Fast Imaging of ELM Structure and Dynamics in DIII-D

Fast-framing images of CIII and D$_{\alpha }$ emission in the low-field-side (LFS) plasma boundary of DIII-D show that ELMs are helical filamentary structures that rotate toroidally. The filaments propagate radially outward at v$_{r}\sim $500 m/s during the nonlinear phase, and result in plasma-wall interactions that are poloidally localized within 15 cm of the midplane. The measured mean poloidal width of the filament is 3 cm, and the ELM toroidal mode number $n$ ranges from 10 to 35. ELM structure and dynamics vary with plasma density, possibly because ELMs are driven by a peeling type of mode in low density plasmas and are driven by a coupled peeling-ballooning mode in high density. At high collisionality ($\nu _{ped}$*=0.50), ELMs begin with an unstable filament or group of filaments at the LFS midplane region. Onset of the ELM-induced radiation in the divertor is delayed by as much as 0.8 ms compared to the midplane signals. In low collisionality ($\nu _{ped}$*=0.25) discharges, the midplane and divertor ELM signals appear simultaneously, possibly suggesting a more poloidally symmetric mode structure.