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 KO3: Hohlraum Physics and Alternative Concepts |
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Chair: John Edwards, Lawrence Livermore National Laboratory Room: Adam's Mark Hotel Governor's Square 15 |
Wednesday, October 26, 2005 9:30AM - 9:42AM |
KO3.00001: Modeling Hohlraum-Based Laser Plasma Instability Experiments N.B. Meezan, R.L. Berger, L. Divol, D.E. Hinkel, O.S. Jones, C. Niemann, E.A. Williams, S.H. Glenzer, L.J. Suter Laser fusion targets must control laser-plasma instabilities (LPI) in order to perform as designed. We present analyses of recent hohlraum LPI experiments from the Omega laser facility. The targets, gold hohlraums filled with gas or SiO$_2$ foam, are preheated by several $3\omega$ beams before an interaction beam ($2\omega$ or $3\omega$) is fired along the hohlraum axis. The experiments are simulated in 2-D and 3-D using the code \textsc{hydra}. The choice of electron thermal conduction model in \textsc{hydra} strongly affects the simulated plasma conditions. This work is part of a larger effort to systematically explore the usefulness of linear gain as a design tool for fusion targets. We find that the measured Raman and Brillouin backscatter scale monotonically with the peak linear gain calculated for the target; however, linear gain is not sufficient to explain all trends in the data. 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] |
Wednesday, October 26, 2005 9:42AM - 9:54AM |
KO3.00002: Gas Filled Halfraums on the First NIF Quad -- Radiation Hydrodynamics, Filamentation, and Spatial Diagnostics S.R. Goldman, J.C. Fernandez, N.M. Hoffman, J.L. Kline, H.A. Rose, E.S. Dodd, J.P. Grondalski, G.D. Pollak, W.J. Powers, M.J. Schmitt, D.G. Braun, D.E. Hinkel, L.J. Suter The Los Alamos gas-filled halfraum series on the first quad of NIF consisted of 4 laser shots at energies close to 15 kJ with nominal cylindrical symmetry. Lasnex modeling of halfraums with CO$_{2}$ as well as propane (C$_{3}$H$_{8})$ gas fill is consistent with peak power Dante experimental detector results. At the peak laser intensities of the shot ($>$ 2 x 10$^{15}$W/cm$^{2})$, filamentation is theoretically predicted, and more detailed simulations have suggested the possibility of conversion of the original f/8 laser beam profile into a beam with f-number as low as f/2. Simulations with laser beam profiles of f/8, f/4, f/3, and f/2 are available for both gas fills. Gated X-ray images of emission at energies above 10 keV probe the laser interaction with gold ablated from the rear wall of the halfraum. For CO$_{2}$, the axial variation in emission lies between the calculated results for the f/4 and f/8 simulations. For propane, the f/4 simulation provides the best fit to the data. Comparison at lower laser intensities, where the laser beam is expected to be f/8, clarifies the qualitatively different axial structuring observed for the two gases. [Preview Abstract] |
Wednesday, October 26, 2005 9:54AM - 10:06AM |
KO3.00003: Performance of foam-filled hohlraums G. Gregori, K. Campbell, L. Divol, S.H. Glenzer, O.L. Landen, J. Schein, C. Sorce, L.J. Suter, R.E. Turner, R.J. Wallace, P.A. Amendt We present time-resolved hohlraum wall hydrodynamics, drive and capsule implosion symmetry measurements in foam-filled holraums driven by 40 beams of the Omega laser facility (University of Rochester) with 14 kJ total energy in a 2.6 ns shaped pulse. Performance of hohlraums filled with silica aerogels at various densities (SiO$_{2}$ at 1, 2 and 4 mg/cc) is compared with LASNEX simulations, showing good agreement with modeling. Integrated x-ray flux measurements (DANTE) show that the radiation temperature inside the gold cavity is marginally affected by the lowest density foams, with low levels of laser backscatter. Similarly, time-of-flight neutron yield data are comparable with measurements taken with empty hohlraums. Targets filled with 1 mg/cc SiO$_{2}$ present good implosion symmetry as well as reduced holraum wall motion, as indicated by soft x-rays framing images taken along the hohlraum axis. We also show that beam pointing can be tuned to control the implosion symmetry. These results are important for assessing performance and benchmarking of inertial confinement fusion target designs for the National Ignition Facility. This work was performed under the auspices of the U.S. Department of Energy by University of California Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. [Preview Abstract] |
Wednesday, October 26, 2005 10:06AM - 10:18AM |
KO3.00004: Ignition hohlraum efficiency as a function of radiation temperature L.J. Suter, S. Haan, M. Herrmann The principal focus of hohlraum research for indirect drive ignition on the National Ignition Facility (NIF) are hohlraums that reach a peak radiation drive of $\sim $300eV. Here we examine the relative performance of ignition hohlraums designed to operate at different radiation temperatures. Current simulations indicate optimized coupling efficiencies, for similar hohlraums with similar capsules, of $\sim $16{\%} at 300eV and $\sim $22{\%} at 250eV. A large part of this rise in efficiency with reduced temperature is an $\sim $25{\%} increase in the ``apparent conversion efficiency''. We quantitatively discuss the underlying reasons for the change in apparent conversion efficiency. The increase in efficiency, coupled with the lower radiation temperature, can reduce the single-quad laser intensity for the lower temperature hohlraums by about a factor of three. This can reduce the LPI linear gain and filamentation figure of merit. Finally, we combine these results with estimates of the required changes in capsule absorbed energy, as a function of temperature, to show the relative operating space of ignition targets at the two temperatures. 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] |
Wednesday, October 26, 2005 10:18AM - 10:30AM |
KO3.00005: Progress in modeling radiation drive in uranium-based cocktail hohlraums O. Jones, J. Schein, M. Rosen, L. Suter, R. Wallace, J. Gunther, K. Campbell, H. Wilkens, A. Nikroo, R. Olson, G. Rochau Although standard gold hohlraums have proven to be effective for generating and confining soft x-rays for indirect drive ICF experiments, it is theoretically possible to make hohlraums more efficient by using mixtures of materials, known as ``cocktails''. Calculations show that by suitably choosing a mixture of two or three materials, one can reduce the radiation energy lost into the hohlraum wall by simultaneously increasing the opacity and decreasing the heat capacity of the wall material relative to a pure gold wall. We report on recent experiments done at Omega in which we measured the radiation drive of cocktail and gold hohlraums with a filtered x-ray diode array. The hohlraums, which were made of gold, uranium-gold cocktail, or uranium-gold-dysprosium cocktail, reached peak radiation temperatures ranging from 270 to 310 eV. The radiation escaping through the laser entrance hole was calculated and compared to the measured values. The measured increase in radiation drive for the cocktail hohlraums compared to gold hohlraums was found to be in good agreement with our modeling. [Preview Abstract] |
Wednesday, October 26, 2005 10:30AM - 10:42AM |
KO3.00006: Indirect Drive Holhraum Performance Using Cocktail Materials Jochen Schein, Ogden Jones, Mordecai Rosen, Siegfried Glenzer, Eduard Dewald, Laurance Suter, Russell Wallace, Janelle Gunther, Otto Landen, Bruce Hammel, Gregory Rochau, Heather Wilkens Indirect drive coupling efficiency between laser energy in and capsule absorbed energy depends to a large degree on the ability to mitigate wall losses in the hohlraum. One approach to do this is based on the use of hohlraum wall materials with overlapping absorption bands to absorb and re-emit the radiation that otherwise would be lost. Albedos and conversion efficiencies for various combinations of these materials, so--called cocktails, have been calculated and a mixture of U, Dy and Au has been determined to be the best candidate. A series of experimental campaigns are performed at the Omega Laser Facility in Rochester to test the performance of cocktails coated hohlraums versus pure Au hohlraums. The campaign uses 20kJ of laser energy to drive well characterized hohlraums and the difference in soft x-ray drive is measured using a soft x-ray spectrometer (DANTE). Experimental results will be presented testing our understanding of high-Z cocktail radiation properties. [Preview Abstract] |
Wednesday, October 26, 2005 10:42AM - 10:54AM |
KO3.00007: Time-Resolved Characterization of Hohlraum Radiation Temperature via VISAR Measurement of Quartz Shock Velocity R.E. Olson, D.K. Bradley, G.A. Rochau, G.W. Collins, R.J. Leeper, L.J. Suter A new technique for time-resolved measurement of hohlraum radiation temperature has been successfully tested in hohlraums with radiation temperatures in the range of 90-170 eV. In these experiments, the hohlraum radiation fields produced ablatively-driven shock waves in quartz samples. A line-imaging velocity interferometer [1] (VISAR) was used to track the quartz shock velocity as a function of time, and an empirical relationship (determined in these experiments) was used to relate the measured shock velocity to the hohlraum radiation temperature. The VISAR results were compared to temperature unfolds of Dante X-ray flux measurements, and to SOP measurements of shock breakout in Al step samples. The experimental results were also compared to the predictions of an integrated 2D rad-hydro code. In these initial tests, a preheat limitation was found for the VISAR technique in hohlraums with temperatures exceeding 170 eV. This limitation is probably related to specific characteristics of the laser hot spots and might be different for other types of hohlraums. The test experiments were done at UR/LLE. The technique should also be useful for hohlraum temperature measurements at other ICF/HED facilities having a VISAR diagnostic capability, such as Z/ZR at SNL or NIF at LLNL. [1] P. M. Celliers et al, Rev. Sci. Instrum. 75, 4916 (2004). [Preview Abstract] |
Wednesday, October 26, 2005 10:54AM - 11:06AM |
KO3.00008: Self-generated magnetic field distributions in multiple-beam produced plasmas Philip Nilson, L. Willingale, M. Kaluza, C. Kamberidis, M.S. Wei, Z. Najmudin, R.G. Evans, A.E. Dangor, K. Krushelnick The importance of self-generated magnetic fields and heat transport inhibition in ignition-scale hohlraums is currently receiving much theoretical attention. In particular, the spatial and temporal evolution of the self-generated magnetic fields and their affect on the plasma evolution inside the hohlraum are not well understood. Megagauss level magnetic fields, attributable to the $\nabla T_e \times \nabla n_e $mechanism, may be sufficiently large inside gas-filled hohlraums to affect the electron energy distribution by magnetizing the electrons $\left( {\omega _c \tau _e >1} \right)$ and reducing the thermal conductivity$\kappa \approx \left( {1+\omega _c^2 \tau _e^2 } \right)^{-1}$, altering the x-ray emission and uniformity inside the hohlraum, laser-beam propagation and pointing to the inner wall surfaces, parametric instabilities, and beam filamentation. Here, we report on recent measurements taken using the VULCAN laser facility at the Rutherford Appleton Laboratory, wherein the magnetic fields and blow-off plasma generated from planar Au and Al solid targets were characterized. [Preview Abstract] |
Wednesday, October 26, 2005 11:06AM - 11:18AM |
KO3.00009: Optimized hohlraum-driven double-shell designs at 750 kJ 3$\omega $ laser absorbed energy for demonstrating ignition on the National Ignition Facility Peter Amendt, Charlie Cerjan, Alex Hamza, Jose Milovich, Harry Robey An effort is underway to redesign indirectly-driven double-shell ignition targets [1] that can accommodate as little as 750 kJ of total absorbed 3$\omega $ laser energy in the hohlraum. The advantages of double-shell ignition include (1) noncryogenic preparation and fielding, (2) expected low levels of laser backscatter with use of a reverse-ramp power profile, and (3) a relatively low threshold ignition temperature ($\approx $ 4 keV) to facilitate requirements on implosion symmetry. A one-dimensional thermonuclear yield of nearly 3.5 MJ for this target is obtained with adequate fall-line behavior or margin to potentially destructive fuel-pusher mix. Integrated two-dimensional hohlraum simulations will be presented and assessed for implosion symmetry and potential backscatter from laser-plasma interactions. [1] P. Amendt et al., Phys. Plasmas 9, 2221 (2002). [Preview Abstract] |
Wednesday, October 26, 2005 11:18AM - 11:30AM |
KO3.00010: Highly-resolved 2D Perturbation Simulations of the NIF 3w low-laser-drive energy (750 kJ) double-shell ignition design. J. Milovich, H. Robey, P. Amendt, M. Marinak Successful target designs driven by laser energies below one MJ are desired for the first ignition campaign on the NIF in 2010. Due to the ease of non-cryogenic preparation and fielding, a double-shell(DS) target indirectly-driven by 750 kJ of laser energy at 3w is being considered as an attractive ignition backup option. However, it is well known that instabilities seeded by interfacial perturbations may pose a major challenge to DS ignition. Previous work ({\it Physics of Plasmas} {\bf 11}, 1552 (2004)) has indicated that the growth of perturbations on the outer surface of the inner shell may potentially disrupt ignition. To control these instabilities new designs will employ a manufactured density-gradient in the bimetallic inner-shell and a material-matching mid-Z nanoporous supporting foam. The difficulties in manufacturing such exotic foams has led to further evaluation of the densities and pore sizes needed for a successful ignition campaign, thereby guiding the ongoing material science R\&D efforts. One key finding is that foams with higher densities than previously considered are now permissible. Furthermore, highly-resolved 2D computations are currently being performed to assess the robustness of these new targets to surface perturbations. Results of these simulations will be presented. [Preview Abstract] |
Wednesday, October 26, 2005 11:30AM - 11:42AM |
KO3.00011: Experimental measurement of interface trajectories in an indirectly-driven planar double-shell geometry H.F. Robey, J.F. Hansen, J.M. Milovich, P. Amendt, H.-S. Park, M. Bono Indirectly-driven double-shell implosions are being investigated as a possible non-cryogenic path to ignition on the NIF. Recent simulations [1], however, have shown that the Au inner shell is highly unstable to high mode number ($>$200) perturbations, possibly leading to breakup of the inner shell. These studies also suggest that this instability can be greatly reduced by tamping the outer surface of the inner shell with a material of intermediate density between that of the high-Z inner shell and the surrounding low-density foam. In order to test the validity of these simulations, a planar double-shell experimental testbed is being developed. Initial halfraum-driven experiments conducted on the Omega laser have investigated the preheat expansion and subsequent shock-driven recompression of two planar double-shell configurations, one with and one without a CH tamper layer between the Au ``inner shell'' and a 50 mg/cc carbon foam. The results of these experiments and comparison with simulations will be presented. [1] J.L. Milovich \textit{et al}., \textit{Phys. Plasmas} \textbf{11}, 1552 (2004). [Preview Abstract] |
Wednesday, October 26, 2005 11:42AM - 11:54AM |
KO3.00012: Operational advantages of double-shell target designs at 750 kJ (3$\omega )$ on the National Ignition Facility Charles Cerjan, Peter Amendt, Christopher Haynam, Kenneth Manes It is well known that several practical operational issues will limit the approach to ignition on any large laser system, especially pulse shaping and optical component damage. Recently proposed double-shell target designs employ a temporal pulse shape that peaks early in time and decreases slowly thereafter [1]. From an operational viewpoint, this pulse shape is easier to produce with a saturated amplifier, which is used in the NIF configuration, and the reduced fluence of this pulse will lead to slower growth of damage sites in the optical components. Thus, in addition to the intrinsic physical advantages of this pulse, enhanced operational latitude will be realized. \newline \newline [1] P. Amendt \textit{et al}., Phys. Plasmas \textbf{9}, 2221 (2002). [Preview Abstract] |
Wednesday, October 26, 2005 11:54AM - 12:06PM |
KO3.00013: Advances in the Manufacture of Omega-scale Double-shell Targets M. Bono, R. Hibbard, D. Bennett, H. Robey, A. Hamza, J. Milovich, P. Amendt The double-shell ignition target design consists of a low-Z outer shell that absorbs hohlraum-generated x-rays, implodes, and collides with a high-Z inner shell containing DT fuel. Efforts are continuing to field scaled ignition-like double shells on the Omega laser facility over a range of inner-shell Z. Previous ignition-like double-shell implosions on Omega used a low-Z CH inner shell [1]. The current target contains a higher-Z glass inner shell of diameter 216 microns, which is supported by SiO$_{2}$ aerogel inside a Br-doped CH ablator shell of diameter 550 microns. Fielding double-shell targets has historically been limited by the ability to successfully fabricate them, but several technological advances have recently been made in the manufacturing process. The inner capsule will be cast in SiO$_{2 }$aerogel of density 50 mg/cc, whose outer contour will be machined concentric to the inner capsule. This piece will then be assembled between two hemispherical ablator shells that mate at a step-joint with an adhesive-filled gap of thickness 100 nm. Three-dimensional tomographs made of each target using an x-ray micro-tomography system will allow precise characterization of the targets. [1] P. Amendt \textit{et al}., Phys. Rev. Lett. \textbf{94}, 065004 (2005). [Preview Abstract] |
Wednesday, October 26, 2005 12:06PM - 12:18PM |
KO3.00014: Materials science for hohlraum-driven ignition double-shell targets Alex V. Hamza, Peter Amendt Designs for ignition double shell targets put stringent requirements on materials and fabrication methods. The inner shell needs to be high-Z and contain 790 atmospheres of DT gas at room temperature. To reduce hydrodynamic instability during implosion the inner shell must be density graded as well. The inner shell must be concentric with the outer shell to with in 2 microns. Foam spacers to maintain concentricity must have density of less than 100 mg/cm$^{3}$. In order to maintain hydrodynamic stability during implosion the cell size of the foam must be small, less than on the order of 500 nm to 1 $\mu $m. In addition the foam must be compositionally matched to the inner shell. In order to maintain symmetry the joint in the outer shell must be without voids and density and opacity matched to the outer shell material. Approaches to and progress toward achieving these requirements will be presented. [Preview Abstract] |
Wednesday, October 26, 2005 12:18PM - 12:30PM |
KO3.00015: Laser-Driven Dynamic Hohlraums: Implosions S.G. Glendinning, M.C. Herrmann, L.J. Perkins The Laser-Driven Dynamic Hohlraum (LDDH) is a direct-drive analogue to the dynamic hohlraums fielded on the Sandia Z machine\footnote{ Matzen, M. K. et al., Phys. Plasmas \textbf{12}, 055503 (2005).}. In the LDDH, a radiatively collapsed shock driven in a xenon-filled capsule confines radiation in a spherically convergent geometry, leading to a small, hot hohlraum. Fielding a second capsule inside the LDDH results in a system which is significantly less vulnerable to mix than a hydrodynamically coupled double-shell implosion. Previously\footnote{ Herrmann, Mark, BAPS, November 2003 (BI2.003).} we have reported on our observations of the radiatively collapsed shock (RCS) and ablation of a silver tracer layer. Since that time, we have used the LDDH to drive implosions in D$_{2}$-filled glass capsules placed inside the LDDH and observed yields of about 4X10$^{9}$ neutrons ($\sim $10{\%} of two-dimensional predictions) and the presence of significant x-ray emission from stagnation of ablated glass with the RCS. We will show our results from implosion experiments and comparisons with simulations. [Preview Abstract] |
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