The propagation of positive streamers in a weak and uniform background electric field

The results of numerical simulations of a positive streamer development in air in a weak and uniform electric field are presented. Streamer dynamics i s considered in an electrode gap of 33 mm in length and the configuration w as 'protrusion on a plate-plate'. This particular configuration was chosen in order to perform direct comparison between simulated results and experim ental data. The electrostatic field in such a system decreases rapidly with increase of the distance from the protrusion (anode) and the region with a weak and uniform background field covers similar to 30 mm of the gap. The parameters of the propagating streamer are studied at six different values of the background field strength: 0.24; 0.30; 0.345; 0.37; 0.43 and 0.50 MV m(-1). Stable streamer development (with constant velocity) takes place in a field of 0.5 MV m(-1) (the stability field) but the streamer is able to cross the gap in a background field of 0.3 MV m(-1). These values are in ex cellent agreement with experimental data. During the stable streamer propag ation, the electron density and plasma conductivity in the discharge channe l and the electric field at its front remain constant. In a background fiel d lower than 0.5 MV m(-1), the discharge front velocity and the electric fi eld at the front decrease linearly with an increase of streamer length. The discharge propagation in the stability field is associated with an increas e of electrostatic energy at the streamer front but it decreases if the str eamer develops in a weaker electric field. This behaviour is accompanied by a constant Joule dissipation at 0.5 MV m(-1) and decreasing energy losses at the streamer front in a weaker background electric field.