Measurement of number density and size distribution of dust in DIII-D during normal plasma operation

Dust particles with submicron radii have been observed in the scrape-off layer (SOL) and divertor region of DIII-D during normal plasma operation. The particles are observed via their relatively large cross section for Rayleigh scattering of the Nd : YAG laser used for Thomson scattering measurements of electron density and temperature. In the outer region of the SOL particles are rare, with an average density of about 6 × 10 3 particles m −3 . The particle density falls sharply going toward the last closed flux surface, being ∼ 0 at and inside the separatrix. Modelling of the observed pulse height distribution indicates an average particle radius of ∼ 80 nm. These measurements indicate that dust created by normal plasma operation is not a likely source of core carbon contamination on DIII-D, nor is it a serious contributor to redeposition of carbon films.