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Title: H-mode plasmas at very low aspect ratio on the Pegasus Toroidal Experiment

H-mode is obtained at $$A\sim 1.2$$ in the Pegasus Toroidal Experiment via Ohmic heating, high-field-side fueling, and low edge recycling in both limited and diverted magnetic topologies. These H-mode plasmas show the formation of edge current and pressure pedestals and a doubling of the energy confinement time to $${{H}_{98y,2}}\sim 1$$ . The L–H power threshold $${{P}_{\text{LH}}}$$ increases with density, and there is no $${{P}_{\text{LH}}}$$ minimum observed in the attainable density space. The power threshold is equivalent in limited and diverted plasmas, consistent with the FM3 model. However, the measured $${{P}_{\text{LH}}}$$ is $$\sim 15\,\,\times $$ higher than that predicted by conventional International Tokamak Physics Activity (ITPA) scalings, and $${{P}_{\text{LH}}}/{{P}_{\text{ITPA}08}}$$ increases as $$A\to 1$$ . Small ELMs are present at low input power $${{P}_{\text{IN}}}\sim {{P}_{\text{LH}}}$$ , with toroidal mode number $$n\leqslant 4$$ . At $${{P}_{\text{IN}}}\gg {{P}_{\text{LH}}}$$ , they transition to large ELMs with intermediate 5 < n < 15. The dominant-n component of a large ELM grows exponentially, while other components evolve nonlinearly and can damp prior to the crash. Direct measurements of the current profile in the pedestal region show that both ELM types exhibit a generation of a current-hole, followed by a pedestal recovery. Large ELMs are shown to further expel a current-carrying filament. Small ELM suppression via injection of low levels of helical current into the edge plasma region is also indicated.
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  1. Univ. of Wisconsin-Madison, Madison, WI (United States)
Publication Date:
Grant/Contract Number:
Accepted Manuscript
Journal Name:
Nuclear Fusion
Additional Journal Information:
Journal Volume: 57; Journal Issue: 2; Related Information: K.E. Thome, M.W. Bongard, J.L. Barr, G.M. Bodner, M.G. Burke, R.J. Fonck, D.M. Kriete, J.M. Perry, J.A. Reusch, and D.J. Schlossberg, "Public Data Set: H-mode Plasmas at Very Low Aspect Ratio on the Pegasus Toroidal Experiment," DOI: 10.18138/1236495; Journal ID: ISSN 0029-5515
IOP Science
Research Org:
Univ. of Wisconsin-Madison, Madison, WI (United States)
Sponsoring Org:
USDOE Office of Science (SC), Fusion Energy Sciences (FES) (SC-24)
Country of Publication:
United States
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; H-mode; spherical tokamaks; equilibrium reconstructions; energy confinement; ELM; ELM dynamics
OSTI Identifier:
Alternate Identifier(s):
OSTI ID: 1327562