Bulletin of the American Physical Society
60th Annual Meeting of the APS Division of Plasma Physics
Volume 63, Number 11
Monday–Friday, November 5–9, 2018; Portland, Oregon
Session PO4: Hydrodynamic Instability |
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Chair: Elizabeth Merritt, Los Alamos National Laboratory Room: OCC B110-112 |
Wednesday, November 7, 2018 2:00PM - 2:12PM |
PO4.00001: Demonstration of Imprint Mitigation in Planar Geometry by a Combination of X‑Ray–Driven Picket-Pulse Shocks and Directly Driven Targets Wolfgang Theobald, Riccardo Betti, Arijit Bose, Suxing Hu, Sean P Regan, Chad A McCoy, Alexis Casner, Luke Ceurvorst, Max Karasik A new scheme was tested to mitigate laser imprint in direct-drive inertial confinement fusion. It is based on that the initial shock wave is driven by x rays generated by a short laser pulse followed by a long main laser pulse that directly drives the target. This hybrid scheme uses a target design with 0.5-μm-thick CH membrane coated with 40 nm of Au to generate the initial shock with x rays. The membrane is located several hundred microns away from the imploding shell. Using x rays to drive the first shock promises to significantly eliminate short and medium wavelength modes from laser imprinting. A proof-of-principle experiment was performed in planar geometry showing promising results. We demonstrate x-ray–driven shocks at peak pressures of up to 8 Mbar in quartz and CH layers for different picket energies and distances of the membrane. Face-on x-ray radiography measurements show an imprint reduction in the hybrid target compared to standard plastic foils, most obviously for short wavelengths. |
Wednesday, November 7, 2018 2:12PM - 2:24PM |
PO4.00002: Laser imprint measurement and mitigation experiments Max Karasik, Jaechul Oh, Andrew J Schmitt, Peter V Heuer, Stephen Philip Obenschain, A. Shvydky, Michael J Rosenberg, Samuel Morse, Elizabeth Hill, Matthias Hohenberger 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 EP and NIF. The high-Z coating imprint mitigation pioneered on Nike [Obenschain et al. PoP 9, 2234 (2002)] has now been demonstrated on EP through an x-ray prepulse to pre-expand the coating. Laser prepulse, while successfully used with ISI smoothing on Nike, appears to seed imprint when using an unsmoothed beam on EP, implying that smoothing on the prepulse will be necessary when moving to implosions on OMEGA. Experiments on Nike will diagnose expansion of metal coating under intensities of prepulse that can be generated with SSD on OMEGA. Experiments on the NIF aim to measure laser imprint in an ignition-scale, low f/# multibeam direct dive to benchmark the simulations and assess the need for imprint mitigation there. Non-uniformity amplified by RM/RT instability is measured using radiography of cone-in-shells. Pre-imposed modulations benchmark amplification of the laser-seeded perturbations. Results indicate significantly higher non-uniformity for ignition-relevant low-adiabat pulses, as expected. The latest measurements and possible mitigation strategies will be discussed. |
Wednesday, November 7, 2018 2:24PM - 2:36PM |
PO4.00003: Small-Scale Mix in Direct-Drive Cryogenic DT Implosions on OMEGA Christian Stoeckl, Timothy J Collins, Reuben Epstein, Valeri N Goncharov, Robert Junquist, Chad Mileham, P. B. Radha, Sean P Regan, Thomas C Sangster, Wolfgang R. Theobald
Mix on scales below the spatial resolution of conventional x-ray imagers (5 to 10 μm) in direct-drive cryogenic DT implosions on OMEGA is studied using time-gated (~20 ps), narrowband, soft x-ray (1865 eV) backlighting. The large opacity difference between DT and the carbon from the plastic ablator, which encapsulates the DT ice layer, makes this radiography setup very sensitive to ablator material mixed into the DT close to peak neutron production. The radiography data shows a strong correlation of the mix signatures with the adiabat (α = Pshell/Pfermi) and the in-flight aspect ratio of the implosion design. Details of the radiography setup including work to improve the brightness of the backlighter and the spatial resolution of the imager will also be presented.
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Wednesday, November 7, 2018 2:36PM - 2:48PM |
PO4.00004: Nonlinear Rayleigh–Taylor Growth in Spherical Geometry Valeri Goncharov Engineering features (stalks, tents, fill tubes, etc.) produce localized mass modulations that are amplified by the Rayleigh–Taylor instability developed during shell acceleration in inertial confinement fusion implosions. Growth of these features leads to shell breakup and injection of ablator and cold fuel material into the target vapor region. To gain physics insight, an analytic model describing the growth of these features has been developed. The model is based on the Layzer-type approach.[1] The talk will discuss the results of the model used to study the perturbation growth in an imploding shell. In particular, interactions between different spherical harmonics (m modes) will be presented. Comparison with numerical simulations will be discussed. [1] V. N. Goncharov and D. Li, Phys. Rev. E 71, 046306 (2005). |
Wednesday, November 7, 2018 2:48PM - 3:00PM |
PO4.00005: Self-Similar Multimode Bubble-Front Evolution of the Ablative Rayleigh-Taylor Instability in Two and Three Dimensions Huasen Zhang, Riccardo Betti, Hussein Aluie, Dov Shvarts The self-similar nonlinear evolution of the multimode ablative Rayleigh–Taylor instability is studied numerically in both two and three dimensions. It is shown that the nonlinear multimode bubble-front penetration follows the agt^2 scaling law with a dependent on the initial conditions and ablation velocity. The value of a is determined by the bubble‑competition theory, indicating that mass ablation reduces a with respect to its classical value for the same initial perturbation amplitude. It is also shown that ablation-driven vorticity accelerates the bubble velocity and prevents the transition from the bubble competition to the bubble-merger regime at large initial amplitudes, leading to higher values of a than in the classical case. These results are applied to explain the hydrodynamic-stability boundary observed in laser direct-drive implosion experiments. |
Wednesday, November 7, 2018 3:00PM - 3:12PM |
PO4.00006: 3D Broadband Bubble Dynamics for the Imprinted Ablative Rayleigh-Taylor Instability Luke Ceurvorst, Alexis Casner, Corentin Mailliet, Shahab Khan, David Martinez, Nobuhiko Izumi, Edouard Le Bel, Igor Igumenshchev, Bruce Allen Remington, Laurent Pierre Masse, Vladimir Smalyuk An experiment was performed at the National Ignition Facility to investigate the three-dimensional ablative Rayleigh-Taylor Instability (RTI). To this end, a 300 µm thick polystyrene disk was irradiated with 450 kJ of 3ω (351 nm) laser light over 30 ns. The initial seeds of the instability were formed using a well-characterized imprinting laser beam both with and without continuous phase plate smoothing. Growth of the optical depth modulations and the acceleration of the foil were observed using time-resolved x-ray radiography. The results closely follow the classical self-similar distribution until late times when the bubble sizes approached the foil thickness. Due to the remarkably long laser drive and travel distance, multiple generations of merging were observed, and measurements of the nonlinear saturation velocities were extended to unprecedented long wavelengths. Smoothing of the imprint beam showed a large decrease in RTI modulations amplitude, though its late-time evolution approached the early-time distribution of the unsmoothed seed measurements, suggesting that the loss of memory of initial conditions has begun. These experiments are of crucial importance for turbulence studies and for benchmarking 3D radiation hydrodynamics codes used in Inertial Confinement Fusion. |
Wednesday, November 7, 2018 3:12PM - 3:24PM |
PO4.00007: Experimental observation of hydrodynamic instability inhibition in a material and density gradient Willow Wan, Evan Dodd, Elizabeth Merritt, Sasikumar Palaniyappan, Tana Cardenas, Joshua P Sauppe, Yuan Ping, Vladimir Smalyuk, Eric Loomis Hydrodynamic instabilities are a significant degradation mechanism in Inertial Confinement Fusion experiments. These instabilities contribute to a loss of symmetry and an undesired intermixing between the fuel and the capsule that reduces the efficiency of the reaction. This effect is especially prominent at interfaces with sudden and large density changes, such as the surface of the heavy inner-shell in a double-shell capsule implosion. Theory suggests that the growth of these instabilities can be mitigated by replacing the sharp density jump with a smooth density gradient, but these models do not account for certain complex effects such as mixed opacities or equations-of-state.
This experiment compares instability growth between a density jump and a density gradient. A hohlraum is used to accelerate an Al ablator into pure Zr foils, Zr foils with a Be tamper, and density-gradient foils that consists of a linear material gradient of Be to Zr. We infer the growth of seed perturbations using x-ray radiography, and compare these results to 2D simulations performed with RAGE, an Eulerian radiation hydrodynamics code with Adaptive Mesh Refinement. |
Wednesday, November 7, 2018 3:24PM - 3:36PM |
PO4.00008: Robustness to Hydrodynamic Instabilities in Indirectly-Driven Layered Capsule Implosions Brian Michael Haines, Richard E Olson, S.A. Yi, Alex Zylstra, Paul A Bradley, John L Kline, Ramon J Leeper We analyze the performance of 3 layered capsule implosions on the NIF1,2 via detailed 2D and 3D simulations. These were high adiabat, moderate convergence ratio implosions with a large amount of unablated ablator mass, features that improve hydrodynamic stability. Nevertheless, while two of these implosions exhibit robustness to asymmetries in experiment and produced neutron yields within 20% of 1D simulations, which do not account for the impacts of hydrodynamics instabilities, the third implosion produced only 14% of the yield from a 1D simulation. We perform a detailed analysis of these three implosions, which suggests that the combination of several large asymmetry seeds result in the significantly degraded performance: a large 30μm fill tube, the presence of microstructure in the HDC ablator, and a higher level of drive asymmetry. Thus, while it is possible to stabilize an implosion through various means, the factors that determine stability cannot be considered independently. Furthermore, when these asymmetries are combined in 2D simulations, they can exhibit destructive interference and underpredict the yield degradation compared to experiment. |
Wednesday, November 7, 2018 3:36PM - 3:48PM |
PO4.00009: Hydro-instabilities experiments with alternate capsule support in indirect-drive implosions on NIF V. A. Smalyuk, H. F. Robey, P. Amendt, T. Bunn, Daniel T Casey, D. S. Clark, S. J. Felker, J. E. Field, Steven W Haan, Bruce A Hammel, A. V. Hamza, Warren Wen-Man Hsing, S. Johnson, Jeremy Kroll, Otto L Landen, Andrew G MacPhee, David A Martinez, Jose L Milovich, A. Nikroo, Louisa Pickworth, Michael Stadermann, Chris Weber, J. Crippen, M. O. Havre, N. Rice It was recognized that the tents had a significant impact of implosion’s stability and new alternative support methods were investigated. The tent-less methods included “fishing pole” reinforced fill tubes, cantilevered fill tubes, and thin-wire “tetra cage” supports. Other concepts used “polar tents” and a “foam-shell” to mitigate effects of the tents. These concepts were investigated in x-ray radiography experiments and compared with perturbations from standard tent support. This talk will review and summarize recent results on these alternate capsule support concepts. |
Wednesday, November 7, 2018 3:48PM - 4:00PM |
PO4.00010: Simulations of the impact of ablator micro-structure on ICF implosions Christopher Weber, Suzanne J Ali, Juergen Biener, Peter M Celliers, Daniel Clark, Steven W Haan ICF implosions using high-density carbon (HDC) ablators have achieved better fusion performance than other ablator types, but the final fuel compression is less than needed for robust fusion burn. One hypothesis for reduced compression is that the internal structure of HDC seeds hydrodynamic instabilities. HDC is composed of anisotropic microcrystalline grains with lower density between grains and may have complex dynamics near the melt transition. Measurements using the two-dimensional VISAR diagnostic at the Omega laser facility revealed velocity modulations on the shock wave that are larger with HDC ablators than with CH or Be. This data is used to construct a density modulation that is applied to simulations of ICF implosions. This talk explores the impact of this micro-structure modulation on the compression and instability growth of DT layered implosions on the National Ignition Facility. |
Wednesday, November 7, 2018 4:00PM - 4:12PM |
PO4.00011: Surface Roughness Impact on Double Shell Target Designs Ryan Sacks, Eric Loomis, Elizabeth Merritt, William S Daughton, David S Montgomery, Douglas Carl Wilson, Joshua P Sauppe, Evan Dodd, Sasikumar Palaniyappan, Tana Cardenas, Willow Wan, John L Kline, Steven Howard Batha, Peter Andrew Amendt, Robert E Tipton, Vladimir Smalyuk, Yuan Ping Double shell targets on the National Ignition Facility (NIF) provide for an alternative path to robust burn1. The target relies upon kinetic energy transfer from an ablator to a high Z and high-density pusher that is adjacent to the fuel. This has the advantage of achieving burn conditions in DT gas with a lower convergence and internal temperature compared to single shell designs. With the multiple layers with large density jumps, hydrodynamic instability growth is a concern. This work will examine the impact of surface roughness on the performance of two different double shell designs. The first is an 1100µm outer radius target inside a cylindrical hohlraum, and the second is a 1510µm outer radius target inside a rugby hohlraum. Due to differences in the maximum temperature in the hohlraum, the targets are driven differently, which affects the overall performance. In addition to impacts of the surface roughness, several mitigation strategies are investigated, including changing the tamper material, foam density, and foam composition.
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Wednesday, November 7, 2018 4:12PM - 4:24PM |
PO4.00012: Impact of Outer Shell Joint Morphology in Double Shell Capsule Designs Joshua Paul Sauppe, Steven Howard Batha, Paul A Bradley, Tana Cardenas, Evan Dodd, Franklin Fierro, Brian Michael Haines, John L Kline, Eric Loomis, Elizabeth Merritt, David S Montgomery, Sasikumar Palaniyappan, Randall Blaine Randolph, Ryan F Sacks, Willow Wan, Douglas Carl Wilson Current double shell target fabrication requires mating two hemispheres with a machined equatorial joint to form an outer shell. Experimental radiographs from sub-scale double shell experiments at the National Ignition Facility (NIF) demonstrate that the morphology of the joint significantly impacts the implosion dynamics. Further, simulations demonstrate that the joint morphology can impart significant asymmetry in the outer shell and is a major factor in yield degradation. Therefore, understanding and mitigating the joint perturbation is important for improving performance. We present simulations of the double shell joint feature using xRAGE, an Eulerian radiation-hydrodynamics code with adaptive mesh refinement, which is well suited to modeling fine-scale engineering features. Post-shot modeling of the sub-scale experiments and preparatory simulations at full NIF energy are discussed. A nominal scale capsule in a cylindrical hohlraum is compared to larger variants designed for a rugby hohlraum, and a design study for a hydro-growth radiography platform to directly image the joint feature is presented. |
Wednesday, November 7, 2018 4:24PM - 4:36PM |
PO4.00013: Deceleration phase Rayleigh-Taylor instability growth of engineered perturbations in cylindrical implosions Sasikumar Palaniyappan, Joshua P Sauppe, Eric Loomis, Dov Shvarts, Arijit Bose, Elad Malka, Derek W Schmidt, Kirk Flippo, Alex Zylstra, Bhuvana Srinivasan, Nomita Vazirani, Paul A Bradley, Steven Howard Batha, John L Kline Deceleration stage Rayleigh-Taylor instability (RTI) is believed to be a significant cause of yield degradation in current inertial confinement fusion implosions, but experimental measurements of RTI growth in convergent geometry are lacking. Recently, we have revived a cylindrical implosion platform at the Omega laser facility to measure deceleration phase RTI growth. Perturbations are engineered on the inner surface of a 500 um long aluminum marker layer that is embedded within a plastic ablator. The targets are filled with CH foam at a density of 60 mg/cc and 300 mg/cc, allowing control of convergence from 2.5 – 6.0. Using x-ray radiography, we have measured the growth of single-mode (mode 10 or mode 20) and two-mode (mode 10 and 20) sine-wave perturbations with initial amplitudes between 2-4 microns. Measurements show growth factors up to ~15. The measurements are compared to simulations of the experiments using the LANL rad-hydro code xRAGE with a laser package. The measurements are consistent with the simulation results. The measurements will also be compared to both the Epstein linear model and the nonlinear Buoyancy-Drag model. |
Wednesday, November 7, 2018 4:36PM - 4:48PM |
PO4.00014: Deceleration-Phase Rayleigh–Taylor Growth Effects on Inferred Ion Temperatures in Room-Temperature, Direct-Drive Implosions Samuel C. Miller, P. B. Radha, Valeri N. Goncharov Performance degradation in direct-drive inertial confinement fusion implosions can be caused by several effects, one of which is Rayleigh–Taylor (RT) instability growth during the deceleration phase. In room-temperature, plastic target implosions, this deceleration-phase RT growth is enhanced by the density discontinuity and finite Atwood numbers at the fuel–pusher interface. For the first time, experiments at the Omega Facility systematically varied the ratio of deuterium-to-tritium within the DT gas fill to change the Atwood number. Ion-temperature variation (ΔTi), as measured by different detectors along different lines of sight during implosions, was smaller in shots that had RT-unstable Atwood numbers (increased RT growth) than those with RT-stable Atwood numbers (and less expected RT growth). Increased levels of short-scale RT growth are suspected to be the cause of reduced ion-temperature variation. Simulations with low-mode-only asymmetries show increased ΔTi whereas the addition of high modes also shows a reduction in this variation, similar to what is believed to occur in experiments. |
(Author Not Attending)
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PO4.00015: Simulations of Gas-Ablator Mix in Symcap Implosions at the National Ignition Facility Jesse Pino, Eduard L. Dewald, Jeff Greenough, Stephan A MacLaren, Joseph E Ralph, Vladimir Smalyuk, Robert E Tipton, Ryan F Sacks Two longstanding questions in ICF physics have been the extent to which capsule ablator material mixes into the burning fusion fuel and degrades performance, and the mechanism by which this mixing occurs. Several recent campaigns at the National Ignition Facility have examined this question through the use of separated reactants. A layer of CD plastic is placed on the inner surface of the CH shell and the shell is filled with a gas mixture of H and T. This allows for a direct measurement of neutron yield proportional to the amount of gas-ablator mixing. More recently, Ge dopant has been added to the CD layer, in order to move the mix into the partially ionized regime, as a stepping stone to higher-Z platforms which hope to use radiation trapping to lower the threshold for ignition. This presentation will examine simulations of ablator-gas mix using two mixing paradigms: a Reynolds-averaged model which assumes fully-developed turbulence, and a Multicomponent Navier-Stokes diffusion-like model. We assess the ability of each model to simultaneously match the Ge doped and undoped capsules, and suggest future improvements to the models. |
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