Ionization of gases by a pulsed electron beam as studied by self-focusing. II. Polyatomic gases

Abstract

In order to analyze data on the self-focusing of a pulsed electron beam in polyatomic gases, the net current Inet in H2, N2, and CH4 was computed self-consistently as functions of time in the pressure range between 5 and 300 Torr of these gases by using swarm parameters. The computational result indicates that the larger dose Dobs, observed by a piled dosimeter on the beam axis, is attributed to the larger Inet, which is mainly determined by a mean ionization time t1 for secondary ionization by the electric field induced by the pulsed beam. When values of Dobs for different gases are compared at the same pressure, the larger Dobs is given by the larger ti. This relationship is demonstrated for several polyatomic gases by estimating ti from various parameters in a function of secondary electron energy or E/p such as the electron drift velocity, the first Townsend ionization coefficient, the ionization cross section, and so on. For the short pulse duration of a Febetron 706, electron–ion recombination processes scarcely affect Inet except at high pressures of some polyatomic gases, while the effect of electron-attachment processes is appreciable in SF6, CCl2F2, and N2O.