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  • 1
    Digitale Medien
    Digitale Medien
    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
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.872443
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  • 2
    Digitale Medien
    Digitale Medien
    Three-dimensional modeling of capsule implosions in OMEGA tetrahedral hohlraums (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 2964-2977 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Tetrahedral hohlraums have been proposed as a means for achieving the highly uniform implosions needed for ignition with inertial confinement fusion (ICF) [J. D. Schnittman and R. S. Craxton, Phys. Plasmas 3, 3786 (1996)]. Recent experiments on the OMEGA laser system have achieved good drive uniformity consistent with theoretical predictions [J. M. Wallace et al., Phys. Rev. Lett. 82, 3807 (1999)]. To better understand these experiments and future investigations of high-convergence ICF implosions, the three-dimensional (3-D) view-factor code BUTTERCUP has been expanded to model the time-dependent radiation transport in the hohlraum and the hydrodynamic implosion of the capsule. Additionally, a 3-D postprocessor has been written to simulate x-ray images of the imploded core. Despite BUTTERCUP's relative simplicity, its predictions for radiation drive temperatures, fusion yields, and core deformation show close agreement with experiment. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.874148
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  • 3
    Digitale Medien
    Digitale Medien
    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
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.873716
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  • 4
    Digitale Medien
    Digitale Medien
    Indirect-drive radiation uniformity in tetrahedral hohlraums (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 3786-3797 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Tetrahedral hohlraums, by which are understood spherical hohlraums with four laser entrance holes (LEH's) placed at or near the vertices of a tetrahedron, are proposed for the National Ignition Facility (NIF) [J. Lindl, Phys. Plasmas 2, 3933 (1995)] and the upgraded OMEGA laser [T. R. Boehly et al., Rev. Sci. Instrum. 66, 508 (1995)]. All but four of the 48 NIF beams can irradiate a tetrahedral hohlraum, assuming that 72 beam ports are provided to accommodate direct drive. On OMEGA, the target chamber provides an exact tetrahedral symmetry, permitting the irradiation of tetrahedral hohlraums with all 60 beams. Hohlraum designs are optimized using a new three-dimensional view-factor program called Buttercup, which traces all beam paths through the hohlraum and calculates the radiation flux on the capsule for different values of the albedo. Good irradiation uniformity (∼2% rms) can be obtained on the capsule at all times during the implosion, even with identical beam temporal histories, in contrast to the case of cylindrical hohlraums where "beam phasing'' is needed. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871511
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  • 5
    Digitale Medien
    Digitale Medien
    Irradiation uniformity for high-compression laser-fusion experiments : 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. 2157-2163 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: High-compression direct-drive laser-fusion experiments require the rms laser-irradiation nonuniformity to be below the 1%–2% level. The combination of two-dimensional smoothing by spectral dispersion (SSD), phase plates, polarization smoothing, and beam overlap are shown to be sufficient to reach this goal. Here is presented a discussion of the mathematical formalism of two-dimensional SSD with numerical calculations illustrating the factors that affect irradiation uniformity. The levels of uniformity that can be achieved on the OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] at the University of Rochester and at the National Ignition Facility (NIF) [Paisner et al., Laser Focus World 30, 75 (1994)] being built at the Lawrence Livermore National Laboratory are examined.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.873501
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  • 6
    Digitale Medien
    Digitale Medien
    Direct-drive laser-fusion experiments with the OMEGA, 60-beam, 〉40 kJ, ultraviolet laser system (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2108-2112 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: OMEGA, a 60-beam, 351 nm, Nd:glass laser with an on-target energy capability of more than 40 kJ, is a flexible facility that can be used for both direct- and indirect-drive targets and is designed to ultimately achieve irradiation uniformity of 1% on direct-drive capsules with shaped laser pulses (dynamic range (approximately-greater-than)400:1). The OMEGA program for the next five years includes plasma physics experiments to investigate laser–matter interaction physics at temperatures, densities, and scale lengths approaching those of direct-drive capsules designed for the 1.8 MJ National Ignition Facility (NIF); experiments to characterize and mitigate the deleterious effects of hydrodynamic instabilities; and implosion experiments with capsules that are hydrodynamically equivalent to high-gain, direct-drive capsules. Details are presented of the OMEGA direct-drive experimental program and initial data from direct-drive implosion experiments that have achieved the highest thermonuclear yield (1014 DT neutrons) and yield efficiency (1% of scientific breakeven) ever attained in laser-fusion experiments. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.871662
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  • 7
    Digitale Medien
    Digitale Medien
    Reduction of laser imprinting using polarization smoothing on a solid-state fusion laser (1999)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 85 (1999), S. 3444-3447 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: We demonstrate a laser beam-smoothing technique known as polarization smoothing. A birefringent optical wedge splits the individual laser beams into two orthogonally polarized beams that, when coupled with a distributed phase plate, produce two speckle patterns shifted with respect to one another. This instantaneously reduces the on-target nonuniformity by a factor of (square root of 2). We measured this reduction optically and its effect is demonstrated in laser-driven targets. © 1999 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.369702
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  • 8
    Digitale Medien
    Digitale Medien
    The upgrade to the OMEGA laser system : Proceedings of the tenth topical conference on high temperature plasma diagnostics (1995)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 66 (1995), S. 508-510 
    Details
    ISSN: 1089-7623
    Quelle: AIP Digital Archive
    Thema: Physik , Elektrotechnik, Elektronik, Nachrichtentechnik
    Notizen: The upgrade to the OMEGA laser system, a 30-kJ, 351-nm, 60-beam direct-drive laser-fusion system is reported here. The system configuration is presented along with the status of its construction and the plans for system activation. The system utilizes rod and disk amplifiers and frequency tripling to produce UV pulses which are smoothed using phase conversion and smoothing by spectral dispersion. Dual driver lines will feed the propagation of two co-axial beams, which have different pulse widths and occupy different portions of the laser aperture. The laser system will be completed in November 1994 and the target area in March 1995. © 1995 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1146333
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  • 9
    Digitale Medien
    Digitale Medien
    Flash x radiography of laser-accelerated foils (1987)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 61 (1987), S. 131-141 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The successful compression of laser-driven pellets to thermonuclear ignition depends on the stability and uniformity of the motion with which dense shells can be imploded. The motion of planar foils accelerated by the Pharos II laser has been studied by two-dimensional, flash x radiography employing pinhole imaging and slitted crystal imaging. The acceleration was driven by a 3–5-ns duration, 1.05-μm laser focused to 3–6×1012 W/cm2 in a millimeter diameter spot, while a second laser beam of shorter duration produced the x-ray flash for imaging purposes. The x-ray images obtained clearly show that the planar foil targets are ablatively accelerated to velocities of 3×106 cm/s while maintaining a density above 3% of solid. The axial extent of the accelerated, high-density material has been observed to be as small as 25% of the distance traveled. The sides of accelerated portions of the foil connect smoothly to the stationary regions removed from the laser illumination. This connection apparently isolates the rear surface from the hot ablation plasma and helps explain the low rear surface temperatures which have been observed. The overall appearance of the accelerated foil is localized and nearly planar at early times when it has moved distances small compared to its diameter. The x-radiographic results are in general agreement with two-dimensional hydrodynamic simulations and with earlier Naval Research Laboratory studies of target motion, ablation pressures, and symmetrization employing other diagnostics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.338845
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  • 10
    Digitale Medien
    Digitale Medien
    Characterization of laser-produced plasma density profiles using grid image refractometry (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 4419-4431 
    Details
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Grid image refractometry (GIR) is proposed as a technique for determining the two-dimensional density profiles of long scale-length laser-produced plasmas. Its distinctive feature is that an optical probe beam is broken up into "rays'' by being passed through a grid before traversing the plasma. The refraction angles of the rays are measured by imaging the plasma at two or more object planes and are integrated to yield the phase front. For cylindrically symmetric plasmas the density profile is then determined using Abel inversion. The feasibility of GIR is illustrated by an experiment in which a thick CH target was irradiated with ∼100 J of 527 nm radiation and diagnosed with a 20 ps, 263 nm probe. The resulting density profile is substantially larger than any that have previously been reported using interferometry and compares quite closely with hydrodynamic simulations.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.860558
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