THE IONIZATION KINETICS AND ELECTRIC-FIELD IN THE LEADER CHANNEL IN LONG AIR GAPS

A kinetic model for the ionization processes in high-temperature air i n a strong electric field is developed. The model is used to study num erically the problem of the relaxation of the plasma properties in atm ospheric-pressure air to a new steady state after an instantaneous cha nge in the gas temperature between 300 and 1000-6000 K. Our simulation shows that the electrical properties of the plasma under the conditio ns of the leader being a long spark nearly follow the local value of t he gas temperature. The reduced field E/N in the leader channel is fou nd to decrease considerably as the gas temperature T increases from 10 00 to 6000 K. The decrease in E/N is caused by the formation of NO mol ecules with low ionization energy at T < 4500 K and by associative ion ization during collisions between O and N atoms, which becomes the dom inant 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 7 = 6000 K for a curre nt of 1 A and a channel radius of 0.1 cm. This agrees well with the ex perimental evidence showing that the leader bridges very long air gaps (about 200 m) at a moderate applied voltage (< 5 MV).