Session NO07: Inertial Confinement: Compression and Burn I

Superior performance of directly driven hydrogen ablator capsules

We investigate the unique physics of using hydrogen as the main ablation material to achieve high nuclear yields from a liquid DT-wetted layer capsule directly driven by the National Ignition Facility’s current laser capabilities. The capsule is composed of a thin plastic shell used to enclose a thick low-density annular 3D-printed matrix layer that contains the liquid hydrogen fuel. Simulations predict high laser absorption fractions consistent with previous polar direct drive (PDD) MJ-class NIF experiments where >95% capsule absorption of the laser drive energy was achieved using a 5 mm diameter plastic capsule with a surface intensity of 2.5x10¹⁴ W/cm². Moreover, superior double-digit hydro-efficiency is predicted when hydrogen is the principal ablation material - higher than any other ablator material having equal shell mass. Simulations using the HYDRA and xRAGE rad-hydro codes show superior ablation pressure from hydrogen for extended times. Development of 3D printing techniques to enable construction of these hybrid capsules is being pursued. Investigation of the heterogeneous nature of the hydrogen/lattice ablator is being performed via planar cyrogenic experiments on Omega. An overview of these multi-Laboratory efforts will be shown.

Coronal Physics Modeling in Wetted-Foam Targets Proposed for Neutron Sources at the National Ignition Facility

The potential application of wetted-foam, direct-drive deuterium–tritium (DT)-filled targets for producing large neutron yields at the National Ignition Facility[1] has stimulated recent multi-laboratory interest in the development of these targets. The DT-wetted foam, ideally the primary ablator, is, however, surrounded by a CH layer that can be the dominant absorption medium for much of the laser pulse. This work used the hydrodynamics code SAGE to model two near-term wetted-foam designs and the design of Ref. 1 in polar-direct-drive geometry, looking at the transition between CH and D₂/DT in the absorption region. It is found that this transition does not occur in an abrupt manner but is spread over a significant time interval—this behavior being understood to result from the absorption being spread over a large range of densities. This understanding is supported by a comparison between observed and simulated time-dependent scattered light in a related experiment (N230131-002)[2] that used a 3-mm-diam, 18-µm-thick CH shell filled with liquid D₂. This material is based upon work supported by the Department of Energy [National Nuclear Security Administration] University of Rochester “National Inertial Confinement Fusion Program” under Award Number DE-NA0004144.

Modeling a Planar Heterogeneous Ablation Experiment on OMEGA

Ablation of heterogeneous material is not well understood and will play an important role in the implosion of targets such as the Polar Direct Drive- Wetted Foam (PDD-WF) concept that employ advanced fabrication techniques. To this end, we have designed and performed an experiment in a simplified planar configuration using the OMEGA cryo-platform which drives a shock through the target by ablating a heterogeneous medium of two-photon-polymerization (2PP) 3D-printed lattice and liquid deuterium (or a warm surrogate target of lattice in aerogel), where shock propagation speed and planarity of the shock structure relative to lattice morphology is measured via VISAR. Results of 2D and 3D xRAGE simulations modeling the experimental setup for a range of lattice sizes will be presented, showing the perturbation of the shock front generated by the 2PP lattice and the healing of the shock front after breakout from the lattice. Simulation results will be compared to experimental data from the shot day.   

Progress towards direct-drive wetted-foam implosions on the NIF and OMEGA

Wetted foam direct-drive inertial confinement fusion (ICF) implosions offer the prospect of improved stability, mitigation of laser-plasma instabilities (LPI), and control over the convergence ratio to achieve high gain without the stringent requirements of solid fuel layering. Recent advancements in two-photon polymerization manufacturing of deterministic and reproducible foam targets have renewed interest in wetted foam as a path forward for ICF and inertial fusion energy. Dedicated experiments to study the physics of wetted-foam spherical targets have been conducted on the National Ignition Facility (NIF). First, implosion of 18-um thick, 3-mm diameter capsules filled with liquid D₂ demonstrated the feasibility of fielding these cryogenic targets while quantifying laser-energy coupling. A transition to lower coupling after the laser burned through the CH shell and into the D₂ matches expectations from radiation-hydrodynamics simulations and bounds the expected coupling in a wetted foam layer, which consists of a CH/D₂ mixture. Further experiments on NIF will diagnose energy coupling in a liquid D₂-filled CH capsule lined with a 135-um thick, 32 mg/cm³ foam. On OMEGA, foam-lined cone-in-shell experiments filled throughout with liquid D₂ are planned to diagnose energy coupling, LPI, and shock propagation in wetted-foam ablators. Preliminary results and plans for future experiments towards the implementation of wetted-foam implosions with interior vapor regions will be discussed   

Impact of Laser-Imprint on Direct-Drive Cryogenic Target Compression at OMEGA Using Statistics-Based Analysis with 2-D DRACO Simulations

We present ensemble comparisons between the measured and predicted areal density and yield from a database of high-resolution imprint simulations covering a wide space of implosions experiments. The 2-D simulation database includes imprint modes up to 150 and various levels of other perturbations (e.g. beam-port geometry, laser power imbalance, ice roughness, etc.), allowing us to quantify the impact of each on the predicted performance as well as obtain trends with respect to design parameters of interest.  Simulations generally show a weak impact from imprint on implosions expected to have moderate stability (inflight adiabat>4, inflight-aspect-ratio<35) and, when compared to experiment, suggest the presence of an additional, significant degradation source(s). In addition to areal density and yield, laser imprint’s impact on other observables is also explored.    

Progress in 3D Modeling of Direct-drive Inertial Confinement Implosions using HYDRA

Modeling of direct-drive (DD) inertial confinement fusion implosions in 3D is essential to understanding the effects of low-, mid-, and high-mode nonuniformity sources on target compression, perturbation evolution, and performance. Cross-beam energy transfer (CBET) has been shown to have a significant effect on both laser-to-capsule energy coupling as well as nonuniformity seeding. A computational platform for modeling DD implosions using the radiation–hydrodynamics code HYDRA has been developed through an LLE–LLNL collaboration. Recently, simulations were performed of OMEGA DD implosions including the as-measured low-mode perturbations sources of beam geometry, target offset, time-dependent laser power imbalance, beam-to-beam mistiming, and beam mispointing. Both CBET and nonlocal thermal conduction were included. These results show excellent agreement with multiple experimental observables, even in the presence of a large ℓ = 1 mode. Future development work will be outlined, including near-term modeling of mid- and high-mode perturbation sources and polar-drive experiments for the NIF.   

Experimentally-Inferred Fusion-Yield Dependencies on Target Specifications for Direct-Drive Implosions on the OMEGA Laser

Recent progress in implosion experiments on the OMEGA laser has considerably improved the prospects for achieving ignition with megajoule-class lasers via direct drive. Those implosions exhibited a significant increase in performance resulting from a statistical approach used to predict the yield degradation from the 1D simulated value.^[1],[2] A new formulation of the statistical model has been developed to identify the experimental fusion-yield dependencies directly on the target specifications—namely, target thickness, ablator material, and laser beam-to-target radius. These new results are used to optimize the target geometry and ablator material to produce the highest fusion yield for OMEGA DT-layered implosions. It is shown that for a given target, the optimum beam radius is about 0.86x the target radius and that higher yields can be achieved by reducing both ice and ablator thicknesses while keeping the outer radius the same as current best performers.

30-40 MJ high yield diamond ablator designs for a NIF 2.6 MJ laser upgrade

Achieving ignition [1-3] and high fusion yields with target gain > unity [4-6] in recent National Ignition Facility (NIF) experiments has enabled accessing a new regime of density and pressure in the laboratory that was only previously accessible by nuclear weapons.  This study explores diamond ablator designs for an enhanced NIF laser capability (2.6 MJ), which are projected to produce 10s of MJ of fusion energy. Initial designs are restricted to laser powers of 450 TW, which puts constraints on the target design.  Since a direct hydrodynamic scaling of current experiments would require higher laser power, i.e., Power ~ Scale^2, we adopt a similar design methodology as that used in current NIF implosions whereby reducing the implosion velocity enables greater areal density and increased fuel burn-up fraction by fielding thicker, and in this case also larger, diamond ablators.   

 

[1] H. Abu-Shawareb, (Indirect Drive ICF Collaboration, Phys. Rev. Lett. 129 (2022).

[2] A. L. Kritcher et al., Phys. Rev. E 106, 025201 (2022)

[3] A. Zylstra, et al., Phys. Rev. E 106, 025202 (2022).

[4] H. Abu-Shawareb et al. (Indirect Drive ICF Collaboration), Phys. Rev. Lett. 132, 065102 (2024)

[5] A. L. Kritcher et al., Phys. Rev. E 109, 025204 (2024)

[6] A. Pak, A. Zylstra et al., Phys. Rev. E 109, 025203 (2024)   

Pre-shot predictions and post-shot analysis of high yield implosions on the National Ignition Facility using LANL's xRAGE code

High yield layered capsule implosions on the National Ignition Facility now routinely demonstrate gain (yield greater than input laser energy) [1]. Nevertheless, predicting the outcome of experiments remains challenging due to a high sensitivity to capsule quality [2] and drive asymmetry [3], particularly in this regime where strong alpha heating amplifies these and other sensitivities [4]. The xRAGE [5,6] code is now being used to provide pre-shot predictions for many of these implosions, supplementing HYDRA [7] and CogSim [8] predictions. xRAGE provides a unique capability to easily, routinely, and accurately model capsule defects and engineering features like the fill tube. We will discuss modeling strategies, results, and lessons learned from the predictions that have been performed so far.

[1] Abu-Shawareb et al, Phys Rev Lett 132:065102, 2024

[2] Haines et al, Phys Plasmas 29:042704, 2022

[3] Kritcher et al, Phys Plasmas 21:042708, 2014

[4] Haines et al, Phys Plasmas 31:042705, 2024

[5] Gittings et al, Comput Sci Discov 1:015005, 2008

[6] Haines et al, Phys Plasmas 24:052701, 2017

[7] Marinak et al, Phys Plasmas 8:2275, 2001

[8] Humbird et al, Phys Plasmas 28:042709, 2021   

Utilizing X-ray Shape Analysis Tools to Tune Integrated Hohlraum and Double Shell Capsule Simulations for Pre-Shot Predictions

The Double Shell Campaign is an inertial confinement fusion (ICF) concept with the goal of achieving volumetric burn instead of hot-spot ignition. For indirect drive ICF, controlling the shape of the implosion to maintain spherical symmetry is imperative for robust performance. To understand target asymmetries originating from the laser-driven hohlraum, we have modeled two previous double shell shots at the National Ignition Facility with the same capsule geometry but different laser setups using xRAGE, LANL’s Eulerian radiation hydrodynamic code.^1,2 We have tuned xRAGE input decks to match the experimental data using the method outlined by Goodarzi et al.³ Here we compare the Legendre shape coefficients of experimental and simulated X-ray radiographs; this allows us to observe the evolution of the inner and outer capsule shells during the implosion. The comparison of both simulated and experimental x-ray radiographs provides the capability for careful modeling of pre-shot predictions to help inform and guide campaign design decisions.   

Designing for early and late time symmetry control of delayed 2-shock pulse shapes relevant to double shell implosions

Current double shell experiments at the National Ignition Facility use a single shock pulse shape is used to drive the implosion. Peak laser power is reached while the shock is only midway through the ablator shell and under these conditions shock symmetry cannot be maintained. To improve shock symmetry significant changes must be made to the pulse shape, such as utilizing a multi-shock pulse. Our recent simulations have also shown that significant reductions in growth of the outer shell assembly joint are predicted with a particular type of 2-shock pulse where the main pulse is delayed until after first shock breakout by over 1 ns. However, to achieve these stability benefits we must use a long pulse duration that creates challenges for late time symmetry control and possibly high backscatter. In this presentation we will present results from integrated hohlraum simulations on various methods to reach adequate symmetry. Our current findings suggest a combination of outer cone repointing, 2-color and 4-color wavelength separation, and hohlraum gas fills above 0.45 mg/cc He4 are needed for main pulse delays of 0.7 ns after first shock breakout. We will also present symmetry sensitivity studies for double shell designs that use these 2-shock pulse shapes.   

Symmetry control using a 2-shock pulse shape in an indirect-drive double shell implosion

Double shell inertial confinement fusion compresses deuterium-tritium fuel to fusion conditions using a multi-shell target with a dense metal inner shell. Performance of these implosions requires approximate spherical symmetry of the implosion for efficient transfer of kinetic energy between shells.  Asymmetries may be seeded by the radiation drive as well as target fabrication artifacts, such as the outer shell assembly joint.

This work discusses the first use of a 2-shock pulse design with an indirectly-driven double shell capsule consisting of an aluminum ablator, a foam layer, and an unfilled tungsten inner shell. The goal of this shot was to delay the main pulse in order to improve symmetry control and mitigate asymmetries caused by the ablator joint. Shape analysis of backlighter images shows an oblate outer shell with a nearly symmetric (slightly oblate) inner shell. These experimental results are in agreement with corresponding hydrodynamic simulations. Results from a corresponding two-axis keyhole experiment which used the 2-shock pulse design will also be discussed.   

Scaling Double Shell Target Designs

Double shell inertial confinement fusion (ICF) targets¹ provide an alternative ignition platform utilizing volume ignition compared to hot spot ignition. The current point design uses a 1.5 MJ reverse ramp laser pulse that has been verified with multiple experiments to produce a predictable shape^2,3 and computationally theorized to obtain a 1 MJ thermonuclear yield. Scaling the current double shell point design to the NIF limit of 2.2 MJ could allow for additional margin built into designs, pushing designs to higher yield in a more robust fashion. This talk will focus on the physics challenges that this higher drive could encounter. Designs for 2 MJ, 2.2 MJ, and a possible NIF upgrade to 2.6 MJ and 3 MJ will be discussed. The physics of the implosion including changes to the kinetic energy transfer, estimates of the Rayleigh-Taylor growth factors, compressed fuel volume, and pusher/fuel areal density will be focused on. 

[1] D. Montgomery et al., PoP 25, 092706 (2018)

[2] R. Sacks et al., PoP 31, 062710 (2024)

[3] Z. Mohamed et al., PoP 31, 052701 (2024)   

Frustraum 1100 campaign on the National Ignition Facility

We report on capsule performance at 5% larger than current ignition-scale, driven by dual conical frustum-shaped hohlraums on the National Ignition Facility. The wall area of the frustraum is ~20% smaller than current cylindrical ignition hohlraums, which leads to an overall 12% more efficient hohlraum and higher radiation temperatures and stagnated ion temperatures for a given peak laser power. The first two layered implosions at this scale entered the burning plasma regime and exhibited an isobaric hotspot. We will also discuss proposed hohlraum geometry, drive profile and capsule thickness changes to increase time-dependent symmetry control and compression while maintaining overall drive efficiency.

 

    

Design of a full-scale High AdiabaT CH ablator implosion (HATCH) for the National Ignition Facility

A series of experiments are scheduled beginning in November 2024 to investigate the potential for improved indirect-drive implosion efficiency in a NIF ignition platform using a CH ablator. We present the design for a cryogenic fuel layered implosion that uses a 1.6 MJ laser pulse to achieve the same peak fuel velocity and total remaining mass at peak velocity as the current 2.1 MJ HDC ablator ignition platform in the same hohlraum. An important difference between this design and previous CH ablator implosions on NIF is a shorter pulse shape for better symmetry control in the low gas-fill hohlraum. This change increases the fuel entropy relative to previous implosions but also improves the predicted stability, including resistance to the tent perturbation. The amorphous nature of the CH is expected to reduce variability relative to HDC implosions due to reproducible dopant levels and a lack of microstructure and voids. Results from integrated and high-resolution capsule simulations will be presented that predict performance and estimate robustness to asymmetry and mix. This work was performed under the auspices of the U.S. DOE by LLNL under contract DE-AC52-07NA27344.