Is CF₃I a good gaseous dielectric? A comparative swarm study of CF₃I and SF₆

This paper deals with the measurement of the electron drift velocity, the longitudinal diffusion coefficient and the effective ionisation coefficient in pure CF₃I and CF₃I mixtures. The E/N range covered was 100-850 Td (1 Td=10^-17 V cm²). The present results were derived from a pulsed Townsend experiment. For pure CF₃I, the values of the electron drift velocity and of the effective ionisation coefficient were found to increase steadily with E/N. The E/N value at which ionisation equals attachment, commonly referred to as the limiting field strength, was found to be E/N_lim=437 Td, which is higher than that of SF₆ (360 Td), a widely used insulating gas. Moreover, the curve for the mixture of 70% CF₃I with N₂ is very similar to that of SF₆ in the neighbourhood of E/N_lim. Thus, from the point of view of these coefficients, CF₃I would seem to be superior in dielectric behaviour to SF₆. Besides, CF₃I is an environmentally friendly gas as opposed to SF₆. Certainly, many more tests should be performed to regard CF₃I as a good dielectric and possible substitute to SF₆.