Bulletin of the American Physical Society
2005 47th Annual Meeting of the Division of Plasma Physics
Monday–Friday, October 24–28, 2005; Denver, Colorado
Session LO1: Hydrodynamic Instabilities |
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Chair: Michael Marinak, Lawrence Livermore National Laboratory Room: Adam's Mark Hotel Governor's Square 10 |
Wednesday, October 26, 2005 2:00PM - 2:12PM |
LO1.00001: Robustness of the Laser M\'{e}gajoule baseline design with graded dopant Jean Giorla, Didier Galmiche, Laurent Masse, Francoise Poggi, Francois Renaud, Patricia Seytor The Laser M\'{e}gajoule facility is under construction near Bordeaux and will deliver up to 2 MJ. The nominal target design has been chosen to provide enough margins towards uncertainties due to laser plasma interaction, radiation asymmetry and hydrodynamic instabilities. The first capsules we studied had uniformly doped plastic ablators, and we have recently begun the study of a plastic ablator with graded Ge dopant. We have estimated the low mode deformations of the DT shell with a linear analysis approach. These deformations are due to the intrinsic x-rays non-uniformity and hydrocoupling, the capsule roughness, and the laser and target 3D-defects. The technological dispersions are taken into account by estimating the final deformation probability distribution with each error sampling according to the specification. 2D-hydrodynamic instability simulations were performed with modes 12 to 120 perturbations at outer ablator interface. The graded dopant drastically lowers the growth factors at DT/CH interface and at the hot spot location. It follows that the graded doped design may support an outer ablator roughness close to 300nm rms(12-120). Sensitivity of hydro instability growth to DT gas density, which depends on the cryogenic temperature, is evaluated with a 1D-mixed model. [Preview Abstract] |
Wednesday, October 26, 2005 2:12PM - 2:24PM |
LO1.00002: Rayleigh-Taylor instability and planar Feedthrough experiments With CHGe on OMEGA Alexis Casner, G. Huser, J-P Jadaud, B. Villette, M. Vandenboomgaerde, D. Galmiche, S. Liberatore, T. Boehly Germanium-doped CH (CHGe) is the nominal ablator for LMJ target design. To investigate its properties we performed indirect drive experiments on the OMEGA laser facility. On each shot foil motion and modulations growth were simultaneously measured by side-on and face-on radiography, while drive was assessed by measuring radiation escaping hohlraum LEH. This allows to better constrain hydrocode simulations of the experimental results. We compare CHGe front and rear-side perturbations growth with those acquired on CHBr in the same experimental configuration. Additionnal shots allow us also to get shock breakout timings and to characterize the X-Ray flux on the foil with CEA Soft X-Ray Spectrometer DMX, whose results will be compared with DANTE measurements through the LEH. [Preview Abstract] |
Wednesday, October 26, 2005 2:24PM - 2:36PM |
LO1.00003: Measurements of Bubble Evolution in the Nonlinear Ablative Rayleigh--Taylor Instability O. Sadot, V.A. Smalyuk, J.A. Delettrez, D.D. Meyerhofer, T.C. Sangster, R. Betti, V.N. Goncharov, D. Shvarts Rayleigh--Taylor instability plays a crucial role in inertial confinement fusion, astrophysics, and other applications. In the present work, 3-D broadband modulations were imprinted by laser nonuniformities on a planer target and were measured at the late nonlinear stage using face-on radiography. The high quality of the results enabled us to determine for the first time the instability evolution in terms of bubble size, amplitude, and velocity distributions. During the evolution, as the bubbles compete and merge, the modulations reach a self-similar regime in which the modulations \textit{$\sigma $}$_{rms}$ and bubble size \textit{$\lambda $} grow as $\sim g t^{2}$, where $g$ and $t$ are the foil acceleration and time, respectively. A phenomenological description of the evolution of the bubble distribution based on the self-similar evolution will be presented. This work was supported by the U.S. Department of Energy Office DOE of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460. [Preview Abstract] |
Wednesday, October 26, 2005 2:36PM - 2:48PM |
LO1.00004: Rayleigh-Taylor and Richtmyer-Meshkov instability experiments in cylindrical geometry at OMEGA Glenn Magelssen, J.R. Fincke, N.E. Lanier, S.H. Batha, N.D. Delamater, R.M. Hueckstaedt, J.M. Taccetti, K.W. Parker, C.J. Horsfield, S.D. Rothman Understanding Rayleigh-Taylor and Richtmyer-Meshkov instabilities for inertial confinement fusion (ICF) capsules is an important goal for the National Ignition Facility (NIF). We have completed Rayleigh-Taylor instability experiments in cylindrical geometry on the University of Rochester laser facility, OMEGA. Unlike previous experiments, for the experiments we will describe the perturbations were placed on the inside of the AL marker that was between an outer epoxy ablator and an inner CH foam region. The foam density was varied to control the amount of deceleration experienced by the marker layer. The cylinder was directly imploded with 50 laser beams. Unlike our outside perturbation, 60 mg/cc experiments that showed significant growth, no instability was seen for similar experiments with inner perturbations. On the other hand, for a foam density of 500 mg/cc we saw significant and similar growth for 0.1, 2.5 and 9 micron wavelength and 0.1 to 1 micron amplitude perturbations. Comparisons between the RAGE code calculations and the experiemnts will be shown [Preview Abstract] |
Wednesday, October 26, 2005 2:48PM - 3:00PM |
LO1.00005: Development of a Platform for High Growth-Factor Rayleigh-Taylor Experiments on the Omega Laser David Braun, David Bradley, Kelly Campbell, Gilbert Collins, John Edwards Current plans for the NIF ignition target call for the use of Beryllium as the capsule ablator. There is a concern however, that the Beryllium microstructure will provide a seed for the growth of Rayleigh-Taylor instabilities. We present here a platform being developed to perform high growth factor RT experiments with Beryllium on Omega. The platform consists of a horizontal halfraum, with the RT target centered on-axis over a hole in the back wall. The RT growth of the sample is backlit and viewed along the halfraum axis through the LEH. The halfraum is driven by a series of staggered 2.4 ns laser pulses, timed to produce a 2-step drive consisting of a 3 ns foot (Tr=100 eV), followed by an extended main drive (Tr=150 eV). Simulations show that very small amplitude initial perturbations have linear RT growth factors of up to 1000 over the 10 ns laser drive. A gas fill is used to avoid Au penetration to the halfraum axis where it can obscure the backlighter view of the RT package. The talk will also compare the predicted Tr drive to that measured in the initial Omega experiments. [Preview Abstract] |
Wednesday, October 26, 2005 3:00PM - 3:12PM |
LO1.00006: Comparison of simulations and experimental results from ICF implosions using capsules of varying surface roughness. R.E. Turner, P.A. Amendt, O.L. Landen, R.J. Wallace, V. Glebov, K. Thorp, G. Pien We have conducted a series of indirect-drive ICF implosion experiments at Omega, using capsules with deliberately roughened surfaces. The 10 atm DD fill capsules had a convergence ratio of 18, higher than previous Nova experiments [M. Marinak et al, Phys. Plasmas 3, 2070 (1996)]; the pre-heat shielded, Ge-doped CH ablators had moderately high ($\sim $200) Raleigh-Taylor growth factors. Each capsule's surface quality was measured using atomic force microscopy. Gated x-ray imaging of the imploded core was used to assure that basic symmetry was maintained, while `best-surface' capsules were used as controls with every experimental run. Neutron yields were observed to decrease as surface roughness increased. Integrated simulations, including mix modeling, have been performed, and are compared to the experimental results. This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. [Preview Abstract] |
Wednesday, October 26, 2005 3:12PM - 3:24PM |
LO1.00007: Validation of LASNEX Calculations of the Beryllium Ablator Microstructure and Stability Experiments Barbara G. DeVolder, S. Robert Goldman, Nelson M. Hoffman, James A. Cobble, Thomas E. Tierney The Omega laser at the University of Rochester is being used to evaluate ablative Rayleigh-Taylor instability growth rates in beryllium, a candidate for capsule ablator material for the National Ignition Facility. Recent experiments (``Late-time Radiography of Beryllium Ablators in Long-pulse Gas-filled Hohlraums,'' J.A. Cobble, invited talk) using a gas-filled hohlraum drive with a composite 6-ns laser pulse have characterized unstable growth of machined sinusoidal surface perturbations on copper-doped beryllium samples, radiation temperatures in the hohlraum, and preheat and shock behavior in the beryllium. The two-dimensional radiation-hydrodynamics code LASNEX has been used to simulate these experiments. With the goal of matching calculations and experimental results, we have tested different models and parameters in the code. We describe a range of calculations, indicating those that are validated by experimental data and assessing possible limitations of the models that may preclude validation. This work was performed under the auspices of the U.S. Department of Energy under Contract No. W-7405-ENG-36. [Preview Abstract] |
Wednesday, October 26, 2005 3:24PM - 3:36PM |
LO1.00008: Rayleigh-Taylor instability experiments in cryogenic deuterium J.F. Hansen, A.R. Miles, H.F. Robey, V.A. Smalyuk, T.C. Sangster, T.R. Boehly, M.J. Bonino, D.D. Meyerhofer We report on experiments under way at the Omega laser, using cryogenic deuterium to study Rayleigh-Taylor instabilities in laser targets. These instabilities are important in astrophysical situations (e.g., mixing of the different shells during a supernova explosion) and in inertial fusion (during the compression stage of a fusion target). They can be studied in small ($\sim $1 mm) shock tubes filled with one heavy and one light material, with an interface between the two materials that is machined to seed the instability. A high-energy laser ($\sim $5 kJ) drives a shock from the heavy to the light material. The evolution of the interface is studied using gated x-ray cameras, where x-ray illumination is obtained from additional laser beams focused on metal backlighter foils. Traditionally the heavy material is CH (1 g/cm$^{3})$ doped with I or Br for improved contrast, while the light material is a low-density ($\sim $0.1 g/cm$^{3})$ C foam. The goal of the current experiments is to determine if contrast can be improved even further by replacing the foam with cryogenic deuterium, which has a density similar to the foam, but a lower x-ray opacity allowing clearer images, including images taken at late times in the evolution. Work performed under the auspices of the Department of Energy by the Lawrence Livermore National Laboratory under contract number W-7405-ENG-48. [Preview Abstract] |
Wednesday, October 26, 2005 3:36PM - 3:48PM |
LO1.00009: K-L turbulence model for Rayleigh-Taylor and Richtmyer-Meshkov instabilities Guy Dimonte, Robert Tipton A turbulence model is developed to describe the self-similar growth of the Rayleigh-Taylor (RT) and Richtmyer-Meshkov (RM) instabilities. The model describes the evolution of the dominant eddies in the mixing zone with a dimension L and a specific energy K $\equiv $ V$^{2}$/2. The equations are based on the successful buoyancy-drag models [1] for RT and RM flows, but constructed only with local parameters so that it can robustly describe multi-dimensional flows with multiple shells of materials. The model has several unknown coefficients that are determined by comparing analytical and numerical solutions with RT and RM experiments [2]. [1] Guy Dimonte, Phys. Plasmas 7, 2255 (2000) [2] Guy Dimonte and M. Schneider, Phys Fluids 12, 304 (2000). [Preview Abstract] |
Wednesday, October 26, 2005 3:48PM - 4:00PM |
LO1.00010: Numerical Study of Temporal Density Variation Effects on Nonlinear Perturbation Evolution in Classical Rayleigh--Taylor Instability D. Li, V.N. Goncharov A two-dimensional compressible Eulerian code was developed to investigate the effect of temporal density variation on the nonlinear bubble evolution of Rayleigh--Taylor instability.\footnote{ V. N. Goncharov and D. Li, Phys. Rev. E \textbf{71}, 046306 (2005).} In this study, a rarefaction wave is generated to accelerate the interface between heavy- and light-density fluids. When the rarefaction wave propagates through the foil, both light and heavy fluid densities decrease in time, and the perturbation evolution at the interface is affected by such decompression. We calculate the perturbation amplitude and interface curvature at the position of bubble and spike and compare the results with the prediction of Layzer-type model.$^{1}$ This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460. [Preview Abstract] |
Wednesday, October 26, 2005 4:00PM - 4:12PM |
LO1.00011: A multi-scale character of the large-scale coherent dynamics in the Rayleigh-Taylor instability Snezhana Abarzhi, Katsunobu Nishihara We report nonlinear solutions for a system of conservation laws describing the dynamics of the large-scale coherent structure of bubbles and spikes in the Rayleigh-Taylor instability for fluids with a finite density ratio. Three-dimensional flows are considered with general type of symmetry in the plane normal to the direction of gravity. The non-local properties of the interface evolution are accounted for on the basis of symmetry theory. It is shown that isotropic coherent structures are stable. For anisotropic structures, secondary instabilities develop with the growth-rate determined by the density ratio. For stable structures, the curvature and velocity of the nonlinear bubble have non-trivial dependencies on the density ratio, yet their mutual dependence on one another has an invariant form independent of the density ratio. Based on the obtained results we argue that the large-scale coherent dynamics in RTI has a multi-scale character. [Preview Abstract] |
Wednesday, October 26, 2005 4:12PM - 4:24PM |
LO1.00012: Distribution of sizes and energy spectrum in the fragmented (``mixed") state induced by Rayleigh-Taylor instability Giora Hazak, Yonatan Elbaz, Alexander Velikovich, John Gardner, Andrew Schmitt, Steven Zalesak A study, based on simulations and experiments as well as analytical derivations, of the fragmented (``mixed'') state induced by the Rayleigh-Taylor instability at the interface between two fluids is presented. The distribution of sizes and the energy spectrum in the fragmented state are derived from the symmetries exhibited by the data. These functions are useful for the construction of a sub-grid model as well as for the understanding of the internal structure of the mixed state. [Preview Abstract] |
Wednesday, October 26, 2005 4:24PM - 4:36PM |
LO1.00013: Atomistic dynamics and the evolution of the Richtmyer-Meshkov instability Vasilii Zhakhovskii, Sergey Zybin, Snezhana I. Abarzhi, Katsunobu Nishihara For the first time the molecular dynamics (MD) approach is applied to study the evolution of the shock-driven Richtmyer- Meshkov instability (RMI), which develops at the corrugated interface separating two Lennard-Jones (LJ) liquids or two solids with different densities. Compared to traditional hydrodynamic simulations, MD has a number of fundamental advantages. It accounts for strong gradients of the pressure and temperature, and captures accurately the heat transfer and the viscous or plastic flow at early (shock passage) as well as late (turbulent mixing) stages of the instability evolution. MD has no limitations for the spatial resolution and does not require the assumption of thermodynamic equilibrium. We analyze the influence of the critical parameters, energy and mass transfer, and the governing stresses on the growth of the interface perturbations, and compare the cases of LJ liquids and solids. In liquids, RMI is driven by the non-uniform velocity shear and vorticity production. In solids, the development of visco-plastic flow, shear stresses, and elastic anisotropy influences significantly the evolution of initial perturbations. [Preview Abstract] |
Wednesday, October 26, 2005 4:36PM - 4:48PM |
LO1.00014: New properties of the interface evolution in the Richtmyer-Meshkov instability Marcus Herrmann, Snezhana Abarzhi, Parviz Moin We report analytical and numerical solutions describing the dynamics of the two-dimensional coherent structure of bubbles and spikes in the Richtmyer-Meshkov instability for fluids with a finite density ratio. The theory accounts for the non-local properties of the interface evolution and the simulations treat the interface as a discontinuity. Good agreement between the analytical and numerical solutions is achieved. To quantify accurately the interface evolution in the observations, new diagnostics and scalings are suggested. The velocity, at which the interface would move if it would be ideally planar, is used to set the flow time-scale as well as the reference point for the bubble (spike) position. The data sampling has high temporal resolution and captures the velocity oscillations caused by sound waves. The bubble velocity and curvature are both monitored, and the bubble curvature is shown to be the relevant diagnostic parameter. The obtained results yield new properties of the evolution of the Richtmyer-Meshkov instability. In the nonlinear regime, the bubbles flatten and decelerate. The flattening of the bubble front indicates a multi-scale character of the coherent dynamics. [Preview Abstract] |
Wednesday, October 26, 2005 4:48PM - 5:00PM |
LO1.00015: Vortex dynamics in incompressible Richtmyer-Meshkov Instability Chihiro Matsuoka, Katsunobu Nishihara We study temporal evolution of an interface in the Richtmyer- Meshkov instability numerically. The interface is treated as a vortex sheet and the Birkhoff-Rott equation is used in order to describe motion of a vortex sheet. We show that redistribution of grid points to equal arclength and the application of the Fourier series expansion for numerical differentiations and integrations make it possible to perform long-time caluculations. Successive profiles of a vortex sheet and the temporal evolution of the sheet strength are presented, and especially the evolution of the sheet strength of a vortex core, defined as a point at which the absolute values of the curvature and strength of a sheet become maximum, is discussed. It is found that the sheet strength of a vortex core takes a maximal value at a finite time and turns to gradually decreasing when the Atwood number in the system is non-zero. [Preview Abstract] |
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