Geodesic oscillations and the weakly coherent mode in the I-mode of ASDEX Upgrade

Abstract

Density fluctuations in I-mode discharges in ASDEX Upgrade are studied. The I-mode specific weakly coherent mode (WCM) appears at the transition from the L to I-mode. The WCM but also the turbulence in general are strongly modulated by a low frequency mode which can be related to the geodesic acoustic mode (GAM). The GAM induces an energy transfer away from the central WCM frequency, indicating an underlying instability responsible for the WCM. During the I-mode magnetic fluctuations close to the WCM frequency are intensified, which can be assigned to the geodesic Alfvénic oscillation. The geodesic Alfvénic oscillation is already present in the L-mode, and does not follow changes of frequency of the WCM, therefore it is not responsible for the WCM.