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Abstract Although the Townsend theory of dielectric breakdown of gases is generally accepted, it is also realized that it does not apply at low gas density. This embraces the range of gas density between a true gas breakdown and vacuum breakdown. Under such conditions, the concept of an average number of collisions per unit path length, fundamental in the Townsend theory, can not be applied. A review of previous theoretical work covering this region is given and it is shown that it is essential to calculate the number of ionizing collisions made by an electron moving from the cathode to the anode before any model of breakdown can be developed. A method of making such a calculation using Monte Carlo computer techniques is described with special reference to mercury vapour. This gas shows are-entrant from of Paschen curve, similar to that exhibited by helium . It is shown that the theory developed for pre-breakdown current growth can explain why some gases exhibit the rementrant form and others do not. However, complete verification is not possible because of lack of experimental data.
Parker et al. (Tue,) studied this question.