Observation of modes at frequencies near the second Alfvén gap in the Tokamak Fusion Test Reactor.

Modes have been observed near the frequency of the second Alfvén gap during off-axis H-minority heating experiments in the circular cross-section Tokamak Fusion Test Reactor. The observation of these modes is surprising in that the second gap, which is generally opened with ellipticity, is expected to be small, of order (r/R)[sup 2]. A model is proposed in which the second gap is opened by the fast ion beta, which is shown to be able to introduce mode coupling, much as toroidal effects introduce mode coupling for Toroidal Alfvén Eigenmodes (TAE). With the low inferred energy of the fast ion tail (30-50 keV), the fast ion bounce resonance condition is assumed to drive the modes. The modes are seen with and without accompanying TAE mode activity. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]