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

Jones, O. S.; Schein, J.; Rosen, M. D.; Suter, L. J.; Wallace, R. J.; Dewald, E. L.; Glenzer, S. H.; Campbell, Kelly; Gunther, Janelle; Hammel, B. A.; Landen, O. L.; Sorce, C.; Olson, R. E.; Rochau, Gregory A.; Wilkens, H. L.; Kaae, J.L.; Kilkenny, J. D.; Nikroo, A.; Regan, S. P.

doi:10.1063/1.2712426

Cited by 50 publications

(21 citation statements)

References 15 publications

(19 reference statements)

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“…Furthermore, the conversion efficiency reaches a fairly constant plateau after 0.5 ns from the beginning of the laser pulse, when the radiative and hydrodynamic losses in the high-Z plasmas are balanced by the laser plasma heating by inverse bremmsstrahlung [19]. The plateau is slightly increasing in time due to the increasing xray albedo [10] that causes a reduction of radiative losses in the bulk target material. 1.0x10 19 1.5x10 19 Intensity (keV/ keV-4pi) E (keV) …”

Section: Discussionmentioning

confidence: 88%

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X-ray conversion efficiency of high-Z hohlraum wall materials for indirect drive ignition

et al. 2008

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We measure the conversion efficiency of 351 nm laser light to soft x-rays (0.1-5 keV) for Au, U and high Z mixtures "cocktails" used for hohlraum wall materials in indirect drive ICF. We use spherical targets in a direct drive geometry, flattop laser pulses and laser smoothing with phase plates to achieve constant and uniform laser intensities of 10 14 and 10 15 W/cm 2 over the target surface that are relevant for the future ignition experiments on NIF. The absolute time and spectrally-resolved radiation flux is measured with a multichannel soft x-ray power diagnostic. The conversion efficiency is then calculated by dividing the measured x-ray power by the incident laser power from which the measured laser backscattering losses is subtracted. After ~0.5 ns, the time resolved x-ray conversion efficiency reaches a slowly increasing plateau of 95% at 10 14 W/cm 2 laser intensity and of 80% at 10 15 W/cm 2 . The M-band flux (2-5 keV) is negligible at 10 14 W/cm 2 reaching ~1% of the total x-ray flux for all target materials. In contrast, the Mband flux is significant and depends on the target material at 10 15 W/cm 2 laser intensity, reaching values between 10% of the total flux for U and 27% for Au. Our LASNEX simulations show good agreement in conversion efficiency and radiated spectra with data when using XSN atomic physics model and a flux limiter of 0.15, but they underestimate the generated M-band flux. I IntroductionIn indirect drive inertial confinement fusion (ICF) experiments, intense laser or charged particle beams heat the interior of high-Z cylindrical cavities called hohlraums to efficiently generate soft x-rays. The role of the soft x-rays is to uniformly produce an ablation drive that compresses the DT filled capsule placed inside the hohlraum, driving it to ignition and burn [1]. Due to the relaxed requirements on laser-beam uniformity and reduced sensitivity to hydrodynamic instabilities the indirect drive scenario is the main approach for future ignition experiments at the National Ignition Facility (NIF) [2,3,4]. In such a hohlraum containing a capsule and heated by laser beams the power balance is given by [5,6]:η CE (P laser -P backscatter ) = P walls +P LEH + P capsule =σT R 4 [(1-α)A wall +A LEH +f capsule A capsule ] (1) where η CE is the laser conversion efficiency into soft x-rays (<2 keV), P laser is the incident laser power, P backscatter is the backscattered light by parametric laser-plasma instabilities, P capsule is the radiation absorbed by the capsule, P LEH is the radiation escaping through the hohlraum laser entrance holes (LEH) and P walls is the radiation loss in the hohlraum walls.Furthermore, σ is the Stefan-Boltzmann constant, T R is the hohlraum radiation temperature (≤300 eV), α is the x-ray albedo of the hohlraum wall [7], defined as the ratio between the re-emitted and incident soft x-ray flux at the hohlraum wall, A wall and A LEH are the areas of the hohlraum wall and LEH's, f capsule is the fraction of incident x-ray flux that is absorbed by the capsule and A ...

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Section: Discussionmentioning

confidence: 88%

“…In a cocktail, the mix has an increased opacity and decreased heat capacity compared to its individual constituents and thus reduced radiation losses into the walls [9,10]. A higher wall albedo not only reduces required laser power, but also improves radiation uniformity at the capsule [1].…”

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confidence: 99%

X-ray conversion efficiency of high-Z hohlraum wall materials for indirect drive ignition

et al. 2008

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“…The hohlraums show 90 % coupling of the incident laser energy and scale with radiation temperature according to radiation hydrodynamic calculations and modeling. Future studies will explore further improvements in hohlraum drive including experiments with optimized wall areas and hohlraum fill den- sities as well as hohlraums with higher-Z wall materials and higher wall opacities [26][27][28], e.g., uranium.…”

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Demonstration of Ignition Radiation Temperatures in Indirect-Drive Inertial Confinement Fusion Hohlraums

Glenzer¹,

MacGowan²,

Meezan³

et al. 2011

Phys. Rev. Lett.

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“…with P and A being the powers incident on and areas of the wall, capsule, and LEH (laser entrance hole), respectively, α cap the capsule albedo and σ the Stefan-Boltzmann constant [22]. The first DU hohlraum experiments during the NIC were reported in Refs.…”

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“…Uranium provides higher opacity and lower specific heat capacity for the hohlraum wall compared to gold [21,22]. Therefore, less laser energy is required to heat the DU hohlraum wall.…”

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Demonstration of High Performance in Layered Deuterium-Tritium Capsule Implosions in Uranium Hohlraums at the National Ignition Facility

Döppner¹,

Callahan²,

Hurricane³

et al. 2015

Phys. Rev. Lett.

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108

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We report on the first layered deuterium-tritium (DT) capsule implosions indirectly driven by a "highfoot" laser pulse that were fielded in depleted uranium hohlraums at the National Ignition Facility. Recently, high-foot implosions have demonstrated improved resistance to ablation-front Rayleigh-Taylor instability induced mixing of ablator material into the DT hot spot [Hurricane et al., Nature (London) 506, 343 (2014)]. Uranium hohlraums provide a higher albedo and thus an increased drive equivalent to an additional 25 TW laser power at the peak of the drive compared to standard gold hohlraums leading to higher implosion velocity. Additionally, we observe an improved hot-spot shape closer to round which indicates enhanced drive from the waist. In contrast to findings in the National Ignition Campaign, now all of our highest performing experiments have been done in uranium hohlraums and achieved total yields approaching 10 16 neutrons where more than 50% of the yield was due to additional heating of alpha particles stopping in the DT fuel. In indirect-drive inertial confinement fusion (ICF) [1,2], laser energy, converted to thermal x rays inside a high-Z cavity (hohlraum), ablatively drives the implosion of a spherical capsule containing a deuterium-tritium (DT) fuel layer. A high-velocity, low-entropy, highly symmetric implosion is required to form a hot spot of sufficiently high density and temperature from a combination of PdV work and alpha particle deposition. Above the ignition threshold, a self-sustained nuclear burn wave is launched igniting the surrounding compressed fuel layer. Ignition and burn is predicted for stagnation pressures above 300 Gbar [3].Experiments during the National Ignition Campaign (NIC) [4] demonstrated high implosion velocities (∼360 km=s) [5], and high areal mass densities of 1.3 AE 0.1 g=cm 2 [6], which were predicted to be sufficient to reach ignition [7]. However, some of them showed evidence for significant amounts of CH ablator material mixing into the hot spot [8,9], degrading the implosion performance. To increase the resistance to hydrodynamic instabilities [10], the "high-foot" drive design was developed [11,12], which has demonstrated an order-of-magnitude improvement in neutron yields with first evidence for additional heating from alpha particle stopping [13]. The high-foot drive raises the hohlraum temperature to 90 eV during the foot, launching a stronger first shock and sets the implosion on a higher adiabat. Among the benefits are a larger ablator density scale length (reducing susceptibility to ablation front RayleighTaylor growth [14]), and a shorter, simplified laser drive (three shocks rather than four). These benefits come at the price of lower fuel areal density, which requires higher implosion velocity compared to the low-entropy design [7] to achieve ignition.The first high-foot DT implosions [12,13,15] were done in gold (Au) hohlraums. These experiments incrementally increased laser peak power P laser and thus total laser energy from 351 TW=1.3 MJ on N13...

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

Cited by 50 publications

References 15 publications

X-ray conversion efficiency of high-Z hohlraum wall materials for indirect drive ignition

X-ray conversion efficiency of high-Z hohlraum wall materials for indirect drive ignition

Demonstration of Ignition Radiation Temperatures in Indirect-Drive Inertial Confinement Fusion Hohlraums

Demonstration of High Performance in Layered Deuterium-Tritium Capsule Implosions in Uranium Hohlraums at the National Ignition Facility

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