The Pump-Out Effect as a Trigger for the Electron to Ion Root Transition in TJ-II

In this paper, we study electron to ion root confinement transitions triggered by Electron Cyclotron Resonant Heating switch-off when the line-averaged electron density is sufficiently high and while Neutral Beam Injection heating is on. We use a Heavy Ion Beam Probe system to document the transition in considerable detail. The transition occurs in two phases. An initial, fast phase is characterized by a rapid, global decay of the plasma potential and the concomitant establishment of an edge radial electric field shear layer. This phase is induced by the pump-out effect, as documented using simultaneous measurements of the evolution of the plasma potential at different radial locations. This initial, fast phase is followed by a slow phase during which the profiles adjust to the new edge and heating conditions via transport.