Fast imaging of edge localized mode structure and dynamics in DIII-D

Abstract

Fast-framing images of CIII and Dα emission in the low-field-side plasma boundary of the DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] show that edge localized modes (ELMs) rapidly eject multiple field-aligned filaments from the plasma edge. The toroidal and poloidal mode numbers of these filaments depend on normalized plasma density, with measured ELM toroidal mode numbers ranging from ⩽10 to 20 in low-density plasmas and 15 to 35 in high-density plasmas. In high-density plasmas with moderate collisionality νped*=0.50, ELMs originate at the low-field-side midplane region and the ion parallel velocity in the scrape-off layer is faster for ELMs with larger Dα divertor emission, suggesting that large ELMs eject higher-temperature ions from deeper within the plasma compared to small ELMs. In low-density plasmas with collisionality νped*=0.25, the midplane and divertor ELM signals appear simultaneously, indicating that ELM behavior depends on collisionality. At all νped*, ELMs drive parallel fluxes to the divertor; in addition, ELMs drive cross-field propagation of filaments, which results in plasma-wall interactions that are poloidally localized within 15cm of the midplane. Using the wall interactions as signatures of the filaments in the scrape-off layer, the measured poloidal width of the filament ranges from 1to5cm.