Intrinsically Grassy ELM Edge Near Peeling Boundary in High Beta Poloidal Regime on DIII-D*

Recent experiments on the DIII-D tokamak have discovered an intrinsically grassy ELM regime operating at reactor-relevant low pedestal collisionalities near the peeling stability boundary, distinct from the typical high-collisionality grassy ELMs near the ballooning boundary. This regime supports a high-confinement, high-β_N core and offers substantial benefits for ITER and future FPPs, addressing critical issues like divertor heat load management and efficient impurity exhaust. These plasmas feature a high-performance Hybrid core (β_N ~ 3.5, β_P ~ 2.0, H₉₈ ~ 1.6, n/n_GW ~ 0.6) coupled with a grassy ELM edge (ELM frequency ~ 400Hz and ELM size △W_ELM/W_ped < 1%) at low edge collisionality (ν^∗_e ~ 0.1). Notably, the divertor heat flux widths in both inter- and intra-grassy ELM phases are about 50% wider than those observed during Type-I inter-ELM phases, displaying a small variation throughout the grassy ELM phases. Accessing this low-collisionality grassy ELM regime involves synthesizing factors such as low pedestal density gradients, high β_P, and widening of density and temperature pedestals. The high β_P induces a strong Grad-Shafranov shift, which stabilizes the ballooning component of the peeling-ballooning modes. Numerical modeling suggests a weak global peeling-ballooning mode, producing much less power flux into SOL compared to large ELM, fosters these grassy ELMs. Unique edge turbulences observed in this regime enhances edge transport, facilitating wide pedestals with gradients below the EPED-KBM scaling predictions. Detachment is achieved through divertor nitrogen injection, however, the increased collisionality correlates with a loss of unique edge turbulences and an increase in ELM size. Since ITER and FPPs can maintain low collisionality pedestals even at high densities, this intrinsically grassy ELM regime offers a promising core-edge integration solution.