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  • 1
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Collisionless electron-temperature-gradient-driven (ETG) turbulence in toroidal geometry is studied via nonlinear numerical simulations. To this aim, two massively parallel, fully gyrokinetic Vlasov codes are used, both including electromagnetic effects. Somewhat surprisingly, and unlike in the analogous case of ion-temperature-gradient-driven (ITG) turbulence, we find that the turbulent electron heat flux is significantly underpredicted by simple mixing length estimates in a certain parameter regime (s(circumflex)∼1, low α). This observation is directly linked to the presence of radially highly elongated vortices ("streamers") which lead to very effective cross-field transport. The simulations therefore indicate that ETG turbulence is likely to be relevant to magnetic confinement fusion experiments. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    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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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 2381-2389 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A first-principles model of anomalous thermal transport based on numerical simulations is presented, with stringent comparisons to experimental data from the Tokamak Fusion Test Reactor (TFTR) [Fusion Technol. 21, 1324 (1992)]. This model is based on nonlinear gyrofluid simulations, which predict the fluctuation and thermal transport characteristics of toroidal ion-temperature-gradient-driven (ITG) turbulence, and on comprehensive linear gyrokinetic ballooning calculations, which provide very accurate growth rates, critical temperature gradients, and a quasilinear estimate of χe/χi. The model is derived solely from the simulation results. More than 70 TFTR low confinement (L-mode) discharges have been simulated with quantitative success. Typically, the ion and electron temperature profiles are predicted within the error bars, and the global energy confinement time within ±10%. The measured temperatures at r/a(approximately-equal-to)0.8 are used as a boundary condition to predict the temperature profiles in the main confinement zone. The dramatic transition to the improved confinement in the supershot regime is also qualitatively explained. Further work is needed to extend this model of core heat transport to include particle and momentum transport, the edge region, and other operating regimes besides the ITG-dominated L mode. Nevertheless, the present model is very successful in predicting thermal transport in the main plasma over a wide range of parameters. © 1995 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 1 (1994), S. 3635-3640 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The ion temperature gradient- (ITG) driven instability, or ηi mode, is studied for discharges with hydrogen, deuterium, or tritium in a toroidal magnetic configuration. Impurity effects on the mode and the instability (impurity mode) driven by the presence of impurity ions with negative density gradient are studied. It is found that the maximum growth rate of the ηi mode scales as M−1/2i for pure hydrogenic plasmas, where Mi is the mass number of the working gas ion. With the inclusion of impurity ions, the growth rate of the ηi mode decreases in all three kinds of plasmas, with a hydrogen plasma still having the highest maximum growth rate, tritium the lowest, and deuterium in between. However, the isotope effects are weaker and scale as M−1/2eff with the presence of impurity ions, where the effective mass number, Meff=(1−fz)Mi+fzMz, with Mz and fz= Zn0z/n0e being the mass number and charge concentration of impurity ions, respectively. For the impurity mode, the scaling is similar to that of the ηi mode without impurity ions. The experimental database shows that the plasma energy confinement time scales as τE∝M1/2i for a wide range of clean plasmas. The correlation of the theoretical results with the experimental confinement scaling is discussed.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The Tokamak Fusion Test Reactor (TFTR) (R. J. Hawryluk, to be published in Rev. Mod. Phys.) experiments on high-temperature plasmas, that culminated in the study of deuterium–tritium D–T plasmas containing significant populations of energetic alpha particles, spanned over two decades from conception to completion. During the design of TFTR, the key physics issues were magnetohydrodynamic (MHD) equilibrium and stability, plasma energy transport, impurity effects, and plasma reactivity. Energetic particle physics was given less attention during this phase because, in part, of the necessity to address the issues that would create the conditions for the study of energetic particles and also the lack of diagnostics to study the energetic particles in detail. The worldwide tokamak program including the contributions from TFTR made substantial progress during the past two decades in addressing the fundamental issues affecting the performance of high-temperature plasmas and the behavior of energetic particles. The progress has been the result of the construction of new facilities, which enabled the production of high-temperature well-confined plasmas, development of sophisticated diagnostic techniques to study both the background plasma and the resulting energetic fusion products, and computational techniques to both interpret the experimental results and to predict the outcome of experiments. © 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 4 (1997), S. 2482-2496 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A physically comprehensive and theoretically based transport model tuned to three-dimensional (3-D) ballooning mode gyrokinetic instabilities and gyrofluid nonlinear turbulence simulations is formulated with global and local magnetic shear stabilization and E×B rotational shear stabilization. Taking no fit coefficients from experiment, the model is tested against a large transport profile database with good agreement. This model is capable of describing enhanced core confinement transport barriers in negative central shear discharges based on rotational shear stabilization. The model is used to make ignition projections from relative gyroradius scaling discharges. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Experiments in the Tokamak Fusion Test Reactor (TFTR) [Phys. Plasmas 2, 2176 (1995)] have explored several novel regimes of improved tokamak confinement in deuterium–tritium (D–T) plasmas, including plasmas with reduced or reversed magnetic shear in the core and high-current plasmas with increased shear in the outer region (high li). New techniques have also been developed to enhance the confinement in these regimes by modifying the plasma-limiter interaction through in situ deposition of lithium. In reversed-shear plasmas, transitions to enhanced confinement have been observed at plasma currents up to 2.2 MA (qa(approximate)4.3), accompanied by the formation of internal transport barriers, where large radial gradients develop in the temperature and density profiles. Experiments have been performed to elucidate the mechanism of the barrier formation and its relationship with the magnetic configuration and with the heating characteristics. The increased stability of high-current, high-li plasmas produced by rapid expansion of the minor cross section, coupled with improvement in the confinement by lithium deposition has enabled the achievement of high fusion power, up to 8.7 MW, with D–T neutral beam heating. The physics of fusion alpha-particle confinement has been investigated in these regimes, including the interactions of the alphas with endogenous plasma instabilities and externally applied waves in the ion cyclotron range of frequencies. In D–T plasmas with q0〉1 and weak magnetic shear in the central region, a toroidal Alfvén eigenmode instability driven purely by the alpha particles has been observed for the first time. The interactions of energetic ions with ion Bernstein waves produced by mode conversion from fast waves in mixed-species plasmas have been studied as a possible mechanism for transferring the energy of the alphas to fuel ions. © 1997 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 1 (1994), S. 1461-1468 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The gyrokinetic and gyrofluid models show the most promise for large scale simulations of tokamak microturbulence. This paper discusses detailed comparisons of these two complementary approaches. Past comparisons with linear theory have been fairly good, therefore the emphasis here is on nonlinear comparisons. Simulations include simple two-dimensional slab test cases, turbulent three-dimensional slab cases, and toroidal cases, each modeling the nonlinear evolution of the ion temperature gradient instability. There is good agreement in both turbulent and coherent nonlinear slab comparisons in terms of the ion heat flux, both in magnitude and scaling with magnetic shear. However, the nonlinear saturation level for ||Φ|| in the slab comparisons shows differences of approximately 40%. Preliminary toroidal comparisons show agreement within 50%, in terms of ion heat flux and saturation level.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 812-835 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Nonlinear gyrofluid equations are derived by taking moments of the nonlinear, electrostatic gyrokinetic equation. The principal model presented includes evolution equations for the guiding center n, u(parallel), T(parallel), and T⊥ along with an equation expressing the quasineutrality constraint. Additional evolution equations for higher moments are derived that may be used if greater accuracy is desired. The moment hierarchy is closed with a Landau damping model [G. W. Hammett and F. W. Perkins, Phys. Rev. Lett. 64, 3019 (1990)], which is equivalent to a multipole approximation to the plasma dispersion function, extended to include finite Larmor radius effects (FLR). In particular, new dissipative, nonlinear terms are found that model the perpendicular phase mixing of the distribution function along contours of constant electrostatic potential. These "FLR phase-mixing'' terms introduce a hyperviscositylike damping ∝k⊥2||Φkk×k'||, which should provide a physics-based damping mechanism at high k⊥ρ which is potentially as important as the usual polarization drift nonlinearity. The moments are taken in guiding center space to pick up the correct nonlinear FLR terms and the gyroaveraging of the shear. The equations are solved with a nonlinear, three-dimensional initial value code. Linear results are presented, showing excellent agreement with linear gyrokinetic theory.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 4096-4104 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Under certain conditions, the electron heat transport induced by electron temperature gradient (ETG) streamers is sufficiently large and sensitive with respect to the normalized electron temperature gradient to represent a possible cause for electron temperature profile consistency ("stiffness"). Here, linear gyrokinetic simulations of toroidal ETG modes in tokamak core and edge plasmas are presented. An algebraic formula for the threshold of the linear instability is derived from the numerical solutions of the linear gyrokinetic equations which recovers previous analytical results in the appropriate limits. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
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