Bulletin of the American Physical Society
51st Annual Meeting of the APS Division of Plasma Physics
Volume 54, Number 15
Monday–Friday, November 2–6, 2009; Atlanta, Georgia
Session NI2: ICF Capsules and Rayleigh-Taylor Instability |
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Chair: David Meyerhofer, University of Rochester Room: Centennial I |
Wednesday, November 4, 2009 9:30AM - 10:00AM |
NI2.00001: Capsule Performance Optimization for the National Ignition Facility Invited Speaker: The overall goal of the capsule performance optimization campaign is to maximize the probability of ignition by experimentally correcting for likely residual uncertainties in the implosion and hohlraum physics used in our radiation-hydrodynamic computational models before proceeding to cryogenic-layered implosions and ignition attempts. This will be accomplished using a variety of targets that will set key laser, hohlraum and capsule parameters to maximize ignition capsule implosion velocity, while minimizing fuel adiabat, core shape asymmetry and ablator-fuel mix. The targets include high Z re-emission spheres setting foot symmetry through foot cone power balance [1], liquid Deuterium-filled ``keyhole'' targets setting shock speed and timing through the laser power profile [2], symmetry capsules setting peak cone power balance and hohlraum length [3], and streaked x-ray backlit imploding capsules setting ablator thickness [4]. We will show how results from successful tuning technique demonstration shots performed at the Omega facility under scaled hohlraum and capsule conditions relevant to the ignition design meet the required sensitivity and accuracy. We will also present estimates of all expected random and systematic uncertainties in setting the key ignition laser and target parameters due to residual measurement, calibration, cross-coupling, surrogacy, and scale-up errors, and show that these get reduced after a number of shots and iterations to meet an acceptable level of residual uncertainty. Finally, we will present results from upcoming tuning technique validation shots performed at NIF at near full-scale. Prepared by LLNL under Contract DE-AC52-07NA27344. \\[4pt] [1] E. Dewald, et. al. Rev. Sci. Instrum. 79 (2008) 10E903. \\[0pt] [2] T.R. Boehly, et. al., Phys. Plasmas 16 (2009) 056302. \\[0pt] [3] G. Kyrala, et. al., BAPS 53 (2008) 247. \\[0pt] [4] D. Hicks, et. al., BAPS 53 (2008) 2. [Preview Abstract] |
Wednesday, November 4, 2009 10:00AM - 10:30AM |
NI2.00002: Shock-Tuned Cryogenic DT Implosion Performance on OMEGA Invited Speaker: Cryogenic-target-compression experiments with low-adiabat (\textit{$\alpha $} $\sim $ 1 to 3) continuous and multiple-picket-drive pulses are performed on the OMEGA laser to understand the physics of fuel assembly in inertial confinement fusion. Continuous-drive-pulse designs require accurate modeling of the compression wave generated by the gradual rise in laser intensity. Uncertainties in the ablator/fuel equation-of-state modeling and laser coupling can lead to uncertainties in the predicted shock coalescence and fuel adiabat. The multiple-picket-drive pulse designs replace the gradual intensity rise of the continuous pulse with two or three narrow pickets (each picket is approximately 100 ps long). These picket designs facilitate shock tuning using an experimental cone-in-shell platform\footnote{ T. R. Boehly \textit{et al.}, Phys. Plasmas \textbf{16}, 056302 (2008).} developed for the National Ignition Campaign. The coalescing shocks are measured with VISAR. The required shock-timing accuracy is achieved by adjusting the energies of the individual pickets. The shock-tuned, multiple-picket-drive pulses have been used to drive cryogenic DT targets to implosion velocities ($>$3 $\times $ 10$^{7}$ cm/s) that exceed those reached using continuous-pulse designs.\footnote{ T. C. Sangster \textit{et al.}, Phys. Rev. Lett. \textbf{100}, 185006 (2008).} This talk will present new multiple-picket shock-timing results and target performance data (yield, areal density, and ion temperature) from shock-tuned implosions on OMEGA. The areal density is inferred from the spectra of knock-on deuterons and the primary neutron down-scattered fraction using a magnetic recoil spectrometer (primary DT neutrons that scatter in the dense fuel end up with energies well below 14 MeV; the ratio of this yield to the primary yield is proportional to the fuel areal density). Properly tuned implosions produce nearly 1-D areal densities. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302. [Preview Abstract] |
Wednesday, November 4, 2009 10:30AM - 11:00AM |
NI2.00003: Asymmetrically-Driven Implosions Invited Speaker: Techniques to achieve uniform, near-spherical symmetry of radiation drive on a capsule in a laser-heated hohlraum have received detailed attention in the context of ICF. However, much less attention has been paid to the understanding of the hohlraum physics in cases where the radiation drive departs significantly from spherical symmetry. In recent work at the OMEGA laser, AWE has carried out a series of experiments to study the implosion dynamics of a capsule irradiated by a \underline {deliberately asymmetric} X-ray drive. The experimental data provide a sensitive test of radiation transport in which drive symmetry is modulated by the use of variable albedo layers and asymmetric laser-beam timing. Data from foam-ball and thin-shell capsule experiments are presented, together with modelling using consecutively linked Lagrangian and Eulerian, as well as single-step ALE, calculational schemes. The thin-shell capsules exhibit much stronger sensitivity to asymmetry than foam balls resulting in the formation of a well-defined polar jet. These data are shown to challenge computational modelling in this highly asymmetric, strongly convergent regime. [Preview Abstract] |
Wednesday, November 4, 2009 11:00AM - 11:30AM |
NI2.00004: Measurements of the down-scattered and TT-neutron spectrum using the Magnetic Recoil Spectrometer Invited Speaker: Proper assembly of capsule mass, as manifested through evolution of fuel areal density (\textit{$\rho $R}), is fundamentally important for achieving hot-spot ignition planned at the National Ignition Facility (NIF).$^{ }$Experimental information about \textit{$\rho $R} and \textit{$\rho $R} asymmetries, $T_{i}$ and yield is therefore absolutely essential for understanding how assembly occurs. To obtain this information, we have built and activated, at OMEGA, a Magnetic-Recoil Spectrometer (MRS) whose objective is to measure the absolute neutron spectrum in the range 5 to 30 MeV, from which \textit{$\rho $R}, $T_{i}$ and yield can be directly inferred for energy-scaled cryogenic DT implosions. This allows for experimental validation of the direct-drive ignition-capsule design prior to the first experiments on the NIF. Another MRS is currently being built for the NIF. In this talk, we present the first ever measurements of the down-scattered neutron spectrum, from which \textit{$\rho $R} was accurately inferred for both CH and cryogenic DT implosions. As much of the OMEGA-MRS R{\&}D is directly applicable to the NIF-MRS, the MRS activities on the NIF are presented as well. In addition, high-resolution measurements of the DT and TT-neutron spectrum have recently been performed to address, for the first time, important science questions regarding non-thermal components in the DT-neutron spectrum, as well as the possible existence of the TT-two-body reaction, at low energies, producing a neutron peak at about 9 MeV. This work was supported in part by the U.S. Department of Energy (Grant No. DE-FG03-03SF22691), LLE (subcontract Grant No. 412160-001G), LLNL (subcontract Grant No. B504974). Contributors: D.Casey, C.Li, F.S\'{e}guin, N.Sinenian, R.Petrasso, MIT, V.Glebov, T.Sangster, D.Meyerhofer, LLE-UR, S.Hatchett, S.Haan, C.Cerjan, M.Eckart, H.Kather, O.Landen, M. Moran, LLNL, K. Fletcher, \textit{Suny Geneseo, and }R. Leeper, SNL. [Preview Abstract] |
Wednesday, November 4, 2009 11:30AM - 12:00PM |
NI2.00005: Strong viscous stabilization of the Rayleigh-Taylor instability at Mbar pressures Invited Speaker: We report experimental results showing significant reductions from classical in the Rayleigh-Taylor instability (RTI) growth rate due to high pressure effective lattice viscosity. Stabilization of the RT instability by ablation and density gradients has been studied for decades. Stabilization by lattice viscosity (material strength) at Mbar pressures is new. Target samples are compressed and accelerated quasi-isentropically by plasma drives, while maintaining the samples in the solid-state. Provided strong shocks are avoided, the higher the applied peak pressure, the higher the sample strength, and hence, the higher the degree of strength stabilization of RTI [1]. This paper will present our results on vanadium (V) using the Omega long-pulse lasers and tantalum (Ta) using the Omega long pulse and EP short pulse laser in combination. The amount of RTI growth is measured by face-on radiography, utilizing a thermally driven He-$\alpha $ backlighter for V and a high energy ($>$ 20 keV) K-$\alpha $ backlighter driven by the EP petawatt laser for Ta. Comparisons with 2D simulations employing constitutive models for solid state strength suggest that we are in the new stabilization regime of very high effective lattice viscosity caused by phonon drag on dislocation motion. This effective lattice viscosity is predicted to increase with pressure, provided the lattice is maintained, making our results relevant to the fields of ICF and high energy density physics in general. Designs that extend this experiment by an order of magnitude in pressure on NIF will also be shown [2]. \\[4pt] [1] B.A. Remington, H.S. Park et al., European Physics Journal, in press (2009).\\[0pt] [2] H.S. Park et al., J. Phys.: Conf. Ser. 112, 042024 (2008). [Preview Abstract] |
Wednesday, November 4, 2009 12:00PM - 12:30PM |
NI2.00006: Investigations into the Seeding of Instabilities due to X-ray Preheat in Beryllium-Based Inertial Confinement Fusion Targets Invited Speaker: The geometry of inertial confinement fusion (ICF) capsules makes them susceptible to various types of hydrodynamic instabilities at different stages during an ICF implosion. From the beginnings of ICF research, it has been known that grain-level anisotropy and defects could be a primary source of instability seeding in solid capsules. This has steered ICF designs to include amorphous materials such as plastic; however, the benefits of low-Z metallic materials, i.e. beryllium, has kept these materials the focus of much research. Recently, experiments were conducted at the Trident laser facility to measure dynamic surface roughening from hard x-ray preheat. M-band emission from laser produced gold plasma was used to heat beryllium targets with different amounts of copper doping to temperatures comparable to National Ignition Facility (NIF) preheat levels. Temporal and spectral x-ray diagnostics were used to estimate the target heating, which was also predicted by multi-dimensional radiation hydrodynamics calculations. Wave profiles of varying complexity due to differences in copper doping were observed with free surface line imaging velocity interferometry. Dynamic roughening measurements were made on the surface away from the plasma at discrete times up to 8 ns after the beginning of the drive pulse using a surface displacement interferometer with nanometer scale sensitivity. Undoped, large-grained targets were measured to roughen between 15 and 50 nm rms depending on variations in x-ray absorption through the target thickness. Fine-grained, copper-doped targets were observed to roughen near the sensitivity limit of the interferometer and approached the Rev2 NIC design point of 0.9 nm. The results of this combined experimental and modeling effort have shed light on the effects of high-Z doping and microstructural refinement on the dynamics of differential thermal expansion and have shown that current NIF capsule designs using beryllium are very effective in reducing preheat related roughening ahead of the first shock. These experiments have raised additional questions, however, such as the possibility of spallation from intense thermal expansion, which will also be discussed. [Preview Abstract] |
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