Electron transport parameters in CO₂: scanning drift tube measurements and kinetic computations

This work presents the transport coefficients of electrons (bulk drift velocity, longitudinal diffusion coefficient, and effective ionization frequency) in CO₂ measured under time-of-flight conditions over a wide range of the reduced electric field, $15Td⩽E/N⩽2660Td,$ in a scanning drift tube apparatus. The data obtained in the experiments are also applied to determine the effective steady-state Townsend ionization coefficient. These parameters are compared to the results of previous experimental studies, as well as to the results of various kinetic computations: solutions of the electron Boltzmann equation under different approximations (multiterm and density gradient expansions) and Monte Carlo simulations. The experimental data extend the range of E/N compared with previous measurements and are consistent with most of the transport parameters obtained in these earlier studies. The computational results point out the range of applicability of the respective approaches to determine the different measured transport properties of electrons in CO₂. They also demonstrate the need for further improvement of the electron collision cross section data for CO₂ taking into account the present experimental data. (paper)