Investigation of a transient energetic charge exchange flux enhancement ('spike-on-tail') observed in neutral-beam-heated H-mode discharges in the National Spherical Torus Experiment

In the National Spherical Torus Experiment (NSTX), a large increase in the charge exchange neutral flux localized around the neutral beam (NB) injection full energy is measured using a neutral particle analyser. Termed the high-energy feature (HEF), it appears on the NB-injected energetic-ion spectrum only in discharges where tearing or kink-type modes (f < 50 kHz) are absent, toroidal Alfven eigenmode activity (f similar to 50-150 kHz) is weak and global Alfven eigenmode (GAE) activity (f similar to 400-1000 kHz) is robust. Compressional Alfven eigenmode activity (f > 1000 kHz) is usually sporadic or absent during the HEF event. The HEF exhibits growth times of Delta t similar to 20-80 ms, durations spanning 100-600 ms and peak-to-base flux ratios up to H = F-max/ F-min similar to 10. In infrequent cases, a slowing-down distribution below the HEF energy can develop that continues to evolve over periods of order 100 ms, a time scale long compared with the typical fast-ion equilibration times. HEFs are observed only in H-mode (not L-mode) discharges with injected power P-b >= 4 MW and in the pitch range chi equivalent to upsilon(parallel to)/upsilon similar to 0.7-0.9; i.e. only for passing particles. Increases of order 10-30% in the measured neutron yield and total stored energy that are observed to coincide with the feature appear to be driven by concomitant broadening of measured T-e(r), T-i(r) and n(e)(r) profiles and not the HEF itself. While the HEF has minimal impact on plasma performance, it nevertheless poses a challenging wave-particle interaction phenomenon to understand. Candidate mechanisms for HEF formation are developed based on quasilinear (QL) theory of wave-particle interaction. The only mechanism found to lead to the large NPA flux ratios, H = F-max/ F-min, observed in NSTX is the QL evolution of the energetic-ion distribution, F-b(E,chi, r), in phase space. A concomitant loss of some particles is observed due to interaction through cyclotron resonance of the particles with destabilized modes having sufficiently high frequencies, f similar to 700-1000 kHz, in the plasma frame that are tentatively identified as GAEs.