The ionization kinetics and electric field in the leader channel in long air gaps

Abstract

A kinetic model for the ionization processes in high-temperature air in a strong electric field is developed. The model is used to study numerically the problem of the relaxation of the plasma properties in atmospheric-pressure air to a new steady state after an instantaneous change in the gas temperature between 300 and 1000 - 6000 K. Our simulation shows that the electrical properties of the plasma under the conditions of the leader being a long spark nearly follow the local value of the gas temperature. The reduced field E/N in the leader channel is found to decrease considerably as the gas temperature T increases from 1000 to 6000 K. The decrease in E/N is caused by the formation of NO molecules with low ionization energy at T < 4500 K and by associative ionization during collisions between O and N atoms, which becomes the dominant ionization mechanism at T > 4500 K. As a result, the value of E/N falls to 10 Td at T = 5000 K and to 1.5 Td at T = 6000 K for a current of 1 A and a channel radius of 0.1 cm. This agrees well with the experimental evidence showing that the leader bridges very long air gaps (about 200 m) at a moderate applied voltage (< 5 MV).