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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.

A fast visible imaging system is installed on the spherical tokamak QUEST to study edge turbulence. The camera uses a complementary metal-oxide semiconductor detector with a maximum resolution of 1024 × 1024 at 7000 frames∕s (fps) and can achieve 775 kfps at a resolution of 128 × 24. In this paper, we present the salient features of the system and its application to study edge turbulence in 8.2 GHz ECRH driven slab plasma, without plasma current. Vertical magnetic field (B(z)) topology is varied with three sets of poloidal field (PF) coils and the variation in the edge turbulence is investigated as a function of the B(z) strength and curvature. Fluctuation amplitude was highest for the shallow PF well. Cross-correlation coefficient shows distinct coherent mode along z direction at the steep density gradient region and it grows with the PF mirror ratio.

A particle image velocimetry technique based on orthogonal dynamic programming is developed to measure the time resolved flow field of the fluctuating structures at the plasma edge and scrape off layer. This nonintrusive technique can provide two dimensional velocity fields at high spatial and temporal resolution from a fast framing image sequence and hence can provide better insights in plasma flow as compared to conventional probe measurements. Applicability of the technique is tested with simulated image pairs. Finally, it is applied to tangential fast visible images of QUEST as a test case to estimate the scrape off layer flow in Ohmic and ECRH driven plasma discharges.

It is well known that the ultra grain refinement can be achieved by sever cold rolling, followed by reversion treatment in metastable austenitic stainless steel plate. In this study, the cold rolling was replaced by cold drawing. This procedure was applied to a metastable austenitic steel (Fe-16Cr-10Ni alloy) thin wire, and then the microstructure development during cold drawing and annealing was investigated. The austenite phase transformed to martensite during the drawing. Vickers hardness of the wire markedly increased with increasing the drawing strain. When the drawing strain reached about 4.5, the wire exhibited martensite single structure and had high hardness of Hv4.4GPa. Annealing of the heavily drawn wire at around 900K for 0.6ks leads to the formation of reversed austenite with the diffusional reversion mechanism. As a result, ultra fine-grained austenitic single structure with the grain size of about 0.6μm was obtained. It was also found that the wire has an excellent combination of a strength and ductility.