Session KP01 - Poster Session III.
POSTER session, Tuesday afternoon, March 23
Exhibit Hall, GWCC
Several distinct classes of energetic particle--driven instability have been observed during neutral beam injection in the Small Tight Aspect Ratio Tokamak (START). Possible interpretations of these observations are given. Shear Alfvén continuous spectra, computed for times of beam--driven wave activity, contain wide spectral gaps, extending up to several times the Alfvén gap frequency, in which shear Alfvén eigenmodes could, in principle, be driven unstable by energetic ions. Eigenfunctions are computed for the lowest frequency gap. Part of the instability drive in START is provided by positive gradients in beam ion velocity distributions, which arise from velocity--dependent charge exchange losses. It is shown that fishbone--like bursts observed at a few tens of kHz can be attributed to kink mode excitation by passing beam ions, while narrow--band emission at several hundred kHz may be due to excitation of fast Alfvén eigenmodes. The possibility of similar instabilities occurring in larger spherical tokamaks is discussed.