Experimental Research On Edge Localized Mode And Turbulence In EAST Tokamak

High confinement mode(H-mode)with edge localized mode(ELM)is the main operation regime of future fusion reactors(such as ITER and CFETR).Edge transport barrier(ETB)in ELMy H-mode is known as pedestal,which can maintain high plasma confinement.Meanwhile,the steep pressure and current density gradient in pedestal region will trigger ELM instability,which can induce extreme transient heat flux and may pose a significant risk to the safe operation of the device.Research over the past decade has shown that pedestal transport and the formation of pedestal structure are still dominated by turbulence.As pedestal structure itself also affects the ELM characteristics,the pedestal turbulence,pedestal structure and ELM are interrelated.The study on them is beneficial for the understanding the edge physics of ELMy H-mode.Besides,active control of ELM is also an important topic for future fusion reactors.For example,resonant magnetic perturbation(RMP)is one of the most effective means of ELM control.Some evidences indicate that pedestal turbulence plays an important role in RMP ELM control experiment.In the thesis,we will firstly study the pedestal turbulence and its influence on edge parameters based on the reflectometry diagnostics on EAST.Then,studies on the pedestal turbulence,pedestal structure and ELM in ELM control experiments(including RMP and gas fuelling)will be presented.During inter-ELM phase of type I ELMs,coherent modes with multi-frequency branches of a range of 40～110 kHz were found and two frequency branches of the modes were studied.One of the modes with 85～110 kHz is electromagnetic coherent mode,the toroidal mode number n=7,the local poloidal wave number in midplane of low field side kθ=-0.3 cm-1 and the rotation in the direction of electron diamagnetic in the laboratory frame.It shows that this instability may be driven by pedestal electron density or density gradient.This mode appeared at about 5ms after ELM crash,which can slowdown the recovery of pedestal density.It means that this mode may drive outward particle transport.The other mode with frequency of about 75 kHz was mainly observed in magnetic fluctuation,while it is very weak in density fluctuation.The toroidal mode number is n=3.It was found that this mode is related to saturation of pedestal electron temperature in the later inter-ELM period,which means this mode may cause electron thermal transport.Based on 8-channel O-mode correlation reflectometry,it was also observed that the instabilities triggered at pedestal gradient region can spread into scrap-off layer(SOL)on EAST.Two main instabilities have been found so far.One of the modes is the electromagnetic coherent mode of 40～90 kHz triggered at pedestal gradient region.When the mode amplitude is strong enough,the same pattern can be observed by reflectometry at SOL.As the mode amplitude increases,the effective deposition width of the particle flux on the upper and outer divertor target plates increases.The other case is a coherent mode of about 230 kHz,which can exist in both pedestal gradient and SOL region.Since the mode amplitude at gradient region is larger,it can be speculated that this mode originates in the gradient region and then spreads into the SOL.Those experimental observation preliminarily confirm the BOUT++simulation prediction.That is the turbulence triggered in pedestal gradient region can spread into the SOL,leading to the enhancement of cross-field transport in SOL and increase of deposition width of heat and particle flux in divertor.During ELM mitigation with the application of n=4 RMP,it was found that mean value of ELM frequency increased by a factor of 2.7 and ELM size decreased.And an obvious density pump-out was observed.The width and gradient of pedestal electron density decrease gradually with RMP.A broadband fluctuation with a frequency range of 20～115 kHz is enhanced during inter-ELM phase.It was found that the growth rate of pedestal density decreased and the density even declined slightly during the broadband fluctuation phase.Meanwhile,an increase of particle flux to divertor target was observed from divertor probes.It means that the broadband fluctuation can drive outward particle transport,which leads to density pump-out.The role of broadband fluctuation in ELM control during RMP ELM mitigation is discussed.In ELMy H-mode plasma heated by NBI and LHW,different normaized densities in the range of 0.4～0,8 are obtained by using gas fuelling.It is found that the plasma thermal energy(Wth)decreased with density.Analysis of density and temperature profiles show that pedestal pressure essentially unchanged while core pressure decreased significantly which leads to decrease of Wth.The decrease in core pressure is mainly due to the decrease of peaking of core temperature.Type Ⅰ ELMs are sustained with increased frequency and reduced amplitude with the increase of density.It was found that time intervals of ELM periods are more irregular.Besides,it was found that sawtooth existed for all the density scans.While the sawtooth characteristics changed significantly at different densities and the reverse radius moves toward to magnetic axis with density,which means the q=1 surface gradually shrinks with density.In future,plasma transport simulation by integrated code need to be done to understand the physics of the decrease of core confinement.