| In the long-pulse discharges with high heating power on the tokamak devices,the divertor targets have to handle the extremely high heat load,which is much higher than the material safety limit.Thus,necessary methods have to be applied to mitigate the heat load,in which detachment operation is the most promising scheme.In tokamaks with metal-wall environment,detachment operation is achieved by seeding low-Z impurities from the divertor region to promote the radiation loss power.However,the low-Z impurities may transport into the pedestal and core region,and induce dilution effect and increase the radiation loss power,which may degrade the energy confinement.How to achieve the compatibility of radiative divertor with high core confinement in long-pulse high-heating-power discharges,is one of the major issues in the present fusion researchesIn the chapter Ⅱ,basic theories and experimental research are introduced to simply describe the basic plasma physics of the divertor or core region.The basic physics of the divertor include the triple states of the divertor plasmas,the impact of drifts on divertor and SOL region,and some novel divertor concepts.Then,the basic transport theories,the global energy confinement scaling,the pedestal structure and the edgelocalized modes(ELM)are also introduced.The chapter Ⅲ presents the research details of the impurity-induced low-n mode,which is observed in the radiative divertor experiments in 2019.This low-n mode has several features,such as the localized distribution and the excitation threshold.It also has strong edge transport capacity,which plays a key role in the small or no ELMs states with divertor detachment.Based on the drift wave and the radiation condensation instability,we develop a coupling model to explain the excitation mechanism of the low-n mode.Several experimental features of the low-n mode are successfully explained,demonstrating the model’s validity.For the future fusion devices,the low-n mode may provide a promising regime to simultaneously mitigate the ELM-induced transient heat load and steady-state heat load with maintaining stable pedestal structure and good core confinement.Due to the relatively open structure of EAST upper divertor,the impurities seeded from divertor region unavoidably enter the core region and degrade the confinement.The chapter Ⅳ presents the research details of the impact of divertor closure on the edge and core plasmas.The research results of the EAST and other foreign devices all support the hypothesis that promoting the divertor closure facilitates detachment by lowering the detachment threshold or reducing the seeding amount of the low-Z impurities.Therefore,the EAST team designs and upgrades the lower divertor,in which the special structure promotes the closure.In this chapter,the basic corner-slot structure of the new lower divertor is presented.The SOLPS-ITER simulation results demonstrated the superiority of the corner-slot divertor by comparing several normal divertor structures.The experimental results of sweeping the strike point in L-mode demonstrate the corner effect,which the corner-slot structure induces.The chapter V presents research details of long-pulse feedback control for detachment in grassy-ELM H-mode based on the new lower divertor.The module of monitoring the stored energy is proposed and developed to ensure the stability of longpulse discharges.By seeding neon,～20s(based on radiative feedback)and～30s(based on floating potential feedback)detachment discharges with H98,y2≥ 1.1 are demonstrated.After seeding neon,the frequency of grassy ELMs decreases and the ECM mode is suppressed.Meanwhile,the electron density increases in the core region but the electron temperature almost has no change.Besides,the core ion temperature almost increases by～40%.By comparing the research results of other devices,it is convincing that seeding low-Z impurities can reduce the ITG-induced turbulence transport.In conclusion,this thesis explores the operation compatibility of radiative divertor with a high confinement.The impurity-driven low-n mode provide an edge transport channel to maintain the pedestal structure and high confinement under radiative divertor operation,which has a potential in future reactors for simultaneous control of steadystate and ELM-induced heat loads on the divertor target plates.EAST upgrades he lower tungsten divertor with a closed corner.The special structure further promotes the divertor closure and mitigates the negative effect of impurity seeding on core confinement,which are demonstrated by simulation and experiments and may inspire the divertor design of future reactor.For long-pulse steady-state discharges,we develop a module of monitoring the stored energy.A long-pulse(～30s)discharge is demonstrated for radiative divertor with a high confinement,which indicates the reliability of divertor detachment as a solution for divertor heat loads for future reactors. |
