Geodesic Acoustic Modes (GAM) are M=0, N=0 axisymmetric toroidal modes combined with M=±l, ±2 poloidal side-bands and driven by the electron and anisotropic ion pressure perturbations with the frequency ω~2_G ≈ (7T_i/2 + 2T_e)/R~2_0m_i . These modes may be important for plasma transport as it has been theoretically discussed ~([2]) and observed in experiments ~([4-7]). Furthermore, they may be useful as a diagnostic tool to indicate L-H confinement transition in tokamaks ~([5]). Eigenmodes in the geodesic frequency range have been experimentally detected under a wide range of conditions in various tokamaks. Typically, GAM oscillations are observed during counter NB injection ~([7,8]), but they may also appear in ohmic discharges ~([6,8]). The nature of the experimentally observed geodesic eigenmodes is still not clear and several different mechanisms might be responsible for their dispersion and manifest themselves in different conditions. Recently, it has been theoretically shown that GAMs may be driven by an electric current with velocity along the equilibrium magnetic field in combination with the diamagnetic drift ~([3]). The instability occurs only when the electron current velocity is higher than the GAM phase velocity of sidebands, V_(e0)>Rqω.
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