A new experimental technique is presented for the study of zonal flows in tokamak plasmas -Upper Hybrid Resonance Doppler Backscattering. The feasibility of the diagnostics is demonstrated allowing a complete characterisation of the temporal and spatial behaviour of geodesic acoustic modes. The experimental results are obtained at the FT-2 tokamak.

Outline • Introduction. ETG mode, UHR back scattering, enhanced Doppler effect, correlative technique.• Measurements results.Two components of frequency and wave number spectra. • Modeling results.Reconstruction of turbulence wave number spectra and phase velocities.• Conclusions.

Direct measurements of micro-, meso-, and macroscale transport phenomena in the FT-2 tokamak are shown to be quantitatively reproduced by global full f nonlinear gyrokinetic simulation predictions. A detailed agreement with mean equilibrium E×B flows, oscillating fine-scale zonal flows, and turbulence spectra observed by a set of sophisticated microwave backscattering techniques as well as a good fit of the thermal diffusivity data are demonstrated. A clear influence of the impurity ions on the fluctuating radial electric field is observed.

The isotope effect in tokamak anomalous transport of energy and particles is a longstanding puzzle for physicists. It was first reported almost 30 years ago and since that time observed in many machines. Already in the TFTR [1] it was shown that coming from hydrogen to deuterium and then to the mixture of deuterium and tritium experimentalists improved the energy transport, which goes down for the heavy isotopes. This effect is promising for fusion applications, but the reason why it happens is still unclear. The typical orbit's widths for circulation of particles in a tokamak go in the opposite direction: the Larmor radius and the banana orbit are larger for heavy isotopes. In terms of turbulent transport, the typical width of the drift-wave turbulent eddy scales like an ion Larmor radius, and therefore for heavier isotopes larger eddies are predicted.Based on these arguments one could expect growing transport with increasing isotope mass, nevertheless, in numerous experiments an opposite direction of effect was observed [2,3]. In addition, it was shown that the isotope effect is much stronger in tokamaks compared to stellarators.Recently it was proposed that the ion mass dependence of the multi-scale turbulence component with long-range correlation could be responsible for the isotope effect in tokamak anomalous transport [4]. Geodesic acoustic mode (GAM), the finite frequency zonal flow, as a global mode, possesses long correlation length in poloidal and toroidal directions and is often reported to be more intensive in deuterium compared to hydrogen discharges [4][5][6][7]. Moreover it is routinely observed in tokamaks, but only rarely reported in stellarators due to its strong Landau damping there. The latter makes the GAM a possible element in explanation of the transport isotope effect.