Influence of High Magnetic Field on Access to Stationary H-modes and Pedestal Characteristics in Alcator C-Mod

Dataset · Tue Oct 16 00:00:00 EDT 2018

DOI:https://doi.org/10.7910/DVN/QBOO7B· OSTI ID:1879092

Tolman, E. A. ^[1]; Hughes, J. W.; Wolfe, S. M.; Wukitch, S. J.; LaBombard, B.; Hubbard, A. E.; Marmar, E. S.; Snyder, P. B.; Schmidtmayr, M.

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1. OSTI

Recent Alcator C-Mod experiments have explored access to and characteristics of H-modes at magnetic fields approaching 8 T, the highest field achieved to date in a diverted tokamak. The H-modes originated from L-mode densities ranging from $$1.1 \times 10^{20} m^{-3}$$ to $$2.8 \times 10^{20} m^{-3}$$, allowing insight into the density dependence of the H-mode power threshold at high magnetic field. This dependence is compared to predictions from the ITPA scaling law (\cite{martin2008power}), finding that the law is approximately accurate at 7.8 T. However, the law underpredicted the high density H-mode threshold at lower magnetic field in previous C-Mod experiments (\cite{ma2012scaling}), \hl{suggesting that the overall dependence of the threshold on magnetic field is weaker than predicted by the scaling law.} The threshold data at 7.8 T also indicates that the onset of a low density branch at this magnetic field on C-Mod occurs below $$1.4 \times 10^{20} m^{-3}$$, which is lower than predicted by an existing model for low density branch onset. The H-modes achieved steady-state densities ranging from $$2.3 \times 10^{20} m^{-3}$$ to $$4.4 \times 10^{20} m^{-3}$$, and higher transient densities, and had values of $$q_{95}$$ from 3.3 to 6.0. This parameter range allowed the achievement of all three types of H-mode routinely observed at lower magnetic field on C-Mod: the stationary, ELM-suppressed enhanced D-alpha (EDA) regime, seen at high densities and high values of $$q_{95}$$; the nonstationary ELM-free regime, seen at lower densities and values of $$q_{95}$$; and the ELMy regime, seen at low density, moderate $$q_{95}$$, and specialized plasma shape. The parameter space in which these regimes occur at 7.8 T is consistent with lower magnetic field experience. Pressure pedestal height at 7.8 T is compared to EPED \cite{snyder2009development, snyder2011first} predictions, and a scaling law for EDA density pedestal height developed between 4.5 and 6.0 T is updated to include fields from 2.7 T to 7.8 T. Overall, this analysis increases confidence in the use of low magnetic field experience to predict some elements of high magnetic field tokamak behavior.

• Research Organization: Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States). Plasma Science and Fusion Center
• Sponsoring Organization: USDOE Office of Science (SC), Fusion Energy Sciences (FES)
• DOE Contract Number: FC02-99ER54512; SC0014264
• OSTI ID: 1879092
• Country of Publication: United States
• Language: English

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70 PLASMA PHYSICS AND FUSION TECHNOLOGY