Turbulent kinetic energy in 2D isothermal interchange-dominated scrape-off layer ExB drift turbulence: Governing equation and relation to particle transport

TL;DR

This paper derives and validates a model linking turbulent kinetic energy, particle transport, and sheath losses in 2D isothermal ExB drift turbulence, providing insights into turbulence-driven particle flux in the scrape-off layer.

Contribution

It introduces an analytical evolution equation for turbulent kinetic energy and a coupled model for particle transport, validated against simulation data.

Findings

Interchange drive is the main source of turbulent kinetic energy.

Sheath losses dominate the sink of turbulent kinetic energy.

The model accurately reproduces simulation data for particle flux.

Abstract

This paper studies the turbulent kinetic energy ($k_\perp$) in 2D isothermal electrostatic interchange-dominated ExB drift turbulence in the scrape-off layer and its relation to particle transport. An evolution equation for the former is analytically derived from the underlying turbulence equations. Evaluating this equation shows that the dominant source for the turbulent kinetic energy is due to interchange drive, while the parallel current loss to the sheath constitutes the main sink. Perpendicular transport of the turbulent kinetic energy seems to play a minor role in the balance equation. Reynolds stress energy transfer also seems to be negligible, presumably because no significant shear flow develops under the given assumptions of isothermal sheath-limited conditions in the open field line region. The interchange source of the turbulence is analytically related to the average turbulent ExB energy flux, while a regression analysis of TOKAM2D data suggests a model that is linear in the turbulent kinetic energy for the sheath loss. A similar regression analysis yields a diffusive model for the average radial particle flux, in which the anomalous diffusion coefficient scales with the square root of the turbulent kinetic energy. Combining these three components, a closed set of equations for the mean-field particle transport is obtained, in which the source of the turbulence depends on mean flow gradients and $k_\perp$ through the particle flux, while the turbulence is saturated by parallel losses to the sheath. Implementation of this new model in a 1D mean-field code shows good agreement with the original TOKAM2D data over a range of model parameters.