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 TO7: Shock Ignition, Diagnostics, and X-ray Sources |
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Chair: Roberto Mancini, University of Nevada Room: Regency VII |
Thursday, November 5, 2009 9:30AM - 9:42AM |
TO7.00001: Design of laser-driven shock ignition targets Andrew J. Schmitt, J.W. Bates, S.P. Obenschain, D.E. Fyfe, S.T. Zalesak Shock ignition target designs can produce high gains at modest driver energies.\footnote{R. Betti, C.D. Zhou, K.S. Anderson, \textit{et al.}, Phys. Rev. Lett. \textbf{98}, 155001} High resolution 2D simulations that include nominal pellet perturbations have shown that gains well over 100 can be generated with targets driven by short-wavelength sub-megajoule KrF lasers.\footnote{A.J. Schmitt, J.W. Bates, S.P. Obenschain \textit{et al.}, Fusion Sci.Tech. \textbf{56}, 377 (2009).} A key feature of shock ignition is the separation of the compression and ignition parts of the target assembly. allowing independent control. We use a simple theoretical model of the assembled pellet to demonstrate how to optimize this separation, and show how it differs from optimized fast ignition and conventional central ignition designs. We also address the issue of high convergence ratios found in previous simulations of shock ignition targets,$^{2}$ which can lead to greater sensitivity of the target gain to low-mode pellet asymmetries. We investigate techniques to minimize both this sensitivity, and present 2D simulations that demonstrate the effectiveness of these strategies. Work supported by US DoE/NNSA. [Preview Abstract] |
Thursday, November 5, 2009 9:42AM - 9:54AM |
TO7.00002: Shock-Ignition Experiments at High Intensity on OMEGA C. Stoeckl, W. Theobald, R. Betti, R.S. Craxton, J.A. Delettrez, O.V. Gotchev, V.Yu. Glebov, F.J. Marshall, D.D. Meyerhofer, W. Seka, T.C. Sangster, C.D. Zhou, J.A. Frenje, R.D. Petrasso Shock ignition is a two-step concept in which a strong spherically convergent shock wave is launched at the end of the laser pulse to ignite the compressed core of a low-velocity implosion. Spherical plastic-shell targets were used in experiments on the OMEGA laser at low implosion velocity. A strong shock wave was sent into the converging capsule using an intensity spike at the end of the laser pulse. Both the neutron yield and the areal density improved significantly with an optimized spike pulse. In a second experiment, the 60 OMEGA beams were split into 40 low-intensity beams used for fuel assembly and 20 delayed beams focused to a high intensity (up to 1 $\times $ 10$^{16}$ W/cm$^{2})$ for shock generation. Preheat and laser absorption during the high-intensity spike were studied using hard x-ray detectors and scattered-light measurements. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement Nos. DE-FC52-08NA28302, DE-FC02-04ER54789, and DE-FG02-05ER54839. [Preview Abstract] |
Thursday, November 5, 2009 9:54AM - 10:06AM |
TO7.00003: Dynamics of Laser-Driven Shock Waves in Solid Targets Y. Aglitskiy, M. Karasik, A.L. Velikovich, V. Serlin, J. Weaver, A.J. Schmitt, S.P. Obenschain, J. Grun, N. Metzler, S.T. Zalesak, J.H. Gardner, J. Oh, E.C. Harding Accurate shock timing is a key issue of both indirect- and direct-drive laser fusions. The experiments on the Nike laser at NRL presented here were made possible by improvements in the imaging capability of our monochromatic x-ray diagnostics based on Bragg reflection from spherically curved crystals. Side-on imaging implemented on Nike makes it possible to observe dynamics of the shock wave and ablation front in laser-driven solid targets. We can choose to observe a sequence of 2D images or a continuous time evolution of an image resolved in one spatial dimension. A sequence of 300 ps snapshots taken using vanadium backlighter at 5.2 keV reveals propagation of a shock wave in a solid plastic target. The shape of the shock wave reflects the intensity distribution in the Nike beam. The streak records with continuous time resolution show the $x-t$ trajectory of a laser-driven shock wave in a 10{\%} solid density DVB foam. [Preview Abstract] |
Thursday, November 5, 2009 10:06AM - 10:18AM |
TO7.00004: Neutron imaging development for inertial confinement fusion experiments Tony Caillaud, Olivier Landoas, Isabelle Thfoin, Franck Philippe, Alexis Casner, Jean-Luc Bourgade, Vladimir Glebov, Frederic J. Marshall, Craig Sangster, Hye Sook Park, Harry Robey, Peter Amendt Various failure mechanisms may limit fuel compression and ignition during Inertial Confinement Fusion (ICF) experiments with MegaJoule class lasers (e.g., the Laser M\'{e}gaJoule: LMJ and the National Ignition Facility: NIF). A Neutron Imaging System (NIS) may be used to determine the asymmetries in the hot core and the surrounding cold fuel shell. To reveal such asymmetries, a NIS must record both a primary (14 MeV) and a down-scattered (5-10 MeV) neutron image with high SNR and an image plane spatial resolution as low as 5 $\mu $m. We report on the continuing development of an NIS diagnostic at the OMEGA laser facility, using coded apertures. A new large neutron camera (150 mm entrance diameter: scaled for LMJ/NIF design) has been activated at OMEGA. This camera will allow 5 $\mu $m resolution for LMJ neutron source. We have tested a set of three detectors that can be used for various NIS diagnostic experiments on OMEGA from low yield (10$^{9}$-10$^{10}$ neutrons) low resolution (32 $\mu $m) measurements at 4 m from the neutron source to high yield (10$^{12}$-10$^{14}$ neutrons) high resolution (15 $\mu $m) measurements at 13 m. The low yield configuration allowed us to record, the first neutron image on an indirect drive shot with pure deuterium filled capsules. [Preview Abstract] |
Thursday, November 5, 2009 10:18AM - 10:30AM |
TO7.00005: Self-generated electric fields in imploding capsules due to shock-front polarization Peter Amendt, Jose Milovich, Scott Wilks, Chikang Li, Rich Petrasso, Fredrick Seguin The generation of strong, self-generated electric fields in direct-drive, inertial-confinement-fusion capsules has been reported using 15 MeV proton radiography [1]. A model incorporating charge separation at a shock front [2] is shown to provide good agreement with the measured proton deflections. A simple proton deflectometry model is developed to further assess the origin of the fields and the sensitivity to thickness of the annular field-generating region in an imploding capsule. The strength of the field in a polarized shock front is calculated to be $\approx $10$^{10}$ V/m or higher, and is competitive with the criterion for runaway electron generation. Collisional particle-in-cell simulations of a shock front are performed to gauge such kinetic effects and their potential role on shock-front broadening. \\[4pt] [1] J.R. Rygg \textit{et al}., Science \textbf{319}, 1223 (2008); C.K. Li \textit{et al}., PRL \textbf{100}, 225001 (2008). \\[0pt] [2] Ya. Zel'dovich and Yu. Raizer, \textit{Physics of Shock Waves} (Dover, Mineola NY, 2002), p. 522. [Preview Abstract] |
Thursday, November 5, 2009 10:30AM - 10:42AM |
TO7.00006: Development of Scintillator Detectors for Fast-Ignition Experiments and Down-Scattered Neutron Measurements on OMEGA V.Yu. Glebov, C. Stoeckl, W. Theobald, T.C. Sangster, K.L. Marshall, M.J. Shoup III, T. Buczek, A. Pruyne, M. Fox, T. Duffy, M.J. Moran, R. Lauck A small signal must be recorded after very large DT or hard x-ray signals in a neutron time-of-flight detector to measure down-scattered neutrons in cryogenic DT implosions or to measure neutron yield in the presence of hard x-ray background from an ultrahigh-intensity laser. Several detectors with plastic and liquid scintillators were developed and tested at the Omega/Omega EP Laser Facility in cryogenic DT implosions and integrated fast-ignition experiments. A gated photomultiplier tube was used to eliminate large DT or hard x-ray signals. The liquid scintillator consists of 0.4{\%} PPO, 0.04{\%} MSB dissolved in xylene and saturated with oxygen. The afterglow (long decay constant) with this scintillator is $\sim $100$\times $ less than conventional scintillators. This is an essential property to mitigate the residual scintillator signal in down-scattered neutron measurements and fast-ignition experiments. Detector designs and responses with the different scintillators will be presented. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement Nos. DE-FC52-08NA28302, DE-FC02-04ER54789, and DE-FG02-05ER54839. [Preview Abstract] |
Thursday, November 5, 2009 10:42AM - 10:54AM |
TO7.00007: Opacity Measurements on Fully Characterized 1.3 MK Titanium Plasmas Robert Heeter, Scott Anderson, Greg Brown, John Castor, Jim Emig, Kevin Fournier, Carlos Iglesias, Stephan Maclaren, Ross Marrs, Marilyn Schneider, Charles Sorce, Brian Wilson, Peter Beiersdorfer X-ray transmission spectra from fully characterized, high temperature plasmas provide validation benchmarks for high-energy-density physics models of interest to stellar astrophysics. This work presents new data from experiments at the Omega laser. 24 beams (at $\approx$ 500J/beam) heat the outer chambers of a 3-part (shine-shielded), 1.6 mm diameter hohlraum. The outer chambers radiatively heat a CH-tamped Ti foil in the center. Other beams drive 2 backlighters: a rear wall burnthrough halfraum, and a Kr-filled CH capsule implosion. Looking through the sample to these backlighters, 2 spectrometers measure the sample transmission from 250-1600 eV and 3000-5500 eV, respectively. The lower energy band encompasses the Rosseland mean opacity, governing radiation flow in stellar interiors. Plasma density of 0.05 g/cc, measured by expansion radiography, agrees with pre-shot calculations. Plasma temperature of 110-125 eV (1.3 MK), inferred from n=1 to n=2 titanium absorption lines, is 10-30\% below predictions. Use of similar techniques on the National Ignition Facility could validate stellar models in stellar core conditions. [Preview Abstract] |
Thursday, November 5, 2009 10:54AM - 11:06AM |
TO7.00008: Anomalous fluorescence line intensity in megavoltage bremsstrahlung Nino Pereira, Marc Litz, George Merkel, Joseph Schumer, John Seely, Jeff Carroll A Cauchois transmission crystal spectrometer intended for laser plasma diagnostics has measured an anomalous ratio between the fluorescence lines in megavoltage bremsstrahlung. When observed in reflection, K$\alpha_1$ fluorescence is twice as strong as the K$\beta$ line, as is usual. However, in forward-directed bremsstrahlung from a 2~MV end point linear accelerator with a tungsten converter, the K$\alpha_1$ and K$\beta$ fluorescence are approximately equal. The anomalous fluorescence line ratio, unity, reflects the large amount of fluorescence generated on the side of the converter where the electrons enter, and the differential attenuation of the fluorescence photons as they pass through the converter to opposite side. Understanding of fluorescence in megavoltage bremsstrahlung is relevant to the explanation of anomalous line ratios in spectra produced by high-energy electrons generated by intense femtosecond laser irradiation. [Preview Abstract] |
Thursday, November 5, 2009 11:06AM - 11:18AM |
TO7.00009: X-ray sources such as metallic liner inside plastic cavities are used to test atomic physics models for LMJ simulations Frederic Girard, L. Jacquet, M. Primout, D. Babonneau, B. Villette, Ph. Stemmler Bright multi-keV K-shell emission sources [1-3] are necessary for ICF studies such as radiography of dense materials. Our recent works with titanium (He$_{\alpha }$ at 4.7~keV) and germanium (He$_{\alpha }$ at 10.3~keV) lined cavities showed very high multi-keV x-ray conversion efficiencies up to 14.0{\%} and 0.8{\%} (respectively) [3]. Cavities with a titanium, copper or germanium liner have been used on the OMEGA laser facility in Rochester to test specifically atomic physics models coupled to hydrodynamic codes. Simulation results differ significantly from experimental conversion efficiencies for titanium liner but they are close together for germanium one. Atomic physics models do have great impact on X-ray output. The need for accurate predictive simulation is important in order to set up relevant radiography sources for experiments on the path to ignition. Multi-keV emission is diagnosed by a full set of diagnostics giving conversion efficiencies, time dependant x-ray power and imaging, time integrated imaging and high resolution spectra of titanium. \\[4pt] [1] D. Babonneau et al., Phys. Plasmas, \textbf{15}, 092702, 2008 \\[0pt] [2] K. B. Fournier et al., Phys. Plasmas, \textbf{16} 052703, 2009 \\[0pt] [3] F. Girard et al., Phys. Plasmas, \textbf{16}, 052704, 2009 [Preview Abstract] |
Thursday, November 5, 2009 11:18AM - 11:30AM |
TO7.00010: A Computational Study of X-ray Emissions from Laser-Irradiated Under-dense High-Z X-ray Sources Jeffrey Colvin, Mark May, Kevin Fournier, Stephen Moon, Howard Scott We generate x-rays from plasmas that are not in local thermodynamic equilibrium (LTE). We first discuss simulations of the x-ray spectral emissions from laser-irradiated very low-density Ge-doped aerogel targets using a 2D radiation-hydrodynamics code incorporating a modern Detailed Configuration Accounting atomic model in non-LTE. We present the details of the computational model and show that, for the $\sim $2 keV long-scale-length sub-critical-density plasmas created in experiments at the Omega laser facility, the simulations get both the measured Ge L-shell emission ($\sim $1-1.5 keV) and the measured Ge K-shell emission ($\sim $10-11 keV) about right, but only by properly accounting for non-local thermal conduction. The older average-atom atomic model is shown to be inadequate for these non-LTE plasmas. We then use the preferred model in the design of larger-scale experiments planned for the National Ignition Facility, in which we will use 350 kJ of laser beam energy to heat a mixture of Ar and Xe gas to peak temperatures $>$ 5 keV. We predict 20{\%} x-ray conversion efficiency into Ar K-shell and Xe L-shell emission. [Preview Abstract] |
Thursday, November 5, 2009 11:30AM - 11:42AM |
TO7.00011: Spectroscopic study of z-pinch K-shell x-ray sources B. Jones, G.A. Rochau, J.E. Bailey, D.J. Ampleford, C.A. Coverdale, S.B. Hansen, C.A. Jennings, M.E. Cuneo, Y. Maron, V. Fisher, V. Bernshtam, A. Starobinets, L. Weingarten, J.P. Apruzese, A. Dasgupta, R.W. Clark, J.W. Thornhill, K.G. Whitney, J.L. Giuliani, J. Davis Fast z-pinches provide intense 1-10 keV photon energy radiation sources. Analysis of time-integrated K-shell x-ray spectra indicates electron temperatures up to 5 keV and ion densities of order 10$^{20}$ cm$^{-3}$ in the stagnated plasma created from wire array implosions on Sandia's Z machine pulsed power driver. Elliptical crystal spectrometers coupled to microchannel plate detectors, fielded previously on Z, have recently provided time-resolved spectral measurements for Al ($\sim $2 keV), stainless steel ($\sim $6.7 keV), and Cu ($\sim $8.4 keV) sources. We discuss implosion and stagnation dynamics, plasma conditions, and spectral analysis in the presence of opacity and Doppler effects. [Preview Abstract] |
Thursday, November 5, 2009 11:42AM - 11:54AM |
TO7.00012: Searching for efficient Z-pinch L-shell Radiators using Cu, Mo, and Ag Single Planar Wire Arrays on 1.6 MA ZEBRA at UNR A.S. Safronova, V.L. Kantsyrev, A.A. Esaulov, N.D. Ouart, M.F. Yilmaz, V. Shlyaptseva, I. Shrestha, K.M. Williamson, M.E. Weller, G.C. Osborne, C.A. Coverdale, C. Deeney The comparative analysis of the results of recent experiments with single planar wire array (SPWA) loads made from Cu, Mo, and Ag performed on 1.6 MA Zebra at UNR is presented. The diagnostic set included a bolometer, x-ray detectors, laser shadowgraphy, x-ray time-gated and integrated imaging, and x-ray time integrated spatially resolved and time-gated spatially integrated spectrometers. The x-ray yields from such mid-$Z_{a}$ SPWAs exceed twice those from low-$Z_{a}$ wire arrays such as Al PWAs, increase with $Z_{a}$, and reach more than 24 kJ for Ag. The size of the x-ray source decreases as Z$_{a}$ increases, which lowers the opacity effects. The maximum L-shell plasma electron temperature increases as $Z_{a}$ increases, up to 1.4 keV for Mo and Ag. These results and the developed models have much broader application, not just only for SPWAs on Zebra, but for higher current Z-pinch generators such as Z at SNL. This work was supported by NNSA under DOE CA DE-FC52-06NA27588, DE-FC52-06NA27586, and in part by DE-FC52-06NA27616. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the US DOE under Contract DE-AC04-94AL85000. [Preview Abstract] |
Thursday, November 5, 2009 11:54AM - 12:06PM |
TO7.00013: X-Ray Emission Trends with Atomic Number and Initial Array Radius from K-Shell Z Pinches C.A. Coverdale, B. Jones, D.J. Ampleford, M.E. Cuneo, C. Deeney, P.D. LePell, K.G. Whitney, J.W. Thornhill, J. Giuliani, J. Davis, J.P. Apruzese, A. Dasgupta Experiments at the pre-refurbished Z facility covered a wide range of K-shell x-ray sources, including Ti (4.7 keV), SS (6.7 keV), and Cu (8.4 keV). The initial wire array radius of single and nested configurations was varied for each of these materials. The total x-ray emission decreased with larger diameter wire arrays for all materials, and nested arrays generally produced higher powers than single arrays. The K-shell emission for Ti behaved as expected based on the K-shell scaling theory [J.W. Thornhill et al., Phys. Plasmas \textbf{1}, 321 (1994)], with decreases in K-shell output observed for $\eta \quad <$2 and for mass $<$ soft mass breakpoint (m$_{bp})$. For SS, the scaling theory would predict inefficient K-shell emission for all the loads fielded ($\eta \quad >$ 2, but mass $<$ soft m$_{bp})$, although the K-shell output increased for larger diameter loads, even with masses $<$ soft m$_{pb}$. Emission from the higher atomic number arrays appears to be dominated by regions of brightly emitting plasma. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000. [Preview Abstract] |
Thursday, November 5, 2009 12:06PM - 12:18PM |
TO7.00014: Analysis of space- and time-resolved Ar K-shell emission in ICF experiments and determination of plasma spatial structure I.E. Golovkin, J.J. MacFarlane, P.R. Woodruff, J.E. Bailey, G.A. Rochau, R.J. Leeper We present a novel method to determine spatial distributions of the plasma temperature, density and relative fuel-pusher material fractions (mix) based on the analysis of space- and time-resolved spectroscopic data obtained in ICF implosion experiments. The determination of space-dependent plasma characteristics provides important constraints for radiation- hydrodynamics calculations and improves the understanding of implosion physics. Determination of the plasma spatial structure is a complex inverse problem with a non-trivial dependence of spectroscopic observables on the plasma distributions. We will present the genetic-algorithm-based analysis of the data and demonstrate that the plasma conditions can be successfully determined. All spectral simulations are performed using a suite of codes contained in the SPECT3D Imaging and Spectral Analysis package. We will discuss the details of the calculations based on the analysis of synthetic experimental data generated with known plasma distributions. [Preview Abstract] |
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