Scaling of the geodesic acoustic mode amplitude on JET

This work aims at establishing the parameter space for the existence of geodesic acoustic modes (GAMs) on JET as well as investigating their driving and damping mechanisms predicted by different theoretical models. This was achieved using an experimental dataset of GAM measurements based on reflectometry with variations mainly on plasma current and line-averaged density. We present clear experimental evidence for the different mechanisms determining the GAM amplitude: turbulence drive, collisional and collisionless damping. Collisional damping is predicted to be dominant in the edge plasma across the explored JET parameter range contrary to our observations revealing that it is only effective at low plasma current, high density. Although the observed GAM suppression at high plasma current is in good agreement with the collisionless models, the estimated damping rates appear to be too small to explain our measurements.