H-mode pedestal and threshold studies over an expanded operating space on Alcator C-Mod

Hubbard, A.; Hughes, J. W.; Bespamyatnov, I.O.; Biewer, T. M.; Cziegler, I.; LaBombard, B.; Lin, Y.; McDermott, R. M.; Rice, J. E.; Rowan, W. L.; Snipes, J. A.; Terry, J. L.; Wolfe, S.; Wukitch, S.J.

doi:10.1063/1.2714297

Cited by 28 publications

(45 citation statements)

References 34 publications

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“…Similar transient increases of edge electron temperature [10] and improved L-mode energy confinement [5] have been reported on CMod. Threshold studies on these tokamaks, as well as DIII-D, showed clearly that edge temperatures, T gradients and required heating power are substantially higher at the L-H transition in the unfavorable vs. favorable ∇B configuration [11][12][13][14].…”

Section: Introductionsupporting

confidence: 78%

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

Whyte

Hubbard

Hughes³

et al. 2010

Nucl. Fusion

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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

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Section: Introductionsupporting

confidence: 78%

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

Whyte

Hubbard

Hughes³

et al. 2010

Nucl. Fusion

Self Cite

281

370

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“…The higher local Te threshold and multi-step dynamics with unfavorable drift were later studied in more detail. 178 These early observations suggests that the I-mode is not an exotic and elusive regime, but is rather a standard behavior under suitable conditions. Figure 28 shows time traces from a typical I-mode and illustrates the salient characteristics.…”

Section: I-modementioning

confidence: 99%

20 years of research on the Alcator C-Mod tokamak

et al. 2014

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The object of this review is to summarize the achievements of research on the Alcator C-Mod tokamak [Hutchinson et al., Phys. Plasmas 1, 1511 and Marmar, Fusion Sci. Technol. 51, 261 (2007)] and to place that research in the context of the quest for practical fusion energy. C-Mod is a compact, high-field tokamak, whose unique design and operating parameters have produced a wealth of new and important results since it began operation in 1993, contributing data that extends tests of critical physical models into new parameter ranges and into new regimes. Using only highpower radio frequency (RF) waves for heating and current drive with innovative launching structures, C-Mod operates routinely at reactor level power densities and achieves plasma pressures higher than any other toroidal confinement device. C-Mod spearheaded the development of the vertical-target divertor and has always operated with high-Z metal plasma facing componentsapproaches subsequently adopted for ITER. C-Mod has made ground-breaking discoveries in divertor physics and plasma-material interactions at reactor-like power and particle fluxes and elucidated a) Paper AR1 1, Bull. Am. Phys. Soc. 58, 21 (2013). b) Invited speaker.

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“…I-mode is typically accessed in the "unfavorable" drift configuration -that is, with ion ∇B drift directed away from the primary X-point [20]. This elevates the H-mode threshold power [40], widening the range over which I-mode can be sustained. This drift configuration has been achieved both with standard field and current in an upper-null shape, and in a lower-null shape with reversed field and current.…”

Section: I-mode Access and Experimental Setupmentioning

confidence: 99%

Edge-localized mode avoidance and pedestal structure in I-mode plasmas

et al. 2014

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I-mode is a high-performance tokamak regime characterized by the formation of a temperature pedestal and enhanced energy confinement, without an accompanying density pedestal or drop in particle and impurity transport. I-mode operation appears to have naturally-occurring suppression of large ELMs in addition to its highly favorable scalings of pedestal structure and overall performance. Extensive study of the ELMy H-mode has led to the development of the EPED model, which utilizes calculations of coupled peeling-ballooning MHD modes and kinetic-ballooning mode (KBM) stability limits to predict the pedestal structure preceding an ELM crash.We apply similar tools to the structure and ELM stability of I-mode pedestals. Analysis of I-mode discharges prepared with high-resolution pedestal data from the most recent C-Mod campaign reveals favorable pedestal scalings for extrapolation to large machines -pedestal temperature scales strongly with power per particle P net /n e , and likewise pedestal pressure scales as the net heating power (consistent with weak degradation of confinement with heating power). Matched discharges in current, field, and shaping demonstrate the decoupling of energy and particle transport in I-mode, increasing fueling to span nearly a factor of two in density while maintaining matched temperature pedestals with consistent levels of P net /n e . This is consistent 2 with targets for increased performance in I-mode, elevating pedestal β p and global performance with matched increases in density and heating power. MHD calculations using the ELITE code indicate that I-mode pedestals are strongly stable to edge peeling-ballooning instabilities. Likewise, numerical modeling of the KBM turbulence onset, as well as scalings of the pedestal width with poloidal beta, indicate that I-mode pedestals are not limited by KBM turbulence -both features identified with the trigger for large ELMs, consistent with the observed suppression of large ELMs in I-mode.

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H-mode pedestal and threshold studies over an expanded operating space on Alcator C-Mod

Cited by 28 publications

References 34 publications

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

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

20 years of research on the Alcator C-Mod tokamak

Edge-localized mode avoidance and pedestal structure in I-mode plasmas

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