Influence of high magnetic field on access to stationary H-modes and pedestal characteristics in Alcator C-Mod

Journal Article · Wed Feb 07 00:00:00 EST 2018 · Nuclear Fusion

DOI:https://doi.org/10.1088/1741-4326/aaa8cc· OSTI ID:1540534

Tolman, E. A. ^[1]; Hughes, J. W.  ^[1]; Wolfe, S. M. ^[1]; Wukitch, S. J. ^[1]; LaBombard, B.  ^[1]; Hubbard, A. E. ^[1]; Marmar, E. S. ^[1]; Snyder, P. B. ^[2]; Schmidtmayr, M. ^[3]

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1. Massachusetts Institute of Technology (MIT), Cambridge, MA (United States). Plasma Science and Fusion Center
2. General Atomics, San Diego, CA (United States)
3. Technische University Wien (Austria)

We report recent Alcator C-Mod experiments have explored access to and characteristics of H-modes at magnetic fields approaching 8 T, the highest field achieved to date in a diverted tokamak. The H-modes originated from L-mode densities ranging from 1.1 x 10²⁰ m^-3 to 2.8 x 10²⁰ m^-3, allowing insight into the density dependence of the H-mode power threshold at high magnetic field. This dependence is compared to predictions from the ITPA scaling law, finding that the law is approximately accurate at 7.8 T. However, the law underpredicted the high density H-mode threshold at lower magnetic field in previous C-Mod experiments, suggesting that the overall dependence of the threshold on magnetic field is weaker than predicted by the scaling law. The threshold data at 7.8 T also indicates that the onset of a low density branch at this magnetic field on C-Mod occurs below 1.4 x 10²⁰ m^-3, which is lower than predicted by an existing model for low density branch onset. The H-modes achieved steady-state densities ranging from 2.3 x 10²⁰ m^-3 to 4.4 x 10²⁰ m^-3, and higher transient densities, and had values of q₉₅ from 3.3 to 6.0. This parameter range allowed the achievement of all three types of H-mode routinely observed at lower magnetic field on C-Mod: the stationary, ELM-suppressed Enhanced D_α (EDA) regime, seen at high densities and high values of q₉₅; the nonstationary ELM-free regime, seen at lower densities and values of q₉₅; and the ELMy regime, seen at low density, moderate q₉₅, and specialized plasma shape. The parameter space in which these regimes occur at 7.8 T is consistent with lower magnetic field experience. Pressure pedestal height at 7.8 T is compared to EPED predictions, and a scaling law for EDA density pedestal height developed between 4.5 T and 6.0 T is updated to include fields from 2.7 T to 7.8 T. Overall, this analysis increases confidence in the use of low magnetic field experience to predict some elements of high magnetic field tokamak behavior.

• Research Organization: Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Fusion Energy Sciences (FES); National Science Foundation (NSF)
• Grant/Contract Number: FC02-99ER54512; SC0014264; 1122374
• OSTI ID: 1540534
• Report Number(s): NF-102160
• Journal Information: Nuclear Fusion, Vol. 58, Issue 4; ISSN 0029-5515
• Publisher: IOP Science
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
pedestal
H-mode
EPED
threshold
EDA H-mode
7.8 T