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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. Zweben5)
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 (nG). 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/nG (or collisionality) is raised, cross-field heat convection begins to compete with parallel conduction to the divertor. At N/nG $ \sim$ 0.5, TE at the separatrix is reduced. As n/nG 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/nG is increased, cross-field edge transport physics progressively changes, ultimately impacting the power balance of the discharge near N/nG $ \sim$ 1.

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IAEA 2003