Experimental Study On Particle Retention And Heat Flux Shielding By Flowing Liquid Lithium Wall In Plasma

Liquid lithium(Li)divertor is one alternative solution to resolve the heat load of the future fusion divertor.The basic principle is to cover the solid material surface with a layer of liquid Li and avoid direct contact between the solid material and high-temperature plasma.The liquid Li divertor could overcome the problems related to thermal shock,irradiation damage,hydrogen isotope retention,recycling of impurity particles,and other issues of the solid target plate.Therefore,the development of liquid Li walls has attracted significant attention.In this paper,the engineering design,upgrading of the liquid Li limiter,the retention of fuel particles under the liquid Li wall,and the thermal load mitigation caused by the vapor shielding effect of liquid Li are introduced briefly from engineering and physical aspects.The future development of the flowing liquid Li(FLiLi)limiter in EAST has been proposed.Firstly,the wettability experiment of liquid Li proves that liquid Li has better wettability on the surface of molybdenum alloy(TZM)material than 316L stainless steel(316L)material.Further,the irradiation resistance and corrosion resistance performance of TZM material are better than 316L stainless steel.Therefore,TZM is selected as the base material of the limiter in the upgrade of the liquid lithium limiter.It has been confirmed that the wettability of liquid Li on TZM material is superior to 316L stainless steel(SS).The resistance capacity of corrosion and irradiation of TZM material is also better than SS.ANSYS thermal analysis of the limiter shows that He cooling can effectively remove the heat load of the limiter target during the discharge.Through the upgrade of the FLiLi limiter,the coverage of liquid Li on the limiter is higher than 85%.It is also found that the FLiLi limiters can effectively reduce recycling,increase plasma storage energy and improve plasma performance.Secondly,the gas balance method is systematically used to study the retention characteristics of fuel particles and their influencing factors under the condition of liquid Li wall in EAST.As a comparison,the retention behavior of fuel particles under Li-coated walls is first explored.It is found that the absorption capacity of the Li-coated wall decreases,and the recycling increases with the discharges.The strong interaction between plasma and Li-coated walls results in higher wall outgassing.It is found that-88%failed to capture D in discharge.In contrast to the Li-coated wall,the retention of fuel particles increases gradually in the FLiLi experiment,and no evident wall saturation phenomenon is noted.It is found that the stronger the interaction between plasma and liquid Li higher the retention amount and retention rate of D.The primary reason is the continuous deposition of a large amount of Li on the first wall surface due to the evaporation and sputtering of Li to achieve the real-time update of Li-coated wall during the FLiLi operation.It is estimated that during the process of the FLiLi limiter,continuous renewal and redeposition Li capture≥80%of the retained D in the Li coated wall of the vacuum chamber.Finally,the heat flux on the FLiLi limiter is moderated by the Li vapor shielding effect of the liquid Li limiter.The liquid Li surface temperature oscillation in the process of liquid Li vapor shielding under type I large edge-localized modes(ELMs)and grassy ELMs are investigated.In the plasma discharge with FLiLi limiter,with the increase of Li radiation,42%of the parallel heat flux is directly dissipated.It is found that the liquid Li surface temperature oscillation amplitude and oscillation frequency are strongly correlated with surface temperature and ELMs type.Large ELMs significantly affect temperature oscillation.The results show that the liquid Li surface temperature oscillation becomes stronger with the increase in the surface temperature of liquid Li to 550℃,primarily due to the enhancement of Li influx from limiter caused by Li evaporation.With the rise in Li radiation,the temperature oscillation on the surface of liquid Li occurs,thereby decreasing the temperature rise rate on the surface of liquid Li.It shows that the surface temperature oscillation caused by the Li vapor cloud can adjust the surface temperature of liquid Li.In summary,the experimental results of the FLiLi limiter on EAST show that the new FLiLi limiter exhibit better wettability,irradiation resistance,and corrosion resistance.The coverage of liquid Li on the limiter surface increases to>85%.The retention characteristics of fuel particles under the condition of the FLiLi wall are investigated.It is confirmed that the continuous flow of liquid Li and the continuous renewal of Li-coated walls are the primary mechanism of stable and controllable fuel particles for a long time.Moreover,the vapor shielding effect and the associated temperature oscillation of the liquid Li surface are observed for the first time in the tokamak device,which effectively controls the liquid Li surface temperature.These engineering and physical studies on the design of liquid Li walls provide a preliminary reference for developing liquid Li divertor in fusion reactors in the future.