Sheared flows and turbulence in fusion plasmas

Abstract

The universality of the observed characteristics of sheared flows points to a general ingredient to explain the damping/driving mechanisms responsible for the development of these flows in the plasma boundary region of fusion devices. Experiments in the TJ-II stellarator showing that the generation of spontaneous sheared flows at the plasma edge requires a minimum plasma density or density gradient, open a unique possibility to characterize the dynamics of sheared flow development in fusion plasmas.The effective viscosity at the plasma edge can be deduced by means of the decay rate of the perpendicular flow measurement once the driving force has been removed. Changes in the plasma rotation and turbulence have been studied when an electric field is externally applied at the plasma edge. The relaxation of flows and radial electric fields has been compared in the edge plasma region of TJ-II stellarator and CASTOR tokamak showing a striking similarity. The findings can help to test neoclassical and anomalous damping mechanisms in fusion plasmas.Finally, the emergence of the plasma edge sheared flow as a function of plasma density can be explained using a simple second-order phase transition model that reproduces many of the features of the TJ-II experimental data while capturing the qualitative features of the transition near the critical point.