First Evidence of the Role of Zonal Flows for theL−HTransition at Marginal Input Power in the EAST Tokamak

Abstract

A quasiperiodic Er oscillation at a frequency of $<4\mathrm{kHz}$, much lower than the geodesic-acoustic-mode frequency, with a modulation in edge turbulence preceding and following the low-to-high ($L\mathrm{\text{−}}H$) confinement mode transition, has been observed for the first time in the EAST tokamak, using two toroidally separated reciprocating probes. Just prior to the $L\mathrm{\text{−}}H$ transition, the Er oscillation often evolves into intermittent negative Er spikes. The low-frequency Er oscillation, as well as the Er spikes, is strongly correlated with the turbulence-driven Reynolds stress, thus providing first evidence of the role of the zonal flows in the $L\mathrm{\text{−}}H$ transition at marginal input power. These new findings not only shed light on the underlying physics mechanism for the $L\mathrm{\text{−}}H$ transition, but also have significant implications for ITER operations close to the $L\mathrm{\text{−}}H$ transition threshold power.