Experimental Research On Transport Of Edge Located Mode In High Confinement Mode In EAST

Edge-localized modes(ELMs)are normally considered to be originated from periodic collapse of the edge pedestal.The inherent complexity makes it difficult to completely study.Especially,type-I ELMs induce the deposition of particle and energy on the divertor target plates and the first wall,which could probably lead to enormous erosion and reducing the lifetime of plasma facing components in large devices.Based on the edge diagnostics with high temporal and spatial resolution,the ELM dynamics and associated transport have been investigated in the Experimental Advanced Superconducting Tokamak(EAST).Precursor and postcursor modes have been studied before and after the ELM events,respectively.Fine structure of the ELMs and their propagation were determined.Furthermore,the evidence of current delivered by ELM filaments was obtained.Besides,the intrinsic rotation of pedestal plasma has been observed at L-H rotation.The momentum flux driven by local turbulence is evaluated to play an important role in this process.The first chapter introduces the relevant background of Tokamak.The confinement mode and experimental scaling relation have been presented.The activity of edge plasma is very important for Tokamak.We introduce the EAST Tokamak which we depend on.And then we summarize the feature of the ELM on EAST.Chapter II presents the diagnostic system including the Reciprocating probe system and the Divertor probe system.Then principle and data processing method have been introduced in detail.Chapter III shows the systematic observation on precursor prior to the ELM,postcursor following ELM and ELM current filaments.A precursor before each ELM event is clearly visible in the frequency spectrum of electrostatic signal.The frequency of the precursor decreases from 130 kHz to 30 kHz within approximately 0.4 ms before the ELM breaks out.The Langmuir tips only pick up the precursor inside the last closed flux surface(LCFS),but do not detect the precursor in the Scrape-off Layer(SOL).This observation indicates that the precursors occurring inside the separatrix are essentially a current oscillation,which raises the distant magnetic fluctuations.Furthermore,the precursor oscillation is not associated with increase of the particle transport into the SOL,which is obvious from the absence of precursor in the ion saturation current.A "post-cursor" quaci-coherent mode with frequency around 50 kHz has been observed following the crash of small edge localized modes(ELMs)by using a reciprocating Langmuir probe system.This mode with strong potential and magnetic perturbations propagates in the electron-diamagnetic drift direction in the laboratory frame.In addition,these quasi-coherent fluctuations appear to be modulated by a MHD mode with lower frequency(-5 kHz).The bicoherece analysis shows there exists nonlinear interaction between the post-cursor mode and the MHD mode.However,we obtained that the information on postcursor is limited.Whereafter,it has been found that the ELM has a complex inner structures.Different types of the ELM have different filament structures.The heat flux induced by the type-I ELM filaments is higher than that induced by the type-III ELM filaments.The convection term of heat flux makes a key contribution to the total heat flux which is induced by the type-? ELM filament.However,the convection term and the diffusion term of the heat flux play the same role for the type-I ELM filament.The current filaments in type-III ELMs have been observed in EAST,only in the far-SOL,as manifested by the significant fluctuations of the poloidal magnetic fields.The detailed 3D trajectory of the magnetic field excursion clearly exhibits a closed elliptical loop structure,which is aligned with the equilibrium magnetic field.In contrast,there is no clear signature of such current filaments inside the separatrix as well as the near-SOL region.Furthermore,it appears that the presence of the current filaments in type-III ELMs has strong correlation with the electric potential difference between the divertor plates connecting the field lines.These findings provide information on the formation and sustainment of current filaments during type-III ELMy H-modes.Chapter IV presents the toroidal momentum and the intrinsic rotation induced by the turbulence Reynolds stress.The benefits of intrinsic rotation for Tokamak performance are well documented.Many factors have effects on intrinsic rotation,but the mechanism of intrinsic rotation is still unclear.Tokamak plasmas exhibit spontaneous co-current toroidal rotation in the case of no apparent external momentum source.Plasma acceleration at the ELM-free period following the L-H transition is observed.Therefore,toroidal momentum transport induced by turbulence is analyzed in this work.Two important contributors for the momentum transport,i.e.the Reynolds stress driven term and the particle transport driven term,are evaluated in detail.The Reynolds stress originated from turbulence is the main contributor to the total momentum flux transport in confinement region during ELM-free H-mode.It drives an inward momentum flux which dominates the momentum loss caused by the outflow of particles.The measurements in the near SOL suggest that the convective term has a dominant effect on total momentum during L-mode.At the L-H transitions,the passive momentum transport was suppressed,which in turn can increase the plasma rotation.Our results identify the role of turbulence driven by Reynolds stress and particle transport in the toroidal momentum redistribution in boundary plasma.Furthermore turbulence driven momentum transport can reasonably predict the acceleration of edge plasma.Finally,conclusions are given and future work is also discussed in Chapter V.