Critical edge gradients and flows with reversed magnetic field in Alcator C-Mod

Both L- and H-mode gradients near the separatrix in C-Mod appear to be set by a ‘critical gradient’ condition [1] -- at fixed values of normalized collisionality, pressure gradient scales as plasma current squared, holding the MHD ballooning parameter, $\alpha_{mhd}$, unchanged. With normal magnetic field direction, lower-null (LSN) L-mode discharges attain higher $\alpha_{mhd}$ values than upper-null (USN). In addition, near-sonic scrape-off-layer flows on the high-field side are co-current directed in LSN and counter in USN, suggesting that such flows may influence critical gradient values. Recently, LSN/USN discharges with reversed magnetic field have been studied, revealing persistent trends: regardless of $B$-field direction, higher $\alpha_{mhd}$ values are attained when $B\times\nabla B$ points towards the x-point -- a generalized condition that results in co-current directed flow in the high-field SOL. The latter observation supports the hypothesis that ballooning-like cross-field transport is responsible for the near-sonic parallel plasma flows observed there [2].\par {\noindent [1] {\it Nucl. Fusion} {\bf 45} (2005) 1658.} \par {\noindent [2] {\it Nucl. Fusion} {\bf 44} (2004) 1047.}