Dust observations in the DIII-D tokamak using a fast camera

Abstract

Naturally occurring and injected dust particles are observed in the DIII-D tokamak in the outer midplane scrape-off-layer (SOL) using a fast-framing visible camera. Sources of naturally occurring dust include material eroded from plasma facing components by transient events such as edge-localized-modes (ELMs) or disruptions. Dust particles ablate when they enter a hot region of plasma near the separatrix, or when an ELM filament intercepts a dust particle already present in the SOL. Modeling shows that line radiation from the ablation cloud dominates thermal radiation from the dust particle when the plasma temperature is greater than ∼10–50 eV for typical edge plasma densities in DIII-D. The size of dust particles is estimated using the observed particle lifetime and theoretical ablation rate of a carbon sphere. Using this method, the lower limit of detected dust diameter is ∼6 mm and particles with inferred diameter ≫1 mm are observed. Dust particle 2D velocities range from approximately 10 to 300 m/s, with larger velocities measured for smaller particles. The observed scaling of the dust velocity as a function of size R d is in rough agreement with the v ∝ R d −1/2 scaling predicted by a simple 1D analytic model of spherical carbon dust particles accelerated due to ion drag in a uniform plasma, for a fixed distance between the dust origin and the observation region. Pre-characterized 2–4 mm diameter diamond dust particles are introduced at the lower divertor in an ELMing H-mode discharge using the divertor materials evaluation system (DiMES), and these particles are found to be at the lower size limit of camera detection using resolution of ∼0.2 cm2 per pixel and exposure time of 330 ms. Injection of pre-characterized 6 mm median diameter dust indicates that the inferred size of naturally occurring dust may be overestimated by up to a factor of 10.