The external kink mode in diverted tokamaks

Journal Article · 16 June 2016 · Journal of Plasma Physics

DOI: https://doi.org/10.1017/S0022377816000568 · OSTI ID:1354788

Turnbull, Alan D. ^[1]; Hanson, Jeremy M. ^[2]; Turco, Francesca ^[2]; Ferraro, Nathaniel M. ^[3]; Lanctot, Matthew J. ^[1]; Lao, Lang L. ^[1]; Strait, Edward J. ^[1]; Piovesan, Paolo ^[4]; Martin, Piero ^[4]

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1. General Atomics, San Diego, CA (United States)
2. Columbia Univ., New York, NY (United States)
3. Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
4. Consorzio RFX, Padova (Italy)

Here, an explanation is provided for the disruptive instability in diverted tokamaks when the safety factor at the 95% poloidal flux surface, q₉₅, is driven below 2.0. The instability is a resistive kink counterpart to the current-driven ideal mode that traditionally explained the corresponding disruption in limited cross-sections when q_edge, the safety factor at the outermost closed flux surface, lies just below a rational value. Experimentally, external kink modes are observed in limiter configurations as the current in a tokamak is ramped up and q_edge decreases through successive rational surfaces. For q_edge < 2, the instability is always encountered and is highly disruptive. However, diverted plasmas, in which q_edge is formally infinite in the magnetohydrodynamic (MHD) model, have presented a longstanding difficulty since the theory would predict stability, yet, the disruptive limit occurs in practice when q₉₅, reaches 2. It is shown from numerical calculations that a resistive kink mode is linearly destabilized by the rapidly increasing resistivity at the plasma edge when q₉₅ < 2, but q_edge >> 2. The resistive kink behaves much like the ideal kink with predominantly kink or interchange parity and no real sign of a tearing component. However, the growth rates scale with a fractional power of the resistivity near the q = 2 surface. The results have a direct bearing on the conventional edge cutoff procedures used in most ideal MHD codes, as well as implications for ITER and for future reactor options.

• Research Organization: General Atomics, San Diego, CA (United States)
• Sponsoring Organization: USDOE
• Grant/Contract Number: FC02-04ER54698
• OSTI ID: 1354788
• Journal Information: Journal of Plasma Physics, Vol. 82, Issue 03; ISSN 0022-3778
• Publisher: Cambridge University PressCopyright Statement
• Country of Publication: United States
• Language: English