1 EX / 1-3 Progress in Performance and Understanding of Steady ELM-free I-modes on Alcator C-Mod

The I-mode has now been demonstrated on Alcator C-Mod to be a distinct, robust confinement regime over very wide parameter ranges. It is attractive in that it combines the high energy confinement and edge thermal barrier of H-mode with the low particle confinement of L-mode, avoiding impurity accumulation and the need for ELMs to expel particles. Divertor heat flux widths SOL are also larger. Most I-mode discharges are ELM-free, and stability analysis confirms pedestals are below peeling-ballooning boundaries. Energy confinement in high power I-modes is close to the H98y2 scaling, but shows little power degradation. L-I power thresholds with unfavourable drift are above L-H mode scalings for favourable drift, and increase with current as well as density. The regime is most robust on C-Mod with upward B×B and the X-point toward the closed lower divertor, and with moderate target density. In these conditions discharges stay in stationary I-mode for many E, at up to the maximum available ICRH power and at least twice the threshold power. At the L-I transition, edge fluctuations in the 60-150 kHz frequency range decrease, correlated with the decrease in edge eff. A higher frequency Weakly Coherent Mode is usually observed on density, magnetic and Te diagnostics, which correlates with particle flux and appears to maintain particle transport, avoiding H-mode transitions. The regime and mode extend to low q95 and *ped. Initial assessments indicate that I-mode may be accessible on ITER with the planned heating power, at reduced density, and that Q=10 could be achievable at higher density. This exercise also highlights some of the key issues remaining to be addressed. Recent CMod experiments have succeeded in raising I-mode densities via fuelling, and further optimization and expansion of the discharge performance and operating space is planned in future campaigns. Multimachine experiments are needed to establish scalings with machine size and dimensionless parameters for a confident extrapolation.