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Title: The external kink mode in diverted tokamaks

Abstract

Here, an explanation is provided for the disruptive instability in diverted tokamaks when the safety factor 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 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 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 >> 2. The resistive kink behaves much like the ideal kink with predominantly kink or interchange parity and no real sign of a tearingmore » 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.« less

Authors:
 [1];  [2];  [2];  [3];  [1];  [1];  [1];  [4];  [4]
  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)
Publication Date:
Research Org.:
General Atomics, San Diego, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1354788
Grant/Contract Number:
FC02-04ER54698
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Journal of Plasma Physics
Additional Journal Information:
Journal Volume: 82; Journal Issue: 03; Journal ID: ISSN 0022-3778
Publisher:
Cambridge University Press
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY

Citation Formats

Turnbull, Alan D., Hanson, Jeremy M., Turco, Francesca, Ferraro, Nathaniel M., Lanctot, Matthew J., Lao, Lang L., Strait, Edward J., Piovesan, Paolo, and Martin, Piero. The external kink mode in diverted tokamaks. United States: N. p., 2016. Web. doi:10.1017/S0022377816000568.
Turnbull, Alan D., Hanson, Jeremy M., Turco, Francesca, Ferraro, Nathaniel M., Lanctot, Matthew J., Lao, Lang L., Strait, Edward J., Piovesan, Paolo, & Martin, Piero. The external kink mode in diverted tokamaks. United States. doi:10.1017/S0022377816000568.
Turnbull, Alan D., Hanson, Jeremy M., Turco, Francesca, Ferraro, Nathaniel M., Lanctot, Matthew J., Lao, Lang L., Strait, Edward J., Piovesan, Paolo, and Martin, Piero. Thu . "The external kink mode in diverted tokamaks". United States. doi:10.1017/S0022377816000568. https://www.osti.gov/servlets/purl/1354788.
@article{osti_1354788,
title = {The external kink mode in diverted tokamaks},
author = {Turnbull, Alan D. and Hanson, Jeremy M. and Turco, Francesca and Ferraro, Nathaniel M. and Lanctot, Matthew J. and Lao, Lang L. and Strait, Edward J. and Piovesan, Paolo and Martin, Piero},
abstractNote = {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 qedge, 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 qedge decreases through successive rational surfaces. For qedge < 2, the instability is always encountered and is highly disruptive. However, diverted plasmas, in which qedge 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 qedge >> 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.},
doi = {10.1017/S0022377816000568},
journal = {Journal of Plasma Physics},
number = 03,
volume = 82,
place = {United States},
year = {Thu Jun 16 00:00:00 EDT 2016},
month = {Thu Jun 16 00:00:00 EDT 2016}
}

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Free Publicly Available Full Text
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  • One promising approach to maintaining stability of high beta tokamak plasmas is the use of a conducting wall near the plasma to stabilize low-{ital n} ideal magnetohydrodynamic instabilities. However, with a resistive wall, either plasma rotation or active feedback control is required to stabilize the more slowly growing resistive wall modes (RWMs). Previous experiments have demonstrated that plasmas with a nearby conducting wall can remain stable to the n=1 ideal external kink above the beta limit predicted with the wall at infinity. Recently, extension of the wall stabilized lifetime {tau}{sub L} to more than 30 times the resistive wall timemore » constant {tau}{sub w} and detailed, reproducible observation of the n=1 RWM have been possible in DIII-D [Plasma Physics and Controlled Fusion Research (International Atomic Energy Agency, Vienna, 1986), p. 159] plasmas above the no-wall beta limit. The DIII-D measurements confirm characteristics common to several RWM theories. The mode is destabilized as the plasma rotation at the q=3 surface decreases below a critical frequency of 1{endash}7 kHz ({approximately}1{percent} of the toroidal Alfv{acute e}n frequency). The measured mode growth times of 2{endash}8 ms agree with measurements and numerical calculations of the dominant DIII-D vessel eigenmode time constant {tau}{sub w}. From its onset, the RWM has little or no toroidal rotation ({omega}{sub mode}{le}{tau}{sub w}{sup {minus}1}{lt}{omega}{sub plasma}), and rapidly reduces the plasma rotation to zero. These slowly growing RWMs can in principle be destabilized using external coils controlled by a feedback loop. In this paper, the encouraging results from the first open loop experimental tests of active control of the RWM, conducted in DIII-D, are reported. {copyright} {ital 1999 American Institute of Physics.}« less
  • The thermal and particle diffusivities driven by resistive fluid turbulence in diverted tokamak edge plasmas are calculated. Diverted tokamak geometry is characterized by increased global shear near the separatrix and the tendency of field lines to linger near the x point. For resistive fluid turbulence, the dominant effect is increased global shear, which causes a reduction in the effective step size of the turbulent diffusion process and corresponding improvements in heat and particle confinement close to the separatrix. Stability of resistive kink modes resonant near separatrix is also ensured by the increased global shear. The relevance of these considerations tomore » the L..-->..H transition and to the edge transport barrier in H-mode plasmas is discussed.« less
  • It is shown that external kink modes in tokamaks can be stabilized by applied radio frequency (rf) waves in the ion cyclotron range of frequencies. This stabilization results from the work done by the plasma against the ponderomotive force exerted by the rf field. For a strongly evanescent surface field (such as the near field of an ion Bernstein wave coupler), the ponderomotive layer is confined to the vicinity of the plasma surface and, if sufficiently strong, influences kink stability in the same way as a close-fitting conducting wall. This effect is illustrated by deriving an analytic condition for stabilizingmore » a single (m,n) mode in the low-beta straight tokamak model by means of a cylindrically symmetric E/sub parallel/ rf field. The effects of poloidally or toroidally nonuniform rf fields are examined in a two-mode coupling perturbation analysis. This stabilization mechanism may be useful in extending the beta limit of ion-Bernstein-wave-heated tokamaks against both ideal and resistive modes.« less
  • It is proposed that the ponderomotive force due to applied ion-cyclotron resonance-frequency waves can stabilize the internal kink mode in tokamaks. The sufficient stability criterion is derived and the necessary power estimated. It is concluded that at the rf power level, present in the Joint European Torus experiment, the ponderomotive force effects are significant and may be responsible for the modification of the sawtooth activity observed in recent experiments.
  • The effects of thermal trapped particles on the internal kink mode are studied using drift kinetic theory. For equal electron and ion temperatures, the trapped particles increase the marginal poloidal beta at [ital q]=1 substantially. For unequal temperatures, drift resonance with the hotter species is destabilizing and can lead to instability below the ideal magnetohydrodynamics threshold. An electrostatic potential is weakly stabilizing for the internal kink. At high beta, fluctuations of trapped particle modes couple to the internal kink mode and can lead to large displacements of the central region.