H-mode and ELM Characteristics at Ultralow Aspect Ratio in the Pegasus Experiment

Operation at low $B_{T} $ and $A<1.3$ allows access to the H-mode regime in the Pegasus experiment using only Ohmic heating.\footnote{ K.E. Thome \textit{et al.}, \textit{Phys. Rev. Lett.} \textbf{116}, 175001 (2016).} Modest plasma parameters in this regime permit detailed probe measurements of the edge pedestal region. H-mode plasmas have standard L-H transition phenomena: a drop in $D_{\alpha } $ radiation; formation of pressure and current pedestals; field-aligned filament ejection during ELMs; and a doubling of $\tau_{E} $ from $H_{98} \sim 0.5$ to $\sim 1$. The L-H power threshold $P_{LH} $ increases monotonically with $n_{e} $, consistent with both the ITPA08 scaling, $P_{ITPA08}_{\mathrm{,}}$ used for ITER and the theoretical FM$^{\mathrm{3}}$ power threshold model. Unlike at high $A$, $P_{LH} $ is comparable in limited and single-null diverted topologies at $A\sim 1.2$, consistent with FM$^{\mathrm{3}}$ predictions. $P_{LH} /P_{ITPA08} $ increases rapidly as $A\to 1$, and is $>10$ for $A<1.3$. Multiple-$n$ modes are observed during ELMs, consistent with excitation of multiple peeling-ballooning modes. Small, Type-III-like ELMs occur at $P_{OH} \sim P_{LH} $ with $n\le 4$. Large, Type-I-like ELMs occur with $P_{OH} >P_{LH} $ and intermediate $5