Multi-beam effects on compressional Alfven eigenmode stability*

Beam-driven sub-cyclotron compressional (CAE) and global (GAE) Alfven eigenmodes are routinely excited in spherical tokamaks such as NSTX and MAST. Early NSTX-U operations demonstrated robust suppression of GAEs with the new, more tangentially injecting neutral beam [1]. This phenomenon motivates the theoretical investigation of similar multi-beam effects on CAE stability in characteristic NSTX-U plasmas. Self-consistent hybrid simulations of CAEs excited by two-beam distributions are performed and compared with an analytic calculation for the perturbative fast ion drive. A stabilization criterion depending on the difference in the central pitch λ₁ - λ₂ and relative beam power fractions P₁ / P₂ is explored. Predictions for multi-beam (de)stabilization of CAEs for NSTX-U are presented and compared with extrapolations to parameter regimes for MAST-U and DIII-D. 

 [1] E.D. Fredrickson et al. Phys. Rev. Lett. 118, 265001 (2017)