Correlation of submicron dust production in DIII-D to impulsive wall heating from ELMs

Abstract

Submicron dust particles are observed by Rayleigh/Mie scattering from ND:YAG laser light during plasma discharges at DIII-D. The observed dust density decreases gradually over the tokamak major radius in the scrape off layer (SOL) (1.0 < Ψ < 1.2 for typical discharges) to the plasma edge. There is a significant increase in dust density with ELMy H-mode discharges relative to L-mode and ELM-free H-mode discharges. The dust density in ELMy H-mode discharges is sensitive to many parameters including the pedestal temperature and ELM frequency and can increase as much as a factor of two with pedestal temperature for similar pedestal pressure and injected power. Dust density decreases with increasing ELM frequency compared to shots with similar stored energy and less frequent ELMs. These measurements suggest the particles are created by impulsive heating from ELMs and the dust density is less sensitive to steady heat flux to the wall.