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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 4954-4959 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In this paper a novel mechanism is identified for the generation of gamma ray flashes observed on the Compton Gamma Ray Observatory satellite. During typical cloud to ground lightning flashes, the electromagnetic pulse can create a self-focused whistler wave channel or duct to guide 10–102/cm−3 of ∼1 MeV electrons (formed by static stratified electric field in clouds at 20 km), to a height of about 30 km where these electrons can create the gamma ray flash by bremsstrahlung. This scenario combines the various observational features of lightning-generated electromagnetic pulses and low altitude energetic electrons to provide a viable nonlinear transport mechanism of energetic electrons to the desired altitude of 30 km for conversion into gamma ray flashes. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 459-462 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The generation of shear flow by drift waves is an important area of investigation. In a recent paper [Chen et al., Phys. Plasmas 7, 3129 (2000)] an elegant formalism was developed for the generation of shear/zonal flows by drift waves in a toroidal plasma. The study of shear flow generation in fluids and plasmas is put into perspective. In this paper it is shown how a simple slab geometry analysis can lead to a similar dispersion relation and highlight the subtle differences between the slab and toroidal geometries. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 3414-3419 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Stimulated Brillouin scattering (SBS) has been reexamined in the strong coupling limit. The three-wave interaction in the strong coupling regime, which gives rise to a purely growing mode for wave numbers k(approximately-greater-than)2k0, where k0 is the wave vector of the pump wave, has been studied. This regime was first found by Cohen and Max [Phys. Fluids 22, 1121 (1979)]. In this regime, for a flowing plasma, a significant portion of the unstable spectrum is found to be blueshifted even for the Mach number equal to unity. For the inhomogeneous case, the convective gain in the strong coupling limit turns out to be identical to the gain in the SBS in the weak coupling limit regime. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2951-2960 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Three-dimensional (3-D) simulations of drift-resistive ballooning turbulence are presented. The turbulence is basically controlled by a parameter α, the ratio of the drift wave frequency to the ideal ballooning growth rate. If this parameter is small [α≤1, corresponding to Ohmic (OH) or low confinement phase (L-mode) plasmas], the system is dominated by ballooning turbulence, which is strongly peaked at the outside of the torus. If it is large [α≥1, corresponding to high confinement phase (H-mode) plasmas], field line curvature plays a minor role. The turbulence is nonlinearly sustained even if curvature is removed and all modes are linearly stable due to magnetic shear. In the nonlinear regime without curvature the system obeys a different scaling law compared to the low-α regime. The transport scaling is discussed in both regimes and the implications for OH, L-mode, and H-mode transport are discussed. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 1273-1278 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A self-consistent set of equations for the fast space–time evolution of fluctuations and the slow space–time evolution of density and flows in a toroidal plasma, relevant for simulations using field-aligned coordinates in thin flux tubes, has been derived. The methodology for the derivation of these equations is outlined for a model set of equations for the plasma edge, specific to resistive ballooning modes but readily adaptable to other instabilities. The derivation proceeds by first writing the axisymmetric and fluctuating equations in the usual toroidal coordinate system. These are then transformed to the twisted coordinate flux-tube system. Most simulations which use twisted flux-tube computational grids transform to the field-aligned coordinate system first and then take averages to obtain the slow evolution. They however miss some terms since the two operations, namely, multiscale separation and coordinate transformation, do not necessarily commute, because of subsidiary assumptions on the box size. In the present formulation, all the relevant neoclassical effects such as the Pfirsch–Schlüter current and the Stringer spin-up as well as the toroidal Reynolds stress are properly included. This set of multiscale equations is appropriate for the study of the formation and evolution of transport barriers. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 3701-3712 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A system of equations that couples the rapidly varying fluctuations of resistive ballooning modes to the slowly varying transport of the density, vorticity and parallel momentum have been derived and solved numerically. Only a single toroidal mode number is retained in the present work. The low-mode (L-mode) phase consists of strong poloidally asymmetric particle transport driven by resistive ballooning modes, with larger flux on the outboard side compared to the inboard side. With the onset of shear flow driven by a combination of toroidal drive mechanisms as well as the Reynolds stress, the fluctuations associated with the resistive ballooning modes are attenuated leading to a strong reduction in the particle transport. The drop in the particle transport results in steepening of the density profile leading to the high-mode (H-mode). © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 4174-4176 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The generation of shear flow driven by a large amplitude drift wave, represented by the Hasegawa–Mima–Charney (HMC) equation, has been investigated. It is shown that, besides a finite amplitude threshold for the shear flow instability to occur, there is also a necessary condition on the aspect ratio of the large amplitude drift wave, due to conservation of the average potential vorticity, that needs to be satisfied. A comprehensive comparison of the shear-flow instability criterion for the HMC equation and the incompressible, invisicid, hydrodynamic equation has been undertaken. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 3065-3072 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The microinstabilities driven by a parallel velocity shear, and a temperature gradient of ions are studied in toroidal plasmas with negative magnetic shear. Both the fluid and the gyrokinetic formulations are investigated. It is found that for a broad range of parameters, the linear growth rates of the modes are lower and the threshold temperature gradient ηicr is higher for plasmas with negative magnetic shear compared to plasmas with positive magnetic shear of equal magnitude. The reduction in the growth rate (with negative shear), although not insignificant, does not seem to be enough to account for the dramatic improvement in the confinement observed experimentally. Other possible physical mechanisms for the improved confinement are discussed. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 9
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The enhancement of stability to ballooning modes from negative shear in tokamaks is shown to be a simple consequence of the orientation of the convective cell with respect to the toroidally outward effective gravity, g↘. For modest positive shear, convective cells remain oriented along g↘ as they map along field lines. In contrast, for negative shear or very positive shear convective cells twist strongly away from g↘ and are less strongly driven. The twist of convection cells is controlled by the profile of the vertical magnetic field along the outer midplane, Bz. Twist is a minimum in regions where Bz is independent of the major radius. Transport should be highest in such locations. Resistive ballooning modes in the tokamak edge are strongly stabilized by modest values of negative shear. Tokamak discharges with finite values of βp develop regions of local negative shear on the outside midplane of the plasma torus. This local negative shear should self-stabilize resistive ballooning modes at finite values of the poloidal beta. This effect may impact the transition to high confinement operation (H-mode). © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 3764-3768 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Analytical and numerical study of the stability of the resistive ballooning modes (RBM) in the scrape-off layer (SOL) of a tokamak plasma is performed. It is shown that the stability of the RBM is controlled by the two parameters λ=(me/mi)1/2νeiqR/vTe—the "effective strength'' of the Debye sheath current, and mˆ=mL0/a the dimensionless poloidal number, where the characteristic scale L0 is given in the main text as a function of the basic plasma edge parameters. For λ(approximately-greater-than)1 the coupling to the sheath is unimportant and the unstable spectrum is virtually unchanged from that in the closed flux region. For λ〈1 the sheath current has a strong destabilizing influence on long wavelength modes. The general case of arbitrary λ and mˆ is studied numerically and the spectrum of the unstable RBM is found. The influence of the diamagnetic effects is also investigated. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
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