Purely alpha-particle-driven toroidal Alfvén eigenmodes (TAEs) with toroidal mode numbers [formula omitted] have been observed in deuterium–tritium (D–T) plasmas on the tokamak fusion test reactor [D. J. Grove and D. M. Meade, Nucl. Fusion 25, 1167 (1985)]. The appearance of mode activity following termination of neutral beam injection in plasmas with [formula omitted] is generally consistent with theoretical predictions of TAE stability [G. Y. Fu et al. Phys. Plasmas 3, 4036 (1996)]. Internal reflectometer measurements of TAE activity is compared with theoretical calculations of the radial mode structure. Core localization of the modes to the region of reduced central magnetic shear is confirmed, however the mode structure can deviate significantly from theoretical estimates. The peak measured TAE amplitude of [formula omitted] at [formula omitted] corresponds to [formula omitted] while [formula omitted] is measured at the plasma edge. Enhanced alpha particle loss associated with TAE activity has not been observed.
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