Search

This paper reports the impact of different confinement regimes on the 2D structure of edge turbulence. An image analysis method based on two-dimensional continuous wavelet transformation is used to localize structures (blobs) in the images and to extract their geometrical characteristics (position, scale, orientation angle and aspect ratio). We study the impact of edge shear-layers on these geometrical aspects of blobs. Results show a reduction in the angular dispersion of blobs as the shear layer is established in the boundary, as well as an increase in the elongation of these structures. Similar behaviour is found in NSTX image sequences when going from L to H mode plasmas. During improved confinement regimes the number of detected blobs decreases. Some indications are found suggesting that the turbulence reduction could be scale-selective in the biasing-induced improved confinement regime of TJ-II stellarator. Perpendicular flow reversal is visualized with the cameras and the time scales for flow reversal are found to be less than 50?µs. Radially propagating structures are found in the SOL with velocities in the range ~1000?m/s and with a poloidally asymmetric spatial distribution.

AbstractTwo-dimensional edge plasma turbulence as measured by high-speed Hαimaging is investigated in the TJ-II stellarator. An image analysis method based on two-dimensional continuous wavelet transformation is introduced. This method detects localized coherent structures (blobs) in the images and extracts their geometrical characteristics (position, scale, orientation angle, and aspect ratio). This paper studies the impact of edge shear layers (both spontaneous and biased induced) on these geometrical aspects of blobs. Results show a reduction in the angular dispersion of k∼ 1.2 to 1.4 cm−1blobs as the shear layer (both spontaneous and biased induced) is established in the boundary, as well as a shift of the aspect ratio histogram toward higher values. The turbulence suppression induced by the biasing seems to be scale selective, more effectively suppressing k∼ 1.4 cm−1, λ ∼ 4.5 cm structures than k∼ 0.7 cm−1, λ ∼ 9.0 cm ones.

Diffusion is essentially the macroscopic manifestation of random (Brownian) microscopic motion. This idea has been generalized in the continuous time random walk formalism, which under quite general conditions leads to a generalized master equation (GME) that provides a useful modelling framework for transport. Here we review some of the basic ideas underlying this formalism from the perspective of transport in (magnetic confinement) plasmas.