Experimental observations of modes with geodesic acoustic character from the core to the edge in the TCV tokamak

Abstract : The geodesic acoustic mode is a coherently oscillating mode, related to the zonal flows that can regulate turbulence in magnetized toroidal plasmas. Modes possessing geodesic acoustic character have been widely observed in the TCV tokamak. A transition has been observed in the course of a single discharge from a continuum regime to a radially extended single-frequency regime. The mode has been also observed and characterized for the first time in the scrape-off layer, primarily by Langmuir probes, suggesting a particle flow to the edge modulated at the mode frequency. These experimental observations are consistent with nonlinear global gyrokinetic simulations, reported in a companion paper (Merlo et al 2017 Plasma Phys. Control. Fusion). These also suggest a possible coupling with radial avalanche phenomena.