Relationship between Edge Gradients and Plasma Flows in Alcator C-Mod

The edge pressure profiles in a tokamak are remarkably `stiff', suggestive of a system near marginal stability. Electromagnetic turbulence appears to be an underlying cause [1]: edge pressure gradients normalized by the square of the poloidal magnetic field strength (i.e., the MHD ballooning parameter, $\alpha _{MHD})$ are invariant in plasmas with the same normalized collisionality. Yet, despite this relationship, x-point topology exerts a surprisingly strong influence; higher $\alpha _{MHD}$ are obtained when \textit{Bx}$\nabla B$ points toward the active x-point [2] -- Is this behavior connected to the strong flows and/or toroidal rotation that is observed in the edge, which changes with x-point topology [3]? Recently, an expanded set of diagnostics has been used to explore edge flows in detail for upper/lower x-points with forward/reversed magnetic fields. Systematic differences in parallel flows and toroidal rotation are evident. Cross-field flows measured by scanning Mach probes reveal an enhanced (diminished) velocity shear layer near the separatrix for \textit{Bx}$\nabla B$ pointing toward (away from) the active x-point. [1] Nucl. Fusion \textbf{45} (2005) 1658; [2] Phys. Plasmas \textbf{15} (2008) 056106; [3] Nucl. Fusion \textbf{44} (2004) 1047.