Stationary high-performance grassy ELM regime in EAST

A stationary, high-performance grassy-ELM regime has been successfully accessed on the EAST tokamak since the 2016 campaign. β_p increase either via increase of heating power or q95 are both found to facilitate a higher grassy ELM frequency. Edge measurement of absolute extreme ultraviolet (XUV) radiation indicates that the affected area by grassy ELMs is localized at the pedestal region and the perturbations there induced by grassy ELMs can be 90% smaller than that by type-I ELMs. Parameter scan demonstrates that grassy ELM regime has good density control capacity, and the access to the grassy ELM regime is independent of the toroidal field direction and low-hybrid-wave (LHW) power. Statistics analysis indicates that grassy ELM regime is highly reproducible in a wide parameter space when edge safety factor q_95 and poloidal beta β_p are simultaneously high enough (q_95≥5.3 and β_p≥1.2). High triangularity δ contributes to the increase of grassy ELM frequency while the requirement on high internal inductance l_i in JET tokamak for grassy ELMs cannot be found in EAST. The operational space in pedestal top collisionality ν_(e,ped)^* for the EAST grassy ELMs is in the range of ν_(e,ped)^*~1-6. The exploration towards lower ν_(e,ped)^* is mainly limited by the available heating power. Evolution of a coherent mode at pedestal top suggests that grassy ELMs most likely generate in the steep-gradient region of pedestal, rather than on the pedestal top or near the separatrix. The grassy ELM regime offers a highly promising approach for further exploring long-pulse high-performance H-mode operations in EAST, with potential application to Chinese Fusion Engineering Test Reactor (CFETR) as the baseline scenario and a primary solution for the control of ELMs.