Dr. Bogdan Teaca, Sr. Lecturer, Applied Mathematics, Coventry University

"Structures in subcritical plasma turbulence"

Abstract: In many plasma systems, including tokamaks, sheared flows perpendicular to the driving temperature gradients can strongly stabilize linear modes. While the system is linearly stable, regimes with persistent nonlinear turbulence may develop, i.e. the system is subcritical. A perturbation with small but finite amplitude may be sufficient to push the plasma into a regime where nonlinear effects are dominant and thus allow sustained turbulence. In this talk methods that account for nonlinear interactions are presented, such as edge tracking, which identifies the boundary between the basins of attraction of the laminar and turbulent states, or non-modal stability theory, which allows us to identify the minimal seed, i.e. the smallest disturbances which can trigger turbulence. We will introduce these concepts for a simple one dimensional plasma interchange model, before showcasing the results obtained for gyrokinetic simulations in tokamak geometries. We will discuss the type of structures and regimes that emerge for different values of the shear. In particular, a traveling wave solution is found as an attractor on the edge manifold for both scenarios. This surprisingly simple advecting solution is qualitatively similar to, but simpler than, the avalanche-like bursts seen in earlier turbulent simulations and provides an insight into how turbulence is sustained in subcritical plasma systems that are in a convectively unstable regime.