Sherwood 2015

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External Kink Mode in Diverted Tokamaks

Author: Alan D. Turnbull
Requested Type: Poster Only
Submitted: 2015-01-19 16:55:00

Co-authors: N.M.Ferraro, L.L.Lao, J.M.Hanson,F.Turco,P.Martin,P. Piovesan

Contact Info:
General Atomics
P.O. Box
San Diego, California   92186-5

Abstract Text:
The current-driven external kink mode is predicted to be unstable in a tokamak when the edge safety factor at the plasma edge, q_{edge}, lies just below a rational value n/m and the current density profile is sufficiently broad [1,2]. In a cylindrical straight tokamak model, the corresponding unstable mode has toroidal mode number n and poloidal mode number m and is strongly peaked at the plasma edge. In a torus, the picture is essentially unchanged, except for additional coupling to neighboring poloidal harmonics. The external kink modes have been observed experimentally as the current in a tokamak is ramped up and q_{edge} decreases. In particular, for the case of m=2 and n=1 with q_{edge}<2, the instability is always encountered. However, analytic models of a diverted plasma have infinite edge q and one would naively expect stability to these current driven kink modes. In reality, q_{95}=2 is a hard limit and has only recently been overcome for short times by active magnetic stabilization [3]. Ideal stability predictions indicate stability for that case despite the observation of gross instability in the absence of feedback stabilization. The conventional view is that the ideal kink mode becomes a tearing mode when the edge rational surface lies inside the finite resistivity plasma edge region. However, it is shown that instead a resistive kink mode is destabilized by the rapidly increased resistivity at the plasma edge. The resistive kink behaves much like the ideal kink with no sign of a tearing component.

[1] V.D. Shafranov, Sov. Phys. JETP (1972)
[2] J.A. Wesson, Rev. Mod. Phys. 1978
[3] P. Piovesan, Phys. Rev. Lett. 2014

*Supported in part by the US DOE under DE-FG02-95ER54309 & DE-FG02-04ER54761.


March 16-18, 2015
The Courant Institute, New York University