Identification of the discharge kinetics in the perfluoro-nitrile C4F7N with swarm and breakdown experiments

Abstract

We follow up on our previous publication on C4F7N and investigate the ion kinetics influencing C4F7N discharges with a pulsed Townsend experiment. The measured current signals at pressures between 6 and 69 kPa, in the range of density-reduced electric fields E/N from 500 to 1050Td, show evidence of three anions: one long-lived anion with a detachment rate coefficient in the order of 10^{-20} m3s-1, a presumably non-detaching anion, and a short-lived anion with a detachment rate coefficient in the order of 10^{-17}m3s-1. We obtain the corresponding rate coefficients for ionization, attachment and detachment in C4F7N. Using these results, we calculate a corrected value of the effective ionization rate coefficient nu_eff, taking into account detachment from negative ions. It results in a corrected value of the density-reduced critical electric field strength of 785+-15Td, much lower than the previously obtained value of 975+-15Td, which is valid only when electron detachment is negligible, i.e. for low pressure and small geometric distance. To find which value is most relevant for electrical insulation, we perform breakdown voltage measurements in homogeneous electric fields in C4F7N at 5 to 65kPa. We compare the measured density-reduced breakdown field strength to that calculated using the streamer criterion with the presently obtained rate coefficients, including electron detachment. We find excellent agreement between measurement and calculation.