Experimental Study of Zonal Flow, Geodesic Acoustic Mode and Turbulence Regulation in Edge Plasmas of the HL-2A Tokamak

The coexistence of intensive low frequency zonal flows (LFZFs) and geodesic acoustic modes (GAMs) has been observed by identifying the spatial characteristics, i.e. the symmetries in poloidal and toroidal directions, and the finite radial wave number, using three-dimensional Langmuir probe arrays in the edge plasmas of HL-2A tokamak. The analysis of three wave coupling with ambient turbulence suggests that the zonal flows are possibly created by three wave coupling mechanism. The radial distribution of LFZF and GAM power clearly shows the transition from GAM dominant to GAM and low frequency zonal flow coexisting structures, moving from LCFS inwards, in ECRH and Ohmic plasmas. The intensity of LFZF and GAM increases with ECRH power. The energy partition among LFZF, GAM, and turbulence indicates that the GAM dominantly regulates turbulence near the LCFS, while both GAM and LFZF modulate turbulence in the inner region of the plasmas. The envelope modulation analyses show that the turbulence envelopes are well correlated and out of phase with the LFZFs and GAMs.