Progress in Performance and Understanding of Steady ELM-free I-modes on Alcator C-Mod

The I-mode regime of operation has been extended in recent Alcator C-Mod campaigns in duration and robustness, over a wide range of parameters. It features an edge thermal barrier, with L-mode like density profiles and particle transport [1]. I-modes are now routinely maintained in stationary conditions for over 10 $\tau _E $. They are usually ELM free, a key advantage given the concern over divertor heat pulses on ITER. Instead, a continuous pedestal fluctuation, the ``weakly coherent mode,'' appears to enhance selectively particle over thermal transport. High performance I-modes are usually obtained with unfavorable ion drift direction, and extend to low q$_{95}$=2.5and $\nu _{ped}^\ast $=1.3. $\tau _E $ is in the range of H-mode (H$_{98,y2}$ up to 1.2), and exhibits less power degradation (W$\sim $P$^{0.7})$. Power thresholds for I-mode are higher than typical L-H scalings, and increase with I$_{p}$ as well as density. The widest power range for I-mode, nearly a factor of two above the L-I threshold, has been obtained in reversed field, lower null discharges at moderate n$_{e}$. The focus of 2012 experiments is on assessing the density range and dependences, important for extrapolating to ITER, and measuring the pedestal profiles and fluctuations in greater detail.\\[4pt] [1] Whyte D G \textit{et al }2010 \textit{Nucl. Fusion} \textbf{50} 105005