Ionization coefficients and sparking voltages in argon-methane and argon-propane mixtures†

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Abstract
Townsend primary and secondary ionization coefficients have been measured in 0.03, 0.3 and 3% of methane and of propane in argon for a total pressure in the range 0.5-500 torr and for a uniform field gap up to 1 cm. The primary ionization coefficient for the Ar-CH4 mixtures remains within 30% of that for pure argon but an optimum concentration of 0.13% of propane in the Ar-C3H8 mixture gives rise to a Penning effect almost comparable in intensity to that observed by Kruithof and Penning (1937) in Ne-Ar mixtures; thus at E/p0 =5 v cm−1 torr−1 at 0°c, the coefficient is increased by a factor of 200 above that for pure argon. Because of the quenching property of hydrocarbons, it is shown that it is possible to reduce the secondary ionization coefficient of argon as much as 107 times; at low and high E/p0 the effectiveness of quenching incroasos with increase of molecular weight whilst in the intermediate rogion 10≲ E/p 0 ≲100, the reverse is the case. At an optimum concentration of 0.03% of propane in argon the sparking voltage is lowered by a factor of 3½ below that for pure argon whilst the presence of methane increases the voltage. When large avalanches are attained, the Townsend equation has to be generalized to take into account space charge effects.