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 RO1: Instability Mitigation/Mix |
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Chair: Valeri Goncharov, University of Rochester Room: Adam's Mark Hotel Governor's Square 10 |
Thursday, October 27, 2005 2:00PM - 2:12PM |
RO1.00001: Radiation Transport in Saturn Targets Used for Polar Direct Drive R.S. Craxton, F.J. Marshall, M.J. Bonino, S.G. Noyes, V.A. Smalyuk Saturn targets, spherical capsules placed within an equatorial low-$Z$ ring, are of interest for polar-direct-drive ignition experiments on the NIF using just the indirect-drive beam ports. Radiation from the ring to the capsule is calculated using a new, view factor-like radiation transport model in the 2-D hydrocode \textit{SAGE} that includes full directional and spectral information. This model provides improved agreement with experimental measurements on OMEGA of the drive uniformity of Saturn targets with a CH ring and a deuterium-filled CH capsule. 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] |
Thursday, October 27, 2005 2:12PM - 2:24PM |
RO1.00002: Polar-Direct-Drive Experiments on OMEGA F.J. Marshall, R.S. Craxton, M.J. Bonino, R. Epstein, V.Yu. Glebov, D. Jacobs-Perkins, J.P. Knauer, J.A. Marozas, P.W. McKenty, S.G. Noyes, P.B. Radha, W. Seka, S. Skupsky, V.A. Smalyuk Polar direct drive (PDD), a promising ignition path for the NIF while the beams are in the indirect-drive configuration, is currently being investigated on the OMEGA Laser System by using 40 beams in six rings that are repointed to more uniformly illuminate the target. The OMEGA experiments are being performed with standard ``warm'' targets (865-\textit{$\mu $}m-diam, 20-\textit{$\mu $}m-thick, 15-atm, D$_{2}$-filled CH shells), with and without the use of an equatorial ``Saturn-like'' toroidally shaped CH ring (nominal dimensions: 2200-\textit{$\mu $}m diam, 300-\textit{$\mu $}m thick). For Saturn targets, the plasma formed around the ring refracts light toward the target equator as the ring plasma expands. The nominal laser drive is a 1-ns flat pulse, $\sim $400 J per beam, employing 1-THz, 2-D SSD with polarization smoothing. Target implosion symmetry is diagnosed with framed x-ray backlighting using additional OMEGA beams and by time-integrated x-ray imaging of the stagnating core. The best results have been obtained with Saturn targets by varying the beam pointing and ring diameter, achieving $\sim $75{\%} of the fusion yield from symmetrically illuminated targets with the same total energy (60 beams, 15.3 kJ). 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] |
Thursday, October 27, 2005 2:24PM - 2:36PM |
RO1.00003: Reduction of Effects of Nonuniform Laser Irradiation in Polar-Direct-Drive Implosions on the NIF I.V. Igumenshchev, R.S. Craxton, P.W. McKenty, J.A. Marozas, S. Skupsky Polar direct drive (PDD) was proposed to perform direct-drive implosion experiments on the National Ignition Facility in its indirect-drive configuration. This requires repointing some of the laser beams toward the equator to reduce irradiation nonuniformities on a target. We present results of a numerical study of optimization techniques that minimize the effects of the irradiation nonuniformities on the cryogenic PDD target performance. These techniques include variation of the beam pointing, individual pulse shapes, spot shapes for each beam, and alternative target geometry.\footnote{ R. S. Craxton and D. W. Jacobs-Perkins, Phys. Rev. Lett. \textbf{94}, 095002 (2005).} Numerical simulations utilize the 2-D hydrodynamic code \textit{DRACO} with the full 3-D laser ray trace. 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] |
Thursday, October 27, 2005 2:36PM - 2:48PM |
RO1.00004: Hydrodynamic stability of direct-drive high-gain inertial-confinement-fusion targets J.W. Bates, A.J. Schmitt, S.P. Obenschain, D. Colombant, S.T. Zalesak, D.E. Fyfe, J.H. Gardner, L.J. Perkins Success with direct-drive high-gain targets for inertial confinement fusion depends critically on understanding (and controlling) the hydrodynamic instabilities that develop during the implosion process. A key task in this analysis is to determine the tolerances for target surface finishes and optically-smoothed laser drive. Because available experimental facilities are unable to directly assess how such nonuniformities affect ignition and yield, numerical simulations coupled with analytical modeling are extremely important. In this presentation, we discuss the status of single-mode stability calculations performed with the FAST radiation hydrocode on NRL's high-gain target designs, and compare our results against ablative theoretical models. The two-dimensional spherical simulations predict that the performance degrades significantly from the one-dimensional ``clean'' case, but may be improved through the use of a spike pre-pulse or ``high-Z'' layer. [Preview Abstract] |
Thursday, October 27, 2005 2:48PM - 3:00PM |
RO1.00005: The effect of metallic overcoats on imprint and perturbation growth in ICF targets Lee Phillips, John Gardner, Stephen Obenschain, Alexander Velikovich, M. Karasik, Y. Aglitskiy A series of 2d simulations using NRL's FAST radiation hydrodynamics code (J.H. Gardner et al., Phys. Plasmas 5, 1935 (1998)) were performed on CH foils accelerated by laser ablation. Without a strategy to counteract the Rayleigh-Taylor instability, the growth of either applied small perturbations or nonuniformities arising from the laser irradiation are amplified and eventually destroy the target. We show that the application of a thin metallic overcoat on the front of the foil can be effective in reducing the perturbation growth rate dramatically, in agreement with experiments conducted at the NRL NIKE laser facility. Also in agreement with these experiments, we show that the growth of perturbations is enhanced if the overcoat is below a critical thickness. Comparison with fluid theory demonstrates that the principal stabilizing mechanism is the increase in ablation velocity resulting from conversion of laser energy to x-rays by the high-Z overcoat. It is found that the degree of stabilization in the simulations is fairly sensitive to initial conditions, and in order to predict the strong stabilization seen in experiment, a realistic initial density profile is required. These results on planar foils are applied to the design of high-gain direct-drive ICF targets. [Preview Abstract] |
Thursday, October 27, 2005 3:00PM - 3:12PM |
RO1.00006: Modeling of Nike Experiments on Acceleration of Planar Targets Stabilized with a Short Spike N. Metzler, A.L. Velikovich, J. Weaver, A.J. Schmitt, D.G. Colombant, S.T. Zalesak, J.H. Gardner A short sub-ns laser pulse (spike) produces a decelerating shock wave and a rarefaction wave immediately behind it, shaping a density gradient in the target. The following main pulse ``rides'' this graded density profile. We have demonstrated how the deceleration of the ablation front following the shock wave suppresses laser imprint and delays perturbation growth in the target [1]. We report the results of 2D numerical modeling of experiments on Nike laser at NRL, with its recently developed short-pulse capability, for a low-energy spike which does not affect the target adiabat. We studied the effect of spike on laser imprint on smooth planar targets and on the growth of perturbations imposed as single-mode ripples on the irradiated surface of the targets. For all cases, delay of the onset and/or suppression of the rate of the mass perturbation growth due to the spike are robust and significant enough to be observable on Nike. [1] N. Metzler \textit{et al}., Phys. Plasmas \textbf{6}, 3283 (1999); \textbf{9}, 5050 (2002); \textbf{10}, 1897 (2003). [Preview Abstract] |
Thursday, October 27, 2005 3:12PM - 3:24PM |
RO1.00007: Nike Experiments on Acceleration of Planar Targets Stabilized with a Short Spike Pulse$^{1}$ J.L. Weaver, A.L. Velikovich, M. Karasik, V. Serlin, S. Obenschain, A.J. Schmitt, N. Metzler, Y. Aglitskiy, J. Oh, A.N. Mostovych, J.H. Gardner Theoretical work has shown that a low energy spike pulse in front of the drive laser pulse can help mitigate the growth of hydrodynamic instabilities in targets for inertial confinement fusion.[1]$^{ }$While other experiments [2] used higher spike pulse energies, this study reports the influence of a lower energy spike and longer spike-main pulse delay on the acceleration of planar CH targets. Time evolution of preimposed sinusoidal ripples on the target surface was observed using a monochromatic x-ray imaging system. Delayed onset and/or suppression of mode growth was found for the spike prepulse shots compared to those with a low intensity foot, in good agreement with predictions from FAST2D simulations. The propagation velocity of the decaying shock wave from the spike pulse was measured with VISAR and was also in good agreement with an analytical prediction.[3] [1] Metzler \textit{et al}., Phys. Plasmas \textbf{6}, 3283 (1999); \textbf{9}, 5050 (2002); \textbf{10}, 1897 (2003);Goncharov \textit{et al}., Phys. Plasmas \textbf{10}, 1906 (2003)~;Betti \textit{et al.}, Phys Plamas \textbf{12}, 042703 (2005)~;[2]Knauer \textit{et al}., Phys. Plasmas \textbf{12}, 056306 (2005)~; [3]Velikovich \textit{et al.}, Phys. Plasmas \textbf{10}, 3270 (2003). $^{1}$Work supported by U. S. Department of Energy [Preview Abstract] |
Thursday, October 27, 2005 3:24PM - 3:36PM |
RO1.00008: 2-D Simulations of Adiabat-Shaped Targets K. Anderson, R. Betti Adiabat shaping has been proposed as a method for simultaneously achieving high compression and high stability in inertial confinement capsule implosions. Adiabat shaping is a concept by which the radial entropy profile in the capsule is modified to allow for low entropy (adiabat) in the inner portion of the shell, giving high 1-D compression and high entropy in the outer portion, which leads to higher ablative stabilization of the Rayleigh--Taylor instability. Two such adiabat-shaping techniques rely solely on temporal laser pulse shaping; the decaying shock\footnote{ V. N. Goncharov \textit{et al}., Phys. Plasmas \textbf{10}, 1906 (2003).} and the relaxation\footnote{ K. Anderson and R. Betti, Phys. Plasmas \textbf{11}, 5 (2004).} methods. Results are presented from a series of 2-D single-mode and multimode laser imprint simulations of cryogenic implosions on the OMEGA laser using the code \textit{DRACO} to compare the effectiveness of these two designs with respect to the Rayleigh--Taylor growth and the initial perturbation seeds to that of capsules with constant entropy profiles. 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] |
Thursday, October 27, 2005 3:36PM - 3:48PM |
RO1.00009: NIF ablator characterization experiments using the OMEGA laser system David Bradley, David Braun, Gail Glendinning, Steve Haan, David Munro, Gilbert Collins, Peter Celliers, John Edwards, Brian Spears The accurate characterization of ablator materials is crucial for successful ignition of indirect-drive ICF capsules. This includes, but is not limited to measurements of preheat and instability growth. The preferred ablator material for the NIF ignition campaign is Cu-doped Be. We are testing our ability to model the hydrodynamic behavior of Cu- doped Be and testing if the microstructure seeds larger than expected Rayleigh Taylor growth. To help diagnose these effects we are developing a platform to perform high growth Rayleigh Taylor experiments on Omega. This talk will present the initial results of our experiments using this platform. This work was performed under the auspices of U.S. Department of Energy by the Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48. [Preview Abstract] |
Thursday, October 27, 2005 3:48PM - 4:00PM |
RO1.00010: Reduced Reshock Growth in a Convergent/Divergent System Steven Batha, J.R. Fincke, J.M. Taccetti, N.D. Delamater, R.M. Hueckstaedt, N.E. Lanier, G.R. Magelssen, K.W. Parker, S.D. Rothman, C.J. Horsfield The interaction of a second shock with an already shocked, Richtmyer-Meshkov unstable interface causes the growth of the instability to increase in a planar geometry. Experiments on the Omega laser, however, have measured reduced growth rates when the second shock is diverging in cylindrical geometry. Illuminating the outside of a 1-mm-diameter cylinder with 18 kJ of laser light creates a strong shock. The shock passes through an embedded Al marker band. The outside surface of the Al is either smooth or has longitudinal perturbations (azimuthally symmetric) of wavelengths 2.5, 6, or 9 $\mu $m. The shock reflects off a hard inner cylinder that controls the timing, planarity, and strength of the rebounding shock (the reshock). Measurement of the mixing zone width after reshock show regrowth independent of the initial surface, contrary to single-shock results. Two possible explanations are considered. Freeze-out of the growth can occur by careful tailoring of the reshock timing. The loss of turbulent energy to the background strain field is also examined. This work was performed under the auspices of the United States Department of Energy under contract No. W-7405-ENG-36. [Preview Abstract] |
Thursday, October 27, 2005 4:00PM - 4:12PM |
RO1.00011: Mitigation of Raileigh-Taylor instability in low-density foam target Takeshi Watari, Mitsuo Nakai, Keiji Nagai, Hiroshi Azechi, Hiroyuki Shiraga, Keisuke Shigemori, Tatsuhiro Sakaiya, Kazuto Otani, Norimitsu Mahigashi, Kazuo Takeda, Shinsuke Fujioka, Kunioki Mima In the recent laser fusion program, foam cryogenic targets have been developed as promising targets which have a great potential to realize an efficient nuclear fusion. The `foam' is porous plastic material which has a microstructure inside. We have studied hydrodynamics of the foam as low-density ablator. We observed the hydrodynamic instability on the foam target. If the low density foam is used as an ablator, the Rayleigh-Taylor (RT) instability growth is expected to be mitigated. If we using low-density foam targets, because of it's low density, we get the lower peak density of the ablation front and longer density scale length at the ablation front than the polystyrene (CH) targets. In order to demonstrate this reduction, we have started an experiment to observe the RT instability and peak density of the ablation front at the target accelerated by using a planar low-density foam target with initial surface perturbation, and this experimental result are compared with those of the experiment with using CH target. This comparison result indicates the mitigation of the RT instability by using low density ablator. [Preview Abstract] |
Thursday, October 27, 2005 4:12PM - 4:24PM |
RO1.00012: Nearly pure tritium filled capsule implosions to measure the time dependence of mix D.C. Wilson, T.C. Sangster, P.S. Ebey, W. Shmayda, M.J. Bonino, A. Nobile, Jr., D. Harding, V. Yu. Glebov, F.J. Marshall We have designed, built, and fielded a unique experiment to measure the time dependence of atomic mix in directly driven plastic capsules. The time history of the burn is measured in two capsules, a reference capsule of 20 $\mu $m thick plastic (CH) and an experimental capsule of the same thickness but with a 1$\mu $m thick deuterated plastic (CD) layer on the inner surface. Both capsules are filled with 10 atm of nearly pure tritium gas containing 0.2 atm {\%} deuterium. Without atomic mix the DT yield of the experimental capsule should be comparable to the reference capsule ($\sim $1 10$^{11}$ with mix). Using the Scannapieco and Cheng model of fully atomic mix we calculated a reference capsule yield of 8 10$^{12}$. First results gave yields of 3 10$^{12}$, confirming the dominant role of atomic mix, but comparison with other plastic capsule implosions suggests that some of the mix is ``chunk,'' or non-atomic in character. In a simple model the ratio of yield rates is proportional to the time dependent amount of atomic mix. Fielding these capsules was a major accomplishment. Because the purest possible tritium was required, a special fill station was built at Los Alamos. Tritium diffusion through the plastic shell required storage and shipment to the University of Rochester at near liquid nitrogen temperatures. [Preview Abstract] |
Thursday, October 27, 2005 4:24PM - 4:36PM |
RO1.00013: Emission spectroscopy as a tool in determining the time evolution of mix in ICF implosion experiments Mark Gunderson, Donald Haynes Jr, Douglas Wilson, George Kyrala A good understanding of the time-dependent evolution of mix is vitally important in the design of ICF implosion and ignition capsules, and emission spectroscopy is an important experimental tool in probing our level of understanding of mix in such systems. By applying a layer of Ti-doped plastic at the pusher-fuel interface that is sufficiently thin (0.1 microns) to be completely mixed into the fuel, we can gain invaluable time-resolved Ti spectral line data indicative of pusher mix into the hot fuel core. Specifically, as the Ti reaches the hot 3 -- 4 keV core of the fuel, a sufficient population of hydrogen-like Ti will be created to result in strong Ti Lyman-alpha emission. From the change in the intensity of this line as a function of time, we can back out the timing and the amount of pusher mix reaching the fuel core. This spectroscopic data in conjunction with neutron and proton yield data and gated imaging data provide an invaluable database that can be used to test and benchmark mix models incorporated into radiation hydrodynamic simulation codes. [Preview Abstract] |
Thursday, October 27, 2005 4:36PM - 4:48PM |
RO1.00014: The Use of Ti K-shell Emission Spectroscopy as a Tracer for Fuel/Ablator Mix in Directly Driven Microballoon Implosions Donald Haynes, Mark Gunderson, Douglas Wilson, George Kyrala, Sean Regan Radiative hydrodynamic simulations of a recent series of directly driven microballoon implosions indicate that the central region of the core reaches temperatures sufficient to populate the hydrogenic ionization stage of titanium. Introducing a very thin (1000 Angstrom) layer of titaniated plastic at the inside surface of the ablator, we use time resolved spectroscopy in the photon energy range of the alpha transitions in hydrogen- and helium-like Ti to determine the time dependence of the intrusion of ablator material into the central core of these implosions. Here, we present the motivation for these experiments, pre-shot modeling and details of the NLTE, Stark-broadened Ti K-shell emission model used in the analysis of these experiments. (LA-UR-05-5606) [Preview Abstract] |
Thursday, October 27, 2005 4:48PM - 5:00PM |
RO1.00015: Spectroscopic analysis of temperature, density and mixing spatial profiles in OMEGA implosion cores L.A. Welser, R.C. Mancini, R. Tommasini, J.A. Koch, J. Delettrez, S.P. Regan, V. Smalyuk The spectroscopic analysis of simultaneous X-ray narrow-band images and X-ray line spectra recorded in argon-doped deuterium-filled ICF implosion experiments can yield information about the temperature, density and mixing spatial profiles in the compressed core. We discuss analysis results from indirect- and direct-drive implosion experiments at OMEGA where X-ray images and line spectra were simultaneously recorded. The images were recorded with several, identical multi-monochromatic imaging instruments that recorded data along quasi-orthogonal lines of sight. The analysis method considers data based on the argon Ly$\alpha $, He$\beta $, and Ly$\beta $ spectral features and their associated Li- and He-like satellite structure. Extracted profiles are compared with gradients from hydrodynamic simulations of the implosion, and results from a passive mixing model. This work is supported by DOE-NLUF Grant DE-FG03-03SF22696, and LLNL. [Preview Abstract] |
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