Transport Bifurcation in a Rotating Tokamak Plasma

TL;DR

This study numerically investigates how flow shear influences turbulent transport in zero magnetic shear tokamak plasmas, revealing a bifurcation that can lead to higher temperature and flow gradients through applied torque adjustments.

Contribution

It demonstrates the existence of a transport bifurcation in rotating tokamak plasmas and characterizes how flow shear affects turbulence thresholds and heat flux sensitivity.

Findings

Flow shear stabilizes linear turbulence but allows subcritical turbulence.

Flow shear raises the nonlinear temperature gradient threshold for turbulence.

A bifurcation enables transition to higher temperature and flow gradients by varying torque.

Abstract

The effect of flow shear on turbulent transport in tokamaks is studied numerically in the experimentally relevant limit of zero magnetic shear. It is found that the plasma is linearly stable for all non-zero flow shear values, but that subcritical turbulence can be sustained nonlinearly at a wide range of temperature gradients. Flow shear increases the nonlinear temperature gradient threshold for turbulence but also increases the sensitivity of the heat flux to changes in the temperature gradient, except over a small range near the threshold where the sensitivity is decreased. A bifurcation in the equilibrium gradients is found: for a given input of heat, it is possible, by varying the applied torque, to trigger a transition to significantly higher temperature and flow gradients.