Comparison of natural grassy ELM behavior in favorable/unfavorable B_t in EAST

The properties of grassy edge-localized modes (ELMs) in EAST in the favorable and unfavorable Bt are statistically studied. Statistical analysis indicates that there is no systematical difference in the frequencies of grassy ELMs under the two different magnetic configurations in the similar parameter spaces. The high-frequency grassy ELM (f_ELM > 1 kHz) in unfavorable Bt is dependent on the high poloidal beta βp and high triangularity δ_u, while the high-frequency grassy ELM (f_ELM > 1 kHz) in favorable B_t appears to rely on the high plasma density. A frequently occurring phenomenon in favorable Bt defined as 'clustered ELM' seems to be the most evident difference in ELM behavior between favorable and unfavorable Bt. Statistical analysis shows that larger plasma-wall outer gap, longer plasma elongation, lower low-hybrid wave heating power and electron cyclotron resonance heating power favor the occurrence of clustered grassy ELMs. Further studies indicate that the generation of clustered grassy ELMs could be correlated with the lower electron temperature in the bulk plasma.