Transport of electrons and propagation of negative streamers in CF₃I-SF₆ mixtures

In this work, we present the electron swarm transport coefficients in CF₃I-SF₆ mixtures as a function of the reduced electric field E/N at room temperature. For E/N≥50 Td the calculations are performed using a multi term theory for solving the Boltzmann equation, while for E/N<50 Td, due to poor convergence of transport data, we apply a Monte Carlo simulation technique. We observed the reduction of the mean energy with increasing E/N for electrons in SF₆, and the occurrence of negative differential conductivity in the bulk drift velocity of electrons in CF₃I-SF₆ mixtures. These kinetic phenomena are explained using the spatially-resolved swarm transport properties, distribution functions, and physical arguments. The calculated swarm transport coefficients are then used as input to the fluid equation-based models to investigate the transition from an electron avalanche to a negative streamer. We investigate how the properties of streamers, including electron density, electric field and streamer velocity, are affected by the introduction of CF₃I to SF₆. Special attention is given to the comparison between the results obtained in the classical fluid model and the corrected fluid model, which assumes the non-local description of electron attachment and electron-impact ionization. We found that the classic fluid model with bulk transport coefficients and the corrected fluid model agree very well.