Numerical and Analytical Interpretation of Rotation and Radial Electric Fields in Collision Dominated Edge Plasmas

Nicolai, A.; Rogister, A.; Daybelge, U.

doi:10.1002/1521-3986(200204)42:2/4<241::aid-ctpp241>3.0.co;2-c

Cited by 5 publications

(25 citation statements)

References 4 publications

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“…The theory is valid in collision dominated plasmas with steep gradients and was able to reproduce the toroidal spin-up in ALCATOR C-MOD [11,12,26].…”

Section: Introductionmentioning

confidence: 99%

Modelling of plasma rotation accounting for neutral beam injection and perturbation coils in JET and TEXTOR

Nicolai¹,

Daybelge²,

Yarim³

2004

Nucl. Fusion

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The impact of the helical perturbations, which can act as a momentum source or sink, on the rotation velocity is calculated on the basis of the ambipolarity constraint and the parallel momentum equation of the revisited neoclassical theory; this theory allows prediction of the parallel and poloidal flow speeds, v , and v , respectively, and therefore the radial electric field E r via the usual radial momentum balance equation. Source terms account for the momentum deposition by neutral beam injection, pressure anisotropization and the j × B force density, the latter two due to Fourier components of (rotating) helical fields. However, the neoclassical theory cannot account for the effect of the electrostatic turbulence on rotation in, e.g. TEXTOR L-modes. This is included by replacing the neoclassical viscosity η 2 by an anomalous one due to turbulence. The main results can be summarized as follows. Using in the case of JET the data of shot #59316 the maximum rotation speed can be reproduced with an accuracy of 10%. A similar result is obtained for the TEXTOR shot #91269. If the angular velocity of the (m = 2, n = 1) Fourier component of the helical field is at low slip frequencies ω p − ω f (ω p is the plasma rotation frequency and ω f the rotation frequency of the helical field) gradually reduced to zero, a localized minimum appears and the gradient of the toroidal velocity becomes around 4 × 10 5 s −1 (JET). However, if the slip frequency is larger than a critical value, the rotation profile of the rotation velocity is not influenced as observed at JET. Although it is possible to create a large velocity shear around the singular surface, this shear is nonetheless limited by the reduction of the central velocity. Therefore, it might not be possible to trigger an ITB by plasma braking at the singular surface.

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“…The theory is valid in collision dominated plasmas with steep gradients and was able to reproduce the toroidal spin-up in ALCATOR C-MOD [11,12,26].…”

Section: Introductionmentioning

confidence: 99%

Modelling of plasma rotation accounting for neutral beam injection and perturbation coils in JET and TEXTOR

Nicolai¹,

Daybelge²,

Yarim³

2004

Nucl. Fusion

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“…The friction with the recycled neutral gas due to charge exchange is, in limiter devices, a strong source of momentum loss, as pointed out in [4]. In the divertor case the recycling zone is shifted partly in front of the plate and the neutral influx is strongly reduced.…”

Section: Introductionmentioning

confidence: 92%

“…Here u θ is the averaged poloidal speed, η 2 is the perpendicular viscosity coefficient and M 1 is a metric quantity of the order unity accounting for elongated cross-sections [4,12]. We note that as in [12] the classical viscosity must be replaced by an anomalous one to reproduce TEXTOR-or JET-data.…”

Section: Ambipolarity Constraint and Parallel Momentum Balancementioning

confidence: 99%

“…As mentioned already, the revisited neoclassical theory accounts in particular for steep temperature and density gradients at high collisionality. Since it is a collision dominated theory, it is based on the fluid equations for particle, momentum and energy conservation [2][3][4][5][6][7]. Accounting for parallel, perpendicular and gyro-viscosity [8,9], we get from the ambipolarity constraint [12] 1 r…”

Section: Ambipolarity Constraint and Parallel Momentum Balancementioning

confidence: 99%

“…We note that as in [12] the classical viscosity must be replaced by an anomalous one to reproduce TEXTOR-or JET-data. The velocities Q and S are defined in [4,12] as well. The charge exchange reactions are characterized by the charge exchange frequency ν cx = σ v cx n 0 .…”

Section: Ambipolarity Constraint and Parallel Momentum Balancementioning

confidence: 99%

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Modelling of plasma rotation accounting for a poloidal divertor and helical perturbation coils

Nicolai¹,

Daybelge²,

Yarim³

2006

Nucl. Fusion

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The impact of helical perturbations on the rotation velocity and thus on the energy confinement is calculated on the basis of the ambipolarity constraint, the parallel momentum equation of the revisited neoclassical theory and a simplified temperature equation. The helical perturbations can act as means for ergodizing the magnetic field and/or as momentum source or sinks, whereas at the separatrix (effective radius r s) of the poloidal divertor a temperature pedestal may arise due to the strong shear flow reducing the transport to a neoclassical level. The neoclassical theory allows the prediction of the parallel and poloidal flow speeds and thus of the 'subneoclassical' heat conductivity χ sub used in the heat conduction equation. This heat conductivity allows us to compute the temperature pedestal and to reproduce the power balance in ALCATOR if one assumes that χ = χ sub in the radial sheath with the thickness of ≈ 0.7 cm, centred around the inflection radius r in , and χ = χ L for r < r in − /2. χ L is the normal L-mode heat conductivity. Source terms account for momentum deposition by neutral beam injection (NBI), by pressure anisotropization and the j × B force density, the latter two due to Fourier components of (rotating) helical fields. Source terms for the power deposition by NBI, Ohmic heating and radiation are also included. The main results can be summarized as follows: At a dynamic ergodic divertor in TEXTOR frequency of 10 kHz, a toroidal velocity gradient of 1.2 × 10 6 s −1 may be achieved which is enough to suppress the ion temperature gradient and thus to generate an ITB. The poloidal divertor suppresses the neutral gas influx and thus effects a (sub)neoclassical sheath with a temperature pedestal of T ped ≈ 400 eV and an increase of the central value by roughly the same amount. In the case of edge localized mode-control with an ergodic layer of ≈ 2.5 cm, generated by the helical coils, the height of the pedestal stays unaffected if in the pedestal region a transition from L-mode confinement to subneoclassical confinement is assumed.

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Rotation dynamics and stability of collisional edge layers in tokamak plasmas

2004

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An earlier neoclassical transport theory for a collisional edge layer with steep radial gradients in the toroidal damping timescale is extended, to include also the faster poloidal damping timescale. In order to determine the stability behaviour of poloidal and toroidal rotations in a tokamak plasma, a two-timescale analysis is applied. It is observed that the poloidal and toroidal spin-up tendencies are strongly coupled by nonlocal interactions, although these evolve essentially on different timescales. Effects of charge exchange interactions, neutral beam injection, and a radial polarization current on both rotation speeds are consistently included in the model.

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Numerical and Analytical Interpretation of Rotation and Radial Electric Fields in Collision Dominated Edge Plasmas

Cited by 5 publications

References 4 publications

Modelling of plasma rotation accounting for neutral beam injection and perturbation coils in JET and TEXTOR

Modelling of plasma rotation accounting for neutral beam injection and perturbation coils in JET and TEXTOR

Modelling of plasma rotation accounting for a poloidal divertor and helical perturbation coils

Rotation dynamics and stability of collisional edge layers in tokamak plasmas

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