Linear gyrokinetic investigation of the geodesic acoustic modes in realistic tokamak configurations
Résumé
In order to provide scaling formulae for the geodesic acoustic mode (GAM) frequency and damping rate, GAMs are studied by means of the gyrokinetic global particle-in-cell code ORB5. Linear electromagnetic simulations in the low-βe limit have been performed in order to separate acoustic and Alfvnic time scales and obtain more accurate measurements. The dependence of the frequency and damping rate on several parameters such as the safety factor, the GAM radial wavenumber, and the plasma elongation is studied. All simulations have been performed with kinetic electrons with a realistic electron/ion mass ratio. Interpolating formulae for the GAM frequency and damping rate, based on the results of the gyrokinetic simulations, have been derived. Using these expressions, the influence of the temperature gradient on the damping rate is also investigated. Finally, the results are applied to the study of a real discharge of the ASDEX Upgrade tokamak.