LLE Review 91

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Highlights

This volume of the LLE Review, covering April to June 2002, features “Modeling Laser-Plasma Interaction Physics Under Direct-Drive Inertial Confinement Fusion Conditions” by J. Myatt, A. V. Maximov, and R. W. Short (p. 93). They report of pF3D, a parallel, three-dimensional laser-plasma interaction (LPI) code developed at LLNL for modeling indirect-drive plasmas, which recently has been modified for use under direct-drive conditions. Unlike indirect drive, modeling direct drive requires simulation of inhomogeneous supersonic flows and density profiles that include at critical surface. The treatment of the critical surface is particularly problematic in codes employing the paraxial approximation for the light waves. Myatt et al. describe the first results of the modified code: realistic simulations motivated by long-scale-length exploding-foil experiments conducted on LLE’s 30-kJ, 351-nm, 60-beam OMEGA laser system and intended to represent future NIF direct-drive conditions.

Additional highlights of research in this issue are

• A. V. Okishev, D. Battaglia, I. A. Begishev, and J. D. Zuegel (p. 103) have developed a new highly stable, diode-pumped, cavity-dumped, compact Nd:YLF regenerative amplifier (regen) of continuously shaped nanosecond pulses with a gain of ~10⁹ for the front-end laser system of OMEGA. High output energy, long-term energy and temporal pulse shape stability, and high-quality beam profile have been demonstrated. Reliability, simplicity, modular design, and compactness are key features of the new diode-pumped regenerative amplifier.
• An experiment recently completed by J. P. Knauer and V. N. Goncharov (p. 108) has tested the ability of a direct-drive ICF laser pulse shape to vary the adiabat within a target shell. A picket pulse was added to a pulse shape designed to implode a cryogenic shell of D₂ with a ratio α of internal pressure to Fermi-degenerate pressure of 5. The effect of a picket is to strengthen the shock in the outer portion of the shell so that the ablation interface has a large α and the fuel maintains its α = 5, resulting in increased stability and improved capsule performance.
• F. J. Marshall, J. A. Delettrez, R. L. Keck, J. H. Kelly, P. B. Radha, and L. J. Waxer (p. 116) describe implosion experiments with enhanced beam balance. Marshall et al. have implemented a new technique that determines the beam peak intensities at target chamber center on a full-power target shot by simultaneously measuring the x-ray flux produced by all 60 beams seen separated on a 4-mm-diam, Au-coated spherical target. Up to nine x-ray pinhole camera images are electronically recorded per shot from which beam-to-beam variations in peak intensity are determined, taking into account view angle and x-ray conversion efficiency. The observed variations are then used to correct the beam energies to produce a more-uniform irradiation. The authors present the results of implosion experiments with enhanced beam balance and comparisons to experiments with standard beam balance.
• W. T. Shmayda, Y. Cao, and J. A. Szpunar (p. 125) present the effects of textures on hydrogen diffusion in nickel. Deuterium and tritium—isotopes of hydrogen—are the primary fuels for inertial confinement fusion (ICF), so determining and controlling their rate of diffusion through containment materials are important to the design of ICF facilities. When polycrystalline metals have texture, the preferential orientation of the metals affects hydrogen absorption and diffusion. Hydrogen permeation results show that there are significant differences among the three main textures of nickel membranes. Plating current density has a strong influence on texture development of nickel deposits. The texture of deposits can be easily manipulated by controlling plating conditions. In the experiments performed by Shmayda et al., textured Ni membranes were prepared using electrodeposition, and the effects of fabrications on their diffusion rates were determined.
• A great deal of interest has been generated by the discovery of superconductivity in a hexagonal magnesium borides not only because of MgB₂‘s high critical temperature and current density but also its lower anisotropy, larger coherence length, and higher transparency of grain boundaries to current flow. R. Sobolewski, P. Kús, A. Plecenik, L. Satrapinsky, and Y. Xu (p. 130) have for the first time fabricated MgB₂ superconducting films on flexible substrates. They describe their process, by which these films could be deposited on large-area foils (up to 400 cm²) and, after processing, cut into any shapes (e.g., stripes) with scissors or bent multiple times, without any observed degradation of their superconducting properties.
• V. S. Smalyuk, P. B. Radha, J. A. Delettrez, V. Yu. Glebov, V. N. Goncharov, D. D. Meyerhofer, S. P. Regan, S. Roberts, T. C. Sangster, J. M. Soures, and C. Stoeckl (p. 133) have inferred the growth of target areal density near peak compression in direct-drive spherical target implosions with 14.7-Mev deuterium-helium ³ (D³He) proton spectroscopy on the OMEGA laser system. The target areal density grows by a factor of ~8 during the time of neutron production (~400 ps) before reaching 123±16 mg cm^-2 at peak compression in an implosion of a 20-μm-thick plastic CH target filled with 4 atm of D³ He fuel.
• J. R. Zurita-Sanchez and L. Novotny (p. 139) have performed a theoretical investigation of a semiconductor quantum dot interacting with a strongly localized optical field, as encountered in high-resolution, near-field optical microscopy. The strong gradients of these localized fields suggest that the higher-order multipolar interaction will affect the standard electric dipole transition rates and selection rules. For a semiconductor quantum dot in the strong confinement limit, Zurita-Sanchez and Novotny have calculated the interband electric quadrupole absorption rate and the associated selection rules, finding that the electric quadrupole absorption rate is comparable with the absorption rate calculated in the electric dipole approximation.