The amplitude behaviour of the geodesic acoustic mode (GAM), a low frequency (5–25 kHz) coherent plasma turbulence-generated Er × B zonal flow oscillation, is studied in the ASDEX Upgrade tokamak using Doppler reflectometry. Across the tokamak edge the GAM displays one or more maxima a few cm wide, coinciding with radial plateaus in the GAM frequency, suggesting nested zonal flow layers. The GAM peak amplitude ranges between 0.2 and 0.8 km s−1, corresponding to several tens of per cent of the mean E × B flow velocity. In limiter configurations, the GAM amplitude is found to decrease inversely with the plasma vertical elongation 1.1 < κ < 1.4, but rises again in X-point divertor configurations at higher κ ∼ 1.6. At low κ the GAM amplitude increases with the safety factor q, consistent with damping effects, but becomes less sensitive to q at high κ. A positive linear dependence on the electron temperature gradient, an indicator of the linear turbulence drive, is also observed.
Observation of geodesic acoustic modes (GAMs) and their radial propagation at the edge of the TEXTOR tokamak