Characterization of fast ion instabilities in DIII-D hybrid discharges*

Significant variations in MHD activity and fast-ion transport are observed in high-beta, steady-state hybrid discharges with a mixture of Electron Cyclotron (EC) waves and Neutral Beam Injection (NBI). For neutral-beam-only heating, many Alfven Eigenmodes (AE), which are likely TAEs/EAEs, are observed at frequencies of 100-250 kHz and caused a ~35% degradation in the neutron rate.  With both NBI and EC, the AE activity is usually suppressed and replaced by bursts of fishbones. The inferred beam-ion transport coefficient during EC is much smaller than the case with NBI only.  A leading explanation for the MHD activity change is because q_min dropped close to unity during EC current drive, which is confirmed by kinetic/MHD hybrid simulations for a pair of hybrid shots. However, experimental cases exist where the MHD activity changes significantly but the q profile apparently does not. A database is being built to find the exact cause(s) for the observed impact of EC on fast-ion instabilities with the goal of utilizing EC to control AE activity in a predictable way. Correlations of fast-ion instabilities with the q profile, the thermal plasma pressure, the fast-ion distribution, and the fast-ion beta will be presented.