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  • 1990-1994  (15)
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Year
  • 1
    Electronic Resource
    Electronic Resource
    Intensity scaling and saturation of stimulated Raman forward scattering (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 1985-1996 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The intensity scaling of stimulated Raman forward scattering has been measured in preformed laser-produced plasmas. It was found that predictions based on linear convective theory were consistent with the amount of scattered light measured when the laser intensity was less than about 1×1015 W/cm2. No increase in the amount of scattered light was found for higher intensities. The measured saturation level is compared to several nonlinear models: pump depletion, coupling to Langmuir and ion-acoustic waves, ponderomotive detuning, wave breaking, and particle trapping. It is found that saturation models based on ponderomotive detuning and on coupling to Langmuir and ion-acoustic waves are consistent with the experiment.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870654
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    Paper (German National Licenses)
    Fulltext
  • 2
    Electronic Resource
    Electronic Resource
    X-ray radiographic imaging of hydrodynamic phenomena in radiation-driven materials—Shock propagation, material compression, and shear flowf| (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 1662-1668 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: One- and two-dimensional, time-resolved x-ray radiographic imaging at high photon energy (5–7 keV) is used to study shock propagation, material motion and compression, and the effects of shear flow in solid density samples which are driven by x-ray ablation with the Nova laser. By backlighting the samples with x rays and observing the increase in sample areal density due to shock compression, the trajectories of strong shocks (∼40 Mbars) in flight are directly measured in solid density plastic samples. Doping a section of the samples with high-Z material (Br) provides radiographic contrast, allowing a measurement of the shock-induced particle motion. Instability growth due to shear flow at an interface is investigated by imbedding a metal wire in a cylindrical plastic sample and launching a shock in the axial direction. Time-resolved radiographic measurements are made with either a slit-imager coupled to an x-ray streak camera or a pinhole camera coupled to a gated microchannel plate detector, providing ∼10 μm spatial and ∼100 ps temporal resolution.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870668
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    Paper (German National Licenses)
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  • 3
    Electronic Resource
    Electronic Resource
    Modeling of Nova indirect drive Rayleigh–Taylor experiments (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 3652-3661 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The growth due to the Rayleigh–Taylor (RT) instability of single-wavelength surface perturbations on planar foils of brominated CH [CH(Br)] and fluorosilicone (FS) was measured. The foils were accelerated by x-ray ablation with temporally shaped drive pulses. A range of initial amplitudes (a0) and wavelengths (λ) have been used. This paper focuses upon foils with small a0/λ, which exhibit substantial growth in the linear regime, and are most sensitive to the calculated growth rate. The CH(Br) foils exhibit slower RT perturbation growth because opacity differences result in a larger ablation velocity and a longer density scale length than for FS. Tabulated opacities from detailed atomic models, OPAL [Astrophys. J. 397, 717 (1992)] and super transition array (STA) [Phys. Rev. A 40, 3183 (1989)] were employed. Unlike previous simulations which employed the average atom (XSN) opacity treatment, parameter adjustments to fit experimental data no longer appear necessary. Nonlocal thermodynamic equilibrium (NLTE) effects do not appear to be important. Other variables which may affect the modeling, such as changes of the equation of state and radiation drive spectrum, were also examined. The current calculational model, which incorporates physically justified choices for these calculational ingredients, agrees with the Nova single wavelength RT perturbation growth data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870900
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    Paper (German National Licenses)
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  • 4
    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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    Paper (German National Licenses)
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  • 5
    Electronic Resource
    Electronic Resource
    X-ray radiographic measurements of radiation-driven shock and interface motion in solid density material (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 2259-2264 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Time resolved x-ray radiographic measurements at high photon energy (∼7 keV) are used to observe radiation-driven shock propagation and interface motion in solid density plastic samples, produced by indirect drive on the Nova laser. Measurements of x-ray transmission through the shock compressed material are used to infer a density of approximately three times solid. In addition, by doping a section of a sample with high-Z material (Br) for radiographic contrast, the shocked particle velocity was measured by observing the motion of the interface between the doped and nondoped materials resulting from acceleration by a shock.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860759
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    Paper (German National Licenses)
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  • 6
    Electronic Resource
    Electronic Resource
    Large growth, planar Rayleigh–Taylor experiments on Nova (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 967-978 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A set of indirect-drive experiments to study large growth Rayleigh–Taylor instability using shaped laser pulses at the Nova laser facility has been conducted. Planar foils of fluorosilicone were accelerated by x-ray ablation. The foil trajectory was measured using edge-on radiography. In separate experiments using face-on radiography, contrast in optical depth was measured as a function of time, from which the evolution of 50 μm wavelength initially sinusoidal surface perturbations was deduced. Holding other parameters fixed, the amplitude of the initial perturbation was varied by up to a factor of 30 in separate shots. The foils with the smallest initial perturbation exhibited growth factors of 75 in contrast. Foils with large initial amplitude perturbation gave growth factors of 6 or less, and displayed the "bubble-and-spike'' shape characteristic of the nonlinear Rayleigh–Taylor instability. Comparisons of two-dimensional computer simulations with both the measured foil trajectory and the perturbation growth show good agreement, provided that a suitable opacity model is chosen. In the linear regime the observed growth rates are 60%–75% of classical, the reduction attributed primarily to ablative stabilization. The observed onset of harmonic generation, signaling the transition into the nonlinear regime, is well predicted by third-order theory.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860113
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    Paper (German National Licenses)
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  • 7
    Electronic Resource
    Electronic Resource
    Production of very large, sub-tenth-critical plasmas for laser-fusion research (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 2898-2905 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The development of a plasma environment that is appropriate for the study of low-gain processes in laser-fusion plasmas is reported. These plasmas have long scale lengths, low densities, and no quarter-critical or critical-density surfaces. The plasmas were created by two-sided irradiation of CH foils with 15 kJ of 351 nm laser light. When the peak electron density in the plasma had decayed to about 2×1020 cm−3, the parabolic density profile was estimated to have a full width at half-maximum of 3 mm and the electron temperature was estimated to be 1 keV. Probe lasers of different wavelengths were used to measure the peak electron density via stimulated scattering processes and odd half-harmonic generation. Sufficiently accurate modeling of the laser-irradiation conditions led to improved agreement between the experiment and two-dimensional (2-D) computer simulations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859923
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    Paper (German National Licenses)
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  • 8
    Electronic Resource
    Electronic Resource
    Laser-driven hydrodynamic instability experiments* (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 2589-2595 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An extensive series of experiments has been conducted on the Nova laser to measure hydrodynamic instabilities in planar foils accelerated by x-ray ablation. Single-mode experiments allow a measurement of the fundamental growth rates from the linear well into the nonlinear regime. Two-mode foils give a first direct observation of mode coupling. Surface-finish experiments allow a measurement of the evolution of a broad spectrum of random initial modes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860695
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    Paper (German National Licenses)
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  • 9
    Electronic Resource
    Electronic Resource
    Three-halves harmonic emission from two-color experiments (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 1228-1232 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Three-halves harmonic emission from a plasma in which the peak plasma density is nc〉n〉nc/4 has been temporally and spectrally resolved. This is achieved by using 0.53 μm wavelength laser light to irradiate a plasma preformed by 0.35 μm wavelength laser light. It is observed that both the backscattered red-shifted and blue-shifted peaks of the 3ω0/2 emission from the 0.53 μm laser disappear at approximately the same time. This result constrains the possible production mechanism for backscattered blue-shifted photons to scattering of the blue-shifted photons or plasmons close to nc/4.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859261
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    Paper (German National Licenses)
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  • 10
    Electronic Resource
    Electronic Resource
    Analysis techniques for directly and indirectly driven hydrodynamic experiments on Nova : 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. 5108-5110 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Over the past five years we have performed an extensive series of experiments on hydrodynamic (Rayleigh–Taylor) instabilities. Typically such experiments involve the time-resolved radiography of an accelerated foil (planar geometry) with the line-of-sight of the radiograph along the direction of motion. In comparing experimental data to simulations, the simplest approach is to extract the modulation in exposure level of such a radiograph versus time and compare the inferred growth rate (assumed to be exponential) with the growth rate in column density (Δρz) from a simulation. We have found that this gives unsatisfactory results. We will discuss the techniques for experimental analysis and post processing of simulations which we use for a more direct comparison of the data with the simulation, including the extraction of Fourier components versus time from the data, the effect of backlighter spectra, emission from sources other than the backlighter, and the nonlinear effects of instrument response.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1143453
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    Paper (German National Licenses)
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