Effect of Aspect Ratio on H-mode and ELM Characteristics

The H-mode confinement regime is achieved at near-unity aspect ratio ($A<1.2)$ in the Pegasus Toroidal Experiment via high-field-side fueling and low edge recycling. Ohmic H-mode is attained in both limited and diverted magnetic topologies. This regime is characterized by: reduced D$_{\alpha}$ emissions; increased core rotation; increased central heating; formation of edge current and pressure pedestals; and measured energy confinement consistent with the ITER98pb(y,2) scaling. The H-mode power threshold, $P_{LH} $, behaves quite differently at low-$A$ when compared with high-$A$ operations. $P_{LH} /P_{LH\_ITPA08}$ increases sharply as $A$ is lowered and no difference in $P_{LH} $ for limited and diverted plasmas is observed at $A\sim 1.2$. No minimum in $P_{LH} $ with density is observed. Some of these results are consistent with the FM$^{3}$ model for the L-H transition.\footnote{Fundamenski \textit{et al}., Nucl. Fusion \textbf{52}, 062003 (2012.)} Two classes of ELMs have been observed. Small, Type III-like ELMs are present at low input power and have $n\le 4$. At $P_{OH} >>P_{LH}$, they transition to large, Type-I-like ELMs with intermediate $5 < n < 15$. The Type III ELM magnetic structures behave opposite that of high-$A$ plasmas, with $n$ much higher, presumably due to the naturally higher $J/B$ peeling mode drive at low-$A$. Long-sought measurements of the $J_{edge} (R,t)$ pedestal collapse during an ELM event show a complex, multimodal pedestal collapse and the subsequent ejection of a current-carrying filament.