Bulletin of the American Physical Society
55th Annual Meeting of the APS Division of Plasma Physics
Volume 58, Number 16
Monday–Friday, November 11–15, 2013; Denver, Colorado
Session NO7: Ablator Studies in ICF |
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Chair: Tom Boehly, University of Rochester Room: Governor's Square 12 |
Wednesday, November 13, 2013 9:30AM - 9:42AM |
NO7.00001: Measuring ablator areal density using Neutron Time of Flight at the National Ignition Facility Joseph Caggiano, Mark Eckart, Chris Hagmann, Robert Hatarik, James McNaney, Daniel Sayre, Vladimir Glebov, James Knauer Measuring ablator areal density at bang time is an important implosion performance metric because excessive ablator areal density (rho.R) is indicative of an inefficent drive. Further, large ablator rho.R values at bang time may be an indication of hydrodynamic instabilities that mix the ablator further into the center of the capsule. ~Pre-bang measurements from the convergent ablator experiments indicate that 15-20{\%} of the starting mass remains, but that is a measurement performed well before bang time - hence, measurements at bang time can help determine the amount and location of the remaining mass. ~NTOF measurements focus on elastic and inelastic neutron scattering features from the carbon present in the ablator material to infer carbon and plastic ablator rho.R, and these measurements will be presented and discussed. The numbers are found to compare favorably with other nuclear diagnostic measurements, but are somewhat discrepant from the X-ray diagnostic measurement using the Ross pair filter technique. This discrepancy may indicate a very non-uniform mass distribution at bang time. [Preview Abstract] |
Wednesday, November 13, 2013 9:42AM - 9:54AM |
NO7.00002: Improved Rocket Efficiency in Direct-Drive Implosions Using Different Ablator Materials D.T. Michel, V.N. Goncharov, I.V. Igumenshchev, D.H. Froula A set of experiments varied the ratio of the atomic number over the atomic mass ($A$/$Z)$ of the ablator to increase both the ablation pressure and the mass ablation rate and improve the rocket efficiency.\footnote{W. M. Manheimer, D. G. Colombant, and J. H. Gardner, Phys. Fluids \textbf{25}, 1644 (1982).} A 20{\%} increase in the implosion velocity was observed when using a Be ablator ($A$/$Z=$ 2.25) compared to C ($A$/$Z=$ 2) and CH ($A$/$Z=$ 1.85) ablators. These measurements are consistent with hydrodynamic simulations that predicted an increase in the hydrodynamic efficiency of 18{\%} for Be and 7{\%} for C compared to CH ablator. A comparable amount of unabsorbed laser power was measured for the three materials ($\sim $30{\%}) that shows that the increase in implosion velocity for Be ablator is a result of the increase in the rocket efficiency, not an increase in absorption. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Wednesday, November 13, 2013 9:54AM - 10:06AM |
NO7.00003: High-Density Carbon (HDC) Ablator for Ignition Capsules D. Ho, S. Haan, J. Milovich, J. Salmonson, G. Zimmerman, L. Benedict, J. Biener, C. Cerjan, D. Clark, E. Dewalds, J. Edwards, L. Berzak Hopkins, A. Mackinnon, M. Marinak, J. McNaney, N. Meezan, S. Ross, R. Tommasini HDC ablators show high performance based on simulations and experiments. HDC capsules have good 1-D performance because HDC has high density (3.5 g/cc), which results in a thinner ablator that absorbs more radiation, and have good 2-D performance because the ablator surface is substantially smoother than plastic ablators. A 25 um thick layer doped with 0.26 at.{\%} of W is sufficient to block the M-band radiation. W can be doped very uniformly in HDC. Simulations using NLTE model for W shows that the capsule can tolerate close to 300 ng of W-doped ablator material in the hot spot. If W is replaced with Si, the entire ablator has to be uniformly doped with 3 at.{\%} of Si. Surprisingly, the hot spot can tolerate about the same amount of ablator mass for the 3 at.{\%} Si-doped HDC as it can for W-doped. The main reason is that Si radiates less and consequently raises the hot spot temperature which in term increases the electron heat conduction. 4, 3, and 2-shock designs and their stabilites will be presented. An undoped HDC Symcap with DT fill reached a record neutron yield of 1.7e15. W-doped HDC Symcap and DT-layered shots will be conducted in Fall. Comparison of simulations with measured data will be presented. [Preview Abstract] |
Wednesday, November 13, 2013 10:06AM - 10:18AM |
NO7.00004: Advantages for a Beryllium Capsule Design for Indirect Drive Inertial Confinement Fusion J.L. Kline, A.N. Simakov, D.C. Wilson, S.A. Yi, J.D. Salmonson, D.S. Clark, J.L. Milovich, M.M. Marinak, D.A. Callahan, S.W. Haan, M.J. Edwards, A. Nikroo, H. Huang, K. Youngblood While much progress has been made towards ignition with plastic (CH) ablators, many challenges remain. Alternate ablators, such as Beryllium, could advance our progress more quickly since these ablators are more efficient. In the case of Beryllium, the lower opacity and higher mass ablation rate leads to higher implosion velocities for the same xray drive. Strategically, comparing and contrasting experimental results for different ablators will provide needed information to determine which material is the best option for achieving ignition. This presentation will be the first in a series of four to outline an updated Beryllium ablator design for ignition experiments. The focus here will be on the advantages, challenges, and path forward for a Beryllium ablator design. [Preview Abstract] |
Wednesday, November 13, 2013 10:18AM - 10:30AM |
NO7.00005: Beryllium Ignition Targets for Indirect Drive NIF Experiments A.N. Simakov, D.C. Wilson, S.A. Yi, J.L. Kline, J.D. Salmonson, D.S. Clark, J.L. Milovich, M.M. Marinak, D.A. Callahan Current NIF plastic capsules are under-performing, and alternate ablators are being investigated. Beryllium presents an attractive option, since it has lower opacity and therefore higher ablation rate, pressure, and velocity. Previous NIF Be designs assumed significantly better hohlraum performance than recently observed (e.g., 7.5 vs. 15-17\% of back-scattered power and 1.0 vs. 0.85 main pulse's power multipliers) and employed less accurate atomic configuration models than currently used (XSN vs. DCA), and thus an updated design is required. We present a new, Rev. 6 Be ignition target design that employs the full NIF capacity (1.8 MJ, 520 TW) and uses a standard 5.75 mm gold hohlraum with 1.5 mg/cm$^3$ of helium gas fill. The 1051 $\mu$m capsule features 180 $\mu$m of layered copper-doped (with the maximum of 3 atom-\%) Be ablator and 90 $\mu$m of cryogenic deuterium-tritium fuel. The peak implosion velocity of 367 $\mu$m/ns results in 4.1 keV of no-burn ion temperature, 1.6 and 1.9 g/cm$^2$ of fuel and total areal densities, respectively, and 20.6 MJ of fusion yield. The capsule demonstrates robust performance with surface/interface roughnesses up to 1.6 times larger that Rev. 3 specs. [Preview Abstract] |
Wednesday, November 13, 2013 10:30AM - 10:42AM |
NO7.00006: Hydrodynamic Stability of Beryllium Ignition Targets for Indirect Drive NIF Experiments S.A. Yi, A.N. Simakov, D.C. Wilson, J.L. Kline, J.D. Salmonson, D.S. Clark, J.L. Milovich, M.M. Marinak, D.A. Callahan Beryllium ablators have the advantage of higher ablation rate, pressure, density and velocity than plastic ablators. We present a new NIF beryllium target design that takes into account recent experimental assessment of hohlraum performance, as well as the new DCA atomic configuration model. Herein, we employ the radiation-hydrodynamics code HYDRA and use 2D capsule-only simulations driven by a frequency dependent source (FDS) to assess the hydrodynamic stability properties of our target capsule. Our FDS has been derived from integrated 2D simulations for a 1.8 MJ, 520 TW laser drive and a 5.75 mm holhraum with a 15\% backscatter loss and a 15\% power loss in the main laser pulse. We show that in our capsule the initial fuel layer surface roughness is the dominant source of instability growth and hotspot distortion. Rarefaction waves transmit the inner ice roughness to the outer surface, where it grows due to the ablative Rayleigh-Taylor (RT) instability. The instability growth is quantified and strategies to mitigate this type of RT instability seeding are discussed. [Preview Abstract] |
Wednesday, November 13, 2013 10:42AM - 10:54AM |
NO7.00007: Optimizing Fuel Adiabat and Thickness for Beryllium Ignition Targets D.C. Wilson, A.N. Simakov, S.A. Yi, J.L. Kline, J.D. Salmonson, D.S. Clark, J.L. Milovich, M.M. Marinak, D.A. Callahan At the same radiation temperature beryllium has a higher mass ablation rate than either plastic (CH) or diamond (C). We take advantage of this by imploding a larger DT mass in beryllium capsules. With a higher initial rho-R, achieving the same final rho-R requires smaller capsule convergence. This in turn leads to more tolerance of instability growth. The capsule convergence can be further decreased by increasing the DT fuel entropy. This is achieved by increasing the power of the first laser pulse from 30 to 60 TW, raising the pressure of the first shock. Beginning with our Rev 6 beryllium ignition design we have varied the fuel thickness between 50 and 130 microns (90 nominal) and increased the adiabat from 1.6 (nominal) to 3.0. While keeping the final laser pulse energy and power fixed, we tuned the laser pulse for each capsule. We report on the capsule sensitivity to surface roughness, using 2D HYDRA capsule only simulations with drives taken from integrated hohlraum simulations. These results guide us in improving capsule robustness to perturbations. [Preview Abstract] |
Wednesday, November 13, 2013 10:54AM - 11:06AM |
NO7.00008: Dependence of Compressed Ablator Conditions on the Shell Adiabat in NIF Implosions S.P. Regan, R. Epstein, T.C. Sangster, D.D. Meyerhofer, C.A. Iglesias, B.G. Wilson, H.S. Park, L.J. Suter, H.A. Scott, O.S. Jones, B.A. Hammel, M.A. Barrios, V.A. Smalyuk, B.A. Remington, G.A. Kyrala, T.J. Murphy, J.L. Kline, P.A. Bradley, N.S. Krasheninnikova, R.J. Kanzleiter, J.D. Kilkenny The x-ray continuum emitted from the hot spot of an inertial confinement fusion implosion around stagnation provides a probe to diagnose the $\rho R$, $n_{\mathrm{e}}$, and $T_{\mathrm{e}}$ of the compressed, Ge-doped CH ablator using the Ge K edge, 1$s$--2$p$ and 1$s$--3$p$ absorption features. Measured x-ray absorption spectra from a low-adiabat ($\alpha =$ $P_{\mathrm{ablator}}$/$P_{\mathrm{Fermi}})$, indirect-drive implosion and a high-$\alpha $, polar-drive implosion are compared. Using the Ge opacity calculations of the \textit{VISTA} code, the inferred quantities are $\rho R\sim $ 0.5 g/cm$^{\mathrm{2}}$, $n_{\mathrm{e}}\sim $ 0.5 $\times $ 10$^{\mathrm{26}}$ cm$^{\mathrm{-3}}$ and $T_{\mathrm{e}}\sim $ 200 eV for the low-$\alpha $ case, while lower compression and higher temperatures are inferred for the high-$\alpha $ case, consistent with radiation--hydrodynamics simulations. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Wednesday, November 13, 2013 11:06AM - 11:18AM |
NO7.00009: Measuring the azimuthal symmetry of the imploding in-flight ablator with a novel axial x-ray shadowgraph platform on the National Ignition Facility Nobuhiko Izumi, S. Glenn, S.R. Nagel, R. Rygg, J.L. Peterson, O.S. Jones, B. Yoxall, T. Parham, S. Burns, O.L. Landen, D. Kalantar, A.V. Hamza, K. Knittel, A.J. Mackinnon, P.M. Bell, R.P.J. Town, D.K. Bradley For x-ray driven implosions, the hohlraum typically has cylindrical symmetry, and the x-ray drive on the capsule is close to axisymmetric. However, realistic targets have azimuthally asymmetric structures (such as a finite number of laser beams, diagnostic holes in the hohlraum wall, and the fuel fill tube attached to the equator of the capsule), all of which can compromise the ability to compress the capsule. Therefore it is important to measure these asymmetries. To measure the time-dependent shape, velocity, and mass, we have developed a new axial x-ray shadowgraph platform at the National Ignition Facility. We set the x-ray backlighter foil on the hohlraum axis (below the laser entrance hole) and observe the backlit image using a gated x-ray camera placed above the hohlraum. The experimental design and detailed results from this new experimental platform will be presented. Prepared by LLNL under Contract DE-AC52-07NA27344. LLNL-ABS-626372 [Preview Abstract] |
Wednesday, November 13, 2013 11:18AM - 11:30AM |
NO7.00010: Measuring center-of-mass velocity on a NIF shot R. Hatarik, J. McNaney, J.A. Caggiano, J.P. Knauer An important diagnostic value of a shot at the National Ignition Facility (NIF) is the resultant center-of-mass motion of the imploding capsule as it contributes to the efficiency of converting LASER energy into plasma temperature. This velocity can be determined with neutron time-of-flight detectors (nToF) by establishing an absolute time reference and using an inferred neutron spectrum with an ion temperature and resulting mean energy. Three nToF detectors based on fast organic scintillators are being used to determine the neutron spectrum and the resultant velocity from three different directions. Results from velocity measurements and their implications will be presented. [Preview Abstract] |
Wednesday, November 13, 2013 11:30AM - 11:42AM |
NO7.00011: Bulk-Velocity Construction from NIF Neutron Spectral Diagnostics J.P. Knauer, R. Hatarik, B.K. Spears, J.M. McNaney, J.A. Caggiano, M. Gatu-Johnson, J. Frenje Target-mass and laser-intensity perturbations may induce a bulk motion of the neutron-emitting region in a National Ignition Facility (NIF) implosion. Most NIF implosions suffer from an incomplete conversion of shell kinetic energy into thermal energy. Four NIF instruments (three neutron time-of-flight detectors and a magnetic recoil spectrometer) accurately measure the neutron spectrum. Four sets of three instruments are available to construct the resultant velocity. A weighted average of the reconstructions is then used as a measure of the bulk velocity. This talk will present the resultant velocity analysis and compare NIF implosion measurements. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Wednesday, November 13, 2013 11:42AM - 11:54AM |
NO7.00012: An experimental and numerical study of top/bottom drive asymmetry on NIF implosions Brian Spears, J. Lindl, J. Edwards, R. Town, T. Ma, A. Pak, D. Eder, A. Kritcher, P. Patel, J. McNaney, J. Knauer, D. Munro, S. Hatchett NIF x-ray and nuclear diagnostics intermittently suggest unintentionally broken top/bottom (mode 1) symmetry. We present the results of a NIF implosion experiment with intentional top/bottom laser power asymmetry. The controlled asymmetric experiment showed agreement with the diagnostic signatures of mode 1 asymmetry as predicted by numerical simulations. Furthermore, the controlled experimental results provide a context for interpreting the historical archive of data on implosions with unintentional asymmetry. This analysis supports our hypothesis that uncontrolled asymmetries have indeed been present on prior NIF implosions. Numerical simulations confirm that these asymmetries impact implosions at levels varying from minor to substantial. We explore the numerical simulations to show the impact of the asymmetry on implosion hydrodynamics and the associated impact on implosion performance metrics including the Ignition Threshold Factor (eXperimental), ITFX, and the Generalized Lawson Criterion. LLNL-ABS-640682. [Preview Abstract] |
Wednesday, November 13, 2013 11:54AM - 12:06PM |
NO7.00013: Effects of EOS adiabat on hot spot dynamics Baolian Cheng, Thomas Kwan, Yi-Ming Wang, Steven Batha Equation of state (EOS) and adiabat of the pusher play significant roles in the dynamics and formation of the hot spot of an ignition capsule. For given imploding energy, they uniquely determine the partition of internal energy, mass, and volume between the pusher and the hot spot. In this work, we apply the new scaling laws [1] recently derived by Cheng et al to the National Ignition Campaign (NIC) ignition capsules and study the impacts of EOS and adiabat of the pusher on the hot spot dynamics by using the EOS adiabat index as an adjustable model parameter. We compare our analysis with the NIC data, specifically, for shots N120321 and N120205, and with the numerical simulations of these shots. The predictions from our theoretical model are in good agreements with the NIC data when a hot adiabat was used for the pusher, and with code simulations when a cold adiabat was used for the pusher. Our analysis indicates that the actual adiabat of the pusher in NIC experiments may well be higher than the adiabat assumed in the simulations. This analysis provides a physical and systematic explanation to the ongoing disagreements between the NIC experimental results and the multi-dimensional numerical simulations.\\[4pt] [1] B. Cheng, T. J.T. Kwan, Y.M. Wang, and S.H. Batha, LA-UR-13-22638, 2013. [Preview Abstract] |
Wednesday, November 13, 2013 12:06PM - 12:18PM |
NO7.00014: Studies of fuel-bulk flows using charged-particle and neutron spectrometry on OMEGA and the NIF M. Gatu Johnson, H. Rinderknecht, M. Rosenberg, H. Sio, A. Zylstra, J. Frenje, C.K. Li, F. Seguin, R. Petrasso, J. Delettrez, V. Glebov, J. Knauer, P. McKenty, T.C. Sangster, B. Appelbe, P. Amendt, C. Bellei, R. Bionta, D. Bleuel, J. Caggiano, D. Casey, J. Edwards, R. Hatarik, S. Hatchett, O. Landen A. MACKINNON, J. MCNANEY, D. MUNRO, J. PINO, S. WILKS, C. YEAMANS, LLNL, J. KILKENNY, A. NIKROO, GA -- Charged-particle and neutron spectra are used to study fuel-bulk flows, which are indicative of implosion asymmetries and inefficient conversion of kinetic energy to thermal energy. We distinguish between (i) collective, directional motion of the burn region, which manifests itself as a directional shift of the fusion-product spectrum, and (ii) radial flow, which appears as an additional broadening of the spectrum relative to expected based on $T_{\mathrm{i}}$ Doppler broadening. In this talk, we will present neutron and charged particle spectra from OMEGA and the NIF, which display the effect of these phenomena and their relation to implosion asymmetry. This work was supported in part by the U.S. DOE, LLNL and LLE. [Preview Abstract] |
Wednesday, November 13, 2013 12:18PM - 12:30PM |
NO7.00015: Comparison of Hot Spot Formation in DT ice layer and DT liquid layer ICF Capsules R.E. Olson, R.J. Leeper Simulations of the implosion and hot spot formation in two DT liquid layer ICF capsule concepts -- the DT wetted CH foam concept and the ``fast formed liquid'' (FFL) concept -- will be described and compared to simulations of standard DT ice layer capsules. The wetted foam and FFL designs allow for flexibility in hot spot convergence ratio through the adjustment of the initial cryogenic capsule temperature and, hence, DT vapor density. 1D simulations are used to compare the drive requirements, the optimal shock timing, the radial dependence of hot spot specific energy gain, and the hot spot convergence ratio in low (DT ice) and high (DT liquid) vapor pressure capsules. 2D simulations are used to compare the relative sensitivities to low-mode x-ray flux asymmetries in the DT ice and DT liquid capsules. It is found that the overall thermonuclear yields predicted for DT liquid layer capsules will be less than yields predicted for DT ice layer capsules in simulations using comparable capsule size and absorbed energy. However, the relative simplicity of the hot spot formation technique might lead to a more robust ignition experiment, a reduction in sensitivity to low-mode x-ray flux asymmetry, and an improvement in the computational prediction of hot spot behavior. [Preview Abstract] |
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