Electron trajectories in ionization gauges

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Abstract
One of the important requirements when optimizing ionization gauge sensitivity is that the most efficient use be made of the ionizing electrons. Recent designs of extractor gauge retain the cylindrical grid structure, external electron emitting filament and coaxial screen cylinder currently used in the Bayard-Alpert gauge. The motion of electrons in this type of electrode system is described and the dependence of gauge sensitivity on the resulting trajectories is analysed. The relationship is illustrated between the average number of electron passes through the grid region and the electron capture probability of the grid, showing the advantage of a high grid transparency. Computer-aided design techniques have been employed to study the influence of electrode spacing and potentials on the path lengths of electrons. The results of this study are presented and show that a considerable variation in gauge sensitivity can occur, depending on the extent to which the electrons are focused into diametric paths across the grid region. Experimental results are given which illustrate this effect and show how the grid-to-screen potentials required for optimum sensitivity depend on both filament-to-grid potential and spacing.

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