Streamer propagation in air

A theory is presented for the development of the first streamer when a positive voltage is abruptly applied to a point in air at atmospheric pressure. The continuity equations for electrons, positive ions and negative ions, including the effects of ionization, attachment, recombination, electron diffusion, and photoionization, are solved simultaneously with Poisson's equation. With an applied voltage of 20 kV across a 50 mm gap, the streamer does not reach the cathode. An intense electric field front propagates away from the point into the gap to a distance of 35 mm in 200 ns. During the next 7.5 mu s the streamer only moves a further 2 mm into the gap, and the electric field at the head of the streamer collapses. Finally, only positive space charge remains which moves away from the point, allowing the field near the point to recover after similar to 10 mu s; free electrons can thus give rise to a secondary discharge near the anode. The electric field distribution is shown to be quite different from that found previously for SF6 in that the electric field in the column of the streamer is generally only a fraction of the critical field for which ionization equals attachment. Streamers for a given applied voltage have a far greater range in air than in SF6. The results presented for air also apply to flue gas mixtures, since the important material properties of both gases are very similar.