Mean E × B flows and GAM-like oscillations in the H-1 heliac

Experimental results on the connection between mean E × B flows and coherent oscillations at the frequency of the geodesic acoustic mode (GAM) in the H-1 heliac are presented. An increase in the mean local radial electric field, Er, is correlated with the development of several coherent modes. As mean Er increases, spectral energy, which is mostly contained in coherent modes, grows. This is followed by the onset of the m = 0, n = 0 finite frequency GAM-like mode. Analysis of the heliac magnetic structure shows that geodesic curvature is considerably stronger in H-1 than in tokamaks. A possible role of geodesic oscillations in the transfer of spectral energy from mean zonal flows into coherent modes leading to the generation of the GAM-like mode is discussed. In the proposed scenario of the L–H transition in H-1 the inverse energy cascade leads to the accumulation of turbulence energy in the mean zonal-flow like structure, until geodesic effects lead to the generation of coherent modes and GAM. The coherent modes' parallel phase velocities are very close to the ion thermal velocity suggesting the possibility of their strong Landau damping. It is suggested that the shear decorrelation mechanism eventually forbids the energy transfer from Er to these modes which reinforces spectral condensation and leads to L–H transition.