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  • 1
    Electronic Resource
    Electronic Resource
    Trapped electron correction to beam driven current in general tokamak equilibria (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 4179-4181 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In the limit that the electron thermal velocity greatly exceeds the fast ion velocity for electrical currents driven by neutral beams, the trapped electron correction to the Ohkawa current and the electron density gradient contribution to bootstrap current are shown to share the same transport coefficient in the banana regime. Therefore, existing analytic expressions for the bootstrap coefficient valid for arbitrary aspect ratio tokamaks can also be used to calculate the trapped electron effect. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872543
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  • 2
    Electronic Resource
    Electronic Resource
    Rotational and magnetic shear stabilization of magnetohydrodynamic modes and turbulence in DIII-D high performance discharges (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1951-1958 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The confinement and the stability properties of the DIII-D tokamak [Plasma Physics and Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159] high-performance discharges are evaluated in terms of rotational and magnetic shear, with an emphasis on the recent experimental results obtained from the negative central magnetic shear (NCS) experiments. In NCS discharges, a core transport barrier is often observed to form inside the NCS region accompanied by a reduction in core fluctuation amplitudes. Increasing negative magnetic shear contributes to the formation of this core transport barrier, but by itself is not sufficient to fully stabilize the toroidal drift mode (trapped-electron-ηi mode) to explain this formation. Comparison of the Doppler shift shear rate to the growth rate of the ηi mode suggests that the large core E×B flow shear can stabilize this mode and broaden the region of reduced core transport. Ideal and resistive stability analysis indicates the performance of NCS discharges with strongly peaked pressure profiles is limited by the resistive interchange mode to low βN≤2.3. This mode is insensitive to the details of the rotational and the magnetic shear profiles. A new class of discharges, which has a broad region of weak or slightly negative magnetic shear (WNS), is described. The WNS discharges have broader pressure profiles and higher β values than the NCS discharges, together with high confinement and high fusion reactivity. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871991
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  • 3
    Electronic Resource
    Electronic Resource
    High and very high modes from energy, particle, and momentum transport models (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 909-926 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Time-dependent simulations of energy, particle, and momentum transport are presented, which show improved confinement similar to the high mode (H mode). The transport model incorporates the suppression of turbulence by sheared flows, which are self-consistently calculated. Constraints on the turbulence model from the time evolution of the temperature, density, and velocity profiles are discussed. A regime similar to the very high confinement mode (VH mode) is found to result at higher heating power due to an increase in the width of the transport barrier. Coneutral beam injection lowers the power required for VH mode substantially due to the toroidal rotation shear. The possible role of edge momentum sources such as ion orbit loss is considered, and a comparison between biased probe-induced and heating-induced H modes is made.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870750
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  • 4
    Electronic Resource
    Electronic Resource
    Poloidal rotation in tokamaks with large electric field gradients (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 159-166 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The ion poloidal flow velocity near the plasma edge in a tokamak has been calculated by extending neoclassical theory to include orbit squeezing, which is the reduction of the ion banana widths due to radial electric field shear. The pressure gradient-driven ion parallel flow is reduced by orbit squeezing, and then no longer cancels the diamagnetic flow in its contribution to poloidal flow. This allows the poloidal flow velocity to be a significant fraction of the ion diamagnetic velocity, which can be much larger than the standard neoclassical value (proportional to the ion temperature gradient). Equations for determining the poloidal flow and radial electric field profiles self-consistently are given. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871105
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  • 5
    Electronic Resource
    Electronic Resource
    Neoclassical ion transport in rotating axisymmetric plasmas (1985)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 28 (1985), S. 3082-3098 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The neoclassical theory of ion transport in rotating axisymmetric plasmas is formulated. The flow speed is allowed to be of the order of the ion thermal speed. It is shown that the ion distribution function becomes Maxwellian, with temperature uniform on a magnetic surface, and the poloidal flow decays, in a few transit or collision times, in general. A drift kinetic equation is derived which is a simple generalization of the drift kinetic equation for nonrotating plasmas. The radial gradient of the toroidal angular velocity appears as a driving term like the temperature gradient. Both gradients drive the transport of toroidal angular momentum and energy, in general; Onsager relations for the two-by-two transport matrix are derived. The off-diagonal transport coefficients are shown to be zero if the magnetic field has up–down symmetry. A simple expression for the enhancement of the ion thermal conductivity in the banana regime, caused by rotation, is derived. The neoclassical viscosity is shown not to be enhanced by rotation in the banana regime, and to be small in the collisionality parameter in the collisional regime, assuming up–down symmetry. In the collisional regime, the thermal conductivity is shown to be suppressed by the effects of rotation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.865350
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  • 6
    Electronic Resource
    Electronic Resource
    Thermal confinement bifurcation and the L- to H-mode transition in tokamaks (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 696-704 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A bifurcation in the thermal confinement of tokamaks, which resembles the L- to H-mode transition, is shown to follow from properties of edge turbulence recently derived by Biglari et al. [Phys. Fluids B 2, 1 (1990)], and the standard neoclassical theory of poloidal rotation. The temperature profiles develop a pedestal at the plasma edge, and the poloidal rotation near the edge is considerably increased, when the heating power exceeds a critical value. The energy confinement time is a discontinuous function of increasing heating power, but is continuous for decreasing power (power hysteresis). Critical values of density and magnetic field are found, which must be exceeded in order for the bifurcation to occur. The scaling of the power threshold with density, magnetic field, and ion mass is similar to what is found experimentally.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859866
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  • 7
    Electronic Resource
    Electronic Resource
    Particle and energy confinement bifurcation in tokamaks (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 1281-1288 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A bifurcation in the particle and energy confinement properties of a tokamak plasma, with properties similar to the experimentally observed L-mode to H-mode transition, is shown to follow from a simple model for the transport. The basic assumptions are that the edge turbulence is suppressed by sheared E×B flow, and the radial ion pressure gradient is approximately balanced by the radial electric field. The particle and thermal diffusivities are assumed to be given by simple nonlinear functions of the radial electric field shear, which is related to the density and pressure gradients. The steady-state density and pressure profiles are found to have large gradient regions near the plasma edge when the product of edge energy flux and particle flux exceeds a threshold value, which corresponds to the H-mode threshold. The particle and energy confinement times are significantly larger when this threshold is exceeded. The confinement times exhibit hysteresis, corresponding to a dependence of quasiequilibrium states on the time history. The width of the edge layer where the gradients are large is determined mainly by the particle source profile, which is assumed to be concentrated near the plasma edge. This edge layer width increases slowly with increasing edge heat flux or heating power, as observed experimentally.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860919
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  • 8
    Electronic Resource
    Electronic Resource
    Poloidal viscosity and radial currents (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 278-279 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Considerable doubt is cast on the L- to H-mode transition mechanism proposed by Shaing [Phys. Rev. Lett. 63, 2369 (1989)]. Contrary to his assertion, a consistent application of the momentum balance equations shows that ion orbit loss current does not drive large poloidal rotation in tokamaks, in steady state.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860445
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  • 9
    Electronic Resource
    Electronic Resource
    Effect of impurity particles on the finite-aspect ratio neoclassical ion thermal conductivity in a tokamak (1986)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 29 (1986), S. 3314-3316 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The effect of finite-aspect ratio on the impurity contribution to neoclassical ion thermal conductivity is studied. A simple modification to the pure-ion case is obtained with the assumption that the single heavy impurity species is in the Pfirsch–Schlüter regime. It is found that the impurity contribution is larger than the usual approximation: Zeff times the pure ion thermal conductivity.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.865847
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  • 10
    Electronic Resource
    Electronic Resource
    Effects of neutral beam injection on poloidal rotation and energy transport in tokamaks (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 3012-3020 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The poloidal mass flow and ion energy transport driven by friction between thermal ions and fast ions resulting from neutral beam injection are investigated. Transport coefficients are derived which relate the poloidal mass flow and the radial ion heat flux to the fast ion friction and the radial temperature gradient. An Onsager symmetry condition shows that the coefficient of the fast ion friction in the radial heat flux is the negative of the coefficient of the temperature gradient in the poloidal mass flow. Numerical results are given for the banana regime. The driven poloidal mass flow can be significantly larger than the standard neoclassical poloidal rotation driven by the ion temperature gradient. The resulting viscous heating of the ions can be larger than the standard neoclassical ion heat conduction term in the ion energy equation. The driven ion energy flux contains a convective contribution, due to the diffusional mixing of the fast ions and thermal ions. This energy flux is inward (a heat pinch) in the case of coinjection, and can be significantly larger than the standard neoclassical energy flux.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.860687
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