Flow topology and Lagrangian conditional statistics in edge plasma turbulence

To understand the role of coherent structures (e.g., vortices, "blobs", and zonal flows) in the plasma edge, we present a study of the relationship between flow topology and Lagrangian statistics in the Hasegawa–Wakatani model. The topology is characterized using the Okubo–Weiss criterion that splits the flow into elliptical (strong vorticity), hyperbolic (strong deformation), and intermediate (transition) regions. The Lagrangian statistics is computed by tracking ensembles of particles over hundreds of eddy turnover times in statistically stationary turbulence. Particle "trapping" and "flights", induced by coherent structures and known to play a key role in anomalous transport, are quantified using residence times statistics. It is shown that the probability density functions (pdfs) of the residence time conditioned to elliptic and hyperbolic regions exhibit algebraic decaying tails. However, in the intermediate regions the pdf exhibits exponentially decay. The pdfs of the Lagrangian velocity, acceleration, and density fluctuations, conditioned to the flow topology, are also computed along with the density flux spectrum, which characterizes the contributions of different length scales.