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 (B z ) and toroidal (B t) fields are investigated as a function of B z /B t (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 (R im) and the source-free (essentially vacuum on the low-field side) regions, respectively. Parabolic relation (k = As 2 + C) is observed between s and k on the entire R–Z plane for a wide range of B z /B t. Progressive shift from the Gaussian statistics beyond R im is observed with increasing B z /B t. From the moments of the probability density functions, possible location for blob generation is inferred to be R im. 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.

Dynamical programming based turbulence velocimetry for fast visible imaging of tokamak plasma

“…However, it has been shown that, at similar n e and T e in TORPEX as compared to our experiment, the mean value of the light emission signal recorded with a tangential fast camera depends linearly on n e at varying neutral hydrogen density. 6,16 Hence, in our case too, it is reasonable to interpret that the intensity fluctuations resembles the density fluctuations to an appreciable extent.…”

“…With the advent of density fluctuation diagnostics with high spatial and temporal resolutions, quantitative analysis of such velocities and derived quantities like Reynolds stress and zonal flows becomes feasible. Turbulence imaging diagnostics have been realized in several forms like the beam emission spectroscopy (BES), 2 fast visible imaging, [3][4][5][6] and microwave reflectometer imaging array. 7,8 Particle Image Velocimetry (PIV) has proven to be an important tool for quantitative estimation of two-dimensional flow structures in experimental fluid mechanics.…”

Invited Review Article: Gas puff imaging diagnostics of edge plasma turbulence in magnetic fusion devices

“…27 Tangential passive imaging of visible light (mainly Hα) was also done in the Q-shu University Experiment with Steady State Spherical Tokamak (QUEST) device in a simple torus configuration, showing blob formation and propagation similar to that in a tokamak SOL. 28 High speed camera imaging of neutral hydrogen recycling near a limiter was done in the TJ-II stellarator, and the 2-D motion of plasma blobs was observed within the shear layer. 29,30 High speed imaging was also done in the TJ-K stellarator using tangential viewing without filters, but with a small depth of field to minimize the effects due to integration along the line of sight.…”

Turbulence Velocimetry of Tangential Fast Imaging Data on QUEST

“…A quantitative evaluation of this effect is carried out under the framework of the TITR code [5,11]. It is seen that the effect is negligible in the edge and SOL of the plasma as compared to the plasma core as the density and hence the intensity is appreciably smaller in that region.…”

“…becomes feasible. Turbulence imaging diagnostics have been realized in several forms like the beam emission spectroscopy (BES) [1], fast visible imaging [2][3][4][5] and microwave reflectometer imaging array [6,7].…”