The role of convective structures in the poloidal and toroidal rotation in tokamak

TL;DR

This paper investigates how convective structures influence plasma rotation in tokamaks, proposing a simplified model that explains the sustained effect of vortices on poloidal and toroidal flows, crucial for high confinement regimes.

Contribution

It introduces a novel simplified model describing the interaction between turbulence-induced vortices and boundary shear flows in tokamak plasmas.

Findings

Vortical structures can sustain shear flows by being absorbed at the boundary.

The model derives a system of equations akin to the Davey-Stewartson system.

Transient convective events can have a lasting impact on plasma rotation.

Abstract

The connection between the poloidal and the toroidal rotation of plasma in tokamak is important for the high confinement regimes, in particular in reactor regime. The sudden onset of closed convection structures in the poloidal section, due to the baroclinic production of vorticity, will sustain a fast increase of the poloidal velocity and a substantial effect on the toroidal rotation. However this is limited to the short time of the onset transition. In real plasma however there is random generation and suppression of convection cells and the sequence of these transient events can prove able to sustain the effect on the toroidal rotation. We formulate a simplified model which consists of a laminar sheared, regular, flow situated at the boundary of a region of drift-wave turbulence. Vortical structures, randomly generated in this turbulent region are spontaneously advected toward the flow and are absorbed there, sustaining with their vortical content, the sheared flow. We examine this dynamics in the wavenumber $\mathbf{k}$ space, using reasonable approximations. We derive a system of equations (which is a class of Davey-Stewartson system) and find that indeed, the vortices advected and absorbed into the layer can preserve its regular, poloidal, flow.