Critical gradients and plasma flows in the edge plasma of Alcator C-Mod

Recent experiments in both L- and H-mode plasmas on Alcator C-Mod have lead to a fundamental shift in our views of edge transport physics: transport in the `near' scrape-off-layer (SOL) region may be more appropriately described in terms of a critical gradient phenomena rather than a diffusive and/or convective transport paradigm. L-mode pressure gradients, normalized by the square of the poloidal magnetic field strength (i.e., $\alpha _{MHD})$ appear invariant in plasmas with the same normalized collisionality, despite vastly different currents and magnetic fields. These data suggest that local gradients are pinned to a `critical gradient' condition, which is sensitive to local collisionality -- a behavior that connects with first-principles electromagnetic fluid drift turbulence simulations [1]. H-mode pedestal gradients are found to follow a nearly identical scaling [2]. Thus, the near SOL, which forms the base of the H-mode pedestal, may play a key role in its creation. Prior to an L-H transition, strong SOL plasma flows are found to set a flow boundary condition for the confined plasma [3]. With favorable \textit{Bx}$\nabla B$ direction (i.e., \textit{Bx}$\nabla B$ pointed toward active x-point) these flows tend to spin the plasma in the co-current direction, perhaps reducing the L-H threshold power. Indeed, we find the edge profiles of the L-mode target plasmas to be fundamentally different, depending on the x-point topology: higher values of $\alpha _{MHD}$ are observed for favorable \textit{Bx}$\nabla B$ direction, independent of the direction of $B$ -- supporting evidence that SOL flows play a role in affecting the observed `critical gradient' value. \newline [1] LaBombard, B., \textit{et al.}, Nucl. Fusion \textbf{45} (2005) 1658 \newline [2] Hughes, J.W., \textit{et al.}, Phys. Plasmas \textbf{13} (2006) 056103 \newline [3] LaBombard, B., \textit{et al.}, Nucl. Fusion \textbf{44 }(2004) 1047.