Session BI2: Indirect Drive ICF

Assessing the prospects for achieving double-shell ignition on the National Ignition Facility using vacuum hohlraums

The goal of demonstrating ignition on the National Ignition Facility (NIF) has motivated a revisit of double-shell (DS) [1] targets as a complementary path to the baseline cryogenic single-shell approach [2]. Benefits of DS targets include room-temperature deuterium-tritium (DT) fuel preparation, minimal hohlraum-plasma-mediated laser backscatter, low threshold-ignition temperatures (4 keV) for relaxed hohlraum x-ray flux asymmetry tolerances [3], and loose shock timing requirements. On the other hand, DS ignition presents several challenges, including room-temperature containment of high-pressure DT (790 atm) in the inner shell; strict concentricity requirements on the two shells; development of nanoporous, low-density, metallic foams for structural support of the inner shell and hydrodynamic instability mitigation; and effective control of perturbation growth on the high-Atwood number interface between the DT fuel and the high-Z inner shell. Recent progress in DS ignition target designs using vacuum hohlraums is described, offering the potential for low levels of laser backscatter from stimulated Raman and Brillouin processes. In addition, vacuum hohlraums have the operational advantages of room temperature fielding and fabrication simplicity, as well as benefiting from extensive benchmarking on the Nova and Omega laser facilities. As an alternative to standard cylindrical hohlraums, a rugby-shaped geometry is also introduced that may provide energetics and symmetry tuning benefits for more robust DS designs with yields exceeding 10 MJ for 2 MJ of 3w laser energy. The recent progress in hohlraum designs and required advanced materials development are scheduled to culminate in a prototype demonstration of a NIF-scale ignition-ready DS in 2007. \newline \newline [1] P. Amendt et al., PoP 9, 2221 (2002). \newline [2] J.D. Lindl et al., PoP 11, 339 (2004). \newline [3] M.N. Chizhkov et al., Laser Part. Beams 23, 261 (2005). \newline In collaboration with C. Cerjan, A. Hamza, J. Milovich and H. Robey.   

Target design for high fusion yield with the double Z-pinch-driven hohlraum.

A key demonstration on the path to inertial fusion energy is the achievement of high fusion yield (hundreds of MJ) and high target gain. An indirect-drive high-yield inertial confinement fusion (ICF) target involving two z-pinch x-ray sources heating a central secondary hohlraum is described by Hammer, Tabak, Wilks, et al. [\textit{Phys. Plasmas} \textbf{6}, 2129 (1999)]. In subsequent research at Sandia National Laboratories, we have developed theoretical/computational models and performed an extensive series of validation experiments to study hohlraum energetics, capsule coupling, and capsule implosion symmetry. We are using these models to design a 0.5 GJ yield z-pinch-driven ICF target that incorporates the latest experience in capsule design, hohlraum symmetry control, and x-ray production by z-pinches. An x-ray energy output of 8-9 MJ per pinch, suitably pulse-shaped, is sufficient for this concept to drive 0.3-0.5 GJ capsules. Integrated 2D hohlraum/capsule LASNEX radiation-hydrodynamics simulations have demonstrated adequate hohlraum coupling, radiation symmetry control, and the successful implosion, ignition and burn of a 0.5 GJ ICF capsule. An important new feature of this target design is mode-selective symmetry control: the use of burnthrough shields offset from the capsule that selectively tune certain low-order asymmetry modes (P$_{2}$, P$_{4})$ without significantly perturbing higher-order modes. This talk will describe the capsule and hohlraum design that have produced 0.5 GJ yields in 2D simulations, as well as provide a preliminary design of the z-pinch load and accelerator requirements necessary to drive the system. In collaboration with M. C. Herrmann, R. W. Lemke, G. R. Bennett, R. B. Campbell, P. J. Christenson, M. E. Cuneo, M. P. Desjarlais, T. A. Mehlhorn, J. L. Porter, D. B. Sinars, S. A. Slutz, W. A. Stygar, E. P. Yu, and J. H. Hammer (LLNL).   

Fill tube radiation hydrodynamics experiments on ignition scale capsules

Recently, experiments on the Z facility at Sandia National Laboratories examined the perturbations created on inertial confinement fusion capsules by the presence of tiny ($\sim $10-40 micron diameter) fill tubes. Tubes similar to these will be used to fill ignition capsules with deuterium tritium fuel on the National Ignition Facility (NIF). Edwards, et. al [Phys. Plas. 12, 056318 (2005)], in calculations assessing the impact of these tubes on ignition capsules, showed that the dominant perturbation from the fill tube was not due to the hydrodynamic perturbation from the hole and tube at the capsule's surface but rather was due to the radiation shadow cast on the capsule surface by the exploding tube. This radiation shadowing leads to a scaling of the perturbation on the capsule that is linear with the tube size. It also implies that the size of the perturbation on the capsule depends on the composition of the tube. The experiments to test these predictions were designed to take advantage of Z's unique capabilities to symmetrically implode ignition scale capsules with drive temperatures near the NIF foot temperature. General Atomics produced smooth, ignition scale capsules, drilled holes in an equatorial plane and attached multiple (up to 4) tubes to each capsule for the experiments. These targets enable the cleanest comparisons of the perturbations created by fill tubes of different sizes and compositions. The capsules and perturbations resulting from the fill tubes were imaged using a 6.151 keV bent crystal imaging system with excellent spatial resolution over a field of view which covered the entire capsule. A careful comparison between calculations and experimental results will be presented and the implications of these results for NIF ignition capsules with fill tubes will be discussed.   

Enhanced hohlraum radiation drive through reduction of wall losses with high-Z mixture 'cocktail' wall materials

Indirect drive efficiency for inertial confinement fusion 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 measured and a mixture of U, Dy and Au has been determined to be one of the best candidates. (A combination of U and Au will be used for the NIF ignition experiments). 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 for radiation temperatures from 180eV to 310eV using 5kJ-20kJ of laser energy to drive well characterized hohlraums. The difference in soft x-ray drive is measured using a soft x-ray spectrometer (DANTE) in combination with backscatter measurements. Experimental results show significant improvement of hohlraum soft x-ray output by increasing the measured flux by up to 8{\%} for the higher radiation temperatures in excellent agreement with analytical and modeling results. The results achieved at Omega will be discussed under the background of increasing the coupling efficiency during NIF ignition experiments through the use of high-Z cocktail walls.   

3$\omega $ Laser Beam Propagation in Inertial Confinement Plasmas*

A study of the relevant laser-plasma interaction processes in a long-scale length high-temperature transparent plasma has been performed using a new target platform to emulate the plasma conditions in an indirect drive fusion target. Recent experiments in this plasma emulator have demonstrated that for ignition relevant conditions (T$_{e}>$3 keV, I $<$ 2x10$^{15}$ W-cm$^{-2})$ the 3$\omega $ laser light propagates through a high-density (5x10$^{20}$ cm$^{-3})$ plasma with a peak transmission of 90{\%}. Experiments have demonstrated an understanding of filamentation in these conditions that is consistent with theory increasing our confidence in our ability to execute the beam conditioning and focal spot designs for future ignition experiments. This target has been well characterized using Thomson-scattering where the peak electron temperature is shown to be 3.5 keV. The electron temperature measurements agree with HYDRA flux-limited radiation hydrodynamics calculations. Using a recently implemented 3$\omega $ transmitted beam diagnostic, the filamentation threshold has been experimentally measured for a beam that employs a continuous phase plate (CPP). For intensities above the threshold for filamentation, the beam was shown to spray. Defocusing the high-power laser beam reduced the backscatter while filamentation was not changed as predicted. Recent experiments investigating the importance of polarization and temporal smoothing of laser beams for propagation in this target platform will be presented. Detailed hydrodynamic and laser-plasma interaction simulations capture the stimulated Brillouin, stimulated Raman, and filamentation thresholds providing significant confidence that our models used for ignition designs can correctly predict the conditions where energy loss and beam propagation through the under dense NIF hohlraum plasmas will be small. \newline \newline ** Collaborators: L. Divol, S. H. Glenzer, J. S. Ross, N. Meezan, S. Prisbrey, S. Dixit.   

Role of Hydrodynamics Simulations for Laser-Plasma Interaction Predictive Capability

Efforts to predict and control laser plasma interactions (LPI) in hohlraum targets for large laser facilities such as the National Ignition Facility (NIF) in the U.S. and the Laser Mega-Joule (LMJ) in France are based on plasma conditions provided by radiation hydrodynamic simulations. Recent experiments provide compelling evidence that codes such as \textsc{hydra} can accurately predict the bulk plasma conditions in laser heated targets. These targets include gasbag and gas-filled hohlraum platforms for studying LPI on the HELEN and OMEGA laser facilities. We find that initially puzzling experimental observations are often caused by bulk hydrodynamic phenomena rather than by speckle-scale LPI phenomena. For example, in $2\omega$ gasbag experiments, shifts in the stimulated Raman backscatter (SRS) streak spectra can be reproduced from the simulated plasma density and temperature profiles. Transmitted light in hohlraum targets is shifted in wavelength due to the rapidly changing density in the interaction-beam channel. Simulations are in good agreement with spatially localized, time-dependent Thomson scattering measurements of the electron and ion temperatures. We use these calculated plasma conditions to explore a simple, linear-gain based phenomenological model of backscatter. Plotting the measured backscatter against the post-processed gain, we find that the backscatter increases monotonically with gain and that the onsets of the backscatter and filamentation instabilities occur near predicted thresholds. These results suggest a role for linear gain post-processing as a metric for assessing LPI risk.