Abstract
The National Spherical Torus Experiment (NSTX, (Ono et al 2000 Nucl. Fusion 40 557)) routinely operates with neutral beam injection as the primary system for heating and current drive. The resulting fast ion population is super-Alfvénic, with velocities 1 < vfast/vAlfven < 5. This provides a strong drive for toroidicity-induced Alfvén 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.
Highlights
The interaction between fast ions and multiple toroidicityinduced Alfven eigenmodes (TAEs [1]) is believed to represent one of the main loss mechanisms for fast ions in ITER [2]
The typical scenarios on the National Spherical Torus Experiment (NSTX) [14] are considerably different from those expected in ITER and future fusion devices, they provide a good test case to improve the present understanding of the basic physics of bursting toroidicity-induced Alfven eigenmodes (TAEs) and induced fast ion loss or redistribution
In this work it has been shown that pairs of TAEs can efficiently couple and generate lower and higher frequency perturbations
Summary
The interaction between fast ions and multiple toroidicityinduced Alfven eigenmodes (TAEs [1]) is believed to represent one of the main loss mechanisms for fast ions in ITER [2]. The modification of the fast ion phase space, which can be further enhanced by the resonance overlap, has been invoked to explain a variety of phenomena observed in TAE experiments These include mode splitting into multiple frequencies [6], or periodic variations of the mode frequency (chirps) [7, 8]. The typical scenarios on the National Spherical Torus Experiment (NSTX) [14] are considerably different from those expected in ITER and future fusion devices (cf sections 2 and 3), they provide a good test case to improve the present understanding of the basic physics of bursting TAEs and induced fast ion loss or redistribution.
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