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

Abstract

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 amore » key tokamak stability limit can be overcome with feedback.« less

Authors:
; ; ; ; ;  [1]; ; ; ; ; ; ; ; ;  [2]; ; ; ; ;  [3] more »; « less
  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)
Publication Date:
OSTI Identifier:
22304075
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 21; Journal Issue: 7; Other Information: (c) 2014 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; AMPLITUDES; CONTROL; DAMPING; DIVERTORS; DOUBLET-3 DEVICE; EDDY CURRENTS; FEEDBACK; KINK INSTABILITY; MAGNETOHYDRODYNAMICS; PLASMA; SAFETY; STABILITY

Citation Formats

Hanson, J. M., E-mail: jmh2130@columbia.edu, Bialek, J. M., Navratil, G. A., Olofsson, K. E. J., Shiraki, D., Turco, F., Baruzzo, M., Bolzonella, T., Marrelli, L., Martin, P., Piovesan, P., Piron, C., Piron, L., Terranova, D., Zanca, P., Hyatt, A. W., Jackson, G. L., La Haye, R. J., Lanctot, M. J., Strait, E. J., and and others. Feedback-assisted extension of the tokamak operating space to low safety factor. United States: N. p., 2014. Web. doi:10.1063/1.4886796.
Hanson, J. M., E-mail: jmh2130@columbia.edu, Bialek, J. M., Navratil, G. A., Olofsson, K. E. J., Shiraki, D., Turco, F., Baruzzo, M., Bolzonella, T., Marrelli, L., Martin, P., Piovesan, P., Piron, C., Piron, L., Terranova, D., Zanca, P., Hyatt, A. W., Jackson, G. L., La Haye, R. J., Lanctot, M. J., Strait, E. J., & and others. Feedback-assisted extension of the tokamak operating space to low safety factor. United States. doi:10.1063/1.4886796.
Hanson, J. M., E-mail: jmh2130@columbia.edu, Bialek, J. M., Navratil, G. A., Olofsson, K. E. J., Shiraki, D., Turco, F., Baruzzo, M., Bolzonella, T., Marrelli, L., Martin, P., Piovesan, P., Piron, C., Piron, L., Terranova, D., Zanca, P., Hyatt, A. W., Jackson, G. L., La Haye, R. J., Lanctot, M. J., Strait, E. J., and and others. Tue . "Feedback-assisted extension of the tokamak operating space to low safety factor". United States. doi:10.1063/1.4886796.
@article{osti_22304075,
title = {Feedback-assisted extension of the tokamak operating space to low safety factor},
author = {Hanson, J. M., E-mail: jmh2130@columbia.edu and Bialek, J. M. and Navratil, G. A. and Olofsson, K. E. J. and Shiraki, D. and Turco, F. and Baruzzo, M. and Bolzonella, T. and Marrelli, L. and Martin, P. and Piovesan, P. and Piron, C. and Piron, L. and Terranova, D. and Zanca, P. and Hyatt, A. W. and Jackson, G. L. and La Haye, R. J. and Lanctot, M. J. and Strait, E. J. and and others},
abstractNote = {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.},
doi = {10.1063/1.4886796},
journal = {Physics of Plasmas},
number = 7,
volume = 21,
place = {United States},
year = {Tue Jul 15 00:00:00 EDT 2014},
month = {Tue Jul 15 00:00:00 EDT 2014}
}