(De-)stabilization of sawteeth by anisotropic fast ions in DIII-D negative/positive triangularity plasmas: modeling versus experiments

Recent DIII-D experiments show, for the first time, that sawtooth stability is strongly affected by anisotropic fast ions from Neutral Beam Injection (NBI) in negative triangularity plasmas, similar to previous observations in positive triangularity plasmas. Fast ions from co-(counter-) current NBI are stabilizing (destabilizing) for sawteeth, resulting in longer (shorter) sawtooth periods. The relative change of sawtooth period and amplitude is more than a factor of two. Non-perturbative toroidal modeling, utilizing the MHD-kinetic hybrid stability code MARS-K, reveals an asymmetric dependence of the stability of the n = 1 internal kink on the injection direction of NBI, being qualitatively consistent with the experimental observation. The MARS-K modeling results suggest that anisotropic fast ions affect the mode growth rate and frequency through both adiabatic and non-adiabatic contributions. The asymmetry of the internal kink mode instability relative to the NBI direction is mainly due to the non-adiabatic contribution of passing fast ions, which stabilize (destabilize) the internal kink with the co-(counter-) current NBI. On the other hand, Finite Orbit Width (FOW) correction to passing particles partially cancels the asymmetry. Trapped particles are always stabilizing due to precessional drift resonances. Modeling also suggests that fast ions affect the internal kink in a similar manner between negative and positive triangularity plasmas, although being slightly more unstable in the negative triangularity plasmas already in the fluid limit. Furthermore, MARS-K modeling indicates that other factors, such as the plasma rotation and drift kinetic effects of thermal plasmas, weakly modify the mode stability as compared to the drift kinetic resonance effects and FOW correction of fast ions.