Bulletin of the American Physical Society
54th Annual Meeting of the APS Division of Plasma Physics
Volume 57, Number 12
Monday–Friday, October 29–November 2 2012; Providence, Rhode Island
Session JO4: Direct Drive |
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Chair: Evan Dodd, Los Alamos National Laboratory Room: 551AB |
Tuesday, October 30, 2012 2:00PM - 2:12PM |
JO4.00001: Improving Implosion Velocity in Cryogenic Deuterium--Tritium Implosions on OMEGA V.N. Goncharov, T.C. Sangster, R. Epstein, S.X. Hu, I.V. Igumenshchev, D.H. Froula, R.L. McCrory, D.D. Meyerhofer, P.B. Radha, W. Seka, S. Skupsky, C. Stoeckl, D.T. Casey, J.A. Frenje, M. Gatu-Johnson This talk will summarize the results on improving performance and the progress in theoretical understanding of cryogenic deuterium--tritium implosions on OMEGA. To increase the implosion velocity, cryogenic layer thickness was reduced over the last year from 65 $\mu$m (which corresponds to $V_{imp} \sim $ 2.7 $\times $ 10$^{7}$ cm/s) down to 40 $\mu$m ($V_{imp} \sim $ 3.1 to 3.5 $\times $ 10$^{7}$ cm/s, depending on ablator thickness). The ablator thickness was varied from 9.2 to 13 $\mu$m. The shell is driven keeping fuel adiabat at $\alpha$ = 1.5 to 3. The experiments have demonstrated that the target yield relative to the 1-D predictions has not changed when the ice thickness is reduced to 55 $\mu$m ($V_{imp} \sim $ 3 $\times $ 10$^{7}$ cm/s), but degradation occurs for thinner ice. This degradation is more pronounced for designs with thicker ablator layers, indicating an enhanced ablator--pusher mix for these designs. Mix-mitigation strategies including high-$Z$ dopants in the ablator materials will be discussed. 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] |
Tuesday, October 30, 2012 2:12PM - 2:24PM |
JO4.00002: Fuel--Ablator Mix from Surface Nonuniformities in Directly Driven Implosions I.V. Igumenshchev, V.N. Goncharov, T.R. Boehly, T.C. Sangster, S. Skupsky Direct-drive ICF targets can have various surface defects/nonuniformities that affect the implosion performance of cryogenic targets. These defects can vary in dimensions from microns to tens of microns and include dust particles, glue spots for target mounting, fill tubes, and manufacturing defects. Two-dimensional hydrodynamic simulations are performed to investigate the effect of surface defects on implosion performance. These simulations show that the defects introduce large amplitude (nonlinear) perturbations to shocks, which compress the targets. The perturbations from defects of the typical size ($\sim $20 to 40 $\mu$m) develop a hole in the target shell, through which the ablator material is injected inside the target and mixed with the fuel. It was found that self-generated magnetic fields can enhance this process. The implosion performance is decreased due to the fuel--ablator mix in the hot spot and due to perturbations of the hot-spot introduced by the injected flow. 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] |
Tuesday, October 30, 2012 2:24PM - 2:36PM |
JO4.00003: Hydrodynamic Stability of Direct-Drive Targets with High-$Z$ Ablators M. Lafon, R. Nora, R. Betti In direct-drive inertial confinement fusion, the perturbations seeded by the laser irradiation nonuniformities and target fabrication defects grow during the acceleration stage of the shell implosion, seeding hydrodynamic instabilities, and degrading target performance. High-$Z$ ablators have been shown to suppress the generation of hot electrons from two-plasmon-decay instability.\footnote{V. A. Smalyuk\textit{ et al.}, Phys. Rev. Lett. \textbf{104}, 165002 (2010).} A set of moderate-$Z$ ablators ranging from carbon to silicon have been used to design both hot-spot and shock-ignition targets at laser energies relevant to the National Ignition Facility. The hydrodynamics of these ablators is studied though single and multimode simulations. Hydro-instabilities exhibit complex behavior in these ablators because of the presence of a double ablation front (thermal and radiative) and a classically unstable interface. The width of the double ablation front growths with $Z$ and the Rayleigh--Taylor instability becomes more localized near the radiative front and the classical interface, while it is fully stabilized at the thermal ablation front. It is shown that ignition target designs with reasonably good hydrodynamic properties using moderate-$Z$ ablators are possible for both shock and hot-spot ignition. 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] |
Tuesday, October 30, 2012 2:36PM - 2:48PM |
JO4.00004: Numerical studies of the use of thin high-Z layers for reducing laser imprint in direct-drive inertial-fusion targets Jason Bates, Andrew Schmitt, Max Karasik, Steve Obenschain Using the FAST code, we present numerical studies of the effect of thin metallic layers with high atomic number (high-Z) on the hydrodynamics of directly-driven inertial-confinement-fusion (ICF) targets. Previous experimental work on the NIKE Laser Facility at the U.S.~Naval Research Laboratory demonstrated that the use of high-Z layers may be efficacious in reducing laser non-uniformities imprinted on the target during the start-up phase of the implosion. Such a reduction is highly desirable in a direct-drive ICF scenario because laser non-uniformities seed hydrodynamic instabilities that can amplify during the implosion process, prevent uniform compression and spoil high gain. One of the main objectives of the present work is to assess the utility of high-Z layers for achieving greater laser uniformity in polar-drive target designs planned for the National Ignition Facility. To address this problem, new numerical routines have recently been incorporated in the FAST code, including an improved radiation-transfer package and a three-dimensional ray-tracing algorithm. We will discuss these topics, and present initial simulation results for high-Z planar-target experiments planned on the NIKE Laser Facility later this year. [Preview Abstract] |
Tuesday, October 30, 2012 2:48PM - 3:00PM |
JO4.00005: Advanced Ablator Target Designs for Direct-Drive Experiments R. Betti, R. Nora, M. Lafon, J.F. Myatt, K.S. Anderson A major concern for direct-drive implosions at the National Ignition Facility (NIF) scale is the DT-fuel preheating by hot electrons produced by the two-plasmon-decay instability. Experiments on OMEGA using thick glass targets\footnote{V. A. Smalyuk \textit{et al}., Phys. Rev. Lett. \textbf{104}, 165002 (2010).} showed that glass SiO$_{2}$ ablators produced a hard x-ray signal from hot electrons that is 40$\times $ lower than in plastic shells for the same laser intensity. These results have stimulated research in new ablator materials with higher $Z$ than plastic CH for direct-drive targets. A set of moderate-$Z$ ablators ranging from carbon to silicon has been used to design both hot-spot and shock-ignition targets at laser energies relevant to the NIF. The hydrodynamics of these ablators is studied through single and multimode simulations. Hydro-instabilities exhibit complex behavior in these ablators due to the presence of a double ablation front (thermal and radiative) and a classically unstable interface. It is shown that target designs with reasonably good hydrodynamic properties using moderate-$Z$ ablators are possible for both shock and hot-spot ignition. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement Nos. DE-FC52-08NA28302 and Office of Fusion Energy Sciences under grant DE-FC02-04ER54789. [Preview Abstract] |
Tuesday, October 30, 2012 3:00PM - 3:12PM |
JO4.00006: Symmetry in a 48 beams Direct Drive configuration Stephane Laffite, Benoit Canaud, Vincent Brandon, Mauro Temporal Symmetry is still one of the major issues of the Direct-Drive (DD) approach to Inertial Confinement Fusion (IOCF). We present here 2D calculations in a 48-beam DD configuration. The objective is to connect the end-of-implosion asymmetries to the laser intensity asymmetries. First, numerical simulations were performed in normal incidence with various applied low-mode asymmetries. Then, we studied the impact on implosion of intrinsic asymmetry for a typical 48-beam configuration. A correlation between fuel radius asymmetries and laser absorption asymmetries is demonstrated. Asymmetry of the laser energy absorption through the whole plasma is found to be a good metric to describe the impact of laser asymmetries on implosion. Also, we show similarities between 2D numerical results and 3D ray-tracing model from [M. Temporal et al, EPJD, 55, 139-145, 2009]. [Preview Abstract] |
Tuesday, October 30, 2012 3:12PM - 3:24PM |
JO4.00007: A Two-Sided Indirect/Direct-Drive Hybrid Target for the National Ignition Facility L. John Perkins, Donald Blackfield, George Zimmerman We report on a new hybrid concept for a high-gain ignition target for the National Ignition Facility that combines the symmetry advantages of indirect-drive assembly with the efficiency of radial-direct-drive shock ignition in a capsule with thick fuel layers. A slow, thick 240-deg spherical shell segment of DT is assembled on a Au guide cone by indirect-drive in a one-sided hohlraum at 0.7MJ/150TW/250eV. It is then shock ignited on the opposite side by direct-drive on a 120-deg fuel segment inside the cone at 0.5MJ/230TW. Given the latter is radial direct-drive it will not require a polar direct drive qualification campaign or new phaseplates and should minimize cross beam transfer. We discuss 2-D LASNEX optimizations of synching the two laser drives, such that the indirect-drive compression pulse should commence some 8ns before the start of the direct-drive pulse. The burn history (rate of fusion energy production) exhibits a double maximum as the smaller, faster direct-drive side ignites and then burns into the main fuel mass on the other side. Critical issues include demonstrating adequate symmetry in a one-sided hohlraum during the low velocity compression phase and minimizing high-Z/DT mix from the Au cone that separates the two in-flight fuel segments during assembly. [Preview Abstract] |
Tuesday, October 30, 2012 3:24PM - 3:36PM |
JO4.00008: Polar-Drive--Ignition Experimental Plan on the NIF D.D. Meyerhofer, D.H. Froula, V.N. Goncharov, I.V. Igumenshchev, S.J. Loucks, P.W. McKenty, R.L. McCrory, P.B. Radha, T.C. Sangster The University of Rochester's Laboratory for Laser Energetics has proposed a multi-campaign series of experiments on the National Ignition Facility (NIF)\footnote{ E. I. Moses\textit{ et al.}, Phys. Plasmas \textbf{16}, 041006 (2009).} to develop polar-drive ignition.\footnote{ S. Skupsky\textit{ et al.}, Phys. Plasmas \textbf{11}, 2763 (2004).} Polar drive is predicted to couple significantly more energy to the compressed core than the baseline indirect-drive approach. This presentation provides an overview of the plan to achieve polar-drive ignition on the NIF and describes the initial experiments that can be performed without changes in the NIF infrastructure. The first proposed experiments will assess the symmetry of moderate convergence ratio, polar-driven capsule implosions. 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] |
Tuesday, October 30, 2012 3:36PM - 3:48PM |
JO4.00009: Two-Dimensional Numerical Evaluation of 1-D Multi-FM SSD Experiments A. Shvydky, P.W. McKenty, M. Hohenberger, G. Fiksel, T.J.B. Collins, J.A. Marozas, J.D. Zuegel, T.C. Sangster Adequate single-beam smoothing is crucial for successful direct-drive target implosions. One-dimensional, Multi-FM smoothing by spectral dispersion (SSD) has been proposed to provide the required level of smoothing for the current NIF ignition point design.\footnote{J. A. Marozas, J. D. Zuegel, and T. J. B. Collins, Bull. Am. Phys. Soc. \textbf{55}, 294 (2010).} Initial validation experiments have been performed on OMEGA EP and have demonstrated the effectiveness of Multi-FM SSD in reducing laser-imprint nonuniformities. \textit{DRACO} simulations of these planar-foil experiments will be performed and results will be compared with experimental data. The simulations will include realistic time-dependent far-field spot intensity calculations that emulate the effect of SSD. The computational mesh of these simulations will be capable of resolving single speckles. Theoretical radiographs will be used to design a proper x-ray filtration for future experiments. 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] |
Tuesday, October 30, 2012 3:48PM - 4:00PM |
JO4.00010: Optimization with \textit{Telios} of the Polar-Drive Point Design for the National Ignition Facility T.J.B. Collins, J.A. Marozas, P.W. McKenty Polar drive\footnote{S. Skupsky\textit{ et al.}, Phys. Plasmas \textbf{11}, 2763 (2004).} (PD) will make it possible to conduct direct-drive--ignition experiments at the National Ignition Facility\footnote{ G. H. Miller, E. I. Moses, and C. R. Wuest, Opt. Eng. \textbf{43}, 2841 (2004).} while the facility is configured for x-ray drive. A PD-ignition design has been developed\footnote{T. J. B. Collins\textit{ et al.}, Phys. Plasmas \textbf{19}, 056308 (2012).} achieving high gain in simulations including single- and multiple-beam nonuniformities, and ice and outer-surface roughness. This design has been further optimized to reduce the in-flight aspect ratio and implosion speed, increasing target stability while maintaining moderately high thermonuclear gains. The dependence of target properties on implosion speed has been examined using the optimization shell \textit{Telios}. \textit{Telios} has the capability to drive complex radiation--hydrodynamic simulations and optimized results over an arbitrarily large parameter space, including ring pointing angles, spot-shape parameters, target dimensions, pulse timing, and relative pulse energies. \textit{Telios} is capable of extracting output from a variety of sources and combining them to form arbitrarily complex, user-specified metrics. 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] |
Tuesday, October 30, 2012 4:00PM - 4:12PM |
JO4.00011: Drive-Symmetry Studies of NIF Exploding-Pusher Experiments P.W. McKenty, R.S. Craxton, A. Shvydky, D.H. Froula, D.T. Michel, J.A. Marozas, T.C. Sangster, D.D. Meyerhofer, R.L. McCrory, J.D. Kilkenny, A. Nikroo, M.L. Hoppe, S. LePape, A.J. MacKinnon, D.H. Munro Polar-drive (PD)\footnote{A. M. Cok, R. S. Craxton, and P. W. McKenty, Phys. Plasmas \textbf{15}, 082705 (2008).} target implosions using DT fuel have been designed and fielded for neutron diagnostic development on the NIF. These implosions are modeled with three separate hydrodynamics codes: \textit{LILAC}, to optimize the 1-D design; \textit{SAGE}, to optimize the pointing uniformity; and \textit{DRACO}, to predict the drive symmetry, neutron yield, and residual fuel motion from 2-D implosion simulations. Recent experimental results, evaluating the overall hydrodynamic assembly, have indicated a significant discrepancy with \textit{DRACO} predictions of the in-flight shell evolution. Several scenarios will be presented in an attempt to understand the source of this discrepancy, therefore providing a clear mitigation strategy for future PD experiments on the NIF. 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] |
Tuesday, October 30, 2012 4:12PM - 4:24PM |
JO4.00012: Optimization of Drive Uniformity in NIF Polar-Drive Implosions Using Gated X-Ray Self-Emission Images R.S. Craxton, P.W. McKenty, P.A. Olson, D.H. Froula, D.T. Michel, S. LePape, A.J. MacKinnon Gated x-ray self-emission images have been obtained in ``exploding-pusher,'' polar-drive, diagnostics commissioning experiments\footnote{A. M. Cok, R. S. Craxton, and P. W. McKenty, Phys. Plasmas \textbf{15}, 082705 (2008).} on the NIF. They show a difference in drive between the pole and equator that serves as a diagnostic of drive uniformity. Modeling of the shell trajectories using the 2-D hydrodynamics code \textit{SAGE}\footnote{R. S. Craxton and R. L. McCrory, J. Appl. Phys. \textbf{56}, 108 (1984).} indicates that the difference between the pole and equator may result from deviations between the modeled and actual on-target phase-plate profiles. The sensitivity of the implosion uniformity to these profiles and to other parameters such as defocus and repointing has been explored with the goal of finding improved polar-drive--irradiation designs. 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] |
Tuesday, October 30, 2012 4:24PM - 4:36PM |
JO4.00013: Effect of Nonlocal Electron Transport in Both Directions on the Symmetry of Polar-Drive--Ignition Targets J.A. Delettrez, T.J.B. Collins, A. Shvydky, G. Moses, D. Cao, M.M. Marinak A nonlocal, multigroup diffusion model for thermal electron transport\footnote{G. P. Schurtz, Ph. D. Nicola\"i, and M. Busquet, Phys. Plasmas \textbf{7}, 4238 (2000).} has been added to the 2-D hydrodynamic code \textit{DRACO}. This model has been applied to simulations of polar-drive (PD) NIF ignition designs. Previous simulations were carried out with a constant flux-limiter model in both the radial and transverse directions. Due to the nonsymmetry of PD illumination, these implosions suffer from low-mode nonuniformities that affect their performance. Nonlocal electron transport in both directions is expected to reduce these nonuniformities. The 2-D thermal electron flux from simulations, using either the nonlocal model or the standard flux-limited approach, will be compared and the effect of the nonlocal transport model on the growth of the nonuniformities and on target performance will be presented. 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] |
Tuesday, October 30, 2012 4:36PM - 4:48PM |
JO4.00014: Polar-Drive Shock-Timing Measurements on OMEGA T.R. Boehly, V.N. Goncharov, S.X. Hu, J.A. Marozas, T.C. Sangster, D.D. Meyerhofer Polar-drive target designs use the nonsymmetric geometry of the NIF to produce uniform direct-drive irradiation of spherical targets. We report on a series of experiments on OMEGA that measure shock timing in polar-drive geometry. The OMEGA laser beams were redirected to mimic the nonradial beam pointing needed on the NIF. The strength and timing of the shocks waves produced by multiple pulses were measured and used to infer the drive on the capsule. The results were compared to simulations and used to assess the utility of models for nonlocal heat transport and cross-beam energy transfer. 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] |
Tuesday, October 30, 2012 4:48PM - 5:00PM |
JO4.00015: Polar-drive experiments with shimmed targets on OMEGA F.J. Marshall, P.B. Radha, M.J. Bonino, J.A. Delettrez, R. Epstein, S. Skupsky, E. Giraldez Polar-drive experiments are being performed on OMEGA in preparation for future ignition attempts using the same method on the National Ignition Facility. This work presents results from a series of experiments employing shimmed shells whose shape (thinner at the target equator) is used to further compensate for the oblique illumination present in the polar-drive beam configuration. Implosion experiments were performed with multiple-picket laser pulses from 40 OMEGA beams driving gas-filled, shimmed shells. The implosions were diagnosed with x-ray backlighting, fusion yield, and reaction particle spectra from which the implosion symmetry, areal density, and core conditions are inferred. The compressed shell shape determined from framed x-ray radiography is compared to that predicted by the 2-D hydrodynamics code \textit{DRACO}. The benefits of using a shimmed target for polar-drive implosions are less oblique illumination, better low-mode implosion symmetry, and are clearly demonstrated by these experiments. 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] |
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