Characterization of filament morphology in NSTX plasmas

Filamentary structures naturally arise from background turbulence in the scrape-off layer (SOL) of plasmas, leading to significant particle and heat transport degrading overall plasma confinement. Filamentary transport can contribute to unacceptably high heat loads on plasma-facing components. As such, understanding the physics of SOL plasma filaments is critical for predicting and mitigating their effects in future fusion devices.

On the National Spherical Torus Experiment (NSTX), SOL filaments were investigated using the gas-puff imaging (GPI) diagnostic. A novel watershed segmentation-based method was developed to identify and track filaments in each frame of the GPI video data. Their evolution was characterized using shape descriptors, as well as translational and angular velocities, derived from their contour coordinates and corresponding fitted ellipses.

The results indicate that as the filament area increases, their shapes become more concave and less circular, suggesting reduced structural stability in larger filaments. A positive trend was found between radial velocity and radial position, indicating outward radial acceleration.

Filament parameters were also compared with bulk plasma and radial profile measurements. Notable trends were found between blob rotation vs collisionality and poloidal velocity vs line-integrated density. These findings contribute to a deeper understanding of blob dynamics and provide valuable insights for refining SOL turbulence models.