Transient Enhancement (`Spike-on-Tail') Observed on NBI Energetic Ion Spectra Using the E$\vert \vert $B NPA on NSTX

An $\sim $ 4x increase in the E$\vert \vert $B Neutral Particle Analyzer (NPA) charge exchange neutral flux localized at the Neutral Beam Injection (NBI) full energy is observed in the National Spherical Torus Experiment (NSTX). Termed the High-Energy Feature (HEF), it appears only at the NBI full energy, exhibits a growth time of $\sim $ 20 - 80 ms, seldom develops a slowing down distribution and arises only in discharges where kink-type modes (f $<$ 10 kHz) are absent, TAE activity (f $\sim $ 10-150 kHz) is weak and CAE/GAE activity (f $\sim $ 400-1200 kHz) is robust. The HEF is observed 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 is suppressed by vessel conditioning using lithium deposition at $\ge $ 100 mg/shot. Coincident increases of $\sim $ 10-30 {\%} in neutron yield and total stored energy during the HEF are driven by plasma profile changes and not the HEF itself. Tentatively, the HEF appears to be caused by a form of CAE/GAE wave-particle resonant interaction.