Experiments on HL-2A, DIII-D and EAST show that turbulence just inside the last closed flux surface acts to reinforce existing sheared E × B flows in this region. This flow drive gets stronger as heating power is increased in L-mode, and leads to the development of a strong oscillating shear flow which can transition into the H-mode regime when the rate of energy transfer from the turbulence to the shear flow exceeds a threshold. These effects become compressed in time during an L–H transition, but the key role of turbulent flow drive during the transition is still observed. The results compare favourably with a reduced predator–prey type model.
Sheared flows and transition to improved confinement regime in the TJ-II stellarator