[Table of Contents]

Plasma and Fusion Research

Volume 5, S2005 (2010)

Regular Articles

Interaction of Mean and Oscillating Plasma Flows Across Confinement Mode Transitions
Garrard D. CONWAY, Emanuele POLI, Tim HAPPEL1) and the ASDEX Upgrade Team
MPI Plasmaphysik, EURATOM-Association IPP, D-85748, Garching, Germany
1)
Laboratorio Nacional de Fusión, EURATOM-Association CIEMAT, 28040, Madrid, Spain
(Received 4 December 2009 / Accepted 12 February 2010 / Published 10 December 2010)

Abstract

The interaction of mean plasma flows (driven by equilibrium E×B forces) and oscillating plasma flows - such as zonal flows (ZFs) and geodesic acoustic modes (GAMs) - (radially localized E × B plasma flows generated by non-linear turbulence interactions) is investigated in the ASDEX Upgrade tokamak using Doppler reflectometry. ZFs and GAMs play an important role in magnetic confinement devices by enhancing the velocity shearing of turbulent eddies which reduces turbulent cross-field transport. Although the GAM is universally observed across the plasma edge gradient region of ohmic and additionally heated low confinement L-mode regimes, it is not seen in the high confinement H-mode. Using slow power ramping of the additional ECRH and NBI heating systems the behaviour of the edge GAM, and its interaction with the mean flow shear, i.e. the radial electric field Er profile, across the L to H-mode transition are studied. As the edge Er well deepens with improving confinement, the associated negative Er shear region narrows and the radial extent of the GAM shrinks. The main feature, however, is the movement of spectral power out of the coherent GAM into large amplitude, low frequency random flow perturbations. It is speculated that these flow perturbations may play a role in the triggering of the confinement transition.

Keywords

turbulence, GAM, H-mode transition, Doppler reflectometry

DOI: 10.1585/pfr.5.S2005

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This paper may be cited as follows:

Garrard D. CONWAY, Emanuele POLI, Tim HAPPEL and the ASDEX Upgrade Team, Plasma Fusion Res. 5, S2005 (2010).