Achieving Low-Collisionalty Small/Grassy ELMs in DIII-D High-Performance Hybrid Scenario Plasmas

Low-collisionality(nu_e*~0.1-0.4) small/grassy ELMs have been achieved in recent DIII-D high-performance hybrid scenario plasmas (beta_N~3.5, beta_p~2.0, H_98~1.6), with ITER similar shape and favorable direction. Stability analysis found that grassy ELMs with high-frequency >400Hz and small ELM energy loss <1.0% are located near the peeling boundary. The high >1.5 and low pedestal top collisionality nu_e*=0.1-0.4 are important in obtaining grassy ELMs on DIII-D. A branch of low-collisionality small ELMs is observed and its ELM size increases with the collisionality increase, which is opposite to the typical type-I ELM scaling. Long stationary phases of grassy ELMs have been achieved by using the n=3 RMP of 1.5kA I-coil current, without occasional large ELMs and without clear density pump out. Additionally, ELM frequency is further increased while the ELM size is further decreased for the RMP-assisted grassy ELMs. Divertor detachment was achieved with divertor N₂ puffing, while high confinement with beta_N>3.0 is maintained. However, the pedestal density increases during detachment, and large ELM occurs. The study sheds light on a promising scenario with high core confinement, good edge impurity exhaust, and acceptant boundary heat load for ITER and future machines.