Study On The Recycling And Comparison Of Grassy ELM In Different Magnetic Configurations On EAST

Achieving 400s long-pulse H-mode is one of the main goals of the EAST tokamak,the uncontrollable density caused by high recycling and the transient heat load caused by large amplitude edge localized modes during the long-pulse H-mode need to be solved urgently.At present,since the lithium coatings can significantly reduce the recycling,the 100s long-pulse H-mode has been achieved with the lithium coating wall,while the challenges of achieving 400s long pulse H mode will increase sharply.Therefore,studying the influence of lithium coatings on the recycling would facilitate the achievement of 400s long-pulse H mode on EAST.Grassy ELM is a kind of high frequency and small amplitude ELM,the low heat flux caused by its burst provides a way to solve the problem of strong heat flux caused by large ELM.Grassy ELMs have been obtained in both favorable and unfavorable toroidal fields B_t with lithium coatings on EAST.The lower SOL density and better performance in the favorable B_t could have potential effects on the grassy ELM behaviors,studying the behaviors of natural grassy ELM under different magnetic configurations would be helpful to the achievement of 400s long-pulse H-mode on EAST.The influence of the lithium coatings on the recycling near the outer mid-plane in EAST tokamak has been studied based on the mid-plane D_αradiation system.Statistics analysis indicates that the cumulative effect of lithium coatings on recycling appears to be most evident in the region closest to the first wall（distance from the first wall～1-1.5cm）.At a distance far from the first wall,the cumulative effect is much weaker.When the cumulative effect is not saturated,the outer mid-plane recycling under fresh lithium wall appears to be lower than that under passivated lithium wall.When the cumulative effect is saturated,there is no significant difference in their effects on the outer mid-plane recycling for either fresh or passivated lithium wall.A database of grassy ELM discharges has been established to systematically study the behaviors of grassy ELM in frequency.Statistical analysis indicates that there is no systematical difference in frequency of grassy ELMs under the two different magnetic configurations in similar parameter space.It is interesting to found that the high frequency grassy ELM(f _ELM>1 k Hz)in unfavorable B_tis dependent on the high poloidal beta β_pand high triangulatityd_u,while the high frequency grassy ELM(f _ELM>1 k Hz)in favorable B_t appears to rely on the high plasma density.The parameter spaces of grassy ELM regime in favorable and unfavorable B_t are compared,the l_i space,for the access to grassy ELM regime in favorable B_t configuration has also been demonstrated to be wider than that in unfavorable B_t configuration,and the operational spaces of n_el,b_p andd_u show no significant difference for the access to grassy ELM regime in the two different magnetic configurations.Furthermore,it was found that clustered grassy ELM is a more frequently-occurred phenomenon in favorable B_t.Statistical analysis shows that larger plasma-wall outer gap,longer plasma elongation,lower low-hybrid wave（LHW）heating power and electron cyclotron resonance heating（ECRH）power favor the occurrence of clustered grassy ELMs.Further studies indicate that the generation of clustered grassy ELMs could be directly correlated with the lower electron temperature in the bulk plasma.In general,studying the influence of lithium coating on recycling when the cumulative effect is or not saturated,the comparison of grassy ELM behavior and parameter space in the different magnetic conguration would be meaningful for the achievement of 400s long-pulse H-mode on EAST.