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 (ICRF) 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 (ELMs) 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 favorable elements of both. The I-mode regime is obtained 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 Lmode. 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=3-6 T, I p =0.7-1.3 MA, q 95 =2.5-5). In general the I-mode exhibits the strongest edge T pedestal 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 ELM-free, stationary pedestals in C-Mod to T ped~1 keV and low collisionality ν* ped~0 .1, as compared to EDA H-mode with T ped < 0.6 keV, ν* ped >1. The I-mode global energy confinement has a relatively weak degradation with heating power; W th ~ I p P heat 0.7 leading to increasing H 98 with heating power.2

Heat-flux footprints for I-mode and EDA H-mode plasmas on Alcator C-Mod

“…Thus the edge profile gradient-scale-lengths for T e are similar in I-mode and H-mode, while the edge gradient-scale-lengths for n e are different, with L ne I-mode, ped ~ 10xL ne H-mode,ped [10]. We compare the I-mode and H-mode heat-flux footprints, both as part of the evaluation of the I-mode regime and for insight into the physics of what determines the widths.…”

“…The I-mode plasmas are of particular interest because of their attractive confinement features [10,11] and because the edge heat and particle transport are more de-coupled than in H-mode, allowing for a large temperature pedestal and high energy-confinement (H 98 <1.2) without a density pedestal and without ELMs regulating the edge pressure. Thus the edge profile gradient-scale-lengths for T e are similar in I-mode and H-mode, while the edge gradient-scale-lengths for n e are different, with L ne I-mode, ped ~ 10xL ne H-mode,ped [10].…”

Major upgrades of the high frequency B-dot probe diagnostic suite on ASDEX Upgrade

“…The so-called I-modes, which have beneficial transport properties [11], are also of interest on ASDEX Upgrade. One of the defining features of the I-mode is the appearance of a mode in the plasma pedestal region, with a characteristic frequency of ~100 kHz.…”

“…5 (b)). The feature is commonly called the weakly coherent mode, or WCM [11]. Note, that the I-mode was "sampled" by the manipulator probe in the inserted position at the major radius of the limiter, while the L-and 3.…”

X-ray observations of medium Z H- and He-like ions with satellites from C-Mod tokamak plasmas

“…C-Mod has access to the following confinement regimes: Ohmic and ion cyclotron range of frequencies heated low confinement mode (L-mode), I-mode [63], and enhanced D α (EDA) [64] and ELM-free high confinement mode (H-mode). Electron densities for this study, determined by Thomson scattering [65], were in the range from 3×10 19 /m 3 to 5×10 20 /m 3 with central electron temperatures (from Thomson scattering and electron cyclotron emission [65]) between 0.8 and 5.5 keV.…”