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E. Tolman, J. Hughes, S. Wolfe, S. Wukitch, B. LaBombard, A. Hubbard, E. Marmar, P. Snyder, M. Schmidtmayr:
"Influence of high magnetic field on access to stationary H-modes and pedestal characteristics in Alcator C-Mod";
Nuclear Fusion, 58 (2018), 04600401 - 04600411.

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 × 1020 m−3 to 2.8 × 1020 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 ([1]), 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 ([2]), 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 × 1020 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 × 1020 m−3 to 4.4 × 1020 m−3, and higher transient densities, and had values
of q95 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 Dα (EDA) regime, seen at high densities and high values of q95; the nonstationary
ELM-free regime, seen at lower densities and values of q95; and the ELMy regime, seen at
low density, moderate q95, 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 [3, 4] predictions, and a scaling law for EDA density
pedestal height developed between 4.5 T 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.

pedestal, H-mode, EPED, threshold, EDA H-mode, 7.8 T