Turbulent-driven low-frequency sheared<i>E</i>×<i>B</i>flows as the trigger for the H-mode transition

Abstract

Experiments on HL-2A, DIII-D and EAST show that turbulence just inside the last closed flux surface (LCFS) acts to reinforce existing sheared ExB 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 favorably with a reduced predator-prey type model.