NBI+RF synergy and its effect on Alfvén Eigenmode stability in NSTX/NSTX-U

The synergy between NBI and RF injection can result in a large distortion of the original NBI distribution function by generating a tail extending to very high energies (E>>100 keV) or by causing increased losses of NB ions that are displaced to loss orbits by the RF field. The modification of the NBI distribution by RF can then lead to different stability properties for Alfvénic modes, making RF a candidate actuator to affect the mode stability. The expected modifications of the NBI distribution on NSTX and NSTX-U as a function of the injected RF spectrum will first be assessed via the ORBIT code. The RF fields from the full wave code TORIC are used to evaluate the RF quasi-linear diffusion coefficients, which can then be used to modify the NBI energetic particle distribution. The impact of the modified NBI distribution on Alfvénic instabilities will then be assessed by quantifying the power exchanged between fast ions and the modes. This modeling exercise would provide initial guidance on the expected effects of the NBI+RF synergy for NSTX-U scenarios, and it would serve as a starting point to design dedicated experiments.