Equality of Higher-Order Diffusion Coefficients between Component and Composite Electron Swarms in Gas

Abstract

When an electron swarm in gas is composed of component electron swarms individually in drift equilibrium, the higher-order diffusion coefficients (HDCs) of the composite electron swarms are equal to those of the component electron swarms. We have derived this equality theoretically and have examined it by numerical simulation. The HDCs are the time derivatives of higher-order cumulants, which are quantities characterizing the shape of a spatial electron distribution. This fact seemingly indicates the dependence of the HDCs of the composite electron swarms on the arrangement of the component electron swarms. However, the equality holds irrespective of the relative positions and electron populations of the component electron swarms. We have given a consistent explanation to these facts. We have also discussed electron swarm development from dispersed or multiple electron sources that may appear in practical experiments.