Pedestal characterization and stability of small-ELM regimes in NSTX

NSTX has observed transition to a desirable small-ELM regime (called type-V), in which the stored energy loss per ELM is less than 1%, by stabilizing type-I ELMs. This regime is accessed in a lower single null configuration with increased edge collisionality (ν * > 1). Coincident with the transition to this regime, a low-frequency (<10 kHz) n = 1 mode is observed at the plasma edge in magnetic and soft x-ray diagnostics, with harmonics up to n = 6 observed in some cases. Low-level density fluctuations associated with this mode are observed using microwave reflectometry, but there is no evidence that the mode is providing sufficient transport to stabilize the type-I ELMs. This mode rotates in the electron diamagnetic direction and has shown a phase inversion on USXR channels, indicating that it is resistive in nature. 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 and closer to the ballooning stability boundary.