Ionization coefficients and sparking voltages in argon and argon-ethane mixtures

Townsend primary and secondary ionization coefficients have been experimentally determined in pure argon and in argon mixed with up to 3% of ethane gas for a total pressure in the range 0·5-700 torr, corresponding to 3less, similarE/p0less, similar2000 v cm−1torr−1at 0°C. Ionization enhancement takes place and it is shown analytically that the major contribution to this is due to the Penning process. At an optimum concentration of about 0·66% of ethane in argon the primary coefficient is increased to forty times its value for pure argon. The presence of common impurities increases the coefficient by up to five times, whilst a similar concentration of electron-attaching gases lowers the coefficient by a factor of 10. Because of the quenching nature of ethane it is shown possible to reduce the secondary ionization coefficient of argon by as much as 107times; with the addition of less than 0·03% of ethane it is possible to increase the secondary coefficient at lowE/p0 by a factor of 10 above that for pure argon. At the optimum concentration the sparking voltage is a factor of 2·2 below that for pure argon, whilst the presence of 0·000 03% of ethane still lowers the sparking voltage of pure argon, by a factor of 1·3. All the sparking voltages lie well below that for pure argon.