Description
Anomalous cross-field transport in the edge and scrape-off layer of magnetically confined plasmas remains a critical challenge for achieving efficient confinement in fusion devices [1]. This region hosts hydrogenic ions, helium ash, injected impurities (e.g. neon, argon), and eroded wall materials, all of which interact with turbulent structures to drive complex, non-diffusive transport behaviour [1] [2]. Coherent vortices and shear flows - particularly ExB shear and zonal flows - are known to regulate transport in fluid and plasma systems [3, 4], yet their precise role in governing impurity dynamics in the plasma edge remains an open question [2].
In this work, we develop a versatile particle-tracer framework coupled to a two-field Hasegawa–Wakatani (HW) turbulence model [5], implemented within BOUT++ [6]. Tracer particles, advanced via the Lorentz force in a Python-based pusher, sample self-consistent drift-wave turbulence across regimes ranging from vortex-dominated to shear-dominated by varying the adiabaticity parameter.
To investigate impurity transport, we first identify coherent structures and their boundaries using three diagnostics: (i) contours of the electric potential field, which reveal the large-scale organisation of vortical structures; (ii) the Okubo–Weiss parameter, which distinguishes vorticity dominated regions from strain dominated shear layers; and (iii) the Lagrangian Averaged Vorticity Deviation (LAVD), which captures coherence from the particle perspective. Together, these measures enable identification of coherent vortices and surrounding shear regions.
Using the identified coherent structures and their boundaries, we identify tracer particles trapped within these regions and track their evolution. This allows us to quantify how particle motion changes when interacting with coherent vortices and shear layers, including trapping, detrapping, and enhanced radial excursions. By analysing particle displacement statistics as functions of adiabaticity and charge-to-mass ratio, we characterise how coherent structures regulate impurity confinement and contribute to convective, non-Fickian transport in drift-wave turbulence.
References
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coherent structures in edge and SOL turbulence. Journal of Plasma Physics, 74(5):679–717, October
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parallel plasma fluid simulations. Computer Physics Communications, 2009.