Secondary ELM filaments in NSTX

Filamentary structures are observed in the scrape-off layer of the National Spherical Torus Experiment during ELMs. While the primary filaments correspond to a direct result of the ELM event, the ‘secondary’ filaments which occur generally later but still within 1 ms of the ELM onset are observed to have the same characteristics as inter-ELM filaments (or blobs): poloidal auto-correlation lengths of ∼4 cm, broadband frequency and poloidal wave number spectra and radial velocities of 1–2 km/s. At the same time, no MHD modes are observed during the phase in which secondary filaments are present.