Detailed Measurements of Poloidal Velocity of Density and Fl oating Potential Fluctuations in the Edge Plasma of the CASTOR Tokamak

The ion saturation current and floating potential are measured with high temporal (1 µs) and spatial (2.5 mm) resolutions within the edge plasma of CASTOR tokamak by two poloidally spaced radial arrays of Langmuir probes. The radial electric field and the phase velocity of plasma fluctuations are determined. It is found that radial profiles of both of these quantities are strongly sheared in the proximity of the last closed flux surface. In the proximity of the VSL, the shearing rate is higher than the characteristic frequency of plasma fluctuation and reduction of fluctuation level is observed.