Session BI2: National Ignition Facility Design, Energetics, Symmetry and Mix

Point design targets, specifications, and requirements for the 2010 NIF ignition campaign

A set of point design targets has been specified for the initial ignition campaign on the National Ignition Facility [G. Miller, E. Moses, and C. Wuest, Opt. Eng. 443, 2841 (2004)]. The targets use an ablator of either Be(Cu) or CH(Ge). They are imploded in a U or Au hohlraum at peak radiation temperature 270 to 300eV. Considerations determining the point design include laser-plasma interactions, hydro stability, laser operations, and target fabrication. Simulations were used to evaluate choices, to define requirements, and to estimate sensitivity to uncertainties. Designs were updated to account for 2009 experimental results. We describe a formalism to evaluate the margin for ignition, in a parameter the Ignition Threshold Factor (ITF). Uncertainty and shot-to-shot variability can be evaluated, as well as sensitivity to systematic uncertainties. The formalism is used to estimate the probability of ignition for each target. In collaboration with J Lindl, D Callahan, D Clark, J Salmonson, B Hammel, L Atherton, R Cook, J Edwards, S Glenzer, A Hamza, S Hatchett, D Hinkel, D Ho, O Jones, O Landen, B MacGowan, M Marinak, E Moses, D Munro, S Pollaine, B Spears, P Springer, L Suter, C Thomas, R Town, S Weber, D Wilson, G Kyrala, M Herrmann, R Olson, R Vesey, A Nikroo, H Huang, and K Moreno.   

Stimulated Raman scatter analyses of experiments conducted at the National Ignition Facility

The recent energetics campaign\footnote{N. B. Meezan {\it et al.}, Phys. Plasmas {\bf 17}, 056304 (2010).} conducted at the National Ignition Facility in Fall, 2009 achieved its two main goals: providing radiation drive and symmetry suitable for subsequent ignition experiments. Many diagnostics were fielded during this campaign, one of which provided a time-resolved wavelength spectrum of light reflected from the target by stimulated Raman scatter (SRS). SRS occurs when incident light reflects off self-generated electron plasma waves. The SRS spectrum of an inner cone quad has provided insight into these experiments. Analyses indicate that synthetic SRS diagnostics better match those of experiments when an atomic physics model with greater emissivity is utilized, along with less inhibited electron transport (higher flux, with, ideally, nonlocal electron transport). With these models,\footnote{M. D. Rosen, this conference.} SRS primarily occurs in a region of the target where nearest-neighbor 23$^{\circ}$ quads significantly overlap the diagnosed 30$^{\circ}$ quad. This increases the gain at lower density (lower wavelength), a feature consistent with experimental results. Other predicted features, such as the direction and spreading of the SRS as well as its intensity, are also in better agreement with experiment. Inclusion of this effect of multiple beams sharing a reflected SRS light wave has resulted in modifications to our laser-plasma interaction codes.\footnote{C. H. Still, this conference.}$^,$\footnote{D. J. Strozzi, E. A. Williams, D. E. Hinkel {\it et al.}, Phys. Plasmas {\bf 15}, 102703 (2008).}$^,$\footnote{R. L. Berger, C. H. Still, E. A. Williams, and A. B. Langdon, Phys. Plasmas {\bf 5}, 4337 (1998); C. H. Still, R. L. Berger, A. B. Langdon, D. E. Hinkel, L. J. Suter, and E. A. Williams, Phys. Plasmas {\bf}, 2023 (2000); D. E. Hinkel, D. A. Callahan, A. B. Langdon, S. H. Langer, C. H. Still, and E. A. Williams, Phys. Plasmas {\bf 15}, 056314 (2008).} These improved capabilities are being tested by making predictions for upcoming National Ignition Campaign experiments. Synthetic SRS spectra, reflectivity levels, and the angular distribution of SRS light will be compared to experimental results.   

Multiple Beam Effects on Backscatter and its Saturation in Experiments with Conditions Relevant to Ignition

The amplification of light when obliquely intersected by laser beams in a plasma has been analyzed for its relevance to the amplification of backscatter in ignition targets. In the targets for the National Ignition Campaign (NIC) where the amplification of forward going beams is now well known and controlled [1], a linear model of the 23 quads of beams that intersect the light scattered from a single quad in the interior of the hohlraum has shown that backscatter re-amplification with additional gain exponents as high as 8 can be produced in some targets designs. This could lead to un-acceptable energy coupling if not mitigated by wave saturation or careful target design. A series of experiments [2], and 2D VPIC and fluid simulations [3] have been carried out to demonstrate the following key aspects of the model. Re-amplification of light with wavelengths corresponding to SRS from individual beams, by a single pump beam has been demonstrated in normalized plasma conditions similar to some ignition target designs. Saturation of this re-amplification has been observed to be in good agreement with 2D PIC models of non-linear kinetic effects such as trapping and subsequent wave front bowing, which limits the scattered energy to $\sim $1{\%} of the pump energy in the cases studied. Re-amplification of light with wavelengths corresponding to SBS by two pumps is shown experimentally to lead to scattered energy that is well above that of a single pump as expected. SRS scatter in ignition scale hohlraums is shown to increase with increased density in the beam crossing region. The relevance of the work to ignition targets will also be discussed. \\[4pt] [1] P. Michel et al., Physics of Plasmas \textbf{17}, 056305 (2010) \\[0pt] [2] R. K. Kirkwood et al., submitted to Phys. Rev. Lett. and in preparation. \\[0pt] [3] Y. Lin L. Yin, B. J. Albright, K. J. Bowers, W. Daughton, and H. A. Rose, Phys. Plasmas, 15, 013109 (2008). and in preparation.   

Quantitative Analysis Of The 2009 National Ignition Facility Ignition Hohlraum Energetics Experiments

A series of thirty experiments on the National Ignition Facility (NIF) to study energy balance and implosion symmetry in reduced- and full-scale ignition hohlraums were shot in late 2009 at energies up to 1.05 MJ. Preliminary [1,2,3] and ongoing analysis of these hohlraums indicates they meet the requirements for ignition. Here we report the findings of quantitative analysis of the ensemble of data that has refined our understanding of those experiments and produced an improved model for simulating ignition hohlraums. Last year we reported the first observation in a hohlraum of energy transfer between cones of beams as a function of wavelength shift between those cones [1,3]. Detailed analysis of hohlraum wall emission as measured through the laser entrance hole has allowed us to quantify the amount of energy transferred versus wavelength shift. We find the change in outer beam brightness to be quantitatively consistent with LASNEX simulations using the predicted energy transfer when we take into account possible saturation of the plasma wave mediating the transfer. The effect of the predicted energy transfer on implosion symmetry is also found to be in good agreement with gated x-ray framing camera images. Hohlraum energy balance, as measured by x-ray power escaping the LEH, is quantitatively consistent with revised estimates of backscatter and incident laser energy combined with a more rigorous NLTE atomic physics model with greater emissivity than the simpler average-atom model used in the original design of NIF targets. \\[4pt] [1] S. H. Glenzer, et al., Science, 327, 1228 (2010). \\[0pt] [2] N. B. Meezan, et al, Phys of Plasmas, 17, 056304, (2010). \\[0pt] [3] P. Michel, et al, Phys of Plasmas, 17, 056305, (2010).   

Symmetry Tuning for Ignition Capsules via the Symcap Technique

Achieving ignition requires control over many parameters for an imploding capsule symmetry being a key parameter to control. The primary technique used to determine the implosion symmetry at the peak of an ignition pulse on the National Ignition Facility (NIF) is the symcap. A symcap is a surrogate capsule that replaces the DT fuel layer by an equivalent mass of ablator to mimic the hydrodynamic behavior of the capsule. The x-ray self-emission signature from the implosion correlates well with an ignition capsule's core shape. Experiments at Omega and NIF demonstrate the ability of this technique to tune capsules' symmetry in ignition relevant conditions. At Omega we used CH and Be symcaps in ignition scale hohlraum to demonstrate tuning at parameters matching the foot of an ignition pulse. These experiments are the first to demonstrate tuning of the capsule by beam phasing in which symmetry control is achieved by varying the relative power between an inner and an outer cone of laser beams having the same basic geometry as the NIF. Experiments at NIF also demonstrate symmetry control using beam phasing with symcaps in full-scale ignition targets with shaped laser pulse having ignition relevant energies, $\sim $840 kJ. The current plans is to extend this technique to cryogenically filled Tritium-Hydrogen-Deuterium (THD) capsule implosions, where deuterium is replace by mostly tritium to reduce the neutron yield. THDs make better emulators for ignition capsules since they have the same convergence and some variable fusion burn.   

Diagnosing and Controlling Mix in NIF Implosion Experiments

Controlling the hydrodynamic growth of capsule perturbations is essential in the optimization of NIF ignition target designs. In simulations, mode numbers up to $\sim $300 can have significant growth on the outer surface of CH capsules.\footnote{B.A. Hammel, et al., High Energy Density Physics, 6 (2010) p.171--178} As a result, ``isolated defects'' on the capsule (e.g. bumps in the CH coating, the fill tube) have the potential to grow enough to penetrate the imploding shell, and produce a jet of ablator material (mass $\sim $ 10's ng) that enters the hot-spot. Although this amount of mix is tolerable, degradation in ignition capsule performance becomes significant at several times this amount. Our predictions of hydrodynamic growth and resulting mix have a level of uncertainty that results from uncertainties in experimental conditions, physical data (e.g. EOS), and the simulation method itself. We are developing an experimental strategy where the final requirements for ignition targets (e.g. surface finish) can be adjusted through direct measurements of mix and experimental tuning. Since the growth can be reduced by controlled reduction of the peak x-ray drive, we can use the \textit{relative} simulated Growth Factors to help set ignition requirements. One method for inferring mix into the hot-spot is through observations of x-ray emission from the ablator material. Internal regions of the CH ablator are doped with Ge in nominal ignition designs, resulting in K-shell emission when it mixes into the hot-spot. We have observed evidence of jets entering the hot-spot in early NIF implosion experiments through the measured x-ray spectra and images, consistent with simulation predictions. Doping other regions of the ablator could provide a corresponding unique indication of mix. In addition, radiographic measurements of high-Z doped layers provide a means of measuring $\rho $R variation in the imploding and compressed capsule.