Investigation of geodesic acoustic mode oscillations in the T-10 tokamak

Geodesic acoustic modes (GAMs) were investigated on the T-10 tokamak using heavy ion beam probe, correlation reflectometry and multipin Langmuir probe diagnostics. Regimes with Ohmic heating and with on- and off-axis ECRH were studied. It was shown that GAMs are mainly the potential oscillations. Typically, the power spectrum of the oscillations has the form of a solitary quasi-monochromatic peak with the contrast range 3-5. They are the manifestation of the torsional plasma oscillations with poloidal wavenumber m = 0, called zonal flows. The frequency of GAMs changes in the region of observation and decreases towards the plasma edge. After ECRH switch-on, the frequency increases, correlating with growth in the electron temperature T-e. The frequency of the GAMs depends on the local T-e as f(GAM) similar to c(s)/R similar to T-e(1/2), which is consistent with a theoretical scaling for GAM, where c(s) is the sound speed within a factor of unity. The GAMs on T-10 are found to have density limit, some magnetic components and an intermittent character. They tend to be more excited near low-q magnetic surfaces.