Electrical ionisation and breakdown of gases in a crossed magnetic field

Abstract
A unified approach to the subject of electrical ionisation and breakdown of gases in the presence of a crossed magnetic field is presented by invoking the equivalent reduced electric field (e.r.e.f.) concept which is a generalisation of the previously accepted equivalent gas density (pressure) principle. The e.r.e.f. concept is derived from basic electron trajectories in a simplified way using a geometrical approach rather than from a fictitious, but later shown to be measurable, equivalent gas density; this method, it is hoped, should appeal to engineers. The e.r.e.f. approach immediately leads to an almost satisfactory interpretation of breakdown characteristics but in order to obtain better agreement with experiment, the influence of a crossed magnetic field on the primary and secondary prebreakdown ionisation processes is considered. Although coaxial systems provide unrestricted electron motion in the E × B direction, and are usually preferred, it is shown that parallel plate electrodes yield as good or even better agreement with theory. For the case of restricted motion due to wall losses which occurs below the Paschen minimum, the usual equations have to be suitably modified. The influence of the crossed magnetic field on the salient minimum breakdown voltage is then dealt with, and by considering in more detail the ionisation-current build-up, the sideways Townsend propagation in the E × B direction is considered. An almost chronological survey of the literature traces the development of the subject from its earliest beginning, and a number of applications are given. The present state of the art is in a concluding section, and likely future developments are outlined.

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