H-mode Characteristics and ELM Dynamics at Near-Unity Aspect Ratio

Ohmic H-mode is achieved at near-unity aspect ratio in the Pegasus Toroidal Experiment through the use of high-field-side fueling in both limited and diverted geometries. This regime is characterized by: increased edge rotation shear; increased central heating; and measured energy confinement consistent with the ITER98pb(y,2) scaling. In limited plasmas the power threshold is $\sim 10\times $ higher than predicted by the high-$A$ empirical tokamak scaling for $n_{G} =0.1-0.6$. No significant reduction in the power threshold has been observed in favorable $\nabla B$ SN plasma when compared to limited plasmas. Two classes of ELMs have been identified to date by their proximity to the power threshold and measured $n$ spectra. Small, Type III-like ELMs are present at input power $P_{OH} \sim P_{th} $ and have $n\le 4$. At $P_{OH} \gg P_{th} $, they transition to large, Type-I-like ELMs with intermediate 5 \textless $n $\textless 15. These general mode numbers are opposite those seen at large $A$ and reflect the increased peeling drive present at low $A. $ The unique operating characteristics available at $A\sim 1$ in Pegasus allow long-sought measurements of the time evolution of the $J_{edge} (R)$ pedestal collapse during an ELM event. They show a complex, multimodal pedestal collapse and the subsequent ejection of a current-carrying filament.