Observation of Modes at Frequencies Near the Second Alfvén Gap in TFTR
Authors: E. Fredrickson, J.W. Van Dam, R.V. Budny, D. Darrow, G.Y. Fu, J. Hosea, C.K. Phillips, and J.R. Wilson
Date of PPPL Report: April 2000
Published in: Phys. Plasmas 7 (October 2000) 4121-4125. (Title and author list changed to: Observation of Modes at Frequencies Near the Second Alfvén Gap in the Tokamak Fusion Test Reactor by E. Fredrickson, R.V. Budny, D. Darrow, G.Y. Fu, J. Hosea, C.K. Phillips, and J.W. Van Dam.)
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)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). The modes are seen with and without accompanying TAE mode activity.