Electron transport and swarm parameters in CO₂and its mixtures with SF₆

Abstract

The pulsed Townsend technique has been used to measure the electron drift velocity, the density-normalized effective ionization coefficient, the density-normalized longitudinal diffusion coefficient NDL, and the `characteristic energy' of electrons DL/K, in CO2 and its mixtures with SF6 over a wide range of the density-reduced field strength E/N, from 100 to 700 Td (1 Townsend = 10−17 V cm2). The SF6 content in the mixture was varied between 2% and 70%. It was observed that for small concentrations (2–5%) of SF6 in the mixtures, the electron drift velocity is relatively close to that for pure CO2. A similar behaviour was observed for the longitudinal diffusion coefficients. In contrast, the influence of SF6 in the mixture is strongly apparent in the values for the effective ionization coefficients. From the latter parameter, the critical field strength E/Ncrit for each SF6 concentration could be derived, and it was found that its value is smaller than that measured for the SF6–N2 mixtures.