Zonal flow in a tokamak pedestal

Kagan, Grigory; Catto, Peter J.

doi:10.1063/1.3077272

Cited by 18 publications

(60 citation statements)

References 19 publications

(24 reference statements)

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“…Thus, the background ion temperature profile must have a pedestal with a scale much larger than that of any density pedestal varying on a ion poloidal gyroradius scale. In addition, weak ion temperature variation with subsonic flow in such a pedestal requires electrostatically restrained ions and magnetically confined electrons thereby impacting zonal flow behavior and altering the neoclassical polarization [6].…”

Section: Discussionmentioning

confidence: 99%

“…For example, in the banana regime the axisymmetric radial electric field of ion poloidal gyroradius pedestal must approximately satisfy edΦ 0 /dψ = −(T i /n)dn /dψ. Then the residual associated with the small amplitude, shorter wavelength, axisymmetric zonal flow potential δΦ will differ substantially from that of reference [15] as shown in reference [6] and discussed in the following paragraphs.…”

Section: Gyrokinetics In the Pedestal And Internal Barriersmentioning

confidence: 99%

“…A detailed evaluation of zonal flow residual is presented in [6]. The two main qualitative features are as follows.…”

Section: Gyrokinetics In the Pedestal And Internal Barriersmentioning

confidence: 99%

“…However, in the pedestal or an internal barrier (or on axis), drift departures from flux surfaces can become comparable to the local scale length ( ρ pi ∇nn~1 with n the plasma density) and the entropy production argument has to be modified to account for the loss of locality due to finite poloidal ion gyroradius effects requiring an equilibrium to be established over the entire pedestal (or barrier). Using the new gyrokinetic variables, we find that entropy production must vanish in the pedestal [6]:…”

Section: Gyrokinetics In the Pedestal And Internal Barriersmentioning

confidence: 99%

“…Weak ion temperature variation with subsonic pedestal flow requires electrostatically restrained ions and magnetically confined electrons. These features result in finite orbit modifications to the zonal flow residual [6].…”

Section: Introductionmentioning

confidence: 99%

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Limitations, insights and improvements to gyrokinetics

et al. 2009

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Section: Discussionmentioning

confidence: 99%

Section: Gyrokinetics In the Pedestal And Internal Barriersmentioning

confidence: 99%

“…A detailed evaluation of zonal flow residual is presented in [6]. The two main qualitative features are as follows.…”

Section: Gyrokinetics In the Pedestal And Internal Barriersmentioning

confidence: 99%

Section: Gyrokinetics In the Pedestal And Internal Barriersmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Limitations, insights and improvements to gyrokinetics

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Kinetic Effects in the Pedestal of a Tokamak

Catto

Kagan

2010

Contrib. Plasma Phys.

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Kinetic effects in the pedestal of a tokamakpedestal, transport, residual, flow 52.25. Fi, 52.25.Dg, 52.55.Fa We consider the effects of finite ExB on the residual and neoclassical ion heat transport and ion flow in the pedestal. For a tokamak pedestal with a density scale of the poloidal ion gyroradius the finite ExB drift modfies the ion orbits, moving the trapped-passing boundary towards the tail of the ion distribution function. IntroductionIn many tokamak plasmas the width of the pedestal is observed to be on the order of a poloidal ion gyroradius.Consequently, finite drift orbit effects take on a special significance. We employ a special version of gyrokinetics that replaces the standard radial gyrokinetic variable by canonical angular momentum [1] to investigate kinetic phenomena in an axisymmetric tokamak pedestal in the banana regime. An entropy production argument for a pedestal in which the density drops substantially on the scale of the ion poloidal gyroradius, finds that in the banana regime the ion temperature is not allowed to vary substantially on such a short scale if the poloidal magnetic field is small compared to the toroidal field [1]. When the toroidal ion flow in the pedestal is subsonic this requires the ExB and ion diamagnetic flows to cancel to lowest order, making the flow of the electrons sonic there. These results mean that in a subsonic banana regime pedestal of poloidal ion gyroradius width, the ions are electrostatically confined and the electrons are confined magnetically [1]. Using this observation we then calculate the collisionless zonal flow residual in the pedestal to demonstrate that it is enhanced over its core value [2,3]. We find that in the pedestal, finite orbit effects due to ExB result in a lower level of anomalous transport.Unlike the core, there is a finite ExB reduction in the trapped particle population. Our result allows us to suggest a model for the pedestal formation. We next proceed by retaining collisions in the pedestal to find that finite ExB drift orbit effects reduce the neoclassical banana regime ion heat conductivity below its core value, and substantially modify the poloidal ion flow. Both the thermal diffusivity reduction and the enhancement in the residual are caused by the trapped ion region being moved towards the tail of the ion distribution function once the ExB drift begins to compete with the poloidal projection of parallel streaming. These calculations ignore ExB shear in the pedestal for simplicity, but work in progress will address this more realistic situation. Pedestal backgroundIn the alternate gyrokinetic technique that has been developed to analyze the tokamak pedestal canonical angularis taken as the gyrokinetic radial variable [1] rather than the radial guiding center location Ψ ≡ ψtreatments. This approach allows strong radial plasma gradients to be treated when considering zonal flow, neoclassical effects, and the turbulent behavior. The nonlinear gyrokinetic equation retains large poloidaldrift and orbit squeezing effects on zo...

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Neoclassical Theory of Pedestal Flows and Comparison with Alcator C‐Mod Measurements

Kagan

Marr

Pusztai

et al. 2012

Contrib. Plasma Phys.

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Neoclassical implications of the strong radial electric field, inherently present in an H-mode tokamak pedestal, are considered. The main ion poloidal flow in the pedestal is predicted to be reduced in magnitude, or even reversed, compared with its core counterpart. The resulting change in the neoclassical formula for the impurity flow is shown to result in improved agreement with boron measurements in the Alcator C-Mod pedestal. In addition, due to the ion flow being modified, the bootstrap current is expected to be enhanced in the pedestal over conventional predictions.

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Zonal flow in a tokamak pedestal

Cited by 18 publications

References 19 publications

Limitations, insights and improvements to gyrokinetics

Limitations, insights and improvements to gyrokinetics

Kinetic Effects in the Pedestal of a Tokamak

Neoclassical Theory of Pedestal Flows and Comparison with Alcator C‐Mod Measurements

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