Study of Chirping Toridicity-induced Alfven Eigenmodes in the National Spherical Torus Experiment
Authors: M. Podesta, R. E. Bell, A. Bortolon, N. A. Crocker, D. S. Darrow, A.Diallo, E. D. Fredrickson, G.-Y. Fu, N. N. Gorelenkov, W. W. Heidbrink, G. J. Kramer, S. Kubota, B. P. LeBlanc, S. S. Medley, H. Yuh
Abstract:
Chirping Toroidicity-induced Alfven Eigenmodes (TAEs) are destabilized during neutral beam
injection on the National Spherical Torus Experiment (NSTX, [M. Ono et al., Nucl. Fusion 40, 557
(2000)]) by super-Alfvenic ions with velocities up to ve times larger than the Alfven velocity. TAEs
exhibit repeated bursts in amplitude and down-chirps in frequency. Larger bursts, so-called TAE
avalanches, are eventually observed and correlate with a loss of fast ions up to 30% over 1 ms.
Frequency, amplitude and radial structure of TAEs are characterized via magnetic pickup coils and a
multi-channel reectometer system. The modes have a broad radial structure, which appears to be
una ected by the large frequency and amplitude variations. However, the large mode amplitude does
impact the modes' dynamics by favoring the coupling among di erent modes. In addition, the coupling
involves kink-like modes and can therefore degrade the thermal plasma con nement. In spite of the non-
linear regime characterizing the TAE dynamics, the measured properties are found to be in reasonable
agreement with solutions from the ideal MHD code NOVA.
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Submitted to: Nuclear Fusion (December 2011)
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