New method for determining the electron swarm parameters in attaching gases

Abstract

This method requires the generation of electrons from a point source at a cathode and the measurement of both the amplification and radial distribution of the electrons arriving at an anode, after drifting and diffusing through the gas in a uniform electric field. At each value of E/p₀ (ratio of electric field to gas pressure) the experimental results are summarised on two curves expressed as a function of p₀d (product of gas pressure and gap separation). The first curve represents the mean-square radial displacement, from an axis drawn through the point source perpendicular to the cathode, and the second represents the current amplification. These curves are easily analysed to give simultaneously the values of the ratio of the radial diffusion coefficient to mobility, D_r/μ, and attachment coefficient η, and the primary ionisation coefficient α. The method has been used to evaluate D_r/μ, η/p₀ and α/p₀ in CF₄, and measurements have been made for 375 ≤ E/p₀ ≤ 52.5 Vm⁻¹ Pa⁻¹ at 0°C.