A comparison of air breakdown criteria for slightly divergent fields

The commonly used air breakdown criteria for slightly divergent fields are compared. The various criteria can be classified into two distinct groups with regard to the basic breakdown mechanism employed. The Townsend (1915) breakdown mechanism is described by a succession of developing electron avalanches between the electrodes. The streamer mechanism of breakdown is based on a single electron avalanche and its inherent exponential growth. The primary assumption in the majority of these breakdown criteria is that the integral of the effective ionization coefficient α¯ over the critical avalanche length Z₀, is equal to a constant K. It is shown that the value of K is not a constant even for gaps with uniform fields. Of the six breakdown criteria considered, the Pedersen (1967) criterion is shown to have the most solid basis in fundamental physics