The role of excited electronic states in ambient air ionization by a nanosecond discharge

The mechanism of air ionization by a single nanosecond discharge under atmospheric conditions is studied using numerical simulations. The plasma kinetics are solved with ZDPlasKin and the electron energy distribution function is calculated with BOLSIG+. The model includes the excited electronic states of O and N atoms, which are shown to play the main role in plasma ionization for n e > 10 16 cm −3 . For electric fields typical in nanosecond discharges, a non-equilibrium plasma ( T e > T gas ) is formed at ambient conditions and remains partially ionized for about 12 nanoseconds ( n e < 10 16 cm −3 ). Then, the discharge abruptly reaches full ionization ( n e ≈ 10 19 cm −3 ) and thermalization ( T e = T gas ≈ 3 eV) in less than half a nanosecond, as also encountered in experimental studies. This fast ionization process is explained by the electron impact ionization of atomic excited states whereas the fast thermalization is induced by the elastic electron–ion collisions.