Session JO4: Hydrodynamic Instability II

Modeling of Low Feed-Through CD Mix Implosions

The CD Mix campaign previously demonstrated the use of nuclear diagnostics to study the mix of separated reactants in plastic capsule implosions at the National Ignition Facility. However, the previous implosions suffered from large instability growth seeded from perturbations on the outside of the capsule. Recently, the separated reactants technique has been applied to two platforms designed to minimize this feed-through and isolate local mix at the gas-ablator interface: the Two Shock (TS) and Adiabat-Shaped (AS) Platforms. Additionally, the background contamination of Deuterium in the gas has been greatly reduced, allowing for simultaneous observation of TT, DT, and DD neutrons, which respectively give information about core gas performance, gas-shell atomic mix, and heating of the shell. In this talk, we describe efforts to model these implosions using high-resolution 2D ARES simulations with both a Reynolds-Averaged Navier Stokes method and an enhanced diffusivity model. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-674867   

Performance of layered DT implosions with adiabat-shaped drives on NIF

Layered DT implosions with adiabat-shaped drives were performed to study the physics of performance degradation due to instability growth and convergence. Both 3-shock and 4-shock adiabat-shaped designs were developed and demonstrated significantly reduced ablation-front instability growth. These new drives with DT fuel adiabat $\sim$ 2.1 and $\sim$ 1.6 respectively, were used in layered DT implosions showing significant improvements in performance compared to implosions during the National Ignition Campaign. Comparison of measured and simulated data will be presented. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Hydrodynamic Instability Growth Measurements at the Ablator-Fuel Interface in Layered ICF Capsule Implosions

Based on the well-established Hydro-growth Radiography (HGR) concept [1-3] we have successfully developed and fielded a new target platform to measure instability growth at the ablator-fuel interface in layered capsule implosions on the NIF. We present the results of a proof-of-principle experiment for which mode 60 perturbations with an amplitude of 4.4 $\mu$m peak-to-valley were laser-machined at the inside of a 0.8-scale plastic ablator capsule. A 55 $\mu$m thick, polycrystalline DT ice layer was grown on top of these perturbations. High quality radiography data were recorded at 4 times, showing the growth of these perturbations in both the linear and non-linear stage. We find good agreement with preliminary HYDRA simulations that include small-scale perturbations introduced by the laser machining. Future directions will be discussed. \\[4pt] [1] V.A. Smalyuk et al., Phys. Rev. Lett. \textbf{112}, 185003 (2014).\\[0pt] [2] D.T. Casey et al., Phys. Rev. E \textbf{90}, 011102 (2014).\\[0pt] [3] K.S. Raman et al., Phys. Plasmas \textbf{21}, 072710 (2014).   

ARES Simulations of a Double Shell Surrogate Target

Double shell targets provide an alternative path to ignition that allows for a less robust laser profile and non-cryogenic initial temperatures [1,2,3]. The target designs call for a high-Z material to abut the gas/liquid DT fuel which is cause for concern due to possible mix of the inner shell with the fuel. This research concentrates on developing a surrogate target for a double shell capsule that can be fielded in a current NIF two-shock hohlraum [4]. Through pressure-density scaling the hydrodynamic behavior of the high-Z pusher of a double shell can be approximated allowing for studies of performance and mix. Use of the ARES [5] code allows for investigation of mix in one and two dimensions and analysis of instabilities in two dimensions. Development of a shell material that will allow for experiments similar to CD Mix is also discussed [6,7]. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344, Lawrence Livermore National Security, LLC. Information Management release number LLNL-ABS-675098. \\[4pt] [1] P.A. Amendt, J.D. Colvin \textit{et al.,} Phys. Plasmas \textbf{9}, 2221 (2002)\\[0pt] [2] P.A. Amendt, H.F. Robey \textit{et al.,} Phys. Rev. Lett. \textbf{94}, 065004 (2005)\\[0pt] [3] H.F. Robey, P.A. Amendt \textit{et al.,} Phys. Rev. Lett. \textbf{103}, 145003 (2009).\\[0pt] [4] Discussions with S.A. MacLaren\\[0pt] [5] R.M. Darlington, Ph.D. Thesis U.C. Davis, UCRL-LR-136160\\[0pt] [6] D.T. Casey, V.A. Smalyuk \textit{et al.}, Phys. Plasmas \textbf{21}, 092705 (2014).\\[0pt] [7] Discussions with A. Nikroo and M. Hope of General Atomics.   

Measurement of inflight shell areal density perturbations in NIF capsule implosions near peak velocity

Quantitative measurements of shell-RhoR perturbations in capsules near peak implosion velocity (PV) are challenging. An external backlighter samples both sides of the shell, unless a re-entrant cone is used (potentially perturbing implosion). Emission from the hot core, after shock-stagnation and prior to PV, has been used as a self-backlighter, providing a means to sample one side of the capsule. Adding high-Z gas ($\sim$ 1{\%} Ar) to the capsule fill in Symcaps ($^{4}$He), has produced a continuum backlighter with significant increase in emission at photon energies $\sim$ 8 keV over nominal fills. From images of the transmitted self-emission, above and below the K-edge of an internally doped Cu layer, we infer the growth at PV of imposed perturbations (100 nm amplitude, mode 40).   

Hydrodynamic Instability Growth in Polar-Direct-Drive Implosions at the National Ignition Facility

Polar direct drive (PDD) is an alternative, direct-drive inertial confinement fusion platform being developed at the National Ignition Facility (NIF). Shell stability of the target is of key importance for an optimized performance. We have begun an experimental campaign to characterize Rayleigh--Taylor (RT) growth and laser imprint in spherical PDD implosions on the NIF. Plastic, cone-in-shell targets with an outer diameter of $\sim 2.2$ mm were imploded, and the RT-amplified shell mass modulations were tracked via measurements of the 2-D optical depth variations using soft x-ray radiography. The RT growth of discrete modes was investigated by machining single-mode, sinusoidal corrugations onto the target surface, which acted as well-characterized seeds. We will present platform characterization and backlighter optimization data as well as experimental results of instability growth in spherical PDD experiments on the NIF. The experimental data will be compared to 2-D \textit{DRACO} simulations and strategies for measuring high $\ell \mbox{-mode}$ perturbations $>300$ and for mitigating imprint in future PDD experiments will be discussed. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

Using radiation temperature to monitor plasma drive in materials strength experiments

Materials strength experiments at the National Ignition Facility generate smooth loading in a material by the plasma drive of a shocked reservoir mounted on the side of a gold \textit{hohlraum}. In these experiments, the loading profile of plasma unloading across a gap and then stagnating at the target is measured with VISAR. Geometric limitations preclude simultaneous measurement of VISAR and the Rayleigh-Taylor (RT) growth that is used to determine strength. We use \textit{hohlraum} radiation temperatures measured with the Dante spectrometer to link the drive measured with VISAR to the stress condition when RT growth is measured. By combining Dante measurements from two different lines of sight with view factor calculations, we infer the radiation drive into the reservoir. With this method, we can account for spatial variations within the \textit{hohlraum} and also reproduce observed variations due to changes in pointing and target orientation. We describe the simplified, physics-based analysis of Dante spectra and the methods of determining radiation drive to the reservoir. We then discuss the effectiveness of this method for inferring drive at the target material.   

Observations of vortex merger and growth reduction in a dual-mode, supersonic Kelvin-Helmholtz instability experiment

The Kelvin-Helmholtz instability (KHI) generates vortical structures and turbulence at an interface with shear flow. This instability is ubiquitous in natural and engineering systems including astrophysical environments and laboratory plasmas. Detailed measurements of modulation amplitude growth reduction and vortex merger evolving from well-defined initial conditions can benchmark hydrodynamic models and theories. This experiment provides the first measurements of the vortex merger rate of well-characterized seed perturbations evolving under the influence of the KHI in a supersonic flow. These data were obtained by utilizing a sustained laser pulse to drive a steady shockwave into low-density carbon foam, introducing shear along a precision-machined plastic interface. The evolution and merger of the modulations was measured with x-ray radiography and reproduced with 2D hydrodynamic simulations. This work is funded by the U.S. Department of Energy, through the NNSA-DS and SC-OFES Joint Program in High-Energy-Density Laboratory Plasmas, grant number DE-NA0001840, and the National Laser User Facility Program, grant number DE-NA0002032, and through the Laboratory for Laser Energetics, University of Rochester by the NNSA/OICF under Cooperative Agreement No. DE-FC52-08NA28302   

Experimental Studies of the Electrothermal and Magneto-Rayleigh Taylor Instabilities on Thin Metal Foil Ablations

The electrothermal instability (ETI) and magneto-Rayleigh Taylor instability (MRT) are important in the implosion of metallic liners, such as magnetized liner implosion fusion (MagLIF). The MAIZE linear transformer driver (LTD) at the University of Michigan generates 200 ns risetime-current pulses of 500 to 600 kA into Al foil liners to study plasma instabilities and implosion dynamics, most recently MRT growth on imploding cylindrical liners. A full circuit model of MAIZE, along with I-V measurements, yields time-resolved load inductance. This has enabled measurements of an effective current-carrying radius to determine implosion velocity and plasma-vacuum interface acceleration. Measurements are also compared to implosion data from 4-time-frame laser shadowgraphy. Improved resolution measurements on the laser shadowgraph system have been used to examine the liner interface early in the shot to examine surface perturbations resulting from ETI for various seeding conditions. Fourier analysis examines the growth rates of wavelength bands of these structures to examine the transition from ETI to MRT.   

Detection and use of HT and DT gamma rays to diagnose mix in ICF capsules

Recent results from Omega capsule implosion experiments containing HT-rich gas mixtures indicate that the 19.8 MeV gamma ray from aneutronic HT fusion can be measured using existing time-resolved gas Cherenkov detectors (GCDs). Additional dedicated experiments to characterize HT-$\gamma $ emission in ICF experiments already have been planned. The concurrent temporally-resolved measurement of both HT-$\gamma $s and DT-$\gamma $s opens the door for in-depth exploration of interface mix in gas-filled ICF capsules. We propose a method to temporally resolve and observe the evolution of shell material into the capsule core as a function of fuel/shell interface temperature (which can be varied by varying the capsule shell thickness). Our proposed method uses a CD-lined plastic capsule filled with 50/50 HT gas and diagnosed using GCDs to temporally resolve both the HT ``clean'' and DT ``mix'' gamma ray burn histories. It will be shown that these burn history profiles are sensitive to the depth to which shell material mixes into the gas region. An experiment to observe these differences as a function of capsule shell thickness is proposed to determine if interface mixing is consistent with thermal diffusion $\left( {\lambda_{ion} \propto {T_{ion}^{2} } \mathord{\left/ {\vphantom {{T_{ion}^{2} } {Z_{ion}^{2} }}} \right. \kern-\nulldelimiterspace} {Z_{ion}^{2} }\rho } \right)$ at the gas/shell interface. Since hydrodynamic mixing from shell perturbations, such as the mounting stalk and glue, could complicate these types of capsule-averaged temporal measurements, simulations including their effects also will be shown.   

Interspecies Ion Diffusion Studies using DT, DT($^{3}$He), and DT(H) Implosions

Anomalous ICF yield degradation has been observed from gas fills containing mixtures (i.e., D($^{3}$He), DT($^{3}$He), D(Ar), and even DT). Interspecies ion diffusion theory has been suggested as a possible cause resulting from gradient-driven diffusion (i.e., pressure, electric potential, and temperature) which forces lower mass ions away from core and higher mass ions toward core. The theory predicts hydrogen addition to deuterium or tritium should result in increased yield compared to expected yield, which is opposite to $^{3}$He addition. At Omega laser facility, we have tested hydro-equivalent fills of DT, DT($^{3}$He), and DT(H) with the assumption that same fuel mass and particle pressure will provide identical convergence. Preliminary results verify a factor of 2 yield reduction relative to scaling when $^{3}$He added to DT. At DT(H) case, however, no significant yield degradation or a slight yield enhancement was observed which agrees with the interspecies ion diffusion theory. Detailed experiment results and simulation are needed to confirm the initial observation.   

Radiochemical Signatures of Interfacial Areal Density and Mix in NIF Implosions

Recent experimental results from the Radiochemical Analysis of Gaseous Samples (RAGS) diagnostic facility fielded at the National Ignition Facility (NIF) have demonstrated $^{13}$N production from charged particle nuclear reactions. This radiochemical product is very sensitive to the fuel-ablator interface areal density. Two specific reactions dominate $^{13}$N production: $^{12}$C(d,n)$^{13}$N and $^{13}$C(p,n)$^{13}$N. The short range of the energetically up-scattered deuterons from the cold DT fuel layer restricts the production to the proximate ablator interface thus providing high sensitivity to the interfacial configuration. Although the proton-mediated reaction is almost equally favorable, the small natural abundance of $^{13}$C suppresses this contribution to $^{13}$N production. Representative HYDRA simulations are used to illustrate these observed effects.   

Buoyancy instability of homologous implosions

Hot spot turbulence is a potential contributor to yield degradation in inertial confinement fusion (ICF) capsules, although its origin, if present, remains unclear. In this work, a perturbation analysis is performed of an analytical homologous solution that mimics the hot spot and surrounding cold fuel during the late stages of an ICF implosion. It is shown that the flow is governed by the Schwarzschild criterion for buoyant stability, and that during stagnation, short wavelength entropy and vorticity fluctuations amplify by a factor $\exp\left(\pi \left|N_0\right| t_s\right)$, where $N_0$ is the buoyancy frequency at stagnation and $t_s$ is the stagnation time scale. This amplification factor is exponentially sensitive to mean flow gradients and varies from $10^3$--$10^7$ for realistic gradients. Comparisons are made with a Lagrangian hydrodynamics code, and it is found that a numerical resolution of ${\sim}30$ zones per wavelength is required to capture the evolution of vorticity accurately. This translates to an angular resolution of ${\sim}(12/\ell)^\circ$, or ${\sim}0.1^\circ$ to resolve the fastest growing modes (Legendre mode $\ell > 100$).   

Sensitivity of mix in Inertial Confinement Fusion simulations to diffusion processes

We explore two themes related to the simulation of mix within an Inertial Confinement Fusion (ICF) implosion, the role of diffusion (viscosity, mass diffusion and thermal conduction) processes and the impact of front tracking on the growth of the hydrodynamic instabilities. Using the University of Chicago HEDP code FLASH, we study the sensitivity of post-shot simulations of a NIC cryogenic shot to the diffusion models and front tracking of the material interfaces. Results of 1D and 2D simulations are compared to experimental quantities and an analysis of the current state of fully integrated ICF simulations is presented.   

Evidence of Systematic Jetting in Nominal Omega Implosions

By means of detailed comparison between narrow-spectrum tracer-emission-images and 2-D radiation-hydrodynamic calculation, we present evidence of a systematic hydrodynamic defect of nominal OMEGA implosions. The defect [1], which arises from a drive asymmetry caused by capsule mounting, distorts the low-mode symmetry and pressure profile of the fuel cavity and also enhances deceleration-phase fuel-shell mixing. It is a critical consideration for interpretations of performance degradation (and for analyses dependent on shape assumptions). The tracer technique is predicted to differentiate the change in fuel cavity structure between the existing and a proposed improvement of the capsule mounting. The influence of the defect on the fuel-shell mixing is also shown to be an essential consideration for analysis of separated reactants experiments. \\[4pt] [1] I.V. Igumenshchev et al. Physics of Plasmas 16: 082701 (2009).