Energetic particle effects as an explanation for the low frequencies of Alfvén modes in the DIII-D tokamak

During beam injection in the DIII-D tokamak, modes with lower frequencies than expected for toroidicity induced Alfvén eigenmodes (TAEs) are often observed. The experimental 'TAE' frequency is often ≈0.8 of the nominal theoretical frequency of the TAE, f_TAE, while the typical frequency of beta induced Alfvén eigenmodes (BAEs) is (0.2-0.4) f_TAE. An analysis is presented of an unstable discharge with a high n stability code, HINST, that includes the effect of energetic ions on mode frequency. The analysis shows that the experimental 'TAE' and 'BAE' could be resonant branches of the TAE and the kinetic ballooning mode, respectively.