The interplay between large scale fluctuations and the local turbulent transport has been investigated in the tokamak ISTTOK [C. A. F. Varandas et al., Fusion Technol. 29, 105 (1996)] plasma boundary region. It has been found that the floating potential fluctuations, dominated by low frequency oscillations, exhibit a significant toroidal correlation at large distances that can be attributed to the geodesic acoustic mode. The level of long-distance correlations is strongly bursty, showing a significant degree of coupling with the local electrostatic turbulent transport. Experimental findings show the key role of multiscale physics in the regulation of transport in the edge region of fusion plasmas.

ISTTOK is equipped with two probe systems that allow the simultaneous measurement of the three-dimensional characteristics of the edge fluctuations with high temporal resolution. Electrostatic fluctuations consistent with the geodesic acoustic mode (GAM) are observed in the edge plasma. The radial, poloidal and toroidal structure of the fluctuations are investigated and good agreement with the GAM theoretical predictions is found. Furthermore, experimental evidence is presented suggesting that the GAM is modulating long-range correlations and the ambient turbulent fluctuations.

The retarding field analyzer (RFA) is a widely used diagnostic tool for the ion temperature measurement in the scrape-off-layer (SOL) of the thermonuclear plasma devices. However, the temporal resolution in the standard RFA application is restricted to the ms timescale. In this paper, a dc operation of the RFA is considered, which allows for the measurement of the plasma ion temperature fluctuations. The method is based on the relation for the RFA current-voltage (I-V) characteristic resulted from a common RFA model of shifted Maxwellian distribution of the analyzed ions, and the measurements of two points on the exponentially decaying region of the I-V characteristic with two differently dc biased RFA electrodes. The method has been tested and compared with conventional RFA measurements of the ion temperature in the tokamak ISTTOK SOL plasma. An ion temperature of T(i) = 17 eV is obtained near the limiter position. The agreement between the results of the two methods is within ∼25%. The amplitude of the ion temperature fluctuations is found to be around 5 eV at this location. The method has been validated by taking into account the effect of fluctuations in the plasma potential and the noise contamination, proving the reliability of the results obtained. Finally, constrains to the method application are discussed that include a negligible electron emission from the RFA grids and the restriction to operate in the exponentially decaying region of the I-V characteristic.

The interplay between mean and fluctuating E × B shear flows has been investigated in the ISTTOK edge plasma. The GAM shearing rate was found to be lower (by a factor of 2-3) than that of the mean flow but comparable to the turbulence decorrelation rate. A competition between GAMs and intermittent-like turbulent transport is observed leading to a dynamic view of plasma transport. External plasma biasing was found to modify the fluctuating shear flow. Negative bias induces a significant increase in the mean E × B shear flows, reducing turbulence and consequently suppressing the GAMs. On the other hand, an increase in the long-range correlation is observed for positive bias associated with a modest increase in the mean shear flow that might be interpreted as an enhanced energy transfer from the turbulence into large-scale, zonal flow-like structures.

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