Session YO6: ICF: Direct, Indirect, and Polar Drive II

Experimental assessment of double shell shape control and predictability at high drive energy

Double shell targets provide an alternative and complimentary path to single shell targets in inertial confinement fusion (ICF). The LANL double shell platform utilizes a 5.75 mm diameter, 10.13 mm long cylindrical hohlraum as the driver for the implosion. The LANL campaign started with experiments at lower laser energy to demonstrate predictive shape control and gradually increased the laser energy to our design goal of 1.5 MJ. We compare the experimentally measured shape and velocity to the preshot predictions to evaluate our ability to model the implosion shape at a laser energy of 1.5 MJ. These particular experiments were designed to be slightly oblate, which is observed in the magnitude of the measured P2/P0 and P4/P0, which range from 2-10{\%} over the time of the measurements. We will also discuss the dominant sources of shape asymmetry.   

Imaging Shape Transfer to a Metal Inner Shell in Double Shell Implosions

The double shell concept for inertial confinement fusion utilizes a low-Z ablator that delivers energy to a high-Z inner shell via collisional transfer. The inner shell then compresses the fuel quasi-adiabatically leading to volumetric ignition. Asymmetries in the outer shell that arise from either the x-ray drive or engineering features can imprint on the inner shell during the collision, and this ultimately impacts the shape of the fuel region. Radiation hydrodynamics codes predict that deviations from round result in reduced performance, setting an upper limit on the allowable levels of low mode asymmetries. We present designs for surrogate double shells using chromium and molybdenum inner shells with deuterated foam fills. These mid-Z inner shells allow for efficient imaging with high-energy photons produced by the ARC backlighter, and the neutrons produced in the deuterated foam fuel will be used to assess mixing of the inner shell into the fuel without the added complexity of fill tube fabrication. Performance degradation due to low mode shape will be assessed in these surrogate designs, and the feasibility of imaging the inner shell shape will be discussed.   

Double Shell Target Design in a 620 Hohlraum

The double shell target provides an alternative platform for reaching a burning plasma regime on the NIF. Concerns regarding target engineering impacts, such as the fill tube, and ablator shape on overall performance of the design requires investigation of ways to mitigate these worries. Moving from a 575 hohlraum with a reverse ramp pulse shape to a larger 620 hohlraum with a higher-adiabat three shock pulse is one path to mitigating these impacts. Average measured P2/P0 and P4/P0 values are 2.8{\%} and 1.7{\%} respectively. Measured values are compared with simulation, and computational fill tube results will be examined. *This work conducted under the auspices of the U.S. Department of Energy, contract number 89233218CNA0000001, release number LA-UR-1926095   

Design of multi-axis keyhole experiments for benchmarking L-band x-ray preheat in double shell implosions

Double shell implosions rely on the efficient transfer of kinetic energy from an outer shell to an inner shell in order to generate the conditions for a burning fusion plasma. With indirect-drive methods used at the National Ignition Facility (NIF) laser heating of the Au hohlraum plasma can generate significant fractions of M-band (2-5 keV) and L-band (9-12 keV) hard x-rays that may affect the shell collision process. The magnitude and symmetry of L-band is especially concerning due to its ability to readily penetrate Al outer shells and deposit its energy within the high-atomic-number inner shell. This deposition and subsequent outward expansion can alter in-flight density profiles and have significant impact on hydrodynamic stability [J.L. Milovich et al., Phys. Plasmas 11 (2004)]. Non-LTE x-ray emission and transport being notoriously challenging to simulate makes L-band preheating of the inner shell a critically important uncertainty that limits our ability to reliably simulate many double shell physics processes. We have measured early time expansion of a W inner shell along the equatorial axis finding about 3x lower expansion velocities than predicted by integrated hohlraum simulations. In this presentation we will discuss these results further as well as designs for upcoming multi-axis preheat symmetry measurements.   

Progress in advancing the Revolver triple-shell direct-drive ignition concept

Recent progress in validating the concept of multi-shell direct-drive ignition has been made. Results of experiments on Omega are consistent with the Revolver ignition requirements of high (\textgreater 90{\%}) coupling of direct-drive laser energy to large Be capsules, the high hydro-efficiency (\textasciitilde 10{\%}) of direct-drive laser energy to payload kinetic energy for small laser beam to capsule ratios (\textasciitilde 1/3), and as-predicted implosion kinetic energy transfer efficiency between concentric colliding shells. The results of recent Omega and NIF experiments will be discussed in relation to the current Revolver triple shell ignition design. Ignition design features including split-quad laser pointing, inner capsule cushion layers to smooth high-mode perturbations and novel fabrication concepts. The goals of current LANL/LLE collaborative experimental efforts include the demonstration of smoothing techniques for both target and laser drive imperfections.   

Evaluation of Ablator-Shell Contouring to Enhance the Performance of NIF Polar-Drive High-Yield Source Experiments

Current experiments at the National Ignition Facility (NIF) are investigating the use of room-temperature, large-diameter, thin-shelled plastic ablator targets to produce high neutron fluxes with total fusion output approaching 100 kJ. Recent results have produced \textasciitilde 25 kJ of fusion output using \textasciitilde 1.1-MJ incident 351-nm light arranged in a polar-drive (PD) beam-pointing configuration. Work done earlier on the OMEGA laser at the University of Rochester's Laboratory for Laser Energetics has demonstrated the efficacy of shell contouring to compensate for oblique laser pointing, which is implicit with polar drive. This work examines the benefit of producing contoured shells to compensate for the polar drive pointings on the NIF. The actual contours are derived from 2-D DRACO simulations modeling the appropriate targets, beam pointings, and phase-plate profiles, as well as accounting for the effects of cross-beam energy transfer between the beams. Initial simulation results are consistent with experimental data obtained on OMEGA resulting in roughly a factor-of-2 improvement in fusion output for identical PD targets with and without shell contouring. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE NA0003856. F.J. Marshall \textit{et al.}, Phys. Plasmas \textbf{23}, 012711 (2016).   

Machine learning design and prediction of polar direct drive experiments at the National Ignition Facility

Inertial confinement fusion (ICF) experiments are often designed using computer simulations that are approximations of reality, and therefore must be corrected to accurately predict experimental observations. We implement a nonlinear technique for calibrating from ICF simulations to experiments called "transfer learning". Transfer learning comes from the machine learning community, in which models trained on one task are partially retrained to solve a separate, but related task, for which there is a limited quantity of data. We use transfer learning to calibrate simulation-based models to experimental data from polar direct drive experiments performed at the National Ignition Facility. The calibrated models enable rapid exploration of design space to identify optimal experiments, and are updated throughout the campaign to continuously improve their predictive accuracy. Prepared by LLNL under Contract DE-AC52-07NA27344. LLNL-ABS-780050.   

A Review of Bigfoot Implosion Data at the National Ignition Facility

We consider the motivations for the high-velocity/high-adiabat approach to indirect drive known as ``Bigfoot,'' and review experiments from 2015 to 2018. We show that performance is a function of symmetry, as expected, and that layered data follows near-1-D scaling(s) for hot-spot pressure\footnote{ K. L. Baker \textit{et al.}, Phys. Rev. Lett. \textbf{121}, 135001 (2018).} and capsule radius.\footnote{ D. T. Casey \textit{et al.}, Phys. Plasmas \textbf{25}, 056308 (2018).} While the design was not intended to achieve high performance, it also reaches high pressure (360 Gbar), yield (2.0~\texttimes ~10$^{\mathrm{16}})$, alpha heating (3.2\texttimes ), and fusion gain $\left[ {1.2\times \sim {\left( Y \right)} \mathord{\left/ {\vphantom {{\left( Y \right)} {\left( {3 \mathord{\left/ {\vphantom {3 {2\;pV}}} \right. \kern-\nulldelimiterspace} {2\;pV}} \right)}}} \right. \kern-\nulldelimiterspace} {\left( {3 \mathord{\left/ {\vphantom {3 {2\;pV}}} \right. \kern-\nulldelimiterspace} {2\;pV}} \right)}} \right],$ at a compression ratio similar to data at a lower-design adiabat (1.5 to 2.5).\footnote{ C. A. Thomas \textit{et al.}, ``Using Indirect Drive Data to Extrapolate in Energy and Scale,'' to be submitted to Physical Review Letters.} We use these results to extrapolate in energy and scale,\footnote{ C. A. Thomas \textit{et al.}, ``Compression in High-Performance Indirect Drive Implosions at the National Ignition Facility,'' to be submitted to Physical Review Letters.} and suggest experiments that could explain current performance (limits). This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE{\-}AC52-07NA27344 and is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Density Measurements of the Inner Shell Release

In inertial confinement fusion implosions, the release of plasma off the inner surface of the target shell after the shock breakout is important to the performance of the design. If the release has a higher density or longer scale length than that predicted by hydrodynamic simulations, the mass increase in the hot spot can decrease its compressibility and reduce performance compared to what is expected from the simulations. Experiments on OMEGA EP at the Laboratory for Laser Energetics were performed to measure the plasma expanding on the back side of a CH shell driven by two UV laser beams with a total of 6 kJ of energy in a 5-ns pulse focused to a 750-?m spot. The peak position of the driven shell was tracked using x-ray radiography streaked over 4 ns. The low-density plasma expanding off the undriven side of the shell after the shock breaks through was measured using the 4? interferometer and angular filter refractometer. Comparison between the experimental data and hydrodynamic simulations indicates that a decompression of the initial neutral CH shell results in an increased expansion of the plasma on the back side of the shell after the shock breaks out. The implication to inertial confinement fusion performance will be discussed.   

Hydroscaling and Alpha Heating in High Adiabat Layered Implosions

High adiabat implosions in inertial confinement fusion (ICF) are designed to be more robust to detrimental plasma and hohlraum physics than their lower adiabat counterparts. They drive a strong first shock into the ablator as well as into the DT fuel, reducing the sensitivity of the integrated system to uncertainties in shock-timing, preheat, and instabilities in the ablator and at the fuel-ablator interface. The higher adiabat enables a short pulse which simplifies hohlraum physics by limiting the extent of difficult to model dynamics such as gold bubble expansion, plasma filling of the hohlraum, and stagnation and interpenetration of the wall, capsule and gas interfaces and subsequent laser propagation through those regions. We report on DT layered implosions used to test the level of alpha heating driven in these high adiabat implosions and the hydroscaling of these implosions between two scales, scaled by x1.125. We present hydroscalings of the hotspot parameters which are shown to scale differently with the scale factor than no alpha heating analytic theory predicts. *This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

High-Volume and -Adiabat Capsule (``HVAC'') Ignition with Layered Gas-filled Capsules in Advanced Hohlraums

Hohlraum designs using rugby [1] and diamond (``Frustraum'' [2]) shapes may enable capsule absorbed energies \begin{figure}[htbp] \centerline{\includegraphics[width=0.25in,height=0.21in]{010720191.eps}} \label{fig1} \end{figure} of 0.5 MJ or more, compared with \begin{figure}[htbp] \centerline{\includegraphics[width=0.10in,height=0.17in]{010720192.eps}} \label{fig2} \end{figure} 0.2 MJ in nominal-sized capsules instandard cylinders. Such an increase in \begin{figure}[htbp] \centerline{\includegraphics[width=0.25in,height=0.21in]{010720193.eps}} \label{fig3} \end{figure} results in more margin to preheat and allows high-adiabat implosions to ignite. Integrated hohlraum simulations show that 3-mm scale DT gas-filled capsules or SYMCAPS may generate \begin{figure}[htbp] \centerline{\includegraphics[width=0.10in,height=0.17in]{010720194.eps}} \label{fig4} \end{figure} 0.1 MJ of yield. Introducing a DT fuel layer into a SYMCAP driven by a 2-shock drive may achieve fuel adiabats \textgreater 5 and allow a \textit{volume} ignition mode to occur - in contrast to hot-spot ignition and propagating burn at low adiabat. Such an alternative ignition mode has lower fuel convergence, less sensitivity to anomalous preheat mechanisms and strengths, and is more robust to hydrodynamic instability. The disadvantage of volume ignition is less yield, mostly due to less fuel loading. Such a platform may enable controlled experiments on the NIF to understand hot-spot ignition thresholds while first establishing a base-camp for volume ignition and energy gains of up to several. [1] P. Amendt \textit{et al.}, PoP \textbf{14,} 056312 (2007); M. Vandenboomgaerde \textit{et al}., PRL \textbf{99}, 065004 (2007); Y. Ping \textit{et al}., Nature Physics (https://doi.org/10.1038/s41567-018-0331-5). [2] P. Amendt \textit{et al}., PoP, to appear.   

Does X matter? Assessing competing hypotheses in NIF implosions

A key element in the analysis of inertial confinement fusion experiments is comparison with simulated models of realized experiments. These models, which can vary in complexity, have a multitude of parameters, such as experimental boundary conditions, physical and numerical settings, and hypothetical degradation mechanisms. The inference of which parameter settings are most likely to be consistent with experimental observations can help guide conclusions about the behavior of current and future designs. In this talk, we will review a Bayesian approach to this problem, which leverages deep learning surrogate modeling of large simulation ensembles, and apply it to a variety of shots performed at the National Ignition Facility, in an attempt to systematically evaluate and assess competing explanations for observed shot behavior. Prepared by LLNL under Contract DE-AC52-07NA27344. LLNL-ABS-779523.   

Quantifying uncertainty in the predicted performance of inertial confinement fusion experiments

State-of-the-art high-throughput simulation studies, combined with experimental data collected at the National Ignition Facility, are enabling the development of statistically calibrated, data-driven models for the performance of indirect-drive inertial confinement fusion experiments. These models match a diverse set of experimental observables over a whole series of NIF laser shots and provide predictions, with uncertainties, over a wide range of experimental design parameters. We will describe our statistical model, and assess the factors driving prediction uncertainty for current NIF implosions. The balance between uncertainty sources such as shot-shot variation, incomplete experimental constraints on physics parameters, and interpolation to new experimental parameters suggests new experimental and simulation studies that reduce prediction uncertainty; we will present some of these and discuss their implications for NIF `BigFoot' implosion studies.   

Development of high-efficiency, non-cryogenic, direct-drive neutron sources on the National Ignition Facility laser

We discuss recent work on the development of high-efficiency, room-temperature, polar-direct-drive neutron sources on the National Ignition Facility laser. Thin-shell ($15-30\,\mu m$), $3-5\,mm$ OD glow-discharge plastic (GDP) capsules filled with $8\,atm$ of DT (65:35) gas are directly driven with $0.5-1.9\,MJ$ of laser energy in a polar direct drive geometry. To date, experimental laser-to-neutron-energy conversion efficiencies of up to $\approx3\%$ have been demonstrated, corresponding to neutron yields in excess of $10^{16}$. Radiation-hydrodynamics simulations with ARES and HYDRA suggest these interactions are neither true exploding-pushers (i.e. low-convergence with shock-driven ion temperatures and most of the shell is ablated away) nor within a traditional inertial confinement fusion regime (i.e. high-convergence with compression and $\alpha$-heating driven ion temperatures). Rather, these experiments appear to exist somewhere in-between in a regime we dub ``compressing-pushers.'' Current experimental and modeling results will be presented along with plans for optimizing the platform under various target and facility constraints.   

Simulations of Double Cone-in-Shell Implosions for an X-Ray Backlighting Source at the National Ignition Facility

A double cone-in-shell plastic (CH) target has been proposed as a short-pulse x-ray source for backlighting a hohlraum-heated iron sample in an opacity platform\footnote{ R. F. Heeter \textit{et al.}, J. Plasma Phys. \textbf{83}, 595830103 (2017).} at the National Ignition Facility. The cones, aligned with the sample viewing direction, prevent x rays from probing the sample before the time of target compression. Simulations of these targets with the 2-D hydrodynamics code \textit{SAGE}\footnote{ R. S. Craxton and R. L. McCrory, J. Appl. Phys. \textbf{56}, 108 (1984).} have been used to optimize the beam pointings and cone parameters. The targets are imploded using beams not required to heat the sample. Careful design of the cone parameters is required to prevent hot imploded plasma from escaping through the cone tip and lengthening the x-ray pulse. Designs are evaluated using a new x-ray diagnostic code, \textit{ORION},\footnote{ A. Sharma, LLE High School Project Report (2018).} which calculates the x-ray output from both the double-cone targets and conventional targets without cones. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.