CXVIII. The sparking-potential of gases

Streamer breakdown: cathode spot formation, Trichel pulses and cathode-sheath instabilities

“…(iv) The streamer arrival to the cathode, forming an active glow-discharge type cathode spot, which is effectively producing the electrons by direct impact ionization in the cathode fall and, consequently, marks a turning point in the streamer-initiated formation of self sustained discharges. At atmospheric pressure the cathode spot develops in several nanoseconds and, as discussed in sections 1-5, the fast-rising displacement current peak (see figures 2, A2, 7, 8, 9, 10, 14, 15, 16, 17(b), [19][20][21] generated during this phase is rather independent on the gas composition, cathode secondary electron emission, and experimental conditions. As indepth discussed in section 4, this stage has been a major bottleneck in the computer simulations of the streamer breakdowns resulting in an arc or spark formation since, as considered in agreement with our opinion in [152] (see section 4) 'It is probably a kinetic description that would be able to precisely simulate the cathode sheath formation when the streamer arrives at the cathode.'…”

“…In 1910, based on the extensive studies of low-pressure gas dischages by measuring the discharge currents by an electrometer and inductive balance, Townsend published his 'Theory of Ionization of Gases by Collision' [17] with the equation describing the observed exponential spatial growth of the ionization with the distance from the cathode attributing, however, the ionization by collisions to the impact of negative and positive ions. The following combination of Townsend's equation with the finding that the discharge current is mainly carried by electrons generated predominantly through the collision processes of electrons with gas atoms and molecules [18,19] resulted in what is currently called the Townsend breakdown theory and in the well known Townsend equation based on the exponential development of current in a series of electron avalanche generations [20] linked through secondary-emission feedback to the cathode. The criterion for the self sustaining discharge resulting from the Townsend equation is that each electron avalanche produces at least one secondary electron at the cathode, so that the discharge is self sustaining:…”

Electrical Discharges in Air at Atmospheric Pressure The Nature of the Positive and Negative Point-to-Plane Coronas and the Mechanism of Spark Propagation

“…For it insures that for each electron in the gap there will be created through some cathode mechanism taken together with cumulative ionization enough new electrons to maintain the current in its existing state. 3 This interpretation has been confused with the sparking condition although it is applicable only to a stable regime occurring after the unstable regime of the spark has passed. It is clear, however, that for any gap if 'Yeao> 1 there will be an increase of the current and charges in transit.…”