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

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 95 = 2 (divertor plasmas). However, passively stable operation can be attained for q( a) and q 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 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 mmore » = 2 poloidal structure that is consistent with ideal MHD predictions. As a result, the experiments contribute to validating MHD stability theory and demonstrate that a key tokamak stability limit can be overcome with feedback.« less
Authors:
 [1] ;  [1] ;  [2] ;  [2] ;  [3] ;  [3] ;  [4] ;  [3] ;  [3] ;  [2] ;  [2] ;  [1] ;  [5] ;  [1] ;  [4] ;  [2] ;  [2] ;  [2] ;  [6] ;  [3] more »;  [2] ;  [1] ;  [3] ;  [2] « less
  1. Columbia Univ., New York, NY (United States)
  2. Consorzio RFX, Padova (Italy)
  3. General Atomics, San Diego, CA (United States)
  4. Oak Ridge Inst. for Science and Education (ORISE), Oak Ridge, TN (United States)
  5. Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
  6. Columbia Univ., New York, NY (United States); Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Publication Date:
Grant/Contract Number:
FC02-04ER54698; AC05-06OR23100; AC02-09CH11466
Type:
Accepted Manuscript
Journal Name:
Physics of Plasmas
Additional Journal Information:
Journal Volume: 21; Journal Issue: 7; Journal ID: ISSN 1070-664X
Publisher:
American Institute of Physics (AIP)
Research Org:
General Atomics, San Diego, CA (United States)
Sponsoring Org:
USDOE Office of Nuclear Energy (NE)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY
OSTI Identifier:
1354831

Hanson, Jeremy M., Bialek, James M., Baruzzo, M., Bolzonella, Tommaso, Hyatt, Alan W., Jackson, Gary L., King, J., La Haye, Robert J., Lanctot, Matthew J., Marrelli, Lionello, Martin, Piero, Navratil, Gerald A., Okabayashi, Michio, Olofsson, K. Erik J., Paz-Soldan, Carlos, Piovesan, Paolo, Piron, Chiara, Piron, Lidia, Shiraki, Daisuke, Strait, Edward J., Terranova, D., Turco, Francesca, Turnbull, Alan D., and Zanca, Paolo. Feedback-assisted extension of the tokamak operating space to low safety factor. United States: N. p., Web. doi:10.1063/1.4886796.
Hanson, Jeremy M., Bialek, James M., Baruzzo, M., Bolzonella, Tommaso, Hyatt, Alan W., Jackson, Gary L., King, J., La Haye, Robert J., Lanctot, Matthew J., Marrelli, Lionello, Martin, Piero, Navratil, Gerald A., Okabayashi, Michio, Olofsson, K. Erik J., Paz-Soldan, Carlos, Piovesan, Paolo, Piron, Chiara, Piron, Lidia, Shiraki, Daisuke, Strait, Edward J., Terranova, D., Turco, Francesca, Turnbull, Alan D., & Zanca, Paolo. Feedback-assisted extension of the tokamak operating space to low safety factor. United States. doi:10.1063/1.4886796.
Hanson, Jeremy M., Bialek, James M., Baruzzo, M., Bolzonella, Tommaso, Hyatt, Alan W., Jackson, Gary L., King, J., La Haye, Robert J., Lanctot, Matthew J., Marrelli, Lionello, Martin, Piero, Navratil, Gerald A., Okabayashi, Michio, Olofsson, K. Erik J., Paz-Soldan, Carlos, Piovesan, Paolo, Piron, Chiara, Piron, Lidia, Shiraki, Daisuke, Strait, Edward J., Terranova, D., Turco, Francesca, Turnbull, Alan D., and Zanca, Paolo. 2014. "Feedback-assisted extension of the tokamak operating space to low safety factor". United States. doi:10.1063/1.4886796. https://www.osti.gov/servlets/purl/1354831.
@article{osti_1354831,
title = {Feedback-assisted extension of the tokamak operating space to low safety factor},
author = {Hanson, Jeremy M. and Bialek, James M. and Baruzzo, M. and Bolzonella, Tommaso and Hyatt, Alan W. and Jackson, Gary L. and King, J. and La Haye, Robert J. and Lanctot, Matthew J. and Marrelli, Lionello and Martin, Piero and Navratil, Gerald A. and Okabayashi, Michio and Olofsson, K. Erik J. and Paz-Soldan, Carlos and Piovesan, Paolo and Piron, Chiara and Piron, Lidia and Shiraki, Daisuke and Strait, Edward J. and Terranova, D. and Turco, Francesca and Turnbull, Alan D. and Zanca, Paolo},
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 q95 = 2 (divertor plasmas). However, passively stable operation can be attained for q(a) and q95 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 q95 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. As a result, 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 = {2014},
month = {7}
}