Pedestal and confinement properties under shape and magnetic topology variation on Alcator C-Mod*

Recent work on Alcator C-Mod has examined the influence of magnetic topology and equilibrium shape on edge pedestal structure and plasma confinement. H-mode pedestal parameters show a striking sensitivity to the ion $\nabla B$ drift direction, relative to the active x-point position, with considerable variability observed when the distance between separatrices is on the order of the pedestal width ($\sim$5mm) or less, {\it i.e.} very near double null (DN). Near DN H-modes can have improved confinement factors ($H_{98}>1$) as a result of elevated pedestal temperature ($T_{\mbox{ped}}$), with the edge regulated by benign small edge-localized modes (ELMs) or continuous modes. Such operational regimes with no large ELMs are desirable for ITER and other future devices. Discharges with L-mode-like particle confinement, yet with $H_{98}\approx1$ and $T_{\mbox{ped}}\approx 1\mbox{keV}$, were maintained steady-state by operating with high current, strong shaping and unfavorable $\nabla B$ drift direction, while holding input power below the L-H threshold to prevent particle barrier formation. The pedestal and confinement properties of these improved ELM-free regimes will be compared to those of typical H-modes.