Abstract: PSFC/JA-06-19

The operational phase space of the edge plasma and its sensitivity to magnetic topology in Alcator C-Mod

B. LaBombard*, N. Smick, M. Greenwald, J.W. Hughes, B. Lipschultz, K. Marr, J.L. Terry

The range of ‘edge plasma states’ (i.e., the combination of densities, temperatures and gradients in the scrape-off layer near the separatrix) accessible to a tokamak are highly restricted. Experiments in Alcator C-Mod suggest that this restriction is a consequence of Electromagnetic Fluid Drift Turbulence (EMFDT), which sets the cross-field transport levels; plasma states are constrained to lie in a narrow region of EMFDT ‘phase-space’ defined by poloidal beta gradient (a_MHD) and normalized plasma collisionality. Recent experiments have investigated this behavior over an increased parameter range in lower- and upper-null (LSN/USN) discharges. L-mode edge pressure gradients are found to exhibit clear sensitivity to magnetic topology, favoring higher a_MHD for LSN compared to USN. These variations may be caused, in part, by the different edge plasma flow patterns in LSN versus USN. Such flows arise from a ballooning-like transport drive and can produce a topology-dependent modulation in the toroidal plasma rotation.

JNM keywords: P0600 Plasma Properties
PSI-17 keywords: Alcator C-Mod, Cross-Field Transport, Edge Plasma, Plasma Flow, Probes
PACS: 52.25.Fi, 52.30.-q, 52.40.Hf, 52.55.F

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