Dusty plasma dynamics in Ar/C2H2 RF discharges

The coupled effects of plasma dynamics, chemical kinetics, and aerosol dynamics in capacitively coupled RF Ar/C2H2 discharges are investigated computationally using a dedicated 1D numerical model. The model combines a fluid description of the plasma with a reactive flow model for the neutral and charged species, and a bimodal particle size distribution model for aerosol dynamics. Simulations of the transient plasma evolution were performed for different C2H2 concentrations in the feed gas (10%, 25%, and 50%). The results demonstrate that dusty plasma effects in Ar/C2H2 discharges differ significantly from silane systems, with positive ion chemistry playing a crucial role in the nucleation kinetics. In particular, three dusty plasma regimes were identified. At low acetylene concentration (10% C2H2), the nucleation is limited, leading to a relatively low particle density (∼108 m−3) and a mean particle diameter of 15 nm after 5 s of discharge duration. In this case, the particle dynamics and the plasma dynamics are decoupled, as indicated by a (numerical) coupling parameter, Pc=zp.Npne, much smaller than unity (Pc ≪ 1). At moderate concentrations (25% C2H2), the particle density is significant (i.e., 1013 m−3), resulting in a local coupling effect (Pc > 1) that affects the space distributions of electron density and temperature in the plasma, the particle cloud being concentrated at the sheath edges. At high C2H2 concentration (50% C2H2), the nucleation rate becomes very high, producing particle densities exceeding 1015 m−3 and a very strong coupling effect, i.e., Pc ≈ 12. In this case, a self-sustained nucleation phenomenon emerges, where particle formation enhances ion density, which in turn promotes further nucleation. Particle growth is then limited, resulting in smaller particles (7 nm) confined in the discharge center and, surprisingly, with only minor effects on the electron density and temperature. This study provides new insights into the modeling of carbon-based dusty plasmas and their applications in nanoparticle synthesis.