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  • 1
    Electronic Resource
    Electronic Resource
    Numerical modeling of effects of power imbalance on irradiation nonuniformities (1990)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 68 (1990), S. 5036-5043 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report on the importance of various sources of power imbalance of the 24 beam OMEGA laser system in contributing to laser irradiation nonuniformity. Long spatial wavelength irradiation nonuniformities on target are investigated numerically by adapting existing uniformity codes to incorporate time-dependent density and temperature profiles. The analysis shows that power imbalance due to slightly detuned (≤100 μrad) frequency conversion crystals and small differences in the ultraviolet (UV)-transport losses (ΔE/E≤5%) makes only a modest contribution to the overall nonuniformity. However, power imbalance from pulse shape distortion was found to create substantial irradiation nonuniformity on the target early in the implosion. The predictions of the present analysis were verified by an experiment in which a well controlled power imbalance was applied. Good agreement was found between the experimentally observed core positions and those predicted by the simulation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.347091
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  • 2
    Electronic Resource
    Electronic Resource
    Single-mode, Rayleigh-Taylor growth-rate measurements on the OMEGA laser system (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 338-345 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The results from a series of single-mode, Rayleigh–Taylor (RT) instability growth experiments performed on the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] using planar targets are reported. Planar targets with imposed mass perturbations were accelerated using five or six 351 nm laser beams overlapped with total intensities up to 2.5×1014 W/cm2. Experiments were performed with both 3 ns ramp and 3 ns flat-topped temporal pulse shapes. The use of distributed phase plates and smoothing by spectral dispersion resulted in a laser-irradiation nonuniformity of 4%–7% over a 600 μm diam region defined by the 90% intensity contour. The temporal growth of the modulation in optical depth was measured using throughfoil radiography and was detected with an x-ray framing camera for CH targets. Two-dimensional (2-D) hydrodynamic simulations (ORCHID) [R. L. McCrory and C. P. Verdon, in Inertial Confinement Fusion (Editrice Compositori, Bologna, 1989), pp. 83–124] of the growth of 20, 31, and 60 μm wavelength perturbations were in good agreement with the experimental data when the experimental details, including noise, were included. The amplitude of the simulation optical depth is in good agreement with the experimental optical depth; therefore, great care must be taken when the growth rates are compared to dispersion formulas. Since the foil's initial condition just before it is accelerated is not that of a uniformly compressed foil, the optical density measurement does not accurately reflect the amplitude of the ablation surface but is affected by the initial nonuniform density profile. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873802
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  • 3
    Electronic Resource
    Electronic Resource
    Measurements of core and pusher conditions in surrogate capsule implosions on the OMEGA laser system : The 39th annual meeting of division of plasma physics of APS (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 1870-1879 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Experiments have been carried out on the 60-beam, 30 kJ OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] as part of an integrated program to diagnose all phases of direct-drive capsule implosions. Laser-imprint levels and Rayleigh–Taylor growth rates associated with the spherical implosions have been inferred from planar-foil radiography experiments. In spherical targets, measurements of the combined effects of imprint and unstable growth at the ablation surface have been carried out using the burnthrough technique [J. Delettrez et al., Phys. Plasmas 1, 2342 (1994)]. Target behavior during the deceleration phase has been investigated using a series of surrogate cryogenic capsules in which the main fuel layer is represented by a Ti-doped CH shell and the hot spot is represented by an Ar-doped deuterium fill gas. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872858
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  • 4
    Electronic Resource
    Electronic Resource
    Signatures of target performance and mixing in titanium-doped, laser-driven target implosions (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 3021-3030 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Absorption-spectroscopy measurements using an embedded titanium layer have been performed on laser implosions of spherical shells. Predicted absorption features formed by the titanium layer were observed: absorption lines due to 1s–2p transitions in titanium ions of incomplete L shell, as well as K-edge absorption in cold titanium. Also observed were oscillations due to EXAFS (Extended X-Ray Absorption Fine Structure) above the Ti K edge. The core temperature and shell areal density and temperature are derived from the measurements and are found to fall short of the values predicted by one-dimensional code simulations. This shortfall is attributed to the lack of irradiation symmetry, which leads to hydrodynamic instability and mixing (smoothing by phase plates was not used in this experiment). Signatures of mixing include the emission of the He-α line of Ti+20 due to titanium migrating to the core, as well as the EXAFS spectrum indicating cold titanium close to peak compression.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872443
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  • 5
    Electronic Resource
    Electronic Resource
    The role of the Rayleigh–Taylor instability in laser-driven burnthrough experiments (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 2342-2349 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Experiments were conducted to confirm that the Rayleigh–Taylor instability is the main process controlling the burnthrough time in imploding spherical experiments. In these experiments the laser irradiates targets overcoated with a parylene layer, in which one or more thin signature layers of moderate- to high-Z material are embedded to signal the penetration of the heat front. Target parameters were varied to study the effect on the burnthrough time of changes to target acceleration, Atwood number, and ablation velocity. The effects of improved laser uniformity through the introduction of smoothing by spectral dispersion are also presented. The results agree well with those obtained from a multimode mix model. This suggests that burnthrough experiments can be used to measure improvements in laser-irradiation or target-fabrication uniformity and to test methods to mitigate the growth of the Rayleigh–Taylor instability.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870631
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  • 6
    Electronic Resource
    Electronic Resource
    Laser-plasma interactions in long-scale-length plasmas under direct-drive National Ignition Facility conditions : The 40th annual meeting of the division of plasma physics of the american physical society (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 2072-2080 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Laser-plasma interaction experiments have been carried out on the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] under plasma conditions representative of the peak of a 1.5 MJ direct-drive laser pulse proposed for the National Ignition Facility (NIF). Plasmas have been formed by exploding 18–20 μm thick CH foils and by irradiating solid CH targets from one side, using up to 20 kJ of laser energy with phase plates installed on all beams. These plasmas and the NIF plasmas are predicted to have electron temperatures of 4 keV and density scale lengths close to 0.75 mm at the peak of the laser pulse. The electron temperature and density of the exploding-foil plasmas have been diagnosed using time-resolved x-ray spectroscopy and stimulated Raman scattering, respectively, and are consistent with predictions of the two-dimensional Eulerian hydrodynamics code SAGE [R. S. Craxton and R. L. McCrory, J. Appl. Phys. 56, 108 (1984)]. When the solid-target or exploding-foil plasmas were irradiated with an f/6 interaction beam at 1.5×1015 W/cm2, well above the NIF f/8 cluster intensity of ∼2×1014 W/cm2, stimulated Brillouin backscattering (SBS) was found to be completely inhibited. A conservative upper limit of direct-backscattered SRS was found to be ∼5% from the solid targets. SRS and SBS are thus unlikely to have a significant impact on target performance at the peak of the NIF direct-drive laser pulse. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873716
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  • 7
    Electronic Resource
    Electronic Resource
    Electrical characteristics of short pulse gated microchannel plate detectors : Proceedings of the 9th topical conference on high temperature plasma diagnostics (1992)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 63 (1992), S. 5072-5074 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Gating of microchannel plates (MCPs) can produce shutter times of less than 40 ps. This is accomplished with high-fidelity electronic circuitry and short pulse (sub 80 ps) high-amplitude electronic drivers. Measurements have shown that the fidelity of the electrical circuitry is one of the limiting factors in producing shorter x-ray gating.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1143495
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  • 8
    Electronic Resource
    Electronic Resource
    High-speed gated x-ray imaging for ICF target experiments (invited) : Proceedings of the 9th topical conference on high temperature plasma diagnostics (1992)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 63 (1992), S. 4813-4817 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We describe the use of gated microchannel-plate detectors as high-speed framing cameras in laser-driven inertial-confinement-fusion experiments. Using an array of pinholes to image the target, detectors capable of generating up to 16 individual frames with ∼90 ps resolution on a single laser shot are now in routine use. The detectors have been used to study the development of intentionally applied perturbations in laser-driven targets. In off-line tests new detectors have demonstrated time resolutions better than 40 ps.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1143571
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  • 9
    Electronic Resource
    Electronic Resource
    Thinshell symmetry surrogates for the National Ignition Facility: A rocket equation analysis (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 2908-2917 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Several techniques for inferring the degree of flux symmetry in indirectly driven cylindrical hohlraums have been developed over the past several years for eventual application to the National Ignition Facility (NIF) [Paisner et al., Laser Focus World 30, 75 (1994)]. These methods use various ignition capsule surrogates, including non-cryogenic imploded capsules [Hauer et al., Phys. Plasmas 2, 2488 (1995)], backlit aerogel foamballs [Amendt et al., Rev. Sci. Instrum. 66, 785 (1995)], reemission balls [Delamater, Magelssen, and Hauer, Phys. Rev. E 53, 5240 (1996)], and backlit thinshells [Pollaine et al., Phys. Plasmas 8, 2357 (2001)]. Recent attention has focussed on the backlit thinshells as a promising means for detecting higher-order Legendre flux asymmetries, e.g., P6 and P8, which are predicted to be important sources of target performance degradation on the NIF for levels greater than 1% [Haan et al., Phys. Plasmas 2, 2490 (1995)]. A key property of backlit thinshells is the strong amplification of modal flux asymmetry imprinting with shell convergence. A simple single-parameter analytic description based on a rocket model is presented which explores the degree of linearity of the shell response to an imposed flux asymmetry. Convergence and mass ablation effects introduce a modest level of nonlinearity in the shell response. The effect of target fabrication irregularities on shell distortion is assessed with the rocket model and particular sensitivity to shell thickness variations is shown. The model can be used to relate an observed or simulated backlit implosion trajectory to an ablation pressure asymmetry history. Ascertaining this history is an important element for readily establishing the degree of surrogacy of a symmetry target for a NIF ignition capsule. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1369412
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  • 10
    Electronic Resource
    Electronic Resource
    National Ignition Facility scale hohlraum asymmetry studies by thin shell radiography : The 42nd annual meeting of the division of plasma physics of the American Physical Society and the 10th international congress on plasma physics (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 2357-2364 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A necessary condition for igniting indirectly driven inertial confinement fusion (ICF) capsules on the National Ignition Facility (NIF) is controlling drive asymmetry to the 1% level [S. W. Haan, S. M. Pollaine, J. D. Lindl et al., Phys. Plasmas 2, 2480 (1995)]. Even flux-asymmetry modes (e.g., Legendre modes P2, P4, P6, and P8) must be reduced by hohlraum design and laser beam pointing. Odd flux-asymmetry modes (e.g., Legendre modes P1, P3, P5, etc.) are theoretically removed by reflection symmetry across the hohlraum midplane [S. M. Pollaine and D. Eimerl, Nucl. Fusion 38, 1523 (1998)], but will be produced by power imbalance, laser beam pointing errors, and target fabrication errors. An experimental campaign is now being conducted on the University of Rochester's Omega laser to measure higher order (P4 and higher) flux asymmetry modes inside hohlraums that approximate the conditions of a NIF hohlraum during the 90 eV early drive phase [S. W. Haan, S. M. Pollaine, J. D. Lindl et al., Phys. Plasmas 2, 2480 (1995)]. These experiments use a new point-projection backlighting technique [O. L. Landen, D. R. Farley, S. G. Glendinning et al., Rev. Sci. Instrum. 72, 627 (2001)] to cast high quality 4.7 keV radiographs of thin 2 mm diameter Ge-doped CH shells designed to enhance sensitivity to drive asymmetries. Distortions in the position of the limb of the shells resulting primarily from drive asymmetries are measured to an accuracy of 2 μm. The linearity and sensitivity of thin imploding shells to flux asymmetry makes it possible to achieve this degree of accuracy, which is sufficient for NIF ignition symmetry tuning. The promising results to date permit the comparison of measured and predicted distortions and, by inference, drive asymmetries for the first eight asymmetry modes. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1364514
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