Evolution Of Zonal Flow’s Power Ratio During L-H Transition And Mode Structure Measurement Of Geodesic Acoustic Mode Density Fluctuation

In this thesis,we research on the power ratio of zonal flow to drift wave turbulence during H-mode to L-mode transition in a tokamak,and toroidal symmetric chracteristics of the density fluctuation caused by Geodesic Acoustic Mode (GAM in short).Zonal flow’s electric potential fluctuation in tokamak is both toroidal and poloidal symmetric,and has finite radial wavenumbers.Theoretical studies have shown that zonal flow have important effects in regulating turbulence amplitude,improving transport and triggering L-H transition and H-L transition.According to the low or high frequency,zonal flow can divide into two branches:Low frequency Zonal flow(LFZF in short),GAM with finite frequency.The key point of this thesis is the change of the power ratio of LFZF to turbulence and toroidal symmetric characteristic of density fluctuation caused by GAM. Both theory model and simulation have shown that zonal flow can gain energy from turbulence directly,reduce the amplitude of turbulence,meantime through flow shearing regulate turbulence,consequently lower the radial correlation length of turbulence.These two processes work together,and suppress turbulence,improve transport.Zonal flow and turbulence together form a self-organize system,named as zonal flow-turbulence system,this concept now is widely accepted.In the research of magnetic confinement fusion,zonal flow plays a very important role.Theoretically Diamond et al use predator-prey model to explain L-H transition and L-I-H transition,of which I means intermediate phase.The model includes zonal flow,mean flow and turbulence,the result is qualitatively consistent with the experiment result.Kagan et al analyse zonal flow in pedestal,the result shows that an increase in the equilibrium radial electric field leads to an increase in the zonal flow residual,and makes the turbulent transport level lower and sharpening the density profile.The experiment of this thesis was done on EAST tokamak and HT-7 tokamak located at Institute of Plasma Physics,Chinese Academy of Sciences.On EAST device we used newly designed probe arrays,including rake probe,two step seven probe array,twelve probe array.We measure at midplane, in toroidal,poloidal and radial direction at the same time during a shot.We confirm that LFZF exists in H-mode,during experiment we control the time of the probe into plasma,get the change of the power ratio of LFZF to turbulence with time. We find during Elmy-free H-mode period the power ratio of LFZF to turbulence is high,while during L-mode period is low.We also study the phase relationship between density fluctuation and radial electric field fluctuation,which indicates that radial electric field fluctuation is ahead of density fluctuation.We also figure out that quasi-periodicity oscillation during Elmy-free H mode period is not LFZF.We also find a high frequency coherent mode during Elmy-free H mode period,whose frequency is about 50kHz,with its second harmonic frequency.On HT-7 tokamak,we use two toroidally separated Langmuir triple-probe arrays to study the toroidal symmetric characteristics of density fluctuation caused by GAM.We get the amplitude of the density fluctuation,it qualitatively agrees with the theoretically predicted amplitude. We use bispectrum analysis and envelope analysis to analyze the nonlinear interaction between density fluctuation caused by GAM and turbulence,and get the anti-phase relation between electric field fluctuation of GAM and the envelope of high frequency turbulence,and π/2 phase relation between density fluctuation of GAM and the envelope of high frequency turbulence.