Modulation behaviour and possible existence criterion of geodesic acoustic modes in tokamak devices

Geodesic acoustic modes (GAMs) represent the oscillating counterpart of zonal flow in tokamak plasma and can affect transport due to their interaction with turbulence eddies. GAMs have been observed in many experiments and modelled under different conditions, but because of their variety of characteristics, we do not yet have a complete picture of their dynamics. It has been demonstrated that optical methods can be efficiently used to describe and predict several characteristics of the GAM radial structures that can be interpreted as 'waves' propagating in the space-time. We exploit complex eikonal theories to investigate the behavior of GAMs that are commonly observed in experiments, and find that their periodic modulation and intermittency can be explained by the properties of the equilibrium temperature profile. Theoretical results obtained in this work are supported by gyrokinetic simulations for several equilibria. Implications for existence criteria and GAM dynamics in different plasma equilibrium conditions are discussed, with particular attention to the edge plasma in low and high confinement modes.