Electron swarm coefficients of CF3I and CF3I-N2 and their comparison with those of SF6

This paper reports the measurement of the electron drift velocity, the longitudinal diffusion coefficient, the ratio between the longitudinal diffusion coefficient and the electron mobility, and the effective ionisation coefficient for electrons in pure CF₃I and CF₃I-N₂ mixtures. A pulsed Townsend technique was used. The E/N range covered was 100-850 Td (1 Td=10^-17 V cm²). The limiting or critical field strength for CF₃I was found to be E/N_lim=437 Td, which is 21% higher than that of SF₆ (E/N_lim=360 Td), a widely used insulating gas. Moreover, the curve of the effective ionization coefficients for the 70% CF₃I-N₂ mixture bears a very similar behaviour to that of pure SF₆ in the vicinity of E/N_lim. Thus, from the point of view of this parameter, CF₃I seems to be superior in dielectric behaviour to SF₆. Admittedly, many other tests are necessary to regard CF₃I as a good dielectric and a likely substitute to SF₆ in high voltage and switchgear applications.