Abstract
An instability near the plasma edge known as the edge harmonic oscillation (EHO) is thought to
enable access to the ELM-free quiescent H-mode (QH-mode) in tokamaks, which is a highly desirable
operational regime for ITER because of the avoidance of periodic ELM heat loads. The EHO has been
hypothesized to be a saturated kink driven unstable by toroidal rotational shear that provides sufficient transport near the plasma edge to keep the edge plasma below the peeling-ballooning stability limit. NSTX has observed unstable modes with similar characteristics to the EHO coincident with transition to a small-ELM regime (called Type-V). These small ELMs do not have a measurable effect on the plasma stored energy (< 1%). Transition to this regime is associated with a downward biased plasma as evidenced by drsep < -5 mm. Soft x-ray emission indicates that these modes are localized just inside the pedestal and are correlated with increased density
fluctuations in the pedestal as measured by microwave reflectometry. The lowest order mode rotates at the plasma rotation frequency, indicating n=1, and harmonics up to n=6 have been observed simultaneously with
the n=1, as determined by the rotation frequency of the higher harmonics. Increased edge collisionality is
required to access Type-V ELMs. Stability analysis during the observed modes indicates instability to n=1-3 with n=3 having the highest growth rate and unstable mode eigenfunctions peaked near the plasma edge. Discharges with Type-V and Type-I ELMs are both calculated to be on the peeling unstable side of the peeling ballooning stability curve, with the Type-V case at higher normalized pressure gradient.