Primary edge localized mode filament structure in the National Spherical Torus Experiment

Edge localized modes (ELMs) are routinely seen in the National Spherical Torus Experiment (NSTX) [M. Ono, Nucl. Fusion 40, 557 (2000)]. These unstable modes give rise to plasma filaments that burst radially outward during the nonlinear phase of the instability, moving across flux surfaces into the scrape-off layer. Fast-frame visible imaging is used in NSTX to study the evolution and characteristics of the post-ELM filaments. These edge filaments, which are well aligned with the local magnetic field, are seen to evolve from a perturbation of the edge that within 40-50 μs develops into the relatively high density/temperature primary filaments. The distribution of primary filaments in toroidal angle is seen to agree with a random model with moderate average toroidal mode numbers. At the same time, gas puff imaging shows that the perturbation of the edge leading to the burst of the ELM into the scrape-off layer is characterized by a broadband increase in fluctuations at much smaller poloidal wavelengths (λ_pol ∼2-12 cm). These two measurements suggest that early development of turbulence may play a role in the development of primary ELM filamentation.