Effect of Fast-ion Distribution Function on Beam Driven Instabilities in NSTX

The deuterium beam distribution function is modified from shot to shot while keeping the total injected power to $\sim$2~MW. The experimental ``knobs'' are the beam energy (90 keV and 60 keV), the beam tangency radius, and the fraction of trapped beam ions, which is modified at a predetermined time by applying $\sim \ \ge 2$~MW of high harmonic fast wave (HHFW) heating. Neutral particle analysis confirms perpendicular acceleration of the beam ions. The neutral beams are injected into a helium L-mode plasma and produce a rich set of instabilities, including TAE modes, instabilities with rapid frequency sweeps or chirps, and strong, low frequency (10-20 kHz) fishbones. Fishbones are excited when $q_0 < 0$ and when the trapped beam-ion fraction increases; they are always present later in the discharge. However, TAE modes are excited only early in the discharge and, under some circumstances, they are suppressed by HHFW heating on a collisional time scale. In contrast with a Dipole experiment,\footnote{D. Maslovsky, B. Levitt and M. E. Mauel, Phys.\ Plasmas {\bf10} (2003) 1549.} the cyclotron heating has no effect on the chirping instabilities.