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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.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 1557-1569 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In previous studies, it was shown that, in the nonlinearly saturated phase of the evolution of drift instabilities in gyrokinetic particle simulations, the saturation levels and especially the particle fluxes are significantly less than predicted by quasilinear theory, and have an unexpected dependence on collisionality. In this paper, a theory is developed that explains these phenomena. The key features of the theory are as follows: The saturation level is determined by a balance between the steady-state electron and ion fluxes. The ion flux is small for levels of the potential below an E×B-trapping threshold and increases sharply once this threshold is crossed. Because of the presence of resonant electrons, the electron flux has a much smoother dependence on the potential. In the 2 (1)/(2) - dimensional ("pseudo-3-D'') geometry, the electrons are accelerated away from the resonance as they diffuse spatially, resulting in an inhibition of their diffusion, and thereby reducing the values of the potential and flux at which the fluxes can balance. Collisions and three-dimensional effects can repopulate the resonance thereby increasing the value of the particle flux.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 1381-1385 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Fluid simulations of long ηi modes are presented that demonstrate that the density profile plays a much more profound role in controlling global energy confinement than would be expected from simple marginal stability considerations. Specifically, local steepening of the density gradient produces an effective transport barrier that allows ion global confinement to improve dramatically. The threshold value ηi =d ln(Ti)/d ln(n) required for instability ηic plays a fundamental role in the formation of the transport barrier—no transport barrier forms for ηic〈1. The implication of these results for understanding ion energy confinement in tokamaks is discussed.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 1420-1424 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The long-time behavior of thermally unstable one-dimensional disturbances in an optically thin ionized gas, subject to external heating and radiative cooling, is investigated. The intermediate-wavelength limit is considered, corresponding to the most rapidly growing isobaric condensation mode. The "reaction–diffusion'' equation, describing the instability dynamics in Lagrangian coordinates, is employed to study the time evolution of small sinusoidal initial perturbations away from the unstable thermal equilibrium. On the relatively short heating–cooling time scale, steep temperature and density fronts develop in the gas, corresponding to spatial coexistence of two locally stable thermal equilibria adjacent to the unstable one on the isobaric heating–cooling curve. It is shown, however, that only one of these states generally survives on the much longer heat conduction time scale. The numerically observed transition to the final "truly'' stable state is interpreted in terms of the interaction between traveling temperature fronts which preserve their identity until annihilation.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 620-626 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An analytical and numerical investigation has been completed of the nonlinear growth and saturation of long-wavelength ion-temperature-gradient-driven turbulence in a sheared magnetic field for the case of an isolated rational surface. The radial correlation length of the turbulence is found to be of order ρiLs/ Lt, with ρi the ion Larmor radius, Ls the magnetic shear length, and LT the temperature scale length. The scaling of the resulting anomalous cross-field diffusivity is χ⊥∝g(ηi)(ρ2i/LTLy) (Ls/LT)2ρivti, where g(ηi) is obtained from the numerical results. In the poloidal direction, the spectrum collapses to the longest wavelength, Ly, available. Explicit results for the function g(ηi) are presented for values of ηi ranging from just above marginal stability ηic to ηi=∞. Although the quasilinear temperature profile is held fixed, the transport rates are very small as a result of local flattening of the temperature profile near the rational surface. The implications of these results for understanding anomalous transport in tokamaks are discussed.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 2591-2599 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The nonlinear growth and saturation of the negative compressibility ηi instability in an inhomogeneous plasma confined by a straight magnetic field is investigated. A set of nonlinear equations that describe the two-dimensional fluid motion along and across (in the direction of ∇⊥Ti ) the local magnetic field is derived. These equations are basically the same as those describing a one-dimensional nonlinear sound wave, but the compressibility is negative for ηi=(overdot)d log(Ti)/log(ni) exceeding a threshold. The nonlinearity in the equations arises from the parallel convective motion of the fluid. The solution of these equations evolves to a final state in which plasma flows are driven into a low temperature region along the local magnetic field and then jet across the magnetic field along ∇⊥Ti in narrow streams. The structure of these streams is consistent with previous two-dimensional simulations. Results for the scaling of the transport due to these streams are presented.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 1768-1774 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Saturation of the collisionless and weakly collisional drift instabilities by nonlinear E×B advection of resonant electrons is considered. The nonlinear E×B advection of the resonant electrons around the O points and X points of the potential shuts off the linear phase shift between the electron density and the potential, and hence the linear growth, and produces residual oscillations at the E×B-trapping frequency. Two analytical solutions of a three-mode model of Lee et al. [Phys. Fluids 27, 2652 (1984)], which describes the saturation of drift waves by this mechanism, are found. The first is an exact solution in the form of a steadily propagating wave of constant amplitude, and is relevant when electron pitch-angle scattering is present. The second is an approximate time-dependent analytical solution, obtained using the method of O'Neil [Phys. Fluids 8, 2255 (1965)], and is relevant to the collisionless case. The predictions that follow from this solution for the saturation level and for the amplitude oscillation frequency are in excellent agreement with the direct numerical solutions of the three-mode system.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 274-277 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The regime of validity of nonlinear gyrokinetic equations is extended to cover uniformly both the usual drift-kinetic and gyrokinetic regimes through the use of an expansion in the parameter ε∼(ρ/λ⊥)e(φ−v(parallel) Az/c)/T. Here, ρ is the gyroradius, λ⊥ is the scale length of the electrostatic and parallel magnetic potentials φ and Az, c is the speed of light, and T is the temperature. This is made possible by a preparatory split of the potentials into gyrophase-dependent and independent parts. For nonlinear fluctuations saturated at mixing-length levels (e.g., with eφ/T∼λ⊥ /L, where L is the equilibrium scale length), ε is of order ρ/L for all scales λ⊥ ranging from ρ to L, and is therefore small in plasmas of fusion interest.
    Type of Medium: Electronic Resource
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