Feedback-assisted extension of the tokamak operating space to low safety factor

Journal Article · Tue Jul 15 00:00:00 EDT 2014 · Physics of Plasmas

DOI:https://doi.org/10.1063/1.4886796· OSTI ID:22304075

Bialek, J. M.; Navratil, G. A.; Olofsson, K. E. J.; Shiraki, D.; Turco, F. ^[1]; Baruzzo, M.; Bolzonella, T.; Marrelli, L.; Martin, P.; Piovesan, P.; Piron, C.; Piron, L.; Terranova, D.; Zanca, P. ^[2]; Hyatt, A. W.; Jackson, G. L.; La Haye, R. J.; Lanctot, M. J.; Strait, E. J. ^[3]; others, and

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1. Department of Applied Mathematics and Applied Physics, Columbia University, New York, New York 10027-6900 (United States)
2. Consorzio RFX, Corso Stati Uniti 4, 35127 Padova (Italy)
3. General Atomics, P.O. Box 85608, San Diego, California 92186-5608 (United States)

Recent DIII-D and RFX-mod experiments have demonstrated stable tokamak operation at very low values of the edge safety factor q(a) near and below 2. The onset of n = 1 resistive wall mode (RWM) kink instabilities leads to a disruptive stability limit, encountered at q(a) = 2 (limiter plasmas) and q{sub 95} = 2 (divertor plasmas). However, passively stable operation can be attained for q(a) and q{sub 95} values as low as 2.2. RWM damping in the q(a) = 2 regime was measured using active MHD spectroscopy. Although consistent with theoretical predictions, the amplitude of the damped response does not increase significantly as the q(a) = 2 limit is approached, in contrast with damping measurements made approaching the pressure-driven RWM limit. Applying proportional gain magnetic feedback control of the n = 1 modes has resulted in stabilized operation with q{sub 95} values reaching as low as 1.9 in DIII-D and q(a) reaching 1.55 in RFX-mod. In addition to being consistent with the q(a) = 2 external kink mode stability limit, the unstable modes have growth rates on the order of the characteristic wall eddy-current decay timescale in both devices, and a dominant m = 2 poloidal structure that is consistent with ideal MHD predictions. The experiments contribute to validating MHD stability theory and demonstrate that a key tokamak stability limit can be overcome with feedback.

• OSTI ID: 22304075
• Journal Information: Physics of Plasmas, Vol. 21, Issue 7; Other Information: (c) 2014 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA); ISSN 1070-664X
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
AMPLITUDES
CONTROL
DAMPING
DIVERTORS
DOUBLET-3 DEVICE
EDDY CURRENTS
FEEDBACK
KINK INSTABILITY
MAGNETOHYDRODYNAMICS
PLASMA
SAFETY
STABILITY