Observations and empirical scalings of the high-confinement mode pedestal on Alcator C-Mod

On the Alcator C-Mod tokamak [Phys. Plasmas 1, 1511 (1994)], radial profiles of electron temperature (T-e) and density (n(e)) are measured at the plasma edge with millimeter resolution Thomson scattering [Rev. Sci. Instrum. 72, 1107 (2001)]. Edge transport barriers in the high confinement regime (H mode) exhibit T-e, n(e) pedestals with typical widths of 2-6 mm, with the T-e pedestal on average slightly wider than and inside the n(e) pedestal. Measurements at both the top and the base of the pedestal are consistent with profiles obtained using other diagnostics. The two primary H-mode regimes on C-Mod, enhanced D-alpha (EDA) and edge-localized mode free, have been examined for differences in pedestals. EDA operation is favored by high edge collisionality nu(*), in addition to high edge safety factor q(95). Scaling studies at fixed shape yield little systematic variation of pedestal widths with plasma parameters, though higher triangularity is seen to increase the n(e) pedestal width dramatically. Pedestal heights and gradients show the clearest dependencies on plasma control parameters. Pedestal n(e) and T-e both scale linearly with plasma current I-P, while pedestal T-e depends strongly on power flowing from the core plasma into the scrape-off layer P-SOL. The electron pressure (p(e)) pedestal and p(e) gradient both scale with (IPPSOL1/2)-P-2. Plasma stored energy W-P scales with pedestal p(e), implying that pedestal scalings may in large part determine global confinement scalings. (C) 2002 American Institute of Physics.