Measurement of ionization and electron transport in methane-argon mixtures

We used a pulsed Townsend technique to measure the effective ionization coe fficients, electron drift velocities and positive ion drift velocities in m ethane-argon mixtures over the combined density-normalized electric field i ntensity, E/N, range from 0.05 to 700 x 10(-17) V cm(2). The mixture studie d contained 0.5, 3, 25, 50 and 75% CH4, including pure methane and pure arg on. We found a well defined dependence of the effective ionization coeffici ent on the amount of CH4 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 velocitie s in the mixture show a pronounced negative differential conductivity regio n, the maxima and minima of which depend on the mixture ratio.