Small amplitude oscillations before the L-H transition in EAST

Before L-to H-mode transition small amplitude oscillations (SAOs), different from the widely known intermediate phase (I-phase), at a frequency of a few kilohertz can be observed on EAST. Under sufficient auxiliary heating, SAOs can transit to the H-mode or I-phase. The edge radial electric field (Er) located inside the separatrix can be observed to deepen after bursts of SAOs. In SAOs, the turbulence level preceding the negative radial electric field and floating potential perturbation about 90 degrees in phase, consistent with the model of zonal-flows and turbulence interaction, is measured by the Langmuir probe at the bottom of the edge Er well. A physical mechanism for SAOs is developed: at a critical gradient in pressure and Er, turbulence increases at the inboard edge of the Er well. The increased turbulence level enhances the radial particle, energy and momentum transport at the plasma edge and increases the amplitude of the zonal flow at the bottom of the Er well due to the increased Reynolds force. The increase in the zonal flow amplitude acts to mitigate the turbulence on the inboard edge of the Er well, driving a limit-cycle oscillation. The poloidal magnetic perturbations of the oscillations are poloidal in-out/updown asymmetric and toroidal symmetric in the SAOs.