Velocity shear, turbulent saturation, and steep plasma gradients in the scrape-off layer of inner-wall limited tokamaks

TL;DR

This study uses 3D turbulence simulations to explain the formation of steep plasma profiles and narrow power decay-lengths in the scrape-off layer of inner-wall limited tokamaks, highlighting the role of shear flows.

Contribution

It provides a theoretical and simulation-based analysis of how sheared flows regulate turbulence and plasma profiles in the tokamak scrape-off layer, linking shear flows to narrow decay lengths.

Findings

Steep plasma profiles arise from radially sheared E×B flows.

Turbulent saturation is regulated by flow-current interactions.

Decay length is approximately equal to turbulent correlation length.

Abstract

The narrow power decay-length ($\lambda_q$), recently found in the scrape-off layer (SOL) of inner-wall limited (IWL) discharges in tokamaks, is studied using 3D, flux-driven, global two-fluid turbulence simulations. The formation of the steep plasma profiles measured is found to arise due to radially sheared $\vec{E}\times\vec{B}$ poloidal flows. A complex interaction between sheared flows and outflowing plasma currents regulates the turbulent saturation, determining the transport levels. We quantify the effects of sheared flows, obtaining theoretical estimates in agreement with our non-linear simulations. Analytical calculations suggest that the IWL $\lambda_q$ is roughly equal to the turbulent correlation length.