Non-linear Dynamics Of Toroidicity-induced Alfven Eigenmodes On The National Spherical Torus Experiment
Authors: M. Podesta, R. E. Bell, N. A. Crocker, E. D. Fredrickson, N. N. Gorelenkov, W. W. Heidbrink, S. Kubota, B. P. LeBlanc, and H. Yu
Abstract: The National Spherical Torus Experiment (NSTX, [M. Ono et al., Nucl. Fusion 40, 557 (2000)])
routinely operates with neutral beam injection as the primary system for heating and current drive. The
resulting fast ion population is super-Alfvenic, with velocities 1 < vfast=vAlfven < 5. This provides a
strong drive for toroidicity-induced Alfven eigenmodes (TAEs). As the discharge evolves, the fast ion
population builds up and TAEs exhibit increasing bursts in amplitude and down-chirps in frequency,
which eventually lead to a so-called TAE avalanche. Avalanches cause large (≤ 30%) fast ion losses
over ~ 1 ms, as inferred from the neutron rate. The increased fast ion losses correlate with a stronger
activity in the TAE band. In addition, it is shown that a n = 1 mode with frequency well below
the TAE gap appears in the Fourier spectrum of magnetic fluctuations as a result of non-linear mode coupling between TAEs during avalanche events. The non-linear coupling between modes, which leads
to enhanced fast ion transport during avalanches, is investigated.
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Submitted to: Nuclear Fusion (April 2011
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