Two-Dimensional Turbulence Analysis Using High-Speed Visible Imaging in TJ-II Edge Plasmas

Two-dimensional edge plasma turbulence as measured by high-speed H imaging 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^-1 blobs 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, ~ 4.5 cm structures than k ~ 0.7 cm^-1, ~ 9.0 cm ones.