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  • 1
    Electronic Resource
    Electronic Resource
    The effects of permeability heterogeneity on miscible viscous fingering: A three-dimensional magnetic resonance imaging analysis (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 7 (1995), S. 468-477 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The three-dimensional evolution of the viscous fingering instability has been visualized directly with magnetic resonance imaging (MRI). Miscible displacement of thin solute bands by aqueous solvent was investigated in packed beds of 30 μm chromatographic particles. Fingering behavior into samples of glycerol and a protein, bovine serum albumin (BSA), with viscosity ratios ranging from 1 to approximately 4, were compared. The three-dimensional morphology and dynamics of fingers were monitored to approximately millimeter spatial resolution using MRI. Linear and nonlinear fingering behavior were observed. Permeability heterogeneities with length scales on the order of the finger wavelength induced complex three-dimensional fingering patterns. Sample and column boundary effects on fingering dynamics were also noted. The differences in fingering behavior observed between albumin and glycerol samples are consistent with the wavelength predictions of linear stability analysis and the large differences in molecular diffusivity. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.868645
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  • 2
    Electronic Resource
    Electronic Resource
    Numerical measurement of turbulent responses in drift-Alfvén turbulence (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 2443-2453 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A drift-Alfvén magnetoturbulence model that augments reduced magnetohydrodynamics with evolution of electron density under parallel compression and fluid advection has been studied numerically. In the Alfvénic regime, measurement of spectral transfer rates, frequency spectra, energy partitions, and the ensemble-averaged turbulent response reveals both Alfvénic and hydrodynamic characteristics. The rms turbulent frequency is Alfvénic, the energies are equipartitioned, and there is a fast, Alfvén-time scale relaxation in the turbulent response. The mean frequency is hydrodynamic, with diamagnetic and eddy straining signatures, and there is an eddy straining decorrelation appearing as a distinct, long time scale branch in the turbulent response. The decay rates and relative fluctuation strengths associated with fast and slow time scale decorrelation are in good agreement with theoretical predictions that posit a Kolmogorov spectrum in the Alfvénic regime. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872225
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  • 3
    Electronic Resource
    Electronic Resource
    Measurements of laser-plasma instability relevant to ignition hohlraums : 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. 1849-1856 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The potential for laser-plasma instability is a serious concern for indirect-drive inertial confinement fusion (ICF), where laser beams illuminate the interior of a cavity (called a hohlraum) to produce x-rays for imploding a fusion capsule symmetrically. The speckled nature of laser beams used in ICF is an important factor in laser-plasma instability processes. For example, models which calculate the spatial growth of convective instability by properly accounting for the laser speckles successfully predict the observed onsets of backscattering due to stimulated Brillouin and Raman scattering instabilities (SBS and SRS). Assuming pump depletion as the only saturation mechanism in these models results in very large predicted levels of SBS and SRS backscattering from the long-scale plasmas expected in ignition hohlraums. However, in the long-scale plasmas studied in the Nova and Trident lasers [E. M. Campbell, Rev. Sci. Instrum. 57, 2101 (1986) and N. K. Moncur et al., Appl. Opt. 34, 4274 (1995)], SRS and SBS are observed to saturate much below the levels expected from pump depletion. While the mechanism of SBS saturation is not understood at present, the observations of SRS saturation are qualitatively understood. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872328
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  • 4
    Electronic Resource
    Electronic Resource
    Laser–plasma interactions in ignition-scale hohlraum plasmas (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2029-2040 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Scattering of laser light by stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) is a concern for indirect drive inertial confinement fusion (ICF). The hohlraum designs for the National Ignition Facility (NIF) raise particular concerns due to the large scale and homogeneity of the plasmas within them. Experiments at Nova have studied laser–plasma interactions within large scale length plasmas that mimic many of the characteristics of the NIF hohlraum plasmas. Filamentation and scattering of laser light by SBS and SRS have been investigated as a function of beam smoothing and plasma conditions. Narrowly collimated SRS backscatter has been observed from low density, low-Z, plasmas, which are representative of the plasma filling most of the NIF hohlraum. SBS backscatter is found to occur in the high-Z plasma of gold ablated from the wall. Both SBS and SRS are observed to be at acceptable levels in experiments using smoothing by spectral dispersion (SSD). © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872000
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  • 5
    Electronic Resource
    Electronic Resource
    Dependence of stimulated Brillouin scattering on focusing optic F number in long scale-length plasmas (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1091-1095 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The onset of stimulated Brillouin scattering (SBS) has been examined by varying the effective F number (lens focal length/incident beam diameter) of the optical system used to irradiate a performed 1 mm long, longitudinally uniform hydrocarbon (CH) plasma. SBS onset occurs at higher average laser intensity for smaller F, in qualitative agreement with theory. Quantitative comparison between theory and experiment, and implications for the National Ignition Facility [Phys. Plasmas 2, 2480 (1995)] are discussed. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871764
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  • 6
    Electronic Resource
    Electronic Resource
    Gas-filled targets for large scale-length plasma interaction experiments on Nova (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 2473-2479 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Stimulated Brillouin backscatter from large scale-length gas-filled targets has been measured on the Nova laser. These targets were designed to approximate conditions in indirect drive ignition target designs in underdense plasma electron density (ne∼1021/cm3), temperature (Te(approximately-greater-than)3 keV), and gradient scale lengths (Ln∼2 mm, Lv(approximately-greater-than)6 mm) as well as calculated gain for stimulated Brillouin scattering (SBS). The targets used in these experiments were gas-filled balloons with polyimide walls (gasbags) and gas-filled hohlraums. Detailed characterization using x-ray imaging and x-ray and optical spectroscopy verifies that the calculated plasma conditions are achieved. Time-resolved SBS backscatter from these targets is 〈3% for conditions similar to ignition target designs. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871208
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  • 7
    Electronic Resource
    Electronic Resource
    Nonparallel linear stability analysis of Long's vortex (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 11 (1999), S. 1114-1126 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A nonparallel linear stability analysis of a family of self-similar vortex cores which includes Long's vortex as a particular member is performed using parabolized stability equations (PSE). The resulting streamwise variation of both the spatial growth rate and the axial wave number of the different unstable modes is compared with the results from a local spatial stability analysis which also takes into account the effects of viscosity and of the streamwise variation of the basic flow, so that the effect of the history of the disturbances on their stability is quantified. It is shown that this last effect is negligible for high Reynolds numbers, but becomes increasingly important as the Reynolds number decreases, especially for very small growth rates. The marching method used to solve the PSE is computationally much faster than the standard methods for solving the nonlinear eigenvalue problem resulting from the local stability equations. As a new result, the local spatial calculations reveal the existence of unstable counter-rotating spiral modes with negative group velocities for Type II Long's vortices (that is, vortices with negative streamwise velocity at the axis), thus showing that these flows are subcritical in Benjamin's sense. This kind of instability does not appear for Type I vortices, which can only sustain non-axisymmetric convective instabilities, and are therefore supercritical. Thus, the spatial stability analysis establishes a fundamental distinction between Type I and Type II Long's vortices. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.869982
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  • 8
    Electronic Resource
    Electronic Resource
    Evidence of plasma fluctuations and their effect on the growth of stimulated Brillouin and stimulated Raman scattering in laser plasmas : 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. 1973-1980 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The reflectivity levels of stimulated Brillouin scattering (SBS) in recent large scale length laser plasma experiments is much lower than expected for conditions where the convective gain exponent is expected to be large [J. C. Fernández et al., Phys. Plasmas 4, 1849 (1997)]. Long-wavelength velocity fluctuations caused during the plasma formation process, or by parametric instabilities themselves, have been proposed as a mechanism to detune SBS in these experiments and reduce its gain [W. L. Kruer et al., Phys. Plasmas 3, 382 (1996); H. A. Rose, Phys. Plasmas 4, 437 (1997)]. Evidence of large-velocity fluctuation levels is found in the time-resolved SBS spectra from these experiments, and correlates with observed changes in the reflectivity of both SBS and stimulated Raman scattering (SRS). Evidence of fluctuations that increase with increasing plasma density is presented, and their effect on the growth of parametric instabilities is discussed. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872868
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  • 9
    Electronic Resource
    Electronic Resource
    Technique for the experimental estimation of nonlinear energy transfer in fully developed turbulence (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 3998-4009 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A new procedure for calculating the nonlinear energy transfer and linear growth/damping rate of fully developed turbulence is derived. It avoids the unphysically large damping rates typically obtained using the predecessor method of Ritz [Ch. P. Ritz, E. J. Powers, and R. D. Bengtson, Phys. Fluids B 1, 153 (1989)]. It enforces stationarity of the turbulence to reduce the effects of noise and fluctuations not described by the basic governing equation, and includes the fourth-order moment to avoid the closure approximation. The new procedure has been implemented and tested on simulated, fully developed two-dimensional (2-D) turbulence data from a 2-D trapped-particle fluid code, and has been shown to give excellent reconstructions of the input growth rate and nonlinear coupling coefficients with good noise rejection. However, in the experimentally important case where only a one-dimensional (1-D) averaged representation of the underlying 2-D turbulence is available, this technique does not, in general, give acceptable results. A new 1-D algorithm has thus been developed for analysis of 1-D measurements of intrinsically 2-D turbulence. This new 1-D algorithm includes the nonresonant wave numbers in calculating the bispectra, and generally gives useful results when the width of the radial wave number spectrum is comparable to or less than that of the poloidal spectrum. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871572
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  • 10
    Electronic Resource
    Electronic Resource
    Energy partitions in saturated compressible electron magnetoturbulence (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 4204-4215 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The two-dimensional stationary turbulence of magnetic field and density fluctuations coupled through the compressibility of parallel electron motion is shown to possess three distinct stationary states under driving of the magnetic field by the time-dependent thermal force and damping by resistivity and collisional diffusivity. The three states are the equipartitioned magnetic state that occurs when short-wavelength fluctuations interact principally through the long-wavelength magnetic field fluctuation, a magnetic energy-dominated state induced by the decorrelation of nonlinear interactions by strong diamagnetic rotation, and an internal energy-dominated state possible when the dissipation of the density is weaker than the resistive diffusion. The equipartitioned and internal energy-dominated states can occur for identical parameters, making the driven/damped turbulent steady state nonunique. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871046
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