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

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

E. A. Tolman, J. W. Hughes, S. M. Wolfe, S. J. Wukitch, B. Labombard, A. E. Hubbard, E. S. Marmar, P. B. Snyder and M. Schmidtmayr
Nuclear fusion, Vol.58(4), 046004
02/07/2018
DOI: 10.1088/1741-4326/aaa8cc
url
https://www.osti.gov/servlets/purl/1540534View
Open Access

Abstract

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 × 1020 m−3 to 2.8 × 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 ([1]), 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 ([2]), 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 × 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 × 1020 m−3 to 4.4 × 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 Dα (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 [3, 4] 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
 7.8 T EDA H-mode EPED H-mode pedestal threshold

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