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  • 1
    Electronic Resource
    Electronic Resource
    A model of laser imprinting : 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. 2062-2068 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Irradiation nonuniformities in direct-drive (DD) inertial confinement fusion experiments generate, or "imprint," surface modulations that degrade the symmetry of the implosion and reduce the target performance. To gain physical insight, an analytical model of imprint is developed. The model takes into account the hydrodynamic flow, the dynamics of the conduction zone, and the mass ablation. The important parameters are found to be the time scale for plasma atmosphere formation and the ablation velocity. The model is validated by comparisons to detailed two-dimensional (2D) hydrocode simulations. The results of the model and simulations are in good agreement with a series of planar-foil imprint experiments performed on the OMEGA laser system [T.R. Boehly, D.L. Brown, R.S. Craxton et al., Opt. Commun. 133, 495 (1997)]. Direct-drive National Ignition Facility's [J.A. Paisner, J.D. Boyes, S.A. Kumpan, W.H. Lowdermilk, and M.S. Sorem, Laser Focus World 30, 75 (1994)] cryogenic targets are shown to have gains larger than 10 when the rms laser-irradiation nonuniformity is reduced by 2D smoothing by spectral dispersion (SSD) used in the current DD target designs.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.874028
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  • 2
    Electronic Resource
    Electronic Resource
    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
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871662
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  • 3
    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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  • 4
    Electronic Resource
    Electronic Resource
    A high-resolution x-ray microscope for laser-driven planar-foil experiments (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 1118-1124 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A soft x-ray microscope (E(approximately-less-than)3 keV) with high spatial resolution (∼3 μm) has been characterized at the University of Rochester's Laboratory for Laser Energetics and used for initial experiments on the Omega laser system [Boehly et al., Opt. Commun. 133, 495 (1997)] to study the hydrodynamic stability of directly driven planar foils. The microscope, which is an optimized Kirkpatrick–Baez-type design, is used to obtain four x-ray radiographs of laser-driven foils. Time-resolved images are obtained with either custom-built framing cameras (time resolution ∼80 ps) or by using short-pulse backlighter beams (Δt(approximately-less-than)200 ps). In the former case, a spatial resolution of ∼7 μm was obtained (limited by the framing camera), while in the latter case a resolution of ∼3 μm was obtained. This paper details the testing, calibration, and initial use of this microscope in the laboratory and on Omega. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872642
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  • 5
    Electronic Resource
    Electronic Resource
    A review of the ablative stabilization of the Rayleigh–Taylor instability in regimes relevant to inertial confinement fusion (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 1379-1389 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: It has been recognized for many years that the most significant limitation of inertial confinement fusion (ICF) is the Rayleigh–Taylor (RT) instability. It limits the distance an ablatively driven shell can be moved to several times its initial thickness. Fortunately material flow through the unstable region at velocity vA reduces the growth rate to (square root of)kg/1+kL−βkvA with β from 2–3. In recent years experiments using both x-ray drive and smoothed laser drive to accelerate foils have confirmed the community's understanding of the ablative RT instability in planar geometry. The growth of small initial modulations on the foils is measured for growth factors up to 60 for direct drive and 80 for indirect drive. For x-ray drive large stabilization is evident. After some growth, the instability enters the nonlinear phase when mode coupling and saturation are also seen and compare well with modeling. Normalized growth rates for direct drive are measured to be higher, but strategies for reduction by raising the isentrope are being investigated. For direct drive, high spatial frequencies are imprinted from the laser beam and amplified by the RT instability. Modeling shows an understanding of this "laser imprinting.''
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870688
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  • 6
    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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  • 7
    Electronic Resource
    Electronic Resource
    Optical and plasma smoothing of laser imprinting in targets driven by lasers with SSD bandwidths up to 1 THz : 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. 2331-2337 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The creation of a plasma atmosphere in laser-target interactions increases the distance between the regions of laser absorption and hydrodynamic instability (ablation front), thus allowing thermal smoothing and a reduction of laser-imprinted modulations that reach the unstable ablation region. The total laser imprinting is reduced with pulse shapes that produce a plasma atmosphere more rapidly and by the implementation of temporal beam smoothing. These effects are measured and found to be consistent with models for the hydrodynamics and optical smoothing by spectral dispersion (SSD). Imprinting is reduced as the laser bandwidth is increased from 0.2 to 1.0 THz. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1352616
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  • 8
    Electronic Resource
    Electronic Resource
    Characterization of an x-ray radiographic system used for laser-driven planar target experiments : Proceedings of the 12th topical conference on high temperature plasma diagnostics (1999)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 70 (1999), S. 647-650 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We have characterized the sensitivity, resolution, and noise of an x-ray radiographic system. Using this information we have formulated a Wiener filter that reduces noise, compensates for detector resolution, and facilitates measurement of perturbations in laser-driven targets. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1149313
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  • 9
    Electronic Resource
    Electronic Resource
    Probing fe(v) by Thomson scattering on W7-X : Proceedings of the eleventh topical conference on high temperature plasma diagnostics (1997)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 68 (1997), S. 732-734 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Thomson scattering is a widely used standard diagnostic for Te,ne measurements in fusion plasmas. Experience on W7-AS has shown, however, that during strong electron cyclotron resonance heating, the scattering spectra can be distorted by superthermal electrons. In addition, model calculations have shown that the distortion depends on the local magnetic field geometry. The Thomson diagnostic planned for W7-X provides for the first time, sufficient access to the vacuum vessel to probe several scattering geometries, allowing for scattering vectors parallel, perpendicular, and oblique with respect to the magnetic field. Furthermore, a detection system with high spectral resolution is foreseen so that there will be a good chance to reconstruct the two-dimensional electron distribution fe(v⊥,v(parallel)). © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1147754
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  • 10
    Electronic Resource
    Electronic Resource
    A charged-particle spectrometer for ρR measurements (abstract)a) : 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. 948-948 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We are developing an advanced electronic charged-particle spectrometer (a magnetic/CCD system) for ICF ρR measurements. Knock-on and other charged fusion products are bent by a magnetic field (about 1 T) in order to avoid line-of-sight x rays from directly impinging on the detector. The detector itself is a stack of four charge coupled devices (CCDs) sandwiched between "ranging filters''. To avoid blooming effects in the CCDs, we plan to use only "thinned'' CCDs, for which each element is ∼20 μm thick. (The CCD is supported by a silicon substrate of thickness ∼200 μm.) The CCDs are sandwiched between four aluminum filters which have two functions. The first filter reduces the background by completely ranging out select charged particles, such as 3.5 MeV α's (range (approximately-equal-to)13 μm). Also, the first filter, with a thickness ∼50 μm, severely attenuates scattered soft x rays and visible light. The following three filters are each ∼300 μm. The total thickness of these filters and CCDs (with support substrate) are about 2000 μm, a thickness which completely stops energetic protons up to (approximately-equal-to) 15 MeV. Equally important, the next three ranging filters downshift the particle energy, and this is reflected in increased energy deposition in the CCD (since dE/dx∼1/E, where E is the kinetic energy). With this spectrometer, a charged particle experiences four separate interactions with the CCDs and ranging filters: this allows the incident particle energy to be overdetermined and the particle to be uniquely identified. Furthermore, because the CCD has small picture elements (∼ 20 μm), pulse pileup can be avoided. A crucial tool in the development of this spectrometer is the MIT Cockcroft–Walton fusion product generator. This generator will be used to test and validate design concepts, study energetic particle trajectories through the magnetic field and through the detector stack, and to test and absolutely calibrate the response of this spectrometer to a variety of 0.5–15 MeV charged particles. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1146216
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