H-mode threshold physics studies on Alcator C-Mod in support of ITER

Comprehensive studies on H-mode threshold physics have been conducted on Alcator C-Mod tokamak, covering many ITER-relevant conditions, $e.g.$ similar magnetic field and density range, metallic wall, and divertor configuration. C-Mod experiments confirm that the density dependence of H-mode threshold power ($P_{th})$ is U-shaped without clear dependence on plasma current, and the local minimum of $P_{th}$ in density ($n_{min})$ decreases as $B_{T}$ is reduced [1]. An effect of divertor geometry on $P_{th}$ was identified, with a dramatic ($\sim $50{\%}) reduction in $P_{th}$ seen in ``slot'' divertor operation accompanied by longer SOL connection length [2]. Experimental results were also compared with a new physics-based model for $P_{th }$[3], showing reasonable agreement of density, $B_{T}$, and divertor geometry dependences with model predictions. A significant implication of this model is that $n_{min}$ occurs as the SOL transitions from sheath-limited to conduction-limited regime, which also seems to agree with experiments. Supported by USDoE award DE-FC02-99ER54512. \\[4pt] [1] Y.Ma, \textit{et al} Nucl. Fusion \textbf{52} (2012) 023010.\\[0pt] [2] Y.Ma, \textit{et al} PPCF \textbf{54} (2012) 082002.\\[0pt] [3] W.Fundamenski, \textit{et al} Nucl. Fusion \textbf{52} (2012) 062003.