Measurement of ionization and electron transport in methane-argon mixtures

We used a pulsed Townsend technique to measure the effective ionization coefficients, electron drift velocities and positive ion drift velocities in methane-argon mixtures over the combined density-normalized electric field intensity, E/N, range from 0.05 to 700 x 10(-17) V cm(2). The mixture studied contained 0.5, 3, 25, 50 and 75% CH(4), including pure methane and pure argon. We found a well defined dependence of the effective ionization coefficient on the amount of CH(4) in the mixture at low E/N, while at the higher E/N end, all the curves merged into a single one. The electron drift velocities in the mixture show a pronounced negative differential conductivity region, the maxima and minima of which depend on the mixture ratio.