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Off-axis fishbone-like instability and excitation of resistive wall modes in JT-60U and DIII-D

  • Author(s): Okabayashi, M
  • Matsunaga, G
  • deGrassie, JS
  • Heidbrink, WW
  • In, Y
  • Liu, YQ
  • Reimerdes, H
  • Solomon, WM
  • Strait, EJ
  • Takechi, M
  • Asakura, N
  • Budny, RV
  • Jackson, GL
  • Hanson, JM
  • La Haye, RJ
  • Lanctot, MJ
  • Manickam, J
  • Shinohara, K
  • Zhu, YB
  • et al.

Published Web Location Commons 'BY' version 4.0 license

An energetic-particle (EP)-driven off-axis-fishbone-like mode (OFM) often triggers a resistive wall mode (RWM) in JT-60U and DIII-D devices, preventing long-duration high-βN discharges. In these experiments, the EPs are energetic ions (70-85 keV) injected by neutral beams to produce high-pressure plasmas. EP-driven bursting events reduce the EP density and the plasma rotation simultaneously. These changes are significant in high-βN low-rotation plasmas, where the RWM stability is predicted to be strongly influenced by the EP precession drift resonance and by the plasma rotation near the q = 2 surface (kinetic effects). Analysis of these effects on stability with a self-consistent perturbation to the mode structure using the MARS-K code showed that the impact of EP losses and rotation drop is sufficient to destabilize the RWM in low-rotation plasmas, when the plasma rotation normalized by Alfvén frequency is only a few tenths of a percent near the q = 2 surface. The OFM characteristics are very similar in JT-60U and DIII-D, including nonlinear mode evolution. The modes grow initially like a classical fishbone, and then the mode structure becomes strongly distorted. The dynamic response of the OFM to an applied n = 1 external field indicates that the mode retains its external kink character. These comparative studies suggest that an energetic particle-driven off-axis-fishbone-like mode is a new EP-driven branch of the external kink mode in wall-stabilized plasmas, analogous to the relationship of the classical fishbone branch to the internal kink mode. © 2011 American Institute of Physics.

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