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The external kink mode in diverted tokamaks

Published online by Cambridge University Press:  16 June 2016

A. D. Turnbull*
Affiliation:
General Atomics Inc, San Diego, California, USA
J. M. Hanson
Affiliation:
Columbia University, New York, USA
F. Turco
Affiliation:
Columbia University, New York, USA
N. M. Ferraro
Affiliation:
Princeton Plasma Physics Laboratory, Princeton, NJ, USA
M. J. Lanctot
Affiliation:
General Atomics Inc, San Diego, California, USA
L. L. Lao
Affiliation:
General Atomics Inc, San Diego, California, USA
E. J. Strait
Affiliation:
General Atomics Inc, San Diego, California, USA
P. Piovesan
Affiliation:
Consorzio RFX, Padova, Italy
P. Martin
Affiliation:
Consorzio RFX, Padova, Italy
*
Email address for correspondence: turnbull@fusion.gat.com

Abstract

An explanation is provided for the disruptive instability in diverted tokamaks when the safety factor $q$ at the 95 % poloidal flux surface, $q_{95}$ , 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 (Shafranov, Sov. Phys. Tech. Phys., vol. 15, 1970, p. 175) when $q_{edge}$ , the safety factor at the outermost closed flux surface, lies just below a rational value $m/n$ . 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 $q_{95}$ , 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 $q_{95}<2$ , but $q_{edge}\gg 2$ . 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.

Type
Research Article
Copyright
© Cambridge University Press 2016 

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The external kink mode in diverted tokamaks
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