Radial homogeneity of geodesic acoustic modes in ohmic discharges with low $B$ in the T-10 tokamak

The electric potential oscillations in the hot plasma zone have been measured directly using a heavy ion beam probe at the frequencies of geodesic acoustic modes in the T-10 tokamak (the major and minor radii are $R = 1.5$ m and $a = 0.3$ m, respectively, and the toroidal magnetic field is $B = 1.5–2.5$ T). In discharges with the lowered magnetic field B = 1.55 T, the diagnostic beam can probe a rather wide radial plasma region ($0.06 < r < 0.28$ m). This made it possible to study the radial structure of geodesic acoustic modes. It has been shown that the frequency and amplitude of geodesic acoustic modes in the region under study are constant over the radius in the whole region of observation. Thus, it has been shown experimentally that the observed frequency of geodesic acoustic modes may not correspond to the predictions of the local theory $(f\sim\sqrt{T_ e})$ in a wide radial range comparable with the plasma minor radius. The numerical simulation of the turbulence dynamics using the solution of Braginskii magnetohydrodynamic equations for a peripheral plasma has showed the formation of geodesic acoustic modes in the observed frequency range owing to the nonlinear interaction between broadband turbulence modes.