I-mode: an H-mode energy confinement regime with L-mode particle transport in Alcator C-Mod

Abstract

An improved energy confinement regime, I-mode, is studied in Alcator C-Mod, a compact high-field divertor tokamak using ion cyclotron range of frequencies (ICRFs) auxiliary heating. I-mode features an edge energy transport barrier without an accompanying particle barrier, leading to several performance benefits. H-mode energy confinement is obtained without core impurity accumulation, resulting in reduced impurity radiation with a high-Z metal wall and ICRF heating. I-mode has a stationary temperature pedestal with edge localized modes typically absent, while plasma density is controlled using divertor cryopumping. I-mode is a confinement regime that appears distinct from both L-mode and H-mode, combining the most favourable elements of both. The I-mode regime is investigated predominately with ion ∇B drift away from the active X-point. The transition from L-mode to I-mode is primarily identified by the formation of a high temperature edge pedestal, while the edge density profile remains nearly identical to L-mode. Laser blowoff injection shows that I-mode core impurity confinement times are nearly identical with those in L-mode, despite the enhanced energy confinement. In addition, a weakly coherent edge MHD mode is apparent at high frequency ∼100–300 kHz which appears to increase particle transport in the edge. The I-mode regime has been obtained over a wide parameter space (B T = 3–6 T, I p = 0.7–1.3 MA, q 95 = 2.5–5). In general, the I-mode exhibits the strongest edge temperature pedestal (T ped) and normalized energy confinement (H 98 > 1) at low q 95 (<3.5) and high heating power (P heat > 4 MW). I-mode significantly expands the operational space of edge localized mode (ELM)-free, stationary pedestals in C-Mod to T ped ∼ 1 keV and low collisionality , as compared with EDA H-mode with T ped < 0.6 keV, . The I-mode global energy confinement has a relatively weak degradation with heating power; leading to increasing H 98 with heating power.