A swarm parameter experiment is introduced, which implements the pulsed Townsend (PT) electrical method with a high degree of automatization. The experimental setup and measurement procedures are described in detail, and a comprehensive definition of the swarm model is given and used for signal analysis. The intrinsic parameters of electron drift currents in the PT method are identified, and novel regression methods are presented for obtaining electron swarm parameters from PT measurements. The setup and methods are verified with measurements in Ar, N 2 and CO 2 , which are focused on the (E/N)-range between dominating electron attachment and weakly dominating ionization. The present data are compared with experimental reference data, and to electron transport coefficients calculated by a Boltzmann solver and simulated by a Monte Carlo method. Excellent agreement was found between the present data and the Monte Carlo results, but there are significant discrepancies to widely used recommended swarm parameters of N 2 and CO 2. Finally, it is proposed to revise some hitherto recommended values of electron transport coefficients.

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Author(s):Franck, Christian M.; Dahl, Dominik A.; Rabie, M.; Haefliger, P.; Koch, M. Publication Date: 2014 Permanent Link:https://doi.org/10.3929/ethz-a-010881816 Originally published in:Contributions to plasma physics 54(1), http://doi.org/10.1002/ctpp.201300030 Rights / License:In Copyright -Non-Commercial Use Permitted This page was generated automatically upon download from the ETH Zurich Research Collection. For more information please consult the Terms of use. ETH LibraryThis is the pre-peer reviewed version of the following article: In this contribution, a new procedure to systematically identify and quantify novel molecular gases with low global warming potential for application in high voltage insulation as gas mixtures is presented. The attention is focused on highly efficient procedures to be able to scan a large number of candidate gases. To identify new molecules, we derived an empirical correlation between the electric strength of a gas and certain molecular properties, like polarizability or dipole moment, which can be calculated by means of density functional theory. The swarm parameters of these pre-selected molecules in mixtures with buffer gases is then quantified, using a newly set-up Pulsed Townsend experiment. The setup operates with a high degree of automation to enable systematic evaluation of gas mixtures not to miss possible synergistic effects. Key element of this PT setup is a new photocathode that works with a high quantum efficiency and long lifetime even when exposed to reactive species during the measurements. Moreover, for an automated operation it is important to know precisely in which range the experiment can be operated, i.e. for example to know up to which electron density space charge effects can be neglected. Finally, the measured swarm parameters need to be translated into breakdown voltage strengths of different electrode arrangements and different applied voltage wave shapes. For this, a model of the the streamer to leader transition in SF6 will be applied to general strong electronegative gases in future studies.

Electron swarm methods are applied for investigating the effects of small amounts (≤ 1.5%) of a strongly electronegative sample gas in the buffer gases Ar, N 2 or CO 2 . A pulsed Townsend method, a Monte Carlo swarm method, and a solution of the Boltzmann equation are used to determine the effective ionization rate constants of the gas mixtures. The sensitivity of the effective ionization rate constant to changes of the mixing ratio is evaluated. Our methods are benchmarked with the analysis of Ar-SF 6 and N 2 -SF 6 mixtures, and subsequently used for the analysis of gas mixtures containing C 3 F 8 . The results based on the recommended C 3 F 8 cross sections are shown to be inconsistent with the experimental data for N 2 -C 3 F 8 and CO 2 -C 3 F 8 mixtures.

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