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.

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.

The complex interaction between large-scale mean EB flows, meso-scale zonal flows and fine-scale micro-turbulence excited due to specific profiles of plasma parameters and leading to anomalous transport is an important area of experimental and theoretical research in magnetically confined plasmas. In this paper the global gyrokinetic full particle distribution particle-in-cell simulations performed for Ohmic discharge of small research FT-2 limiter tokamak are for the first time validated against experimental data obtained both with a set of standard tokamak diagnostics and by sophisticated microwave backscattering techniques characterizing the tokamak turbulent dynamics and transport phenomena at micro, macro and intermediate scales. As a result of the simulation carried out with the ELMFIRE code an overall reasonable agreement between the numerical expectations and the experimental estimations of the electron and ion effective heat conductivity profiles obtained from the primary experimental profiles is achieved. The agreement obtained at the macro-level was strengthen at the microlevel and for intermediate turbulent scales using the Doppler reflectometry (DR), radial correlation reflectometry (RCR) and Doppler Enhanced Scattering (ES) microwave diagnostics. The turbulence dynamics at the micro-scale measured by DR at FT-2 were compared to the ELMFIRE predictions using synthetic DR diagnostics. As a result, not only the experimental DR spectra frequency shift, but its width and even the shape of the spectra was well reproduced by synthetic diagnostic indicating comparable rotation and spreading of the selected turbulent density fluctuations. The turbulence radial correlation length dependence on its frequency provided by the modeling appears to be in reasonable agreement to that given by RCR. The fluctuation poloidal rotation velocity profile obtained by the DR is very close to that obtained from the synthesized DR spectrum and also close to that given by ES and to the value produced by ELMFIRE for plasma EB drift velocity. The large spectral width of the DR and ES spectra is explained by fast and strong variation of radial electric field observed in the modeling. Giant oscillations of the field at a frequency of approximately 30-50 kHz much smaller than the typical drift wave frequency, but much larger than the inverse energy confinement time, observed by DR and ES as well, are attributed to the geodesic acoustic mode (GAM). This attribution is supported by comparison of the oscillation frequency dependence on the radial position with analytical prediction for the GAM frequency accounting for the impurity contribution. The dispersion of radial electric field estimated from the ES is found to be comparable to the huge E r dispersion provided by the gyrokinetic modeling. Moreover the GAM correlation length and phase velocity determined using ES technique appears to be close to those obtained by the ELMFIRE. The interrelations of GAM and drift wave turbulence in experiment and gyrokinetic modeling are compared....