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Steady I-mode Plasmas in Alcator C-Mod

I-mode operation on the Alcator C-Mod tokamak combines a strong edge thermal transport barrier with L-mode levels of particle and impurity transport, allowing access to very high performance discharges with low pedestal collisionality, high central temperatures up to 9 keV, and without large ELMs or other intermittent edge instabilities. In recent experiments, C-Mod Imodes have been extended to quasi-steady-state. I-modes with normalized energy confinement quality factor H_98~1.0 to 1.2 can be maintained with input power up to nearly two times the threshold power, with the largest accessible range in closed divertor geometry at modest triangularity. Simple extrapolations at fixed field imply that ITER could access I-mode with available power, and stay in I-mode with alpha-dominant heating. Detailed pedestal fluctuation measurements reveal changes in the turbulence, with decreases in the power at some size scales and growth of a weakly coherent mode (WCM) (k~1.5 cm -1 , f/f ~ .3) which propagates in the electron diamagnetic direction in the plasma frame. The WCM, which has density, temperature and magnetic signatures, appears to play a key role in pedestal density and impurity regulation. Stability analysis shows that the typical I-mode pedestal is well away from the peeling-ballooning boundary.The distribution of divertor exhaust power depends on ion drift direction, with nearly equal power flows to the inner and outer strike points in single-null configurations.