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  • 1
    Electronic Resource
    Electronic Resource
    Radially global gyrokinetic simulation studies of transport barriers (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1959-1966 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Improvements in tokamak transport have recently been obtained in a variety of operational modes through the formation of transport barriers, or good confinement radial zones. Here global nonlinear three-dimensional toroidal gyrokinetic simulation is used to study three effects that are linearly stabilizing and may cause the formation of transport barriers, namely, sheared toroidal rotation, reversed magnetic shear, and peaked density profiles. The effect of toroidal shear flow on ion heat diffusivity is found to be relatively weak compared to mixing-length expectations based on linear calculations. In contrast, it is found that weak or negative magnetic shear (s〈1/2) in combination with a peaked density profile relative to the temperature profile greatly suppresses ion-temperature-gradient-driven turbulence in the central region of global nonlinear simulations. Similar features are seen experimentally in reversed magnetic shear tokamak plasmas. There is some nonlocal penetration (∼20–30ρi) of the turbulence into the subcritical core region. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871992
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  • 2
    Electronic Resource
    Electronic Resource
    Calculation and interpretation of analytic high-beta poloidal equilibria in finite aspect ratio tokamaks (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 266-274 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The validity of the analytic large aspect ratio, high-β equilibria developed by Cowley et al. [Phys. Fluids B 3, 2066 (1991)] is extended to include finite aspect ratio equilibria with q2(very-much-greater-than)1, where q is the safety factor. These high-β equilibria have two regions. Most of the volume lies in the "core region,'' where ψ=ψ(R). The flux surfaces close in the "boundary layer region,'' which has thickness δ. The solutions are valid when δ/a∼O ((square root of)ε/βq2) is small, where a is the minor radius. Thus, finite ε is allowed when q2 is large. The equilibria are completely specified by the midplane profiles of pressure p(R) and poloidal magnetic field BP(R) and the shape of the plasma boundary, all of which can be measured experimentally. Note the departure from customary specification of p(ψ), q(ψ), or F(ψ). A fast numerical code, requiring a few seconds to execute, has been written to compute and illustrate the analytic high-β equilibria. The qualitative features of high-βP tokamaks are discussed in detail. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871852
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  • 3
    Electronic Resource
    Electronic Resource
    Nonlinear electromagnetic gyrokinetic equations for rotating axisymmetric plasmas (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 2682-2692 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The influence of sheared equilibrium flows on the confinement properties of tokamak plasmas is a topic of much current interest. A proper theoretical foundation for the systematic kinetic analysis of this important problem has been provided here by presenting the derivation of a set of nonlinear electromagnetic gyrokinetic equations applicable to low-frequency microinstabilities in a rotating axisymmetric plasma. The subsonic rotation velocity considered is in the direction of symmetry, with the angular rotation frequency being a function of the equilibrium magnetic flux surface. In accordance with experimental observations, the rotation profile is chosen to scale with the ion temperature. The results obtained represent the shear flow generalization of the earlier analysis by Frieman and Chen [Phys. Fluids 25, 502 (1982)], where such flows were not taken into account. In order to make it readily applicable to gyrokinetic particle simulations, this set of equations is cast in a phase-space-conserving continuity equation form.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870595
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  • 4
    Electronic Resource
    Electronic Resource
    Trapped ion mode in toroidally rotating plasmas (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 3384-3400 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The influence of radially sheared toroidal flows on the Trapped Ion Mode (TIM) is investigated using a two-dimensional eigenmode code. These radially extended toroidal microinstabilities could significantly influence the interpretation of confinement scaling trends and associated fluctuation properties observed in recent tokamak experiments. In the present analysis, the electrostatic drift kinetic equation is obtained from the general nonlinear gyrokinetic equation in rotating plasmas [M. Artun and W. M. Tang, Phys. Plasmas 1, 2682 (1994)]. In the long perpendicular wavelength limit krρbi(very-much-less-than)1, where ρbi is the average trapped ion banana width, the resulting eigenmode equation becomes a coupled system of second order differential equations for the poloidal harmonics. These equations are solved using finite element methods. Numerical results from the analysis of low and medium toroidal mode number instabilities are presented using representative Low confinement mode (L-mode) experimental data followed by a discussion of numerical results. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871173
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  • 5
    Electronic Resource
    Electronic Resource
    Integral eigenmode analysis of shear flow effects on the ion temperature gradient mode (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 4072-4080 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Previous numerical and analytic kinetic studies have investigated the influence of velocity shear on the ion temperature gradient (ITG) mode. These studies relied on a differential approximation to study mode structures with k⊥ρi(very-much-less-than)1. A recently developed gyrokinetic integral code is here used to explore the effects of sheared flows on the ITG mode for arbitrary values of k⊥ρi in sheared slab geometry. It is found that both the mode structure and eigenfrequencies predicted by the integral code can differ from the results obtained by the differential approach, even in the kyρi(very-much-less-than)1 limit. Although some trends predicted by the differential approximation are recovered by the integral approach, there are some significant differences. For example, the slight destabilizing effect observed for small values of the perpendicular velocity shear at k⊥ρi(very-much-less-than)1 is amplified when the integral approach is applied. In dealing with the higher radial eigenmodes, which can often exhibit the largest growth rates, it is emphasized that their finer radial structure usually dictates that the integral equation analysis is required. Results from the integral code are presented together with comparisons with results from the differential approach.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860575
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  • 6
    Electronic Resource
    Electronic Resource
    Gyrokinetic analysis of ion temperature gradient modes in the presence of sheared flows (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 1102-1114 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The linearized gyrokinetic equation governing electrostatic microinstabilities in the presence of sheared equilibrium flows in both the zˆ and yˆ directions has been systematically derived for a sheared slab geometry, where in the large-aspect-ratio limit zˆ and yˆ directions correspond to the toroidal and poloidal directions, respectively. In the familiar long perpendicular wavelength regime (k⊥ρi〈1), the analysis leads to a comprehensive kinetic differential eigenmode equation that is solved numerically. The numerical results have been successfully cross-checked against analytic estimates in the fluid limit. For typical conditions, the ion temperature gradient (ηi) modes are found to be stabilized for yˆ direction flows with a velocity shear scale comparable to that of the ion temperature gradient and velocities of a few percent of the sound speed. Sheared flows in the zˆ direction taken alone are usually destabilizing, with the effect being independent of the sign of the flow. However, when both types are simultaneously considered, it is found that in the presence of sheared zˆ-direction flow, sheared yˆ-direction flow can be either stabilizing or destabilizing depending on the relative sign of these flows. However, for sufficiently large values of v'y the mode is completely stabilized regardless of the sign of vzv'y. The importance of a proper kinetic treatment of this problem is supported by comparisons with fluid estimates. In particular, when such effects are favorable, significantly smaller values of sheared yˆ-direction flow are required for stability than fluid estimates would indicate.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860118
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