Session PO7: ICF: Hydrodynamics

A Survey of Different Perturbation Amplification Mechanisms in the Early Stages of Inertial Confinement Fusion Implosions

Hydrodynamic instability growth during shell acceleration put severe constraints on target designs in inertial confinement fusion (ICF) experiments. These instabilities are seeded during the early stages of an ICF implosion when shocks launched by intensity pickets and a main drive pulse propagate through the shell. In addition to the well-known mechanisms of early-time perturbation amplification caused by the Richtmyer–Meshkov and Rayleigh-Taylor instabilities at the ablator–DT interface, compression waves launched as a result of shock interaction with material interfaces also contribute to perturbation amplification. This talk will summarize several mechanisms contributing to the early-time perturbation evolution relevant to various ICF target designs. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

A Study of Internal Perturbation Evolution in Inertial Confinement Fusion Implosions

Performance degradation in direct-drive inertial confinement fusion (ICF) implosions can be caused by several effects, one of which is Rayleigh--Taylor (RT) instability growth. Defects in ICF targets like inner-surface voids and surface roughness create seeds for RT growth during the initial phase of implosions. Perturbations created by these defects are propagated along acoustic waves that reverberate within the shell. The presence of an ablator--ice interface creates reflected rarefaction and compression waves that can amplify these initial seeds and perturbations. The reflected rarefaction wave launched by the interface in picket-pulse designs has been shown to create an acoustic trap for perturbations near the outer edge of the shell that can create instability seeds later in the implosion. A comprehensive understanding of the evolution of these particular waves and perturbations is required to characterize the influence of these internal defects. The interplay of shell defects and acoustic wave propagation and its impact on implosion performance will be presented.   

Mixing at the Fuel--Ablator Interface in Backlit OMEGA Cryogenic Implosions

OMEGA cryogenic target implosions show a performance boundary correlated with acceleration-phase shell stability. Direct evidence that this is caused by Rayleigh$-$Taylor fuel--ablator mixing was previously obtained using a backlighter driven by a short pulse generated by OMEGA EP. The radiographic shadow cast by the shell shortly prior to stagnation shows significantly more absorption than post-processed clean simulations predict, evidence of ablator--fuel mix for an unstable implosion ($\alpha $~\textasciitilde ~1.9, IFAR~$=$~14). We show comparison of synthetic radiographs from \textit{DRACO }simulations investigating imprint and other mechanisms such as isolated surface perturbations and uncertainties in the mass ablation rate for reproducing experimental signatures of mix. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Commissioning of a wire-point projection backlighting platform for Rayleigh-Taylor instabilities experiments on Omega EP

A novel HED experimental platform was fielded at OMEGA EP to study the highly nonlinear phase of the Rayleigh-Taylor Instability (RTI) in scaled laboratory conditions relevant for the physics of young Supernova Remnant. This platform is a scale-up version of preliminary experiments performed at smaller drive energy on LULI2000. The long pulse beams of EP were used to drive the RTI whereas its evolution is probed by transverse point-projection radiography used thin titanium wires irradiated by one short pulse beam (50 ps, 700 J, 75 \textmu m focal spot). Here we will discuss the design of the experiment with radiative magneto-hydrodynamics code FLASH, the radiographies acquired as well as their sensitivity to experimental details such as target alignment or backlighter hard x-ray spectrum. Based on these results, prospects for improvements for future experiments on MJ scale facilities are presented, in association with possible novel advanced high -resolution x-ray diagnostics.   

\textbf{Optimization of Capsule Dopant Levels~to~Improve Fuel Areal Density}

The Capsule Dopant Study, a series of implosions at the National Ignition Facility, seeks to investigate improvements in fuel areal density of ICF implosions. Each experiment is fielded with a different amount of dopant in the ablator, while preserving peak velocity, coast time, remaining mass and fuel adiabat. The Scale 0.9 CH implosions are utilized in this study, as these implosions are near-round with small swings in symmetry across burn. Further, these implosions are fairly stable to ablation front perturbations from the perspective of capsule-only simulations. This study then tests the hypothesis that higher dopant levels reduce growth factors at the fuel-ablator interface, and that an increase in dopant does not compromise the stability of the fuel-ablation front. Analyses from the campaign will be presented along with comparisons to post-shot simulations.   

\textbf{Scanning the Si dopant level in DT-layered CH capsule implosions at the National Ignition Facility}

Across all ablator designs a reduction of the DT ice $\rho $r compared to predictions from hydrodynamic simulations is observed. We hypothesize that this is due to mixing at the ablator ice interface caused by preheating from Au M-shell fluorescence emitted by the hohlraum wall. The project presented here tests this hypothesis by varying the Si dopant level in the preheat shielding layer between 0.8 and 4.2 atomic {\%} with the goal of identifying an optimum Si dopant fraction that reduces the discrepancy between experiments and simulations. For this study a series of 0.9scale CH layered-DT capsule implosions (inner radius $=$ 840 $\mu $m) was fielded in low gas-fill (0.6 mg/cc) hohlraums. A diagnostic-rich configuration was implemented to identify and study diagnostic signatures of ablator-ice mix. The results of this project are important to improve stability at the ablator ice interface in future implosion designs.   

Influence of the dopant structure of CH capsule on the development of mix at the fuel-ablator interface in ICF implosions on the NIF.

NIF implosions show, across the different platform currently used, a lack of shell compression as compared to the numerical predictions. Different hypothesis could explain such disagreement. The growth of high mode mix at the fuel-ablator interface is one of them. To analyze the impact of high mode mix at the fuel-ablator interface on the shell compression the Atwood number at this interface is varied by modifying the atomic fraction of the Si doping of a set of similar CH capsules. The DT fuel mix fraction varies accordingly with the Atwood number from no mix to entirely mixed. We present here the experimental results and the corresponding numerical simulations of this dopant scanning. We show how the shell compression varies with the fuel-ablator mix. The other sources of degradation are discussed as well and compared to the high mode mix.   

\textbf{Visualizing hydrodynamic mix at capsule stagnation using spectroscopic marker layers in ICF implosions on the NIF}

Hydrodynamic instabilities in ICF implosions can cause ablator material mixing into the hot spot plasma and shell rho-r variations which adversely impact the hot spot assembly and subsequent nuclear performance. To better understand this process in indirectly driven capsule implosions at the NIF, we have added a thin Ge dopant layer at the gas-ablator and ice-ablator interface in a series of non-DT layered symmetry capsule (Symcap) and DT layered implosions. This provides a spectroscopic marker for visualizing hydrodynamic mix during capsule stagnation. Comparison of the high-resolution, spectrally filtered Symcap hot spot images with those from `standard candle' Symcap implosions that do not have a Ge dopant allows us to measure how far ablator material is mixed into the hot spot. Complementary x-ray spectroscopy measurements constrain the thermodynamic state of the mixed ablator material. While the Symcap experiment shows enhanced x-ray emission from both the tent and fill-tube locations, the subsequent Ge- and Si-doped layered DT experiments show enhanced x-ray emission dominated by the fill-tube perturbation.   

Evaluating the Effects of Imperfections in Laser Illumination and Target Geometry on Direct-Drive Cryogenic DT Implosions on OMEGA

Laser-direct-drive experiments with layered cryogenic DT targets on OMEGA are used to evaluate the impact of imperfections in laser illumination and target geometry on the implosion performance. Some effects do not disturb the observable spherical symmetry of the implosion, like shock timing, preheat, and imprint, while others, like laser irradiation and ice layer nonuniformities, will show signatures in spatially resolved observables. A comprehensive set of diagnostics including neutron detectors, x-ray self-emission, and x-ray backlighting techniques is used to distinguish between different causes for the observed performance degradation. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Laser imprint measurement and mitigation experiments in spherical geometry on OMEGA and NIF

NRL in collaboration with LLE is conducting a broad effort in laser imprint measurement and mitigation, with experiments on the Nike KrF laser and Nd:glass lasers Omega and NIF. An experimental platform is being developed on OMEGA to test imprint mitigation in spherical geometry using a high-Z coating technique pioneered in planar experiments on Nike [Obenschain et al. PoP 9, 2234 (2002)]. As demonstrated in our Omega EP experiments [M. Karasik, et al., to be published], a smooth prepulse is required to pre-expand the coating for order-of-magnitude imprint reduction. A low intensity, ISI-smoothed prepulse was used in the original Nike experiments, while an x-ray prepulse was used in the EP experiments. The OMEGA experiments aim to test both pre-expansion methods - an SSD-smoothed, few-ns duration laser prepulse as well as an x-ray prepulse, in spherical geometry. The NIF experiments intend to measure and mitigate laser imprint for direct drive at ignition scale. Areal mass non-uniformity amplified by RM/RT instability is measured using radiography of spherical shells.   

Direct-drive laser imprint experiment measuring shock velocity perturbations at Nike$^*$

Using a high resolution 2D VISAR$^{a,b}$, we have performed a laser imprint experiment measuring velocity modulations in shock wave produced by the Nike laser. The 2D VISAR takes snapshot images of the velocity field across the shock front that is imprinted and then decouples from the ablation surface before the target gets accelerated. Hence the VISAR experiment measures imprint effects without relying on Rayleigh-Taylor amplification during the acceleration providing a complementary means to study laser imprint. In this campaign, the measurements were made on shocks driven in planar CH targets by various numbers of overlapped Nike beams to deliver a broad range of illumination uniformities. Each Nike beam utilizes 1 THz bandwidth ISI beam smoothing to achieve a more uniform illumination than any other ICF laser. We also made measurements on shocks driven in high-Z coated CH targets to explore the high-Z imprint-reduction strategy pioneered in radiography experiments on Nike. The current experiment has observed the multibeam irradiation effect on imprinting when varying the number of overlapping beams and confirmed less velocity perturbations in the shock when using a high-Z overcoat. $^a$ Celliers et al, Rev Sci Instr 81, 035101 (2010). $^b$ Oh et al., APS DPP, GP11.119 (2018).   

High-resolution imaging of the Rayleigh-Taylor Vortex Breakdown

The Rayleigh-Taylor instability (RTI) is a well-known, extensively studied, hydrodynamic instability in High Energy Density Systems that arises in inertial confinement fusion and supernovae explosions. Although the RTI is heavily studied, the structures produced by simulations at late times have never been experimentally observed. Previous experiments conducted at the National Ignition Facility (NIF) utilized diagnostics with insufficient spatial resolution to observe the fine-scale structure that is predicted to occur along the RT spikes. The Crystal Backlighter Imager (CBI) produces an x-ray radiograph of the fine-scale features expected in these RT unstable systems with unprecedented clarity. By adapting a well-characterized RT- unstable NIF platform to accommodate the CBI, the resolution of the system radiographed has already improved twofold. Continued efforts to optimize the platform to highlight the fine-scale features along the RT spike tip are presently underway. The simulations and results from the first experiments of this kind will be discussed.   

Vortex-sheet modeling of hydrodynamic instabilities on oblique interfaces in the HED regime

Fluid mixing plays an important role in high energy-density systems such as inertial confinement fusion and may be traced back to the growth of hydrodynamic instabilities due to perturbed interfaces separating different species undergoing shear or accelerations. Hydrodynamic instabilities can be understood from the interfacial vorticity generation and its subsequent evolution. Our work is based on previous experimental measurements of the growth of perturbations that are tilted by a certain angle with respect to an incoming shock wave. While previous vorticity-based models investigated the early-time linear regime of perturbation growth on tilted interfaces, where the effect of the Atwood number can often be neglected, we demonstrate the importance of the Atwood number contributions to the evolution of the interface in the transition from early- to late-time. In particular we show that the interface morphology is significantly affected by Atwood-number effects from the asymmetrical growth of the bubble and spike. We use the vortex-sheet paradigm to describe RT growth after the arrival of a rarefaction wave from laser turn-off in the experiments. The low computational cost of the model enables us to show that the non-linear regime occurs sooner for larger tilt angles.   

Simulations of isolated defects in planar plastic foils

Experiments at the Naval Reearch Laboratory have been performed to examine the hydrodynamic response of laser irradiated planar foils that include isolated, structured defects. In particular we have examined the evolution of long channel-like structures of varying widths and depths in accelerated planar CH foils driven by 4 nsec pulses at about $5\times 10^{13} W/cm^2$ with the Nike laser. The Nike KrF laser overlaps up to 44 beams with state-of-the-art echelon-free ISI optical smoothing with 1 THz bandwidth, providing extremely smooth irradiation of the flat targets. The experiments were diagnosed with both face-on and side-lit radiography using monochromatic curved-crystal imagers. This measured evolution involves growth of both the isolated defect and the background perturbations (laser imprint and surface finish irregularities) by the Richtmyer-Meshkov and Rayleigh-Taylor instabilities. We will present simulations of both 2D and 3D FASTrad3D radiation-hydrodynamic modeling of these experiments and discuss both the observed similarities and differences.