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Plasma Physics (Physics/ECE/NE 922) Seminar
H-mode and ELM Studies at Near-Unity Aspect Ratio
Date: Monday, October 12th
Time: 12:00 pm
Place: 2241 Physics Bldg
Speaker: Kathreen Thome, Engineering Physics
Abstract: 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 Dα emissions; increased core rotation; increased central heating; formation of edge current and pressure pedestals; and improved energy confinement. The H-mode power threshold, PLH, behaves quite differently at low-A when compared with high-A operations. PLH/PLH_ITPA08 increases sharply as A is lowered and no difference in PLH for limited and diverted plasmas is observed at A ~ 1.2. No minimum in PLH with density is observed. Some of these results are consistent with the FM3 model for the L-H transition [1]. Two classes of ELMs have been observed. Small, Type III-like ELMs are present at low input power and have n ≤ 4. At POH >> PLH, they transition to large, Type-I-like ELMs with intermediate 5 < n < 15. The Type III ELM magnetic structures behave opposite to that observed in 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 Jedge(R,t) pedestal collapse during an ELM event show a complex, multimodal pedestal collapse and the subsequent ejection of a current-carrying filament.
Host: Engineering Physics
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