DIII-D high beta hybrid with high frequency ELMs

Recent high beta (β_N ~ 3.5), high confinement (H_98y2 ~ 1.3) DIII-D hybrid experiments observed an increase in the ELM frequency at higher plasma density. These experiments showed increased confinement quality in low-rotation hybrid plasmas by increasing the plasma density (line-averaged density of ~7x10¹⁹ m^-3) and toroidal current (Ip = 1.2 MA). Discharges with higher scrape-off layer density showed an increase in the ELM frequency and a loss of type-I ELMs. Stability analysis with BOUT++ indicates that the plasma is unstable to a resistive ballooning mode near the separatrix. The modeling shows that this mode becomes unstable at a low pedestal pressure and high separatrix density/collisionality, leading to the high frequency ELMs. These high frequency ELMs prevent the pedestal pressure from increasing to the high levels needed for type-I ELMs to form. This operating regime appears to be an extension to higher beta and confinement of the quasi-continuous exhaust (QCE) mode discovered on ASDEX-Upgrade [1].

[1] Harrer, G.F., et al. Phys. Rev. Let. 129, 165001 (2022)