Modelling of carbon dust formation by cluster growth in argon plasmas

In tokamaks with carbon plasma-facing components, one can observe the presence of nano-sized dust particles. Understanding such dust particle formation is a prerequisite to any attempt to limit or avoid this dust that may be responsible for tritium retention and pollution of the plasma. We report on coupled modeling of carbon chemistry and dust particle nucleation, growth, and transport in a plasma discharge. The chemical model used for carbon cluster dust growth is described in detail. The results are consistent with measurements made at LPIIM from low-pressure argon DC discharges in a stainless steel reactor with a graphite cathode [C. Arnas, C. Dominique, P. Roubin et al., J. Nucl. Mater. 337-339 (2005) 69], serving as a proxy for the tokamak plasma edge. The time evolution of the 'large' dust particles consists of a nucleation phase followed by an accretion phase. These reach a dust grain size of 40 nm on a timescale comparable to the experimental observations (minutes to hours).