Edge sheared flows and the dynamics of blob-filaments

Abstract

The edge and scrape-off-layer (SOL) region of a tokamak plasma is considered, with emphasis on sheared flow generation and the dynamics of blob-filaments. Both numerical simulations and experimental data analysis are employed. The simulations use the fluid-based 2D curvature-interchange model embedded in the SOLT code. A blob-tracking algorithm based on 2D time-resolved images from the gas puff imaging diagnostic has also been developed and applied to NSTX, Alcator C-Mod and simulation data. The algorithm is able to track the blob motion and changes in blob structure, such as elliptical deformations, that can be affected by sheared flows. Results of seeded blob simulations and quasi-steady turbulence simulations are compared with the experimental data to determine the role of plasma parameters on the blob tracks and to evaluate the exchange of momentum between the blobs and flows. The simulations are shown to reproduce many qualitative and quantitative features of the data including size, scale-length and direction of perpendicular (approximately poloidal) flows, the inferred Reynolds acceleration and residual stress, poloidal reversal of blob tracks, and blob trapping and/or ejection. Mechanisms related to blob motion, SOL currents and radial inhomogeneity are shown to be sufficient to explain the presence or absence of mean and oscillating zonal sheared flows in selected shots.

The role of turbulence–flow interactions in L- to H-mode transition dynamics: recent progress

“…The eddy tilt direction will likely depend on the eddy location with respect to the radial profile of the seed shear flow. Taking advantage of advanced 2D GPI imaging capabilities, the topology, orientation and movement of blobs in the inside and outside the LCFS has been systematically studied in NSTX and Alcator C-Mod OH plasmas [88,89]. A finite average Reynolds stress was found just inside and outside the LCFS in the NSTX data, with large standard deviations potentially consistent with oscillating zonal flows.…”

Comparison of edge turbulence imaging at two different poloidal locations in the scrape-off layer of Alcator C-Mod

“…If the E×B velocity is sheared, it will shear and possibly tear apart the blob structure [26]. The motion of the centroid of a blob is also affected by a velocity shear layer which can act to accelerate or decelerate it depending on the relative signs of vorticity of the shear layer and the blob [27]. Inhomogeneities in the background plasma modify both the blob motion and structure [27,28].…”

Turbulent transport across shear layers in magnetically confined plasmas

“…It is widely accepted that poloidal shear flows increase the particle and energy confinement in fusion devices [2][3][4][5][6]. Recently, the extraordinary role of shear layers for blob generation has been identified [7][8][9][10][11][12]. A picture of blob ejection depending on the shearing rate has been presented in Ref.…”