Initiation and progress of breakdown in the range to the left of the paschen minimum

Mechanisms of gas breakdown initiation and development in the range to the left of the Paschen minimum are investigated in this paper. Experiments were conducted for gas pressures from 10^-9 to 10^-1 bar and inter-electrode gaps from 0.1 to 1 mm. The obtained results were subjected to the Chauvenet´s criterion for discarding spurious values, the U-test for checking if they belonged to a common statistical distribution, and the chisquared test for testing their adherence to one of the considered theoretical distributions (normal, Weibull or double-exponential). The statistical analysis shows that the region left of the Paschen minimum can be divided into three subregions. It is established that the so called anomalous Paschen effect refers to breakdown occuring by way of the edge mechanisms in the first subregion, just next to the Paschen minimum. In the second subregion, breakdown emerges through the vacuum avalanche mechanism, and in the third subregion it occurs through the vacuum cathode emission mechanism.