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

Abstract

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 ti 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 ti for a mixture given by Eq. (1); ti 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 σi of additives, and the retardation of secondary electrons by additives for acceleration by E. When a gas having small σi and large effect of slowing down is added, the self-focusing is strongly enhanced due to the large increase of ti. This increase of ti 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.