponds to a particular 0.2-mm diameter 300 eV Cu-doped Be capsule design that absorbs about 160 kJ of x-ray energy [12]. Figure 6 also shows that the calculated (using Lasnex with STA opacities) time-dependent ratio of the wall loss of the cocktail to that of gold is 0.825 by the end of the pulse (17.5% reduction in wall loss).…”

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confidence: 81%

Proof of principle experiments that demonstrate utility of cocktail hohlraums for indirect drive ignition

et al. 2007

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This work is a summary of experiments, numerical simulations, and analytic modeling that demonstrate improved radiation confinement when changing from a hohlraum made from gold to one made from a mixture of high Z materials ("cocktail").First, the results from several previous planar sample experiments are described that demonstrated the potential of cocktail wall materials. Then a series of more recent experiments are described in which the radiation temperatures of hohlraums made from uranium-based cocktails were directly measured and compared with a gold reference hohlraum. Once cocktail hohlraums with minimal oxygen contamination were made, an increase in radiation of up to ~7 eV was measured, which agrees well with modeling.When applied to an indirectly-driven fusion capsule absorbing ~160 kJ of x-ray energy, 2 this data suggests that a hohlraum made from a suitably chosen uranium-based cocktail would have about 17% less wall losses and require about 10% less laser energy than a gold hohlraum of the same size. 3Increasing the hohlraum coupling efficiency (ratio of capsule absorbed energy to laser energy) for indirectly-driven inertial confinement fusion experiments at the National Ignition Facility (NIF) is desired because it would allow one to drive an ignition capsule with reduced laser energy. The radiation temperature a hohlraum achieves is the result of a balance between sources and sinks ( Fig. 1). This radiation energy balance is described by the following equation [1].Here, E cap is the x-ray energy absorbed by the fusion capsule in the center of the hohlraum, (E Laser -E Scatter ) is the laser energy delivered inside the hohlraum with backscatter losses accounted for, η CE is the fraction of that energy that is converted to xrays, E wall is the x-ray energy lost into the hohlraum wall, and E LEH is the x-ray energy that escapes out the laser entrance holes (LEHs). In this work, we show that we can reduce E wall by replacing a standard gold hohlraum with one made from a combination of high Z materials. For a fixed E cap , which is set by the ignition capsule design, reducing the wall losses allows one to reduce the amount of laser energy required for ignition, which increases the lifetime of NIF laser optics and thus has a large impact on facility operating costs. Alternatively, at fixed laser energy, reducing wall losses allows one to drive a capsule with more absorbed energy, thus increasing margin for ignition.The x-ray losses into the hohlraum wall are well modeled as a radiation ablation front diffusing into a cold wall, the so-called Marshak wave [2]. The starting point for that theory is the one-dimensional diffusion equation that describes the conservation of energy for a radiating fluid. In words, that equation states that the time derivative of the energy density is equal to the gradient of the diffusive energy flux. If we neglect the 4 radiation component of the energy density and the material component of the diffusive energy flux, we get ! " "t #ewhere ρ is the density, e is t...

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confidence: 81%

Proof of principle experiments that demonstrate utility of cocktail hohlraums for indirect drive ignition

et al. 2007

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“…Rather, another option is to modify the geometry of the hohlraum from a cylinder to a rugby shape, providing more volume over the equator as needed. Previous ignition designs, both for double shells [6] and single shells [15], have included gas fills or low-Z liners to control hohlraum flux asymmetry by suppressing the radial inward expansion of the (cylindrical) hohlraum wall. A feature of these hohlraum designs is the risk of laser backscatter, potentially resulting in lower x-ray drive and yet another source of drive asymmetry.…”

Section: High-gain Double-shell Design With Vacuum Hohlraumsmentioning

confidence: 99%

Assessing the prospects for achieving double-shell ignition on the National Ignition Facility using vacuum hohlraums

et al. 2007

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The goal of demonstrating ignition on the National Ignition Facility (NIF) has motivated a revisit of double-shell (DS) targets as a complementary path to the cryogenic baseline approach. Expected benefits of DS ignition targets include non-cryogenic deuterium-tritium (DT) fuel preparation, minimal hohlraum-plasma mediated laser backscatter, low thresholdignition temperatures (≈ 4 keV) for relaxed hohlraum x-ray flux asymmetry tolerances, and minimal (two-) shock timing requirements. On the other hand, DS ignition presents several formidable challenges, encompassing room-temperature containment of high-pressure DT (≈ 790 atm) in the inner shell, strict concentricity requirements on the two shells (< 3 µm), development of nano-porous (<100 nm) low-density (<100 mg/cc) metallic foams for structural support of the inner shell and hydrodynamic instability mitigation, and effective control of hydrodynamic instabilities on the high-Atwood number interface between the DT fuel and the high-Z inner shell. Recent progress in DS ignition designs and required materials-science advances at the nanoscale are described herein. Two new ignition designs that use rugby-shaped vacuum hohlraums are presented which utilize either 1 MJ or 2 MJ of laser energy at 3ω. The capability of the NIF to generate the requested reverse-ramp pulse shape for DS ignition is expected to be comparable to the planned high-contrast (≈100) pulse-shape at 1.8 MJ for the baseline cryogenic target. Nano-crystalline, high-strength, AuCu alloy inner shells are under development using electrochemical deposition over a glass mandrel, exhibiting tensile strengths well in excess of 790 atm. Novel, low-density (85 mg/cc) copper foams have recently been demonstrated using 10 mg/cc SiO 2 nano-porous aerogels with suspended Cu particles. A prototype demonstration of an ignition DS is planned for 2008, incorporating the needed novel nano-materials science developments and the required fabrication tolerances for a realistic ignition attempt after 2010.

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“…The drive we are trying to match is the one outlined by Callahan et al, 17 and is similar to the one outlined by Olson et al 13 We believe the two key criteria that must be met to assure surrogacy of the drive environment are: matching the radiation drive temperature ͑T rad ͒ next to the ablator material; and matching the amount of Au M-band radiation sampled by the pole. Of the two, the M-band radiation is the hardest and most important to match.…”

Section: Methodsmentioning

confidence: 97%

“…However, mitigation of the Be microstructure is a process that continues to evolve with our understanding of its material properties with the result that details of the foot of the ablative drive have changed several times. 7,[13][14][15][16][17] This mitigation technique must maintain a balance between having sufficient shock strength to melt the Be and keeping the entropy low in the fuel during the implosion. The final tuning of the drive will be carried out empirically during the ignition campaign, by directly measuring the shocks as they enter the fuel layer.…”

Section: Measurements Of Preheat and Shock Melting In Be Ablators Durmentioning

confidence: 99%

Measurements of preheat and shock melting in Be ablators during the first few nanoseconds of a National Ignition Facility ignition drive using the Omega laser

et al. 2009

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A scaled Hohlraum platform was used to experimentally measure preheat in ablator materials during the first few nanoseconds of a radiation drive proposed for ignition experiments at the National Ignition Facility [J. A. Paisner et al., Laser Focus World 30, 75 (1994)]. The platform design approximates the radiation environment of the pole of the capsule by matching both the laser spot intensity and illuminated Hohlraum wall fraction in scaled halfraums driven by the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. Back surface motion measured via VISAR reflecting from the rear surface of the sample was used to measure sample motion prior to shock breakout. The experiments show that the first ∼20 μm of a Be ablator will be melted by radiation preheat, with subsequent material melted by the initial shock, in agreement with simulations. The experiments also show no evidence of anomalous heating of buried high-Z doped layers in the ablator.

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

Cited by 26 publications

References 10 publications

Proof of principle experiments that demonstrate utility of cocktail hohlraums for indirect drive ignition

Proof of principle experiments that demonstrate utility of cocktail hohlraums for indirect drive ignition

Assessing the prospects for achieving double-shell ignition on the National Ignition Facility using vacuum hohlraums

Measurements of preheat and shock melting in Be ablators during the first few nanoseconds of a National Ignition Facility ignition drive using the Omega laser

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