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

Physics of Plasmas Volume 21, Issue 7, July 2014, Article number 072107 Feedback-assisted extension of the tokamak operating space to low safety factor (Article) Hanson, J.M.a , Bialek, J.M.a, Baruzzo, M.b, Bolzonella, T.b, Hyatt, A.W.c, Jackson, G.L.c, King, J.d, La Haye, R.J.c, Lanctot, M.J.c, Marrelli, L.b, Martin, P.b, Navratil, G.A.a, Okabayashi, M.e, Olofsson, K.E.J.a, Paz-Soldan, C.d, Piovesan, P.b, Piron, C.b, Piron, L.b, Shiraki, D.af, Strait, E.J.c, Terranova, D.b, Turco, F.a, Turnbull, A.D.c, Zanca, P.b a Department of Applied Mathematics and Applied Physics, Columbia University, New York, NY 10027-6900, United States b Consorzio RFX, Corso Stati Uniti 4, 35127 Padova, Italy c General Atomics, P.O. Box 85608, San Diego, CA 92186-5608, United States d Oak Ridge Institute for Science and Education, Oak Ridge, TN 37830-8050, United States e Princeton Plasma Physics Laboratory, P.O. Box 451, Princeton, NJ 08543-0451, United States f Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831, United States View additional affiliations View references (58) 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 95 = 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. The experiments contribute to validating MHD stability theory and demonstrate that a key tokamak stability limit can be overcome with feedback. © 2014 AIP Publishing LLC.