Poloidal velocity shear and decorrelation of the edge fluctuations in tokamak

Abstract

Summary form only given, as follows. Summary form only given. The effect of the sheared E/spl times/B poloidal plasma rotation on the edge turbulence has received much attention because of its potential impact on the high confinement regimes in magnetic fusion research. Such a sheared flow is theoretically predicted to lead to reduced fluctuation levels and transport. A Langmuir probe array has been used to study the fluctuations of potential, density and temperature in the edge of the SINP tokamak in which a naturally occuring E/spl times/B velocity shear layer exists near the limiter. Both the poloidal and radial correlation lengths have been found to drop around the shear layer position, indicating a decorrelation of the fluctuations as predicted by existing theories. The auto-correlation times of the potential and density fluctuations are observed to decrease at this position, as reported by other workers earlier. We have investigated the effect of temperature fluctuations and find that the temperature fluctuations also exhibit the same effect The coherences between all the different pairs of density, potential and temperature fluctuations drop near the shear layer. The power spectral density of the potential fluctuations indicate a broadening in both k and /spl omega/. The phase between the density and potential fluctuations is modified by the shear. The drop of particle flux at this position is thus attributed to not only reduced fluctuation amplitudes or reduced coherences but also to the modification of the phase as expected from theory. The reduced particle and heat transports due to the above factors manifest in the steepening of density and temperature profiles with time as has been observed experimentally. The high confinement mode in tokamaks shows similar characteristics in exaggerated form and could thus be related to the same mechanism.