Dependence of compressional Alfv\'{e}n eigenmode instability on fast-ion density and phase space distribution on DIII-D

Instability of compressional Alfv\'{e}n eigenmodes (CAEs), driven through Doppler-shifted cyclotron resonance with beam ions, is studied in an experiment on DIII-D that varies beam current at constant beam voltage. A mode excited by an off-axis co-I$_{\mathrm{P}}$ injected beam is observed with a pair of toroidally separated magnetic field sensing loops to be propagating counter to the beam direction with n$=$-5 and f \textasciitilde 0.57f$_{\mathrm{c}}$. The mode is unstable at high beam current and observed to stabilize as the current drops below a threshold. This would seem consistent with a simple theoretical expectation for CAEs to be unstable at beam densities above a dissipation-based threshold [Belova 2017]. However, TRANSP modeling shows that the fast-ion phase space distribution changes in a complex way over the lifetime of the mode. The initial destabilization of the mode is observed to be delayed \textasciitilde 10ms after beam turn-on. The complicated evolution of mode stability during the beam current scan motivates an in-depth analysis of the changing fast-ion distribution to determine the exact features controlling the instability. Analysis is underway to compare mode stability with expectations from the orbit-averaged resonance equation evaluated for particles in the fast-ion population.