Access to and performance of I-mode plasmas on Alcator C-Mod

The I-mode regime of operation features an edge thermal transport barrier, without a particle barrier. Stationary conditions are thus achieved without impurity accumulation, and usually without ELMs. In contrast to the EDA H-mode regime on Alcator C-Mod, it is readily accessed at low q$_{95}$ and low collisionality, both relevant for ITER. Analysis of a dataset of 400 discharges at q$_{95}$ $\sim$ 3 shows normalized energy confinement in I-modes reaches or exceeds that in most H-modes, up to H$_{98} =$ 1.2. Confinement and pedestal temperature increase with input power. In some cases I-mode is maintained up to the maximum available power (5 MW ICRF) while in others a transition to H-mode limits the performance. Understanding and extending the conditions for entering and staying in I-mode is thus critical for extrapolation of the regime. Experiments have extended the regime both to lower densities and to higher densities and powers through gas puffing into established I-modes. Results from an expanded database of C-Mod discharges will be presented, along with details of I-mode profiles and fluctuations, including GAMs and a weakly coherent mode, which are providing insights into the physics of the regime.