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  • 1
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The predictions of gyrokinetic and gyrofluid simulations of ion-temperature-gradient (ITG) instability and turbulence in tokamak plasmas as well as some tokamak plasma thermal transport models, which have been widely used for predicting the performance of the proposed International Thermonuclear Experimental Reactor (ITER) tokamak [Plasma Physics and Controlled Nuclear Fusion Research, 1996 (International Atomic Energy Agency, Vienna, 1997), Vol. 1, p. 3], are compared. These comparisons provide information on effects of differences in the physics content of the various models and on the fusion-relevant figures of merit of plasma performance predicted by the models. Many of the comparisons are undertaken for a simplified plasma model and geometry which is an idealization of the plasma conditions and geometry in a Doublet III-D [Plasma Physics and Controlled Nuclear Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159] high confinement (H-mode) experiment. Most of the models show good agreements in their predictions and assumptions for the linear growth rates and frequencies. There are some differences associated with different equilibria. However, there are significant differences in the transport levels between the models. The causes of some of the differences are examined in some detail, with particular attention to numerical convergence in the turbulence simulations (with respect to simulation mesh size, system size and, for particle-based simulations, the particle number). The implications for predictions of fusion plasma performance are also discussed. © 2000 American Institute of Physics.
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  • 2
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In response to the comment on theory and simulation of light filamentation in laser-produced plasma the authors believe that the criticism overstates the outcome of choosing one form of thermal conduction over another. (AIP)
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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 1732-1741 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Pulsed reflectometry using both ordinary (O) and extraordinary (X) modes can provide time- and space-resolved measurements of the electron density, the magnitude of the magnetic field, the magnetic shear as a function of radius, and information on density and magnetic fluctuations. Such a diagnostic also yields the current profile from the curl of the magnetic field. This research addresses theoretical issues associated with the use of pulsed reflectometry with particular emphasis on applications in the Sustained Spheromak Physics Experiment (SSPX) at the Lawrence Livermore National Laboratory [E. B. Hooper et al., "Sustained Spheromak Physics Experiment," in Proceedings of the 17th International Atomic Energy Agency (IAEA) Fusion Energy Conference, Yokohama, Japan, October 19–24, 1998, Lawrence Livermore National Laboratory Report UCRL-JC-132034 (September 29, 1998)]. Simulation results are presented for O- and X-mode mixed-polarization reflectometry and linear mode conversion in two spatial dimensions. The profile reconstruction algorithms depend on Wentzel–Kramers–Brillouin–Jeffreys (WKBJ) formulae for group delays and linear mode conversion, which agree reasonably well with direct numerical solutions of the wave equation. Reconstructions of the electron density and modulus of the magnetic field are relatively robust in the presence of two-dimensional electron density and magnetic perturbations of the plasma.
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  • 4
    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.
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  • 5
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 3402-3407 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Particle simulations and the analytical solution of coupled mode equations are used to demonstrate that the mode coupling of a primary ion acoustic wave produced by stimulated Brillouin backscatter with a secondary finite-amplitude ion wave propagating at a finite relative angle can reduce the amplitude of the primary ion wave and the backscatter. The simulations and analysis give results that are in qualitative agreement with experimental observations [Baldis et al., Phys. Rev. Lett. 77, 2957 (1996)] of the partial suppression of stimulated Brillouin backscattering (SBBS) when there are simultaneous, overlapping SBBS processes at a finite relative angle, which may influence SBBS reflectivities in other current and future laser-fusion experiments with multiple overlapping beams. © 1998 American Institute of Physics.
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  • 6
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 3408-3415 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Particle simulations and solutions of coupled mode equations are used to analyze the energy transfer between two equal-frequency, opposed laser beams resonantly interacting with ion acoustic waves in a plasma drifting at the sound speed. The simulations and analysis illustrate the dependence of the energy transfer and the ion wave dynamics on laser intensities and detuning, and the time dependence of the phenomena. The simulation results are in qualitative agreement with experimental observations in the NOVA laser facility [E. M. Campbell et al., Rev. Sci. Instrum. 57, 2101 (1986)] at the Lawrence Livermore National Laboratory. This work is part of a continuing examination of possible resonant crossed-beam interactions in flowing plasmas and their potential effects on the fusion performance of current and future laser-fusion experiments with multiple crossing beams, e.g., proposed experiments in the National Ignition Facility [National Tech. Info. Service Document Nos. DE95017671-DE95017673 and DE95017676-DE95017700 (J. A. Paisner, E. M. Campbell, and W. J. Hogan, The Natl. Ignition Facility Project, UCRL-JC-117397 and UCRL-PROP-117093, May, 1994)]. © 1998 American Institute of Physics.
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  • 7
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 1356-1357 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The authors disagree with the assertions in Ref. 1. The authors think that the expression for the electron heat flux is fairly general and the appropriate electron damping rage, eq. (56) of Ref. 2 is correct. (AIP)
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  • 8
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 1287-1302 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The Landau-fluid model is a recently introduced fluid-moment closure scheme [G. W. Hammett and F. W. Perkins, Phys. Rev. Lett. 64, 3019 (1990)] that was designed to include kinetic dissipative effects like Landau damping in fluid calculations. The fluid-moment hierarchy is terminated by assuming linear relationships among the retained moments in Fourier-transform space, with coefficients determined by matching the plasma response to that obtained from a kinetic analysis. This paper generalizes the technique to the full range of ion and electron collisionality and applies it to a new fluid simulation code constructed to study laser filamentation in underdense plasmas [Berger et al., Phys. Fluids B 5, 2243 (1993)]. By matching the ion-acoustic complex frequency derived from the fluid model with that predicted by collisional, Fokker–Planck, and kinetic analyses, the specific heat ratio, thermal conductivity coefficient, and viscosity coefficient for ions and the thermal conductivity coefficient for electrons are determined as functions of the wave number k. For frequencies much less than the pump frequency this leads to a fourth-order polynomial dispersion relation whose spectrum includes damped ion-acoustic waves as well as filamentation modes whose stability depends on the pump strength. An analytic instability threshold condition on the laser intensity is derived from which the relative importance of ponderomotive and thermal drives can be assessed. Expressions for the linear susceptibilities in the presence of a finite-amplitude pump are also given, which might prove useful for understanding spectral linewidths for Thomson scattering.
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  • 9
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 2243-2258 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A desire to interpret recent experiments on filamentation with and without beam-smoothing techniques led to the development of a three-dimensional fluid model that includes the effects of nonlocal electron transport and kinetic ion damping of the acoustic waves. The damping of the electron-temperature perturbations that drive thermal filamentation by nonlocal electron conduction, valid in the diffusive limit, is supplemented in the present model by electron Landau damping in the collisionless limit when the wavelength of the perturbation is much less than the electron–ion scattering mean-free path. In this collisionless limit, Landau damping of the "temperature'' fluctuations makes ponderomotive forces universally more important than thermal forces. Simulations in plasmas of current interest illustrate the relative importance of thermal and ponderomotive forces for strongly modulated laser beams. Although thermal forces may initiate filamentation, the most intense filaments are associated with ponderomotive forces. The present simulations of filamentation model well the density perturbations observed in experiments [Young et al., Phys. Rev. Lett. 61, 2336 (1988)]. In addition, a simple criterion is obtained analytically and supported by simulations for stabilization of filamentation by laser beam-smoothing techniques such as induced spatial incoherence and random phase plates [Eq. (1)].
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  • 10
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: By using HELM1D, a numerical, one-dimensional Helmholtz equation solver, we have studied the scattering phase shifts of S-polarized O modes in fluctuation reflectometry both within and beyond the Born approximation. Comparisons are made between these numerical results and analytical expressions obtained through the use of the Born approximation. As the fluctuation amplitude increases, various peaks of the phase response evolve with distinct growth rates. This leads to steepening and distortion of the scattering phase shift due to a single wave-number fluctuation. The simultaneous coexistence of large amplitude fluctuations with nearby wave numbers complicates the task of resolving the response at each wave number. © 1995 American Institute of Physics.
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