Operational Phase Space of the Edge Plasma in Alcator C-Mod

Earlier C-Mod experiments [1] uncovered some clear connections between the operational space of the near scrape-off layer (SOL) and that expected from electromagnetic fluid drift turbulence theory -- at fixed values of normalized collisionality (`diamagnetic parameter', $\alpha$d, pressure gradients increase with plasma current squared, holding the MHD ballooning parameter, $\alpha$MHD , unchanged. Thus, the edge plasma state occupies a narrow band within this two-parameter phase-space; both L and H-mode profiles appear controlled by a critical-gradient transport paradigm. Here, we examine a wider parameter range in lower and upper single null (LSN/USN) discharges and with an improved edge diagnostic set: Langmuir-Mach probes on high and low-field SOLs and D$\alpha$ imaging on the low-field SOL. Preliminary results indicate a sensitivity of L-mode gradients to magnetic topology, favoring higher sensitivity of L-mode gradients to magnetic topology, favoring higher $\alpha$MHD for LSN compared to USN. These observations potentially connect to the different SOL plasma flows and velocity shears that arise in LSN versus USN [2]. [1] B. LaBombard, et al., submitted to Nuclear Fusion [2] Nucl. Fusion \textbf{44} (2004) 1047, Phys. Plasmas \textbf {12} (2005) 056111.