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 UI1: ICF Ignition |
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Chair: Denise Hinkel, Lawrence Livermore National Laboratory Room: Adam's Mark Hotel Plaza Ballroom ABC |
Friday, October 28, 2005 9:30AM - 10:00AM |
UI1.00001: Exploring New Methods For Increasing Coupling Efficiency in Indirect Drive Ignition Targets for the National Ignition Facility (NIF) Invited Speaker: Coupling efficiency, the ratio of capsule absorbed energy to driver energy, is a key parameter in ignition targets. The hohlraums originally proposed for the NIF coupled $\sim $11{\%} of the absorbed laser energy to the capsule as x-rays. We describe here a second generation of hohlraum targets which has higher coupling efficiency, $\sim $17.5{\%}. Because an ignition capsule's ability to withstand 3D effects increases so rapidly with absorbed energy, the additional coupling can significantly increase the likelihood of ignition. The new targets include laser-entrance-hole (LEH) shields as a principal method for increasing coupling efficiency while controlling symmetry in indirect-drive ICF targets. The LEH shields are high Z disks placed inside the hohlraum to block the capsule's view of the cold LEH's. The LEH shields can reduce the amount of laser energy required to drive a target to a given temperature via two mechanisms: 1) keeping the temperature high near the capsule pole by putting a barrier between the capsule and the hole, 2) because the capsule poles do not have a direct view of the cold LEH's, good symmetry requires a shorter hohlraum with less wall area. Current integrated simulations of this class of target couple $\sim $140 kJ of x-rays to a target from $\sim $800 kJ of absorbed laser light and produce over 10 MJ of thermonuclear yield. In addition to shields, this design utilizes a low density (1mg/cc) aerogel foam to provide symmetry control. This replaces the He-H fill gas in previous targets and provides significant operational advantages. We describe the new targets, provide a quantitative design analysis of the radiation hydrodynamics properties as well as estimates of the laser-plasma interaction environment and properties. We also describe the diagnostic challenges presented by these targets and how we propose to meet those challenges on NIF. [Preview Abstract] |
Friday, October 28, 2005 10:00AM - 10:30AM |
UI1.00002: Radiation-driven hydrodynamics of long pulse hohlraums on the National Ignition Facility*,** Invited Speaker: The first hohlraum experiments have been performed at the National Ignition Facility (NIF) in support of indirect drive Inertial Confinement Fusion (ICF) and High Energy Density Physics. Vacuum hohlraums have been irradiated with laser powers up to 8 TW, 1-9 ns pulse lengths and energies up to 17 kJ to activate several hohlraum drive diagnostics, to study the radiation temperature scaling with the laser power and hohlraum size, and to make contact with hohlraum experiments performed at the NOVA and Omega laser facilities. The vacuum hohlraums yield low laser backscattering and hot electron fractions, and the hohlraum radiation temperature measured with a newly activated 18 channel Dante soft x-ray power diagnostic agrees well with two-dimensional LASNEX calculations. Using the unique feature of NIF to deliver long steady laser drives, these hohlraum experiments have also validated analytical models and LASNEX calculations of hohlraum plasma filling as evidenced by time-resolved hard x-ray imaging and coronal hohlraum radiation production measured by Dante. Analytical modeling used to estimate hohlraum radiation limits due to plasma filling is in agreement with measurements and predicts for full NIF system with peak powers up to 500 TW peak radiation temperatures that are considerably higher than required in ICF designs. \newline \newline * Work performed in collaboration with L.J. Suter, O.L. Landen, J. Schein, K. Campbell, M.S. Schneider, J. Holder, S.H. Glenzer, J.W. McDonald, C. Niemann, A.J. Mackinnon, D.H. Kalantar, C. Haynam, S. Dixit \newline **This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-48. [Preview Abstract] |
Friday, October 28, 2005 10:30AM - 11:00AM |
UI1.00003: Gas-filled hohlraum experiments at the National Ignition Facility Invited Speaker: A joint team from the National Laboratories at Los Alamos (LANL) and Lawrence Livermore (LLNL) has fielded the first gas-filled hohlraum experiments at the National Ignition Facility (NIF) laser, with the available four beams arranged as a single {\it f/8} beam. The gas-fill in this LANL design provides plasma pressure to tamp the hohlraum gold wall to avoid filling, the same technique used in ignition designs. A shaped laser pulse 8~ns in duration was used, with a low-power foot and a late peak of 7~TW, a contrast ratio exceeding 100 (the highest on NIF so far), and a total energy of 14~kJ. Deployed measurements include laser energy and power; back-scattered light spectrum, power and energy directly into the focusing lenses; back-scattered laser light energy outside the lenses; soft x-ray drive spectrum and power, and; gated framing-camera images of the hohlraum self-emission with x-ray energy $>$~10~keV. Our main results and conclusions are: (1)~This is the first experimental demonstration that a low-Z fill can keep the interior of a laser-driven hohlraum open long enough to ensure efficient coupling of ignition-relevant laser pulses. (2)~When backscattering losses are accounted ($\approx$~25\% reflectivity due to stimulated Brillouin scattering [SBS]), we have the radiation-hydrodynamics predictive capability necessary to understand the energy balance in such hohlraums quantitatively, as well as other details of the hohlraum-plasma evolution. (3)~Laser-plasma instabilities (LPI) can lead to considerable laser reflectivity levels, with a significant and measurable deleterious impact on hohlraum energetics. Thus, continued development of LPI predictive capability and understanding is needed. (4)~These experiments provide evidence that Stimulated Raman back-scattering losses (SRS) may be minimized with a proper choice of plasma conditions. [Preview Abstract] |
Friday, October 28, 2005 11:00AM - 11:30AM |
UI1.00004: Progress in Polar-Direct-Drive Simulations and Experiments Invited Speaker: Polar direct drive (PDD)\footnote{ S. Skupsky \textit{et al}., Phys. Plasmas \textbf{11}, 2763 (2004).} will allow ignition experiments on the National Ignition Facility (NIF) while it is configured for x-ray drive. Optimal drive uniformity is obtained via a combination of beam repointing, pulse shapes, spot shapes, and/or target design. This talk describes progress in developing PDD designs including the ``Saturn'' target\footnote{ R. S. Craxton and D. W. Jacobs-Perkins, Phys. Rev. Lett. \textbf{94}, 095002 (2005).} concept that improves drive uniformity by adding a low-$Z$ ring around the target to refract light toward the target equator. These concepts are being evaluated on the OMEGA Laser System and with two-dimensional hydrodynamic simulations.\footnote{ R. S. Craxton \textit{et al}., Phys. Plasmas \textbf{12}, 056304 (2005).} Forty OMEGA beams, arranged in six rings to emulate the NIF x-ray drive configuration, implode 20-\textit{$\mu $}m-thick, 865-\textit{$\mu $}m-diam CH shells, filled with 15 atm of D$_{2}$. Diagnostics include framed x-ray backlighting and time-integrated x-ray imaging. Saturn target experiments have resulted in $\sim $75{\%} of the yield from energy-equivalent symmetrically irradiated implosions. Two-dimensional PDD simulations are performed using both the \textit{SAGE} (Eulerian) and \textit{DRACO} (ALE) codes. The simulations are in good agreement with the experiments. PDD simulations for the NIF show modest gains and high areal densities. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460. Contributors: F. J. Marshall, R. S. Craxton, I. V. Igumenshchev, P. B. Radha, S. Skupsky, P. W. McKenty, T. J. B. Collins, R. Epstein, M. J. Bonino, D. Jacobs-Perkins, D. D. Meyerhofer, T. C. Sangster, J. P. Knauer, V. A. Smalyuk, V. Yu. Glebov, S. G. Noyes, W. Seka, and R. L. McCrory. [Preview Abstract] |
Friday, October 28, 2005 11:30AM - 12:00PM |
UI1.00005: First Detailed Diagnosis of Double Shell Collision under Realistic Implosion Conditions Invited Speaker: Double shell implosions provide a non-cryogenic path to inertial confinement fusion [ICF]. In the double shell target the laser energy is absorbed in an outer shell that is accelerated inward which then, after the laser is off, collides with an inner shell that implodes against the deuterium fuel. However the design of these ICF capsules depend on a many step process to achieve the ignition. One of these processes is the symmetric collision of the outer shell with an inner shell. This requires that the shells must be illuminated and built symmetrically. In reality the targets are complicated and the construction is not symmetric, due to the seam that our current assembly method requires. Furthermore, in order to diagnose the symmetry of the implosion and the hydrodynamics, radiography of the shells are required. This places a significant requirement on the x-ray energy in the backlighter. Using the OMEGA laser, we have designed an illumination strategy that uses 40 beams in an offset geometry, leaving 20 beams to perform radiography from two different directions. This places an artificially significant non-symmetric illumination that may not exist in final targets shot on the NIF. We will present the first measurement of the time history of a collision of two shells in a double shell capsule. We will briefly review the illumination geometry, give the results of the measurements of the trajectory of the outer and inner shells, and compare the results with calculations. We will also present data on the measured symmetry of the outer and inner shell implosions, and if time permits we will present some of the data on the development of the seam in the capsule and its effect on the inner shell implosion. This is the first measurement of such a collision in a spherical geometry and is of great interest to double shell implosions, and to the verification of the code we used. [Preview Abstract] |
Friday, October 28, 2005 12:00PM - 12:30PM |
UI1.00006: Pathway to a lower cost high-repetition IFE ignition facility Invited Speaker: We have identified an attractive path to develop the science and technology for fusion energy based on direct-drive pellet designs that substantially reduce the needed laser energy. A power plant based on laser fusion will require pellet energy gains of about 100 to overcome inefficiencies in the laser and power generation. For directly-driven targets this probably requires energies of at least one MegaJoule. However, many of the key science and engineering tasks could be accomplished with ignition and lesser gains. If one increases the pellet implosion velocity from the nominal 300 km/sec in high gain designs to 400 to 500 km/sec, one can obtain ignition and moderate gains at substantially reduced laser energy. This higher velocity can be obtained by increasing the distance over which the pellet shell is accelerated. But this approach leads to thin large-diameter pellet shells and the implosion is more likely to be disrupted by hydrodynamic instability. One can alternately obtain higher velocity by increasing the laser irradiance and thereby produce higher ablation pressure. This approach allows high-velocity implosion of relatively thick-shelled smaller-diameter targets that are much more resistant against hydrodynamic instability. The Krypton Fluoride (KrF) laser has substantial advantages towards implementing this approach. Its 248 nm deep-UV wavelength and very broad bandwidth suppress the laser-plasma instability that limits usable peak irradiance. The short laser wavelength also gives higher pressure and more efficient absorption. Our one-dimensional simulations using a KrF driver predict energy gains of 20 with 250 kJ laser energy and gains above 50 at 500 kJ. These pellet designs employ mechanisms that increase hydrodynamic stability. This approach opens the opportunity for a relatively small high-repetition KrF-based laser fusion facility that would be useful for developing and testing fusion energy science and technologies. Progress in the analysis of the pellets and implications for a faster-track IFE program will be discussed. This work was supported by the U.S. Department of Energy, NNSA. Work in collaboration with the researchers in the NRL Laser Fusion Program and the High Average Power Laser (HAPL) Program. [Preview Abstract] |
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