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

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.