Ionization of gases by a pulsed electron beam as studied by self-focusing. III. He, Ar, and O2 mixtures

The self-focusing of a pulsed electron beam from a Febetron 706 in He, Ar, and O₂ is enhanced by 10% addition of polyatomic gases at total pressure higher than 50 Torr. The enhancement by addition of N₂ to He is stronger than that expected from pure N₂. The effect of additives to Ar appears abruptly from a certain pressure dependent on additives. According to the conclusion in part II, this enhancement is caused by the increasing of the mean ionization time t_i of secondary ionization due to electric field E induced by the pulsed beam. Such inference is demonstrated in the present paper on the basis of the time t_i for a mixture given by Eq. (1); t_i for a mixture is determined by the ionization potential of additives relative to that of a host gas, the initial slope of the energy dependence of the total ionization cross section σ_i of additives, and the retardation of secondary electrons by additives for acceleration by E. When a gas having small σ_i and large effect of slowing down is added, the self-focusing is strongly enhanced due to the large increase of t_i. This increase of t_i is also demonstrated by the increasing of 1/(wα) by addition of additives (w is the electron drift velocity and α is the first Townsend ionization coefficient); the values of both parameters for some cases can be known either from papers of experimental data and the Boltzmann equation analysis or by the average electron method.