Experimental study of SF6/N2 and SF6/CCl2F2 mixtures by the steady-state Townsend method

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Abstract
Prebreakdown ionization currents have been measured in SF6/N2 and SF6/CCl2F2 mixtures and respective gases by the steady‐state Townsend method over the range of reduced fields 70≤E/p20≤200 V cm−1 Torr−1. Relevant collisional processes to be included in the formulation of the current‐growth equation are briefly reviewed; electron detachment was accordingly neglected in the present current‐growth analysis. A numerical simulation has been carried out to quantify the errors expected when numerically retrieving the transport coefficients. A two‐parameter model, for which the initial current I0 is considered known, is found to give the best performance. Nonlinear regression analysis using Marquardt’s procedure is performed for evaluation of the ionization (α), attachment (η), and effective ionization (ᾱ) coefficients. The limit fields are obtained from the condition ᾱ(E/p*)=0. The coefficients for the respective gases compare well with available data in the literature with the exception of the attachment coefficient in SF6 which shows a more pronounced decrease than the general trend as E/p increases. In the case of SF6/N2 mixtures, good agreement is found for the effective ionization coefficients. The attachment coefficients do not saturate but rather increase with the SF6 partial density. In SF6/CCl2F2 mixtures, the variation of ᾱ is found to be nonlinear with the percentage mixture ratio. The resulting limit field exhibits small positive and negative synergistic effects, 6% and 3%, respectively; at 25% SF6 content, the limit field is approximately 5% greater than that of pure CCl2F2. For SF6/N2 mixtures, however, the limit field never exceeds that of SF6.

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