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

Journal Article · Mon Jul 07 00:00:00 EDT 2014 · Physics of Plasmas

DOI:https://doi.org/10.1063/1.4886796· OSTI ID:1354831

Hanson, Jeremy M. ^[1]; Bialek, James M. ^[1]; Baruzzo, M. ^[2]; Bolzonella, Tommaso ^[2]; Hyatt, Alan W. ^[3]; Jackson, Gary L. ^[3]; King, J. ^[4]; La Haye, Robert J. ^[3]; Lanctot, Matthew J. ^[3]; Marrelli, Lionello ^[2]; Martin, Piero ^[2]; Navratil, Gerald A. ^[1]; Okabayashi, Michio ^[5]; Olofsson, K. Erik J. ^[1]; Paz-Soldan, Carlos ^[4]; Piovesan, Paolo ^[2]; Piron, Chiara ^[2]; Piron, Lidia ^[2]; Shiraki, Daisuke ^[6]; Strait, Edward J. ^[3] ; Terranova, D. ^[2]; Turco, Francesca ^[1]; Turnbull, Alan D. ^[3]; Zanca, Paolo ^[2]

---

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)

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.

• Research Organization: General Atomics, San Diego, CA (United States)
• Sponsoring Organization: USDOE Office of Nuclear Energy (NE)
• Grant/Contract Number: FC02-04ER54698; AC05-06OR23100; AC02-09CH11466
• OSTI ID: 1354831
• Journal Information: Physics of Plasmas, Vol. 21, Issue 7; ISSN 1070-664X
• Publisher: American Institute of Physics (AIP)
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY