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  • 1
    Electronic Resource
    Electronic Resource
    High-gain direct-drive target design for laser fusion (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 2298-2301 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A new laser fusion target concept is presented with a predicted energy gain of 127 using a 1.3 MJ KrF laser. This energy gain is sufficiently high for an economically attractive fusion reactor. X rays from high- and low-Z materials are used in combination with a low-opacity ablator to spatially tune the isentrope, thereby providing both high fuel compression and a reduction of the ablative Rayleigh–Taylor instability. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.874063
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  • 2
    Electronic Resource
    Electronic Resource
    Richtmyer–Meshkov-like instabilities and early-time perturbation growth in laser targets and Z-pinch loads : The 41st annual meeting of the division of plasma physics of the american physical society (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 1662-1671 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The classical Richtmyer–Meshkov (RM) instability develops when a planar shock wave interacts with a corrugated interface between two different fluids. A larger family of so-called RM-like hydrodynamic interfacial instabilities is discussed. All of these feature a perturbation growth at an interface, which is driven mainly by vorticity, either initially deposited at the interface or supplied by external sources. The inertial confinement fusion relevant physical conditions that give rise to the RM-like instabilities range from the early-time phase of conventional ablative laser acceleration to collisions of plasma shells (like components of nested-wire-arrays, double-gas-puff Z-pinch loads, supernovae ejecta and interstellar gas). In the laser ablation case, numerous additional factors are involved: the mass flow through the front, thermal conduction in the corona, and an external perturbation drive (laser imprint), which leads to a full stabilization of perturbation growth. In contrast with the classical RM case, mass perturbations can exhibit decaying oscillations rather than a linear growth. It is shown how the early-time perturbation behavior could be controlled by tailoring the density profile of a laser target or a Z-pinch load, to diminish the total mass perturbation seed for the Rayleigh–Taylor instability development. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873986
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  • 3
    Electronic Resource
    Electronic Resource
    Effects of radiation on direct-drive laser fusion targets : The 41st annual meeting of the division of plasma physics of the american physical society (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 2046-2054 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The role played by radiation in the radiation-preheated direct-drive laser fusion target design is discussed. The soft x-rays emitted during the foot of the laser pulse—at a few 1012 W/cm2—preheat the low-opacity foam ablator which helps to control the Rayleigh–Taylor instability. The foam opacity is, however, thick enough to stop that radiation, keeping the fuel on a low adiabat. Radiation effects are also important in the blow-off corona of the target because they establish a long scale-length plasma. This may help to shield the ablation region from the nonuniformities in the laser absorption. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.874026
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  • 4
    Electronic Resource
    Electronic Resource
    Experimental investigation of short scalelength density fluctuations in laser-produced plasmas (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 2114-2125 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The technique of near forward laser scattering is used to infer characteristics of intrinsic and controlled density fluctuations in laser-produced plasmas. Intrinsic fluctuations are studied in long-scalelength plasmas where it is found that the fluctuations exhibit scale sizes related to the intensity variation scales in the plasma forming and interaction beams. Stimulated Brillouin forward scattering and filamentation appear to be the primary mechanism through which these fluctuations originate. The beam spray resulting from these fluctuations is important to understand, since it can affect symmetry in an inertial confinement fusion (ICF) experiment. Controlled fluctuations are studied in foam and exploding foil targets. Forward scattered light from foam targets shows evidence that the initial target inhomogeneities remain after the target is laser heated. Forward scattered light from an exploding foil plasma shows that a regular intensity pattern can be used to produce a spatially correlated density fluctuation pattern. These results provide data which are being used to benchmark numerical models of beam spray.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.874056
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  • 5
    Electronic Resource
    Electronic Resource
    Illumination uniformity of spherical targets using kilojoule-scale lasers with optical smoothing (1990)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 67 (1990), S. 2303-2309 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Illumination uniformity in laser-pellet interactions is investigated for small kilojoule-range laser systems that are capable of symmetrically illuminating spherical targets. We address a variety of options that might be used to control the illumination uniformity on such targets, including the number of beams, pulse energy, laser wavelength, temporal pulse shape, and beam spatial profile shape. We consider only laser beams that are optically smoothed using methods such as induced spatial incoherence or random phase plates. The parameters that are most important for providing good uniformity are identified. We find that the most important parameters governing the uniformity are the number of beams and the shape of the beam profile. Configurations with a small number of beams (e.g., six) are much more sensitive to the beam profile; using techniques such as splitting the focal spots provides an additional degree of freedom that can significantly increase the illumination uniformity. Additionally, we show that the hydrodynamics of the plasma is important in determining the uniformity, although it is not so easily controlled. Quantitatively, we find nonuniformities (measured by peak-to-valley ablation pressure differences) can be less than 10% for six-beam kilojoule-level 1/4 -μm laser wavelength systems, and of the order of a few percent for comparable 24-beam systems.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.345525
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  • 6
    Electronic Resource
    Electronic Resource
    Measurements of laser-imprinted perturbations and Rayleigh–Taylor growth with the Nike KrF laser : The 38th annual meeting of the Division of Plasma Physics (DPP) of the American Physical Society (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 1969-1977 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Nike is a 56 beam Krypton Fluoride (KrF) laser system using Induced Spatial Incoherence (ISI) beam smoothing with a measured focal nonuniformity 〈ΔI/I〉 of 1% rms in a single beam [S. Obenschain et al., Phys. Plasmas 3, 1996 (2098)]. When 37 of these beams are overlapped on the target, we estimate that the beam nonuniformity is reduced by 37, to (ΔI/I)≅0.15% (excluding short-wavelength beam-to-beam interference). The extraordinary uniformity of the laser drive, along with a newly developed x-ray framing diagnostic, has provided a unique facility for the accurate measurements of Rayleigh–Taylor amplified laser-imprinted mass perturbations under conditions relevant to direct-drive laser fusion. Data from targets with smooth surfaces as well as those with impressed sine wave perturbations agree with our two-dimensional (2-D) radiation hydrodynamics code that includes the time-dependent ISI beam modulations. A 2-D simulation of a target with a 100 Å rms randomly rough surface finish driven by a completely uniform beam gives final perturbation amplitudes similar to the experimental data for the smoothest laser profile. These results are promising for direct-drive laser fusion.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872560
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  • 7
    Electronic Resource
    Electronic Resource
    X-ray emission from plasmas created by smoothed KrF laser irradiation (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 3438-3447 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The x-ray emission from plasmas created by the Naval Research Laboratory Nike KrF laser [Phys. Plasmas 3, 2098 (1996) ] was characterized using imaging and spectroscopic instruments. The laser wavelength was 1/4 μm, and the beams were smoothed by induced spatial incoherence (ISI). The targets were thin foils of CH, aluminum, titanium, and cobalt and were irradiated by laser energies in the range 100–1500 J. A multilayer mirror microscope operating at an energy of 95 eV recorded images of the plasma with a spatial resolution of 2 μm. The variation of the 95 eV emission across the 800 μm focal spot was 1.3% rms. Using a curved crystal imager operating in the 1–2 keV x-ray region, the density, temperature, and opacity of aluminum plasmas were determined with a spatial resolution of 10 μm perpendicular to the target surface. The spectral line ratios indicated that the aluminum plasmas were relatively dense, cool, and optically thick near the target surface. The absolute radiation flux was determined at 95 eV and in x-ray bandpasses covering the 1–8 keV region. The electron temperature inferred from the slope of the x-ray flux versus energy data in the 5–8 keV region was 900 eV for an incident laser energy of 200 J and an intensity of ≈1013 W/cm2.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871496
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  • 8
    Electronic Resource
    Electronic Resource
    The Nike KrF laser facility: Performance and initial target experiments (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2098-2107 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Krypton-fluoride (KrF) lasers are of interest to laser fusion because they have both the large bandwidth capability ((approximately-greater-than)THz) desired for rapid beam smoothing and the short laser wavelength (1/4 μm) needed for good laser–target coupling. Nike is a recently completed 56-beam KrF laser and target facility at the Naval Research Laboratory. Because of its bandwidth of 1 THz FWHM (full width at half-maximum), Nike produces more uniform focal distributions than any other high-energy ultraviolet laser. Nike was designed to study the hydrodynamic instability of ablatively accelerated planar targets. First results show that Nike has spatially uniform ablation pressures (Δp/p〈2%). Targets have been accelerated for distances sufficient to study hydrodynamic instability while maintaining good planarity. In this review we present the performance of the Nike laser in producing uniform illumination, and its performance in correspondingly uniform acceleration of targets. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871661
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  • 9
    Electronic Resource
    Electronic Resource
    Influence of bulk diffusion on the adsorption of hard spheres on a flat surface (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 3813-3820 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Irreversible adsorption of hard spheres onto a solid surface is analyzed by using a generalization of the "random sequential adsorption'' (RSA) model: "diffusion random sequential adsorption'' (DRSA). In addition to the irreversible nature of the adsorption process and exclusion effects of the adsorbed configurations of hard spheres, the new model also considers the interactions between adsorbed particles and particles from the bulk, diffusing toward the surface. It is shown, in particular, that this affects the structure of adsorbed configurations for coverages different from the jamming limit coverage θ∞. Surprisingly, θ∞ appears to be identical for configurations generated by RSA and DRSA algorithms. Moreover, the structures of the configurations, as characterized by the radial distribution function g(r), are also identical at the jamming limit, whereas they differ for lower coverages. The coupling between the bulk diffusion process and the "adsorption process'' is also analyzed as a function of the coverage. New approximate boundary conditions at the interface for the adsorption–diffusion equations are derived and the adsorption rate is calculated for a particular example.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.462963
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  • 10
    Electronic Resource
    Electronic Resource
    Observation of Rayleigh–Taylor growth to short wavelengths on Nike (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 565-570 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The uniform and smooth focal profile of the Nike KrF laser [S. Obenschain et al., Phys. Plasmas 3, 2098 (1996)] was used to ablatively accelerate 40 μm thick polystyrene planar targets with pulse shaping to minimize shock heating of the compressed material. The foils had imposed small-amplitude sinusoidal wave perturbations of 60, 30, 20, and 12.5 μm wavelength. The shortest wavelength is near the ablative stabilization cutoff for Rayleigh–Taylor growth. Modification of the saturated wave structure due to random laser imprint was observed. Excellent agreement was found between the two-dimensional simulations and experimental data for most cases where the laser imprint was not dominant. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873201
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