| Energy issues are limited by the depletion of natural resources and the environmental pollution it brings,so nuclear fusion energy,represented by magnetically confined fusion,is getting more and more attention.Currently,the prevailing trend in magnetically confined fusion is the high confinement mode(H-mode)operation of tokamak devices.However,H-mode is often accompanied by edge-localized modes(ELMs),which can lead to an unacceptable transient heat load on the plasma-facing components(PFCs).Therefore,a no-ELMs regime with high energy confinement,namely the improved confinement mode(I-mode),is studied in this dissertation.I-mode,featuring high energy confinement comparable to H-mode and moderate particle confinement comparable to L-mode,can be a potential candidate for future fusion devices due to the lack of ELMs.The existence of particle transport in I-mode plasma is favorable for avoiding impurity accumulation in the plasma core and helium ash removal.Therefore,I-mode is also one of the operational options available for future fusion reactor experiments.This dissertation is based on the Experimental Advanced Superconducting Tokamak(EAST)and Doppler reflectometer.A temperature perturbation,named edge temperature ring oscillation(ETRO),can be observed in most edge diagnostics during stationary I-mode.In order to explore the stationary I-mode microphysical mechanism to maintain the stationary I-mode discharge,we have conducted a more in-depth study of the ETRO.The ETRO is an azimuthal symmetry and radially localized structure and the ETRO is a ring structure obtained by the inversion of the radiation signal.ETRO is sustained by the alternating transitions between an electron diamagnetic drift turbulence and an ion diamagnetic drift turbulence.Noted that ETRO is not geodesic acoustic mode(GAM).During the L-I transition,it can be observed that ETRO appears after the disappearance of GAM,and the frequency of ETRO is much smaller than that of GAM.In addition,the frequency of ETRO is linearly proportional to the temperature at the pedestal top,and the frequency of ETRO is related to the radiation power and impurity concentration.There is a distinct impurity concentration threshold for the ETRO amplitude,and when this threshold is exceeded,a stronger ETRO can be excited.Generally,I-mode will transit to H-mode after increasing the auxiliary heating power.In order to maintain the stationary I-mode discharge and avoid the plasma entering H-mode,we have carried out a more detailed study of the I-H transition.Pedestal burst instabilities(PBIs),featuring alternative turbulence suppression and bursts in the pedestal,can be clearly observed by most of the edge diagnostics(such as DR,bolometer,SXR,ECE,Mirnov probe,Dα,and DivLPs)during the I-H transition in the EAST tokamak.The relative density perturbation caused by PBI is about 6%-8%,which is much smaller than that of large ELMs.Prior to each PBI,a significant increase in density gradient in the pedestal can be distinguished.Then the turbulence burst is generated,accompanied by the relaxation of the density profile.Considering that PBI is just an intermittent turbulence-driven process,both the deposition range and the intensity of the induced particle flux are much smaller than that caused by large ELMs.And the significant increase of the chord-averaged density and the density gradient during the PBI phase implies that the PBI phase is a gradual process of density pedestal establishment.Statistic analyses show that the pedestal normalized density gradient R/Ln triggering the first PBI has a threshold value,mostly in the range of 22-24.Those results suggest that a PBI triggering instability is probably driven by the density gradient.In addition,the explosion of PBIs is accompanied by an outbreak of turbulence in the direction of electron retro-magnetic drift,according to which the process of PBIs is divided into three stages:the electron turbulence outburst phase,the mixing phase,and the high confinement phase.The PBI outburst is followed by a rapid collapse of the electron density gradient,after which a slow recovery begins.Therefore,the PBIs process is actually a quasi-periodic transition process of the pedestal region between two states of particle confinement improvement and deterioration.Throughout the PBI process,the averaged density is gradually and slowly increasing,which indicates that the I-H mode transition process is a multiple progressive process of density pedestal establishment.The appearance of PBIs and the rapid increase of the electron density gradient before the PBIs will help to identify the precursors of I-H mode transition and establish the foundation for the steady-state long-pulse I-mode operation.The Super I-mode was innovatively discovered in the EAST long pulse discharge by the weak coherent mode(WCM)and ETRO observed in the Doppler reflectometer.It was verified by electron density profile(similar to L-mode)and electron temperature profile(similar to H-mode)and confirmed as Super I-mode.The H98,y2 is almost 1.2,and the βp is approximately 1.5 in Super I-mode.The enhancement in Super I-mode confinement performance is associated with a significant improvement in the boundary(I-mode boundary transport barriers)and core(internal transport barrier,ITB).During Super I-mode,electron thermal transport barriers are observed at both ρ=0-0.4 andρ=0.9-1.0,i.e.,Super I-mode=I-mode+e-ITB. |
