This review assesses the status of these developments and the progress made towards a unified understanding of the GAM behaviour and its role in plasma confinement. In recent years there have been major advances in all areas of GAM research: measurements, theory, and numerical simulations.

GAMs can also be driven by energetic particles (so-called EGAMs) or even pumped by a variety of other mechanisms, both internal and external to the plasma, opening-up possibilities for plasma diagnosis and turbulence control. GAMs impact the plasma confinement via velocity shearing of turbulent eddies, modulation of transport, and by providing additional routes for energy dissipation. GAMs are supported by the plasma compressibility due to magnetic geodesic curvature-an intrinsic feature of any toroidal confinement device. They are recognized as the non-stationary branch of the turbulence driven zonal flows which play a critical regulatory role in cross-field turbulent transport. Geodesic acoustic modes (GAMs) are ubiquitous oscillatory flow phenomena observed in toroidal magnetic confinement fusion plasmas, such as tokamaks and stellarators.

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