Bidirectional tornado modes on the Joint European Torus
Journal article, 2007
In discharges on the Joint European Torus [P. H. Rebut and B. E. Keen, Fusion Technol. 11, 13
(1987)] with safety factor q(0)<1 and high-power ion cyclotron resonance heating (ICRH),
monster sawtooth crashes are preceded by frequency sweeping “tornado modes” in the toroidal
Alfvén eigenmode frequency range. A suite of equilibrium and spectral magnetohydrodynamical
codes is used for explaining the observed evolution of the tornado mode frequency and for
identifying temporal evolution of the safety factor inside the q=1 radius just before sawtooth
crashes. In some cases, the tornado modes are observed simultaneously with both positive and
negative toroidal mode numbers. Hence, a free energy source other than the radial gradient of the
energetic ion pressure exciting these modes is sought. The distribution function of the
ICRH-accelerated ions is assessed with the SELFO code [J. Hedin et al., Nucl. Fusion 42, 527
(2002)] and energetic particle drive due to the velocity space anisotropy of ICRH-accelerated ions
is considered analytically as the possible source for excitation of bidirectional tornado modes.
© 2007 American Institute of Physics.