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

Investigation of the critical edge ion heat flux for L-H transitions in Alcator C-Mod and its dependence on B_T

This paper presents investigations on the role of the edge ion heat flux for transitions from L-mode to H-mode in Alcator C-Mod. Previous results from the ASDEX Upgrade tokamak indicated that a critical value of edge ion heat flux per particle is needed for the transition. Analysis of C-Mod data confirms this result. The edge ion heat flux is indeed found to increase linearly with density at given magnetic field and plasma current. Furthermore, the Alcator C-Mod data indicate that the edge ion heat flux at the L-H transition also increases with magnetic field. Combining the data from Alcator C-Mod and ASDEX Upgrade yields a general expression for the edge ion heat flux at the L-H transition. These results are discussed from the point of view of the possible physics mechanism of the L-H transition. They are also compared to the L-H power threshold scaling and an extrapolation for ITER is given