Electron swarm characteristics in Ar:NF3mixtures under steady-state Townsend conditions

The effect of small amounts of electronegative NF3 molecules on the electron swarm characteristics in Ar:NF3 mixtures is studied numerically under steady-state Townsend conditions. Two different methods of calculation are used: the solution of the two-term Boltzmann equation (BE) and the Monte Carlo (MC) simulation. The BE analysis is performed for an established electron energy distribution function. The MC calculations give the distance at which the shape of the distribution function approaches that resulting from the BE analysis. It is shown that the losses of electrons in the attachment processes play a significant role in the formation of the electron energy distribution function. In a wide range of the reduced electric field strength E/N and NF3 concentrations the distribution function with a maximum at finite energy is established. It is also shown that the diffusion-modified drift velocity Vd is a non-monotonous function of E/N and that the so-called field-determined part of the Vd is negative in a wide range of E/N values. These effects are explained in terms of the specific energy dependence of the cross section for electron attachment to the NF3 molecules and the momentum transfer cross section of Ar.