Edge Stability of Small-ELM Regimes in NSTX

NSTX has observed low-frequency (f $<$ 10 kHz) unstable modes with characteristics similar to the edge harmonic oscillation (EHO) coincident with transition to a small-ELM regime where the ELMs reduce the plasma stored energy by less than 1\%. Soft x-ray emission indicates that these modes are localized just inside the density pedestal. Microwave reflectometry shows density fluctuations in the pedestal at the mode frequency. Toroidal Mirnov analysis indicates n=1 for the lowest order mode, which rotates at the plasma rotation frequency, with higher harmonics observed simultaneously in some cases. Stability analysis during the observed modes indicates instability to n=1-4 with unstable mode eigenfunctions peaked near the plasma edge. Transition to this regime is associated with a downward biased plasma ($\delta$$_r$$^s$$^e$$^p$ $<$ -5 mm) and increased edge collisionality ($\nu$$^*$$_p$$_e$$_d$ $>$ 1). Increased pedestal pressure and bootstrap current are observed in the small-ELM regime, with the peak in the bootstrap current moved inward from $\Psi$$_N$ = 0.96 to 0.94.