Formation of the radial electric field profile in WEST tokamak
Résumé
The shear of the radial electric field at the edge is widely accepted to be responsible for turbulence reduction
in edge transport barriers and thought as a key ingredient of the improved confinement of H-mode plasmas.
Nevertheless, a full understanding of how its profile builds up at the edge is still lacking. It can either be
formulated as the result of a competition between several mechanisms that generate and damp flows or as the
result of a non-ambipolar particle flux, which enhances radial charge separation imposing a radial electric field
profile. Among possible mechanisms, one can think of turbulence generated flow via Reynolds stress, ion orbit
losses, toroidal magnetic ripple and the effect of neutral friction at the edge. Based on previous results obtained
on Tore Supra, the radial profile of the perpendicular flow expected in WEST can be separated in three spatial
areas. Inside ρ = 0.8, the radial electric field is dominated by losses of thermal ions in the magnetic ripple
while between 0.7 < ρ < 0.95, a competition between this latter and the generation of large scale flows by
turbulence appears as a possible explanation of the measured poloidal asymmetry of the mean perpendicular
velocity. In addition, edge conditions such as contact points and parallel dynamics in the scrape-off-layer
(SOL) influence the edge profiles beyond ρ = 0.9 (3). This contribution presents unexpected differences in the
radial electric field profile observed in WEST between Lower Single Null (LSN) and Upper Single Null (USN)
configurations and a study of the competition between specific mechanisms playing a role in the formation
of this profile.
Domaines
Physique [physics]Origine | Fichiers produits par l'(les) auteur(s) |
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