GENE simulations of GAM-turbulence interactions in FT-2 tokamak

In an experiment conducted in the FT-2 tokamak, intermittencies in the GAM (Geodesic Acoustic Mode) and turbulence intensities were observed over a 13 ms time interval. Notably, the changes in the density and temperature profiles were insignificant. Two reflectometers recorded a 21 % suppression in turbulence fluctuations in the radial region x/a = 0.4 to 0.8 during the GAM period. To gain a deeper understanding of the mechanisms behind the energy transfer between GAMs and turbulence, several local linear simulations and one global linear and nonlinear simulation were performed using the gyrokinetic code GENE. The global simulations covered the radial range x/a from 0.2 to 0.9. Linear simulations were conducted to identify the dominant instability and its growth rate at various radial locations between x/a = 0.3 and 0.8. The nonlinear simulation results were analyzed to elucidate the interplay between GAMs and turbulence within the FT-2 tokamak. The global nonlinear simulation revealed intermittencies in the GAM activity around the rational surface at x/a = 0.54 with a safety factor q = 3. The nonlinear coupling between GAMs and turbulence was investigated by calculating the cross-bicoherence and Reynolds force at x/a = 0.54. The temporal behavior of these quantities was compared with the observed intermittency.