Session UO5: HED: Mix, Shocks

Overview of the MARBLE Mix and Burn Campaign

MARBLE is a separated reactants campaign on the NIF to investigate the effects of heterogeneous mix on thermonuclear burn. In MARBLE experiments, a two-shock laser-driven implosion compresses Si-doped plastic capsules filled with deuterated plastic foam and cryogenic hydrogen-tritium gas fills. Embedded in the foam are ``macro-pores,'' engineered voids in the foam of known sizes and locations, which allow for control over the levels of heterogeneity prior to hydrodynamic mixing. In MARBLE implosions, the ratio of DT to DD neutron yield is measured, from which the degree of atomistic mix can be deduced. The MARBLE team has successfully demonstrated for the first time an ability to control plasma heterogeneity and study in a quantitative way the effects on thermonuclear burn. These data provide a unique means of validating mix and burn models in multi-physics ICF design codes such as the LANL xRAGE code.   

Observation of persistent species temperature separation in inertial confinement fusion mixtures

The injection of contaminant mass into the fuel region of ICF implosions is a primary factor preventing ignition. We report results of unique separated reactants implosion experiments[1] which indicate that the amount of contaminant inferred in ICF experiments is routinely underestimated. Our experiments[2] study the limiting case of pre-mixed chunks of contaminant in the fuel and we interpret the results with the aid of detailed 3D simulations that resolve mixing lengthscales. At conditions relevant to mixing regions in high-yield implosions, we observe that contaminant does not achieve thermal equilibrium with the fuel and this temperature separation persists throughout the burn phase. The assumption of thermal equilibrium is made in nearly all computational modeling of high-yield implosions as well as methods used to experimentally infer levels of contaminant present. [1]B.M. Haines et al., submitted, 2019. [2]T.J. Murphy et al., J. Phys.: Conf. Ser. 717, 012072, 2016   

Results from Marble Experiments Using an Argon/Tritium Fill Gas for Studying the Effect of Heterogeneous Mix on Thermonuclear Burn

The Marble\footnote{T J Murphy {\it et al,} J Phys:Conf Series {\bf 717}, 012072 (2016).} campaign on NIF quantifies the effect of heterogeneous mix on thermonuclear burn for comparison to a probability distribution function (PDF) burn model.\footnote{J R Fincke, unpublished; J R Ristorcelli, Phys Fluids 29, 020705 (2017).} MARBLE utilizes plastic capsules filled with deuterated plastic foam and tritium-containing gas. Indirect-drive experiments in which the Marble capsules were filled with a hydrogen/tritium gas mix and driven with either a single strong shock or using a 2-shock drive have been completed. The ratio of DT to DD neutron yield for these shots is consistent with that for uniform atomic mix regardless of the initial morphology of the foam. Recent experiments, in which the hydrogen-tritium fill gas is replaced with an argon-tritium mixture, has shown the expected decrease in DT/DD yield ratio with non-uniform initial foam morphology.   

Characterization of shock interaction with single-void in foam on OMEGA

The Marble campaign on NIF is performing experiments to quantify the effect of heterogeneous mix on inertial confinement fusion yield. The platform utilizes spherical foams with varying pore size to affect the heterogeneity of the mix. To verify accurate modeling of this complex target, it is necessary to understand the interaction between the driving shock and a single pore/void. The Marble void collapse platform on OMEGA addresses this need through two axis radiography of a void in a foam-filled shock tube. Pre- and post- shock interaction void shapes are extracted using computer vision analysis techniques, and subsequently compared to results from radiation-hydrodynamic simulation. LA-UR-19-26057.   

Modeling Shock Wave Speed in MARBLE Foam

The MARBLE campaign at Los Alamos National Laboratory (LANL) is a series of separated reactant ICF experiments employing plastic foams with engineered macro-pores designed to investigate heterogeneous material mixing during spherical implosions. We discuss the results of companion MARBLE Void Collapse experiments performed on OMEGA. These experiments were designed to validate the radiation-hydrodynamics modeling of shock propagation through foams with macropores. Foam-filled shock tubes were directly-driven by lasers on one end with x-ray radiographs generated at various times, enabling the direct measurement of shock speed, shock front shape, and shock/interface dynamics, which is not possible in a spherically convergent geometry. The pore sizes were varied to investigate the effects on shock speed. Additionally, the effect of neopentane fill gas on shock speed was investigated. We employed xRAGE, a LANL Eulerian radiation-hydrodynamics code, to perform the simulations and study the material effects. Our simulations are in good agreement with the experiments. We present the conditions necessary for accurate simulation of these experiments and discuss modeling implications.   

\textbf{Preheat Effect on Macro-Pore MARBLE Foams}

The Marble capsules consist of deuterated foam whose pores are filled with tritium gas. Initial pore sizes are varied to adjust mix morphology. During the capsule implosion, hohlraum-drive X-rays may increase foam temperature, and thereby reduce pore size. Quantifying the amount of preheat will reduce uncertainty in the Marble macro-pore simulations. An indirectly-driven preheat platform was developed at the Omega laser facility. The platform measures the expansion of a thin plastic disk using point-projection x-ray radiography as a measure of preheat. Experimental data shows that hohlraum-driven X-ray preheat is on the order of a few eV, however shock-driven heating can be stronger than X-ray preheat. Based on this result, high-Z Argon gas was added to the NIF Marble capsule to reduce shock-driven heating.   

Development, Fabrication, and Characterization of NIF MARBLE

The NIF MARBLE platform, designed to study the effects of Mix and Burn morphology, has pushed the boundaries of target fabrication by generating a demand for foam filled gas tight capsules with controlled dimensional porosity. This platform requires several unconventional but highly effective fabrication and assembly techniques. In order to understand how these relatively new techniques affect the dimensional form and composition of the capsule, six characterization methods are used. Techniques include BET specific surface area analysis, confocal micro X-ray fluorescence, scanning electron microscopy, quantitative nuclear magnetic resonance, gravimetric density analysis, and computed tomography. A chronological overview of the fabrication of NIF MARBLE capsules and foam followed by a discussion of the characterization strategies used to define the capsule and foam will be presented.   

Seeding and Modeling Mix in High-Energy-Density Physics Experiments

How one should model the mixing of materials by chaotic or turbulent motions remains an open issue. Contributing to the problem is the open question of what information must be retained of the initial conditions in order to model the late-time state effectively, particularly since the mixing may depend on fine-scale structure many orders of magnitude smaller than the larger scales of an experiment. At OMEGA-EP, where spherical crystal x-ray optics have enabled effective resolutions below 10 $\mu$m, we have been able to prepare repeated surfaces with specified broadband modal content, and to observe the evolution of the modal content into nonlinearity, resolving spectrally the dynamics of the surface evolution. Surveying results from these campaigns, we demonstrate that focused HED experiments can make a broad contribution to the essential issues of mix modeling, by elucidating the role of interfaces and their evolution under various conditions. *This work was conducted by Los Alamos National Laboratory, managed by Triad National Security LLC under U.S. DOE contract 89233218CNA000001.   

Colliding shock waves induced by supersonic to subsonic transition of radiation in silica aerogel

We have designed an experiment to be carried out at the Omega EP laser facility to study the behavior of colliding shock waves in SiO$_{\mathrm{2}}$ aerogel. The target package is comprised of a silica foam cylinder with tantala (Ta$_{\mathrm{2}}$O$_{\mathrm{5}})$ foam caps, which is exposed to a radiation drive of \textasciitilde 100 eV created by a laser-driven hohlraum. The tantala caps limit the radiation flux to enter the SiO$_{\mathrm{2}}$ foam cylinder radially, generating a converging Marshak wave in the cylinder. As the supersonic radiation wave slows down to the silica's speed of sound, a radially converging shock wave is generated. The interaction of the converging shock wave with itself at the center of the cylinder generating the conditions of interest. The goal of this experiment is to demonstrate that the interaction of shock waves produced by the supersonic-to-subsonic radiation transition can be accurately simulated with the radiation hydrodynamic code KULL.   

Analysis for Self-Emission from Spherical Shock Experiments

Measurements of the self-emission produced by a strong spherically converging shock wave driven by the OMEGA laser are analyzed using a Bayesian model-fitting procedure. Various models are used, including semi-analytic and hydrodynamics codes, and the results are compared. The primary measurements being used to constrain the models include DD neutron production and x-ray self-emission. The systems studied include solid density targets made of deuterated plastic and gas density exploding-pusher targets designed to be hydrodynamic in nature. The model fitting gives insight into the physics mechanisms that dominate the self-emission from these types of experiments. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Results from the NIF Re-Shock platform for studying Rayleigh-Taylor and Richtmyer-Meshkov instabilities in a planar geometry

We present results from experiments at the National Ignition Facility (NIF) studying the nonlinear Richtmyer-Meshkov and Rayleigh-Taylor instabilities of a multiply-shocked plasma interface in a planar geometry. Compared to ``re-shock" experiments in classical shock tubes, laser-driven systems present new opportunities to precisely vary a range of parameters, including the initial perturbation of the unstable interface, the densities of the mixing materials, and the strengths of the shockwaves. In our experiments we have used this flexibility to obtain data for different re-shock strengths, and Atwood numbers. The platform further includes the ability to diagnose both the extent of the penetration of the heavy fluid into the light fluid as well as the light fluid into the heavy, which aids with the accuracy of the mix width measurement. We present a comparison of the effect of the various experimental conditions on the observable mix width in both, our data from the NIF experiments, and the hydrodynamics simulations that have been developed to simulate these experiments.   

Demonstration of co-propagating shock formation with a hybrid drive for Mshock experiments at NIF

The LANL MShock campaign is developing a platform capable of studying RM growth and transition to turbulence in the ICF-relevant regime of thin layers driven with multiple, varying strength shocks. Reshocking an imperfect interface with a counter-propagating shock increases the RM growth rate and hastens the system's transition to turbulence. Shocking a layer multiple times from the same side should change the layer instability growth and mixing properties, but the effects co-propagating reshock have not been studied. The next step of the NIF MShock campaign is to study the isolated physics of RM evolution on a single interface, under the influence of co-propagating shocks. We present results demonstrating the ability to experimentally generate two co-propagating shocks on NIF in a shock-tube target using a hybrid direct/indirect drive scheme; direct drive generates the first shock and indirect drive generates the second shock. This scheme nominally allows independent control of shock strengths and shock timings, even at the long time scales required for instability growth experiments.   

A technique for producing co-propagating shocks on the National Ignition Facility

We describe a technique for generating planar co-propagating shocks using the National Ignition Facility laser. The case of co-propagating shocks is an important and understudied aspect of the effect of multiple shocks (termed ``reshock'') on a material interface. This kind of behavior is of computational interest for mix modeling, which seeks to understand and predict the physics of relevant systems such as inertial-confinement fusion capsules, and many astrophysical processes. Previous studies involving reshock have overwhelmingly involved counterpropagating shocks, and our technique provides an avenue for investigating the action of co-propagating shocks. This talk will focus on the computational design of the method, which involves sequential direct and indirect drives on a single planar ablator, and the details of correctly modeling the drive.   

Developing New X-ray Diagnostic Methods for HED Hydrodynamic Experiments

HED Hydrodynamic and Laboratory Astrophysics experiments are notoriously difficult to diagnose with anything other than x-rays, either by self-emission or backlighting. LANL has been working on several methods to enhance and extend x-ray diagnostics for our laser-driven HED experiments. These include stereoscopic image reconstruction and multi-frame image reconstruction using Bayesian techniques to reduce noise, remove parallax, and produce 3D images for better statistics and deeper understanding of the images. We present the results of these techniques as applied to the HED hydrodynamic experiments performed by LANL.