Statistical features of coherent structures at increasing magnetic field pitch investigated using fast imaging in QUEST

Abstract

Fluctuations in slab plasma produced by electron cyclotron waves in an open magnetic configuration with vertical (Bz) and toroidal (Bt) fields are investigated as a function of Bz/Bt (field pitch) using a high speed visible camera in the spherical tokamak QUEST. Higher order moments (skewness s and kurtosis k) of spatio-temporal fluctuations are investigated. Background fluctuations, generation of coherent convective structures (blobs) and their propagation are analysed at the intensity gradient (Rim) and the source-free (essentially vacuum on the low-field side) regions, respectively. Parabolic relation (k = As2 + C) is observed between s and k on the entire R–Z plane for a wide range of Bz/Bt. Progressive shift from the Gaussian statistics beyond Rim is observed with increasing Bz/Bt. From the moments of the probability density functions, possible location for blob generation is inferred to be Rim. Direct correspondence is observed with the prominence of the blobs and increase in stochastic forces with the increase in field pitch. Distribution of waiting time suggests that the blob generation may be a Poisson process. Accelerated cross field transport is observed for reasonably large-sized blobs.