Electron swarm coefficients and critical field strength of the gaseous ternary mixtures CF₃I-SF₆-N₂ and CF₃I-SF₆-CO₂

Abstract

The electron drift velocity, longitudinal diffusion and effective ionization coefficients in the gaseous ternary mixtures CF3I-SF6-N2 and CF3I-SF6-CO2 have been measured using a pulsed Townsend apparatus over the density-normalized electric field intensity, E/N, from 230 to 470 Td and 230 to 430 Td, respectively (1 Td = 10−17 V cm2). Several ternary gas mixture compositions containing 10% to 50% CF3I or SF6 and either N2 or CO2 have been studied. Also, we have measured the critical field strength, E/Ncrit, for all of these gaseous mixtures, finding out that the 40% CF3I–40% SF6–20% N2 and 40% CF3I–40% SF6–20% CO2 have an E/Ncrit value smaller by only 4.0% and 1.0%, respectively than that of pure SF6 (360 Td). Provided that both CF3I, CO2 and N2 are environmentally friendly, its addition to SF6 may be highly advantageous over other binary SF6 mixtures with gases having relatively high warming potentials, since these mixtures have only 40% SF6 and behave, from the swarm physics point of view as good candidates to ameliorate impact of SF6 when released into the atmosphere. An interesting synergetic effect has been found for the CF3I-SF6 mixture when N2 added and has been interpreted in terms of the efficient low-energy electron scattering of N2 at low collision energies up to 2 eV.