Geodesic acoustic mode frequencies in experimental tokamak equilibria

The geodesic acoustic mode (GAM) frequency is calculated for experimental equilibrium data from ASDEX Upgrade and NSTX discharges using an eigenvalue solver. For this purpose, a special algorithm is applied to generate precise, smooth flux-surfaces suitable for calculation of local equilibria from the coarse grained output of the equilibrium solver CLISTE. Since realistic magnetic geometries allow for more than one mode with the properties of a GAM, two-fluid turbulence computations are discussed to clarify the influence of the turbulence on the linear mode spectrum. Using equilibrium data from ASDEX Upgrade this approach is shown to be superior to the use of approximated flux-surface shapes and leads to better agreement with the experimental results.