Summary of recent studies of beam-driven BAE and chirping modes in DIII-D

Abstract

In high beta DIII-D plasmas with intense neutral beam injection, beta induced Alfven eigenmodes (BAE modes) are observed. These instabilities cause concentrated losses of >50% of the fast ions and thus are of concern for future devices. The authors have now observed BAE modes and resultant fast ion loss in full field (2.0 T) discharges where the ratio of parallel velocity to Alfven speed is v||/vA0.3. In a few discharges, they have also observed a new instability, a 'chirping' mode. These modes have frequencies between 50 and 200 kHz that 'whistle' down a factor of two in a single 2 ms burst. They occur in plasmas with relatively large values of fast ion β((βf) 1%), Alfven speed (v||/vA0.5) and plasma rotation (frot> 20 kHz). In contrast to the usual Alfven modes, which are fluid modes of the background plasma, the chirping instabilities seem to be beam modes that are nearly stationary in the plasma frame.