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 × 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.