Characteristics of an axisymmetric magnetic oscillation closely related to L-H transition on experimental advanced superconducting tokamak (EAST)

An axisymmetric magnetic oscillation (AMO) of 5–10 kHz in the L-mode plasma edge has been studied on an experimental advanced superconducting tokamak. The analysis shows that this oscillation only occurs at the plasma very close to the L-H transition. The structure of magnetic oscillation is axisymmetric (n=0) toroidally and demonstrates an m¼2 standing wave in the poloidal direction. The density fluctuation of AMO measured by reflectometry at the midplane is nearly in phase, indicating that the AMO is not a radial traveling wave.The AMO shows an obvious modulation effect on the background magnetic/density fluctuation, and as a result, the edge density/density gradient and particle flux onto the divertor are also modulated by the AMO. The frequency scaling of the AMO is f m. The bicoherence analyses show a strong nonlinear interaction between the AMO and the background turbulence, and other nonlinear interactions between the AMO and three other coherent fluctuations with frequencies (4, 11, and 15 kHz) are also found.