Transient energetic charge exchange flux enhancement observed in NSTX neutral-beam-heated H-mode discharges

Large increases in the E$\vert \vert $B Neutral Particle Analyzer (NPA) charge exchange neutral flux localized at the Neutral Beam Injection (NBI) full energy are observed in the National Spherical Torus Experiment (NSTX). Termed the High-Energy Feature (HEF), it appears only at the NBI full energy, exhibits growth times $\sim $ 20-80 ms, seldom develops a slowing down distribution and arises only in discharges where NTM modes (f $<$ 30 kHz) are absent, TAE activity (f $\sim $ 30-150 kHz) is weak and GAE/CAE activity (f $\sim $ 400-1200 kHz) is robust. The HEF occurs only in H-mode discharges with P$_{b} \quad \ge $ 3 MW and v$_{\vert \vert }$/v $\sim $ 0.7-0.9; i.e. only for passing ions. The HEF appears to be caused by a GAE wave-particle interaction that modifies of the NB fast ion distribution, f$_{i}$(E,v$_{\vert \vert }$/v,r). This proposed mechanism was studied using the SPIRAL code that imports a TRANSP-calculated f$_{i}$(E,v$_{\vert \vert }$/v,r) distribution and evolves it under drive from GAE wave-particle resonances.