Zonal flow energy ratio evolution during L-H and H-L transitions in EAST plasmas

The essential role of zonal flow in the L-H transition and the suppression of turbulence have been studied with a long range correlation technique using Langmuir probe arrays in EAST tokamak. Two toroidally localized probe arrays are used to measure the zonal flow during L-H transition and H-L back transition. The energy ratio of the low frequency zonal flow to the total drift wave turbulence is calculated. During ELM-free H mode, the energy ratio is higher than that in L mode, which reveals the important role of zonal flows in regulating turbulence amplitude in L-H transition.