H-mode access and characteristics at near-unity aspect ratio on the Pegasus Toroidal Experiment

The high-confinement (H-mode) operating regime is achieved at near-unity aspect ratio (A [special character omitted] 1.2) in the Pegasus Toroidal Experiment. Ohmic H-mode is attained via high-field-side fueling and low edge recycling in both limited and diverted magnetic topologies. This features of this regime are: formation of a quiescent edge; reduced Dalpha emissions; increased core rotation; formation of edge current and pressure pedestals; measured energy confinement consistent with the IPB98(y,2) scaling [1]; and increased heating in the electron and ion channels.;The H-mode power threshold, PLH, behaves quite differently at low-A when compared with high- A operations. The threshold has been studied in both limited and favorable single null diverted plasmas in Pegasus, with PLH =10--20x higher than projected by the conventional ITPA08 scaling [2]. This continues and emphasizes the trend indicated from other low-A tokamaks, MAST and NSTX, that increasingly more power than predcted by the scaling is required as A decreases. Unlike at higher- A, no minimum PLH with density is observed. Also in contrast to higher-A tokamaks, where P LH is [special character omitted]2x higher in limited plasmas than diverted plasmas, the threshold in approximately the same in both limited and favorable single null diverted Pegasus plasmas. Some of these PLH results are consistent with the FM3 model for the L-H transition [3].;Two classes of edge localized modes (ELMs) have been observed. Small, Type III-like ELMs are present at low Ohmic input power POH and have toroidal mode number n ≤ . At POH >> PLH, they transition to large, Type-I-like ELMs with intermediate 5 < n < 15. These ELM magnetic structures behave opposite that of high-A plasmas, with n lower, presumably due to the naturally higher J/B peeling mode drive at low-A. The unique edge plasma parameters afforded by near-unity A operations allow long-sought measurements of the edge current profile dynamics during an ELM. Such measurements with a multi-channel magnetic probe array show a complex, multimodal pedestal collapse and the subsequent ejection of a current-carrying filament. [1] Nucl. Fusion, 39, 2175, (1999). [2] Martin et. al, J. Phys: Conf. Ser., 123, 012033, (2008). [3] Fundamenski et. al, Nucl. Fusion, 52, 062003, (2012).