Investigation of geodesic acoustic modes and related zonal flows at TEXTOR

This paper attempts to give an overview on the study of the geodesic acoustic mode (GAM) and the related velocity oscillation of the zonal flow (ZF) at TEXTOR. The reported results are obtained from poloidal cross correlation reflectometry. The advantages of the diagnostic combined with a multi-antenna set-up, as it is installed at TEXTOR, are shortly reviewed and compared with other diagnostics used in experiments on GAMs. Furthermore, the GAM properties in ohmic discharges are reported as frequency scaling with local temperature, poloidal structure and radial distribution. GAM induced density fluctuations and related velocity oscillations are measured for a large radial range. In the context of the latter the collisionality on the GAMs at the plasma edge is investigated. The measurement of the GAM related velocity fluctuations shows that the ZF occurs in two rings at least, separated by a rational surface. The velocity oscillation allows us to estimate the effect of GAMs on the shearing rate. The influence on the shearing rate is quantitatively investigated. The GAM induced shearing rate is not able to overcome the decorrelation rate of the ambient turbulence (AT). Also the relation between AT and GAMs is analyzed. Therefore the AT is characterized and the turbulence driving the GAMs is discussed. A relation between the density scale length and the pronounced occurrence of GAMs is obtained. This is related to a narrow region of k (perpendicular to) rho(s) values, which is characteristic for drift ballooning modes. The spectrum of the AT is analyzed in both ZF rings. A significant change in the shape of the low frequency turbulence is obtained. This may be connected with a transition from drift ballooning modes to ITG modes. The latter seems to drive the GAMs less effectively.