Experimental Study On ELM Burn-through Induced By Small ELMs With Divertor Detachment In EAST

The divertor detachment is one of the most promising solutions to effectively reduce the steady-state heat flux on the target plate for future fusion devices.However,a large amount of particles and energy carried by ELMs could burn through the neutrals front and reach the target plate,causing the divertor plasma reattached during the ELM burst,which is usually called the ELM burn-through.The re-attached hot plasma could damage the divertor materials and bring large risk to the high-performance operation of future fusion reactors.The ELM burn-through induced by large ELMs in divertor region,especially near the strike point,has been observed in the high-confinement(H-mode)operations with divertor detachment of EAST tokamak.Nevertheless,there is a lack of a clear understanding as to whether small ELM will induce serious ELM burnthrough.Based on the radiative divertor experiments in small grassy ELM regime in EAST,a corresponding database on small ELM burn-through has been built up in the paper.Furthermore,a systematic study on the ELM burn-through induced by small grassy ELMs has also been presented in the paper.In this study,the particle flux fall-off width λjs before detachment and the temperature Te on the target plate,both based on the measurement of divertor Langmuir probes(Div-LPs)in EAST,are important parameters to evaluate the degree of detachment and the occurrence of the ELM burn-through induced by small ELMs.Firstly,the radial extension width of large and small ELMs on the target plate,which is mapped on the outer mid-plane,has been evaluated by the correlation analysis method.A statistical study on the saturated ion flux density(js)and the floating potention(Vf)has revealed that the affected areas of small ELMs on the divertor target plate is much smaller than that of large ELMs.Evidenced by the measurements of js,the radial extension of large ELMs can be observed even from a far SOL region of 3-4λjs or more,while that of the small ELMs is restricted to a range of no more than λjs.However,the measurement of Vf indicates a larger radial extension up to 1.5λkjs of small ELMs.Therefore,for the study of ELM burn-through,only the region within 1.5λjs on the divertor target needs to be considered for small ELMs.Secondly,based on the preliminary analysis of the ELM burn-through database,it has been found that the occurrence of small ELM burn-through is highly correlated with the width of the detached area mapped on the outer mid-plane and the ELM frequency.When the divertor falls into detachment with detached area larger than λjs,the small ELMs could be mitigated or even suppressed,avoiding the burn-through of small ELMs.When the divertor detachment only occurs near the strike point while the ELM frequency is high enough,there is also no small ELM burn-through.However,when the degree of divertor detachment is low and the ELM frequency is not high simultaneously,the burn-through of small ELMs could be observed.Under this condition,the small ELM burn-through is sensitive to the change in the electron temperature and correlated with the competition between ionization and recombination of the divertor plasma.Further analysis reveals that the ELM burnthrough is mainly triggered by the accumulated particles and energy of multiple small ELMs,not by that of a single small ELM.There is also no occurrence of small ELM burn-through in the natural detachment case with fELM>1000Hz.Finally,a statistical study on the discharges with good measurement in the ELM burn-through database has been carried out.The analysis of the detached area and ELM frequency indicates that only when the divertor detachment is achieved within a limited region of Ldetached<0.4 λjs and the ELM frequency is smaller than 800 Hz,the ELM burn-through induced by small ELMs could occur.This paper presents a systematic study on the ELM burn-through induced by small grassy ELMs in EAST,which will contribute to the integrated control of transient and steady-state heat loads of a fusion reactor operating in small ELM regime,such as CFETR.Furthermore,active ELM control techniques are also scheduled to be applied in some other fusion reactors,such as ITER.Considering that full ELM suppression with these methods still remains uncertainties,this study could also facilitate to evaluate the maximum amplitude of ELMs at which they do not burn through the detached plasma in these reactors.