Modeling time-dependent Alfvénic drive in DIII-D beam blip experiments

Recent DIII-D experiments have focused on the effects of beam modulation on the stability of Alfvénic eigenmodes (AEs), with beam blips having durations comparable to the slowing down time of the injected fast ions. Consequently, the AE drive is modified on these timescales, providing interesting conditions to stress-test theoretical models.

While previous studies have focused on wave-particle interactions with a steady-state distribution function, a comprehensive analysis should also consider the effects of a distribution function that changes over time on similar timescales to the AE growth and saturation. The time variation of the AE linear growth rate is studied here using a time-dependent distribution function implemented in the NOVA-C code. The time-dependent linear growth rate also helps determine stability thresholds for the regulation of AE saturation by zonal flows.

The impact of wave-particle nonlinearities on mode saturation has been investigated using the resonance broadened quasi-linear (RBQ) code. When the distribution function is modified due to wave-particle interactions on shorter timescales compared to the time required by the distribution function to slow down across a broadened resonance in velocity space, the RBQ code can be used to analyze the nonlinear AE amplitude over/under-shoot oscillations during the saturation process.