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(EX/D2-1) Density Limit and Cross-Field Edge Transport Scaling in Alcator C-Mod

B. LaBombard¹⁾, M. Greenwald¹⁾, R.L. Boivin²⁾, B.A. Carreras³⁾, J.W. Hughes¹⁾, B. Lipschultz¹⁾, D. Mossessian¹⁾, C.S. Pitcher⁴⁾, J.L. Terry¹⁾, S.J. Zweben^5)
 
1) MIT Plasma Science and Fusion Center, Cambridge, MA, USA
²⁾ General Atomics, San Diego, CA, USA
³⁾ Oak Ridge National Laboratory, Oak Ridge, TN, USA
⁴⁾ Institute for Aerospace Studies, University of Toronto, Toronto, Canada
⁵⁾ Princeton Plasma Physics Laboratory, Princeton, NJ, USA

Abstract.  Experiments in Alcator C-Mod have uncovered a direct link between the character and scaling of edge transport and the empirical Greenwald density limit (n_G). In low to moderate density discharges, the scrape-off layer (SOL) exhibits a two-layer structure: a near SOL ($\sim$5 mm zone) with steep density and temperature gradients and a far SOL with flatter profiles. In the far SOL, the transport fluxes exhibit large transport events (`bursts' which carry particles to main-chamber structures. In the near SOL, transport fluxes appear to be less `bursty' particle diffusivities in this region is found to increase strongly with local plasma collisionality. As n/n_G (or collisionality) is raised, cross-field heat convection begins to compete with parallel conduction to the divertor. At N/n_G $\sim$ 0.5, T_E at the separatrix is reduced. As n/n_G approaches $\sim$1, regions inside the separatrix exhibit flatter profiles with `bursty' transport behavior; cross-field heat convection to main-chamber structures becomes comparable to the radiated power. Thus as n/n_G is increased, cross-field edge transport physics progressively changes, ultimately impacting the power balance of the discharge near N/n_G $\sim$ 1.

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