The fully nonlinear (full-$f$) four-dimensional TEMPEST gyrokinetic continuum code correctly produces the frequency and collisionless damping of geodesic-acoustic modes (GAMs) and zonal flow, with fully nonlinear Boltzmann electrons for the inverse aspect ratio $ϵ$ scan and the tokamak safety factor $q$ scan in homogeneous plasmas. TEMPEST simulations show that the GAMs exist in the edge pedestal for steep density and temperature gradients in the form of outgoing waves. The enhanced GAM damping may explain experimental beam emission spectroscopy measurements on the edge $q$ scaling of the GAM amplitude.

• Received 30 May 2007

DOI:https://doi.org/10.1103/PhysRevLett.100.215001