SUMMARY OF RECENT STUDIES OF BEAM-DRIVEN BAE AND CHIRPING MODES IN DII-D

In high beta. DIII-D plasmas with intense neutral beam injection, beta induced Alfven eigenmodes (BAE modes) are observed. These instabiliti es cause concentrated losses of >50% of the fast ions and thus are of concern for future devices. The authors have now observed BAE modes an d resultant fast ion loss in full field (2.0 T) discharges where the r atio of parallel velocity to Alfven speed is upsilon(parallel to)/upsi lon(A) similar or equal to 0.3. In a few discharges, they have also ob served a new instability, a 'chirping' mode. These modes have frequenc ies between 50 and 200 kHz that 'whistle' down a factor of two in a si ngle 2 ms burst. They occur in plasmas with relatively large values of fast ion beta ((beta(f)) greater than or similar to 1%), Alfven speed (upsilon(parallel to)/upsilon(A) less than or similar to 0.5) and pla sma rotation (f(rot) > 20 kHz). In contrast to the usual Alfven modes, which are fluid modes of the background plasma, the chirping instabil ities seem to be beam modes that are nearly stationary in the plasma f rame.