Session BO4: Inertial Confinement Fusion I

Shell asymmetry-driven hot-spot generation issues in high convergence ratio implosions on the National Ignition Facility

Much of the conceptual understanding, theory, and design of ICF implosions has been developed assuming a one-dimensional (1D) implosion [e.g. Lindl, J., Phys. Plasmas, \textbf{2}, 3933-4024 (1995); Betti, R., et al., 17, 058102 (2010)]. But what if the typical ICF implosion is not 1D? In this talk we present an overview of data and simulation results from recent high performance implosions on NIF that imply highly distorted implosions and an associated non-ideal hot-spot generation issue, \textit{even in cases where the bang-time emission (in x-rays and neutrons) from the implosion appears 1D.} We present a simple extension of a semi-analytic dynamic implosion model that captures the key effect of localized thin-regions in an implosions shell (fuel$+$remaining ablator), via a leaking hot-spot picture, and discuss what the model implies about the physics we can't directly diagnose in our suite of implosions.   

Design Options for the High-Foot Ignition Capsule Series on NIF

Several options exist for improving implosion performance in the High-Foot series of ignition capsules on NIF. One option is to modify the fill tube used to supply DT to the capsule. Simulations indicate that a gold-coated glass tube may reduce implosion hydro effects and allow fielding a larger diameter tube capable of supporting the capsule, eliminating the need for the nominal tent support. A second option adds a fourth shock to the implosion history. According to simulation, this extra shock improves fuel confinement and capsule performance. A third option studies the feasibility of holding the DT fuel in liquid form in a foam layer inside the shell. This ``wetted foam'' concept, advanced by Olson, has existed for several years and may allow some control over the convergence of the capsule during implosion.   

Hohlraum Drive and Asymmetry in High Foot Implosions on NIF

The strategy in the high foot campaign on NIF has been to take reasonably small steps away from a working design, which means that we have a very rich database to understand both capsule and hohlraum performance. Over the course of the campaign, we have changed the laser power and energy, used both gold and depleted uranium hohlraums, and varied the thickness of the ablator. Each of these changes has an impact on the hohlraum drive and drive asymmetry, as measured by the implosion shape. In this talk, we will discuss what we have learned about hohlraum performance and residual kinetic energy resulting from drive asymmetry in the high foot database.   

Improved hohlraums for high foot implosions

Recent High Foot implosions[1] at the National Ignition Facility (NIF), where the laser power is high early in time, have resulted in record neutron yields. In these implosions, there is evidence of low mode radiation drive asymmetries impacting both in-flight and hot spot symmetry. Simulations suggest these asymmetries reduce neutron yield 2-4x, and thus improving the hohlraum should ameliorate implosion performance. To improve symmetry, a hohlraum 1.18x larger with a lower gas fill density has been designed and is being tested. This larger hohlraum with intermediate fill density has performed well for the shorter pulse lengths driving implosions with high-density carbon (HDC) ablators [2]. The challenge here is to maintain the predictability shown by simulation at the longer pulse lengths necessary for plastic ablators. Upcoming shots provide the first tests of drive symmetry and deficit as well as laser backscatter in these larger hohlraums with an intermediate gas fill density using the longer High Foot pulse. Results will be presented and compared to design. \\[4pt] [1] Hurricane \textit{et al}\textbf{., }\textit{Nature} \textbf{506}, 343-348 (2014).\\[0pt] [2] P. A. Amendt, D. D. Ho, O. S. Jones, \textit{et al}., submitted to Phys. Rev. E, 2015.   

High Foot Implosion Experiments in Rugby Hohlraums

The rugby hohlraum design is aimed at providing uniform x-ray drive on the capsule while minimizing the need for crossed beam energy transfer (CBET). As part of a series of experiments at the NIF using rugby hohlraums, design improvements in dual axis shock tuning experiments produced some of the most symmetric shocks measured on implosion experiments at the NIF. Additionally, tuning of the in-flight shell and hot spot shape have demonstrated that capsules can be tuned between oblate and prolate with measured velocities of nearly 340 km/s. However, these experimental measurements were accompanied by high levels of Stimulated Raman Scattering (SRS) that may result from the long inner beam path length, reamplification of the inner SRS by the outers, significant (CBET) or a combination of these. All rugby shots results were achieved with lower levels of hot electrons that can preheat the DT fuel layer for increased adiabat and reduced areal density. Detailed results from these experiments and those planned throughout the summer will be presented and compared with results obtained from cylindrical hohlraums. This work performed under the auspices of U.S. Department of Energy by Lawrence Livermore National Lab under Contract DE-AC52-07NA27344.   

Physics and Designs of Ignition Capsules Using High-Density Carbon (HDC) Ablators: Robust Designs, Stability, and Shock Mergers

We present six ignition designs using W-doped HDC ablators with, respectively, 2, 3, and 4-step increases in Tr. Fuel adiabat $\alpha $ ranges between 1.5 and 4. The 4-step design has the lowest $\alpha $ of 1.5 but has the highest ablation front Rayleigh-Taylor (RT) growth. Consequently, the overall robustness of the 4-step design is inferior to the intermediate-$\alpha $ 3-step design, assuming typical currently measured surface roughness spectrum. As the foot level is increased further and the shocks merge inside the fuel, the fuel adiabat is raised to 4. The RT growth and mix are reduced but the 1D margin is decreased making it overall more susceptible to surface roughness. The 2-step $\alpha = $ 2.5 design turns out to be the most robust against surface roughness and still can deliver very high 1D yield of 14.5 MJ. Systematic evaluation of the robustness of these capsules with respect to low-mode radiation asymmetries, will also be discussed. Different paths to achieve low-convergence-ratio implosions (i.e. high velocity and high $\alpha $ as one option versus low velocity and low $\alpha $ as another option), while still giving respectable neutron yield will be presented. Finally, we discuss how the performance of these doped capsules changes; if the Au wall of the hohlraum is replaced by U.   

Simulations of the 3-Shock HDC gas-filled hohlraum experiments at the NIF

We describe simulation efforts to design and field a series of high-density-carbon [1] (HDC) capsule tuning experiments in 1.6 mg/cc gas-filled hohlraums at the National Ignition Facility (NIF), culminating in two DT-layered shots. The radiation-hydrodynamics code HYDRA coupled to an off-line power transfer model was employed to ascertain the optimal laser pulse that minimizes radiation asymmetries and implosion adiabat for a given stability margin. We found that these HDC targets have similar sensitivity as their CH ``high-foot'' [2] counterparts when laser cone-fraction and power as well as ablator thickness are varied, leading to comparable implosions. A point of divergence, however, is the measured neutron down-scatter-ratio (DSR) that typically gauges the degree of compression obtained in a DT implosion, with HDC targets having approximately half the CH value. Concerted efforts are underway to understand and ascertain the causes of this discrepancy. Simulations and comparisons with data will be presented. \\[4pt] [1] D. Ho et al, BAPS.2012.DPP.GO4.13.\\[0pt] [2] O. Hurricane et al, Nature 506, 343 (2014).   

Laser/x-ray coupling in the first NIF beryllium implosions

The x-ray flux driving a capsule is currently overestimated in standard Hydra high-flux model (Rosen \textit{et al.,} HEDP \textbf{7},180 (2011)) calculations of gas-filled hohlraums. Jones \textit{et al.} (Phys. Plasmas,\textbf{19},056315 (2012)) introduced time dependent multipliers to reduce the laser drive and achieve an appropriate radiation drive on NIF capsules. Using shock velocities from VISAR capsule experiments, symmetry capsule implosion times with truncated laser pulses, and time dependent DANTE X-ray flux measurements from 1D and 2D convergent ablator implosions, we derived a set of time dependent flux multipliers for the first NIF cryogenically layered beryllium capsule implosion. The similarity between these multipliers for both plastic and beryllium capsules suggests that they are primarily correcting for improper modeling of the hohlraum physics, with possibly some residual contribution from capsule modeling deficiencies. Using Lasnex we have adjusted hohlraum physics and resolution in an attempt to model these implosions without drive multipliers.   

Shell and CORE Symmetry of beryllium capsule implosions at the National Ignition Facility

We will present results of the Be experimental campaign on the implosion symmetry properties of Be capsules at the National Ignition Facility. The experiments measured the inflight and core implosion symmetry. Images of the x-ray emission from the core around bang time provide a measure the symmetry near peak compression [1]. Inflight symmetry of the ablator before stagnation is measured using a backlight imaging [2] techniques. A Cu backlighter was used to measure the transmissions of the Cu doped Be shells. 2D symmetry is used to infer the drive and velocity uniformity and help adjust the time dependent ratio of the inner to the outer laser beam powers, to achieve proper implosion symmetry. Results show inner beam propagation is not degraded compared to CH ablators, corroborated by laser backscatter measurements. Variations in shape compared to CH ablators also provides information about the cross beam energy transfer used to adjust the equatorial shape and thus infer information about the differences in plasma conditions near the laser entrance holes. Experimental results and modeling implosion shape for Be capsules will be presented with comparisons to CH ablators. [1] G.A.Kyrala, RSI 81,10E316(2010). [2] J.R.Rygg, et al., PRL. 112,195001(2014).   

Measurements of hydrodynamic instability growth in beryllium capsules at the National Ignition Facility

Beryllium is an ablator material that is predicted to improve resilience to capsule hydro-instability growth in ICF implosions. Beryllium creates a higher ablation velocity at NIF-relevant radiation temperatures, due to its lower opacity. As a result, beryllium capsules are predicted to have enhanced ablative stabilization of Rayleigh-Taylor instabilities. Thus, beryllium capsule implosions are expected to suffer less performance degradation due to capsule hydro-instabilities. A hydro-growth radiography (HGR) experiment is planned for September 2015 to test this hypothesis. The HGR experiment will measure the ablation front instability growth of a beryllium capsule using backlit radiography. Here, we present an analysis of the capsule stability properties for the first beryllium target recently fielded on NIF, and compare to the results of the HGR experiment.   

Wetted Foam Liquid Fuel ICF Target Experiments

We are developing a new NIF experimental platform that employs wetted foam liquid fuel layer ICF capsules. We plan to use the liquid fuel layer capsules in a NIF experimental campaign to explore the relationship between hot spot convergence ratio (CR) and the robustness of hot spot formation. DT or D2 Liquid Layer ICF capsules allow for flexibility in hot spot convergence ratio via the adjustment of the initial cryogenic capsule temperature and, hence, DT vapor density.\footnote{R.E. Olson and R.J. Leeper, Phys. Plasmas 20, 092705 (2013).} Our hypothesis is that the predictive capability of hot spot formation is robust and 1D-like for a relatively low CR hot spot (CR$=$15), but will become less reliable as hot spot CR is increased to CR\textgreater 20. Simulations indicate that backing off on hot spot CR is an excellent way to reduce capsule instability growth and to improve robustness to low-mode x-ray flux asymmetries. In these initial experiments, we are testing our hypothesis by measuring hot spot size, neutron yield, ion temperature, and burn width to infer hot spot pressure and compare to predictions for implosions with hot spot CR's in the range of 12 to 25. Larger scale experiments are also being designed, with the longer-term objective of developing a liquid fuel layer ICF capsule platform with robust thermonuclear burn, modest CR, and significant $\alpha $-heating with burn propagation.   

Plans for Double Shell Experiments on NIF

Double-shells are an alternative approach to achieving indirect drive ignition. These targets consist of a low-Z ablatively-driven outer shell that impacts a high-Z inner shell filled with DT fuel. In contrast to single-shell designs, double-shell targets burn the fuel via volume ignition, albeit with a lower gain. While double-shell capsules are complicated to fabricate, their design includes several beneficial metrics such as a low convergence pusher (C.R. $<$ 10), low implosion speed ($\sim$ 250 km/s), a simple few-ns laser drive in a vacuum hohlraum, less sensitivity to hohlraum asymmetries, and low expected laser-plasma instabilities. We present preliminary double-shell capsule designs for NIF using a cryogenic gas DT fill which are optimized for yield and minimized for fall-line mix. Challenges will be discussed, as well as uncertainties and trade-offs in the physics issues compared to single-shells. A development path for double-shell experiments on NIF will be presented.   

Design Options for Polar-Direct-Drive Targets: From Alpha Heating to Ignition

Polar direct drive (PDD)\footnote{S. Skupsky\textit{ et al.}, Phys. Plasmas \textbf{11}, 2763 (2004).} makes it possible to perform direct-drive--ignition experiments at the National Ignition Facility while the facility is configured for x-ray drive. We present the first PDD ignition-relevant target designs to include the physical effects of cross-beam energy transfer (CBET) and nonlocal heat transport, both of which substantially affect the target drive. These effects are complementary: CBET reduces target drive, while nonlocal heat transport increases the drive (relative to flux-limited models). Previous ignition designs\footnote{T. J. B. Collins \textit{et al.}, Phys. Plasmas \textbf{19}, 056308 (2012).} incorporated these processes in only an approximate way through use of an \textit{ad-hoc} flux limiter applied to the classical expression for heat conduction. In the PDD configuration, a multiwavelength detuning strategy was found to be effective in mitigating the loss of coupling caused by CBET, allowing for implosion speeds comparable to those of previous designs. Target designs are found that span the region from alpha-particle heating to ignition. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

A New Intermediate Far-Field Spot Design for Polar Direct Drive at the National Ignition Facility

New far-field spot shapes were required and subsequently designed for the intermediate phase plates that will be fielded at the National Ignition Facility for polar-direct-drive laser-coupling experiments. Two-dimensional \textit{DRACO} simulations using the new far-field spot design, coupled with appropriate ring energies and beam pointing angles, achieve a high neutron yield-over-clean (YOC) ratio with a clean hot-spot radius averaging 50 $\mu $m and a convergence ratio (CR) above 17 when performed with a 1300-$\mu $m plastic shell target driven by a 700-kJ double-picket pulse. This meets the original design objectives of maintaining a clean hot spot with a CR of 17.~ The presented far-field spot shapes are based on an ignition polar-direct-drive configuration\footnote{ T. J. B. Collins \textit{et al}., Bull. Am. Phys. Soc. \textbf{59}, 150 (2014).} modeled with the iSNB nonlocal thermal transport model.\footnote{ D. Cao \textit{et al}., Bull. Am. Phys. Soc. \textbf{59}, 353 (2014).} In addition, the use of Multi-FM\footnote{ J. A. Marozas, J. D. Zuegel, and T. J. B. Collins, Bull. Am. Phys. Soc. \textbf{52}, 145 (2007).} during the first two pickets does not hinder performance, but instead slightly improves the neutron yield. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

Polar-Direct-Drive Shock-Timing Measurements at the National Ignition Facility

The adiabat of an inertial confinement fusion implosion affects both the compressibility and stability of the target being imploded and is determined by a series of shock waves that precede the implosion. We report on experiments at the National Ignition Facility that measure the strength and timing of shocks in surrogate CH targets driven by two $\sim 400\mbox{-ps}$ pulses in the polar-direct-drive (PDD) configuration. The results facilitate optimizing the drive pulses to produce the requisite implosion adiabat on future cryogenic implosions, but more importantly will assess the efficacy of the PDD technique. Since we measure shock velocities simultaneously at the pole and equator, these velocities provide a measure of the drive uniformity created by the PDD configuration. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.