Positron transport in CF4 and N2/CF4 mixtures

Abstract

In this paper we present our recently compiled set of cross sections for positron scattering in CF4. Using this set of cross sections as an input in our multi term Boltzmann equation analysis of positron transport in CF4 and N2/CF4 mixtures, we calculate various transport properties as a function of the reduced electric field E/n0. Values and general trends of the mean energy, drift velocity and diffusion coefficients as a function of E/n0 are reported here. Emphasis is placed on the explicit and implicit effects of positronium (Ps) formation on the drift velocity and diffusion coefficients. Two important phenomena arise. Firstly, for certain regions of E/n0 the bulk and the flux components of the drift velocity and longitudinal diffusion coefficient are markedly different, both qualitatively and quantitatively. Secondly, and contrary to previous studies in positron swarm studies, there is negative differential conductivity (NDC) in both the bulk and flux drift velocity. The variation of the drift velocity with E/n0 and origin of the NDC are addressed using physical arguments. Cross sections and transport data for positrons in CF4 and N2/CF4 mixtures presented in this work are compiled and evaluated with an aim of improving the efficiency of positron traps.