Simulations of positive streamers in air in different electric fields: Steady motion of solitary streamer heads and the stability field
Francisco, H.L. (Hani), Teunissen, H.J. (Jannis), Bagheri, B. (Behnaz), Ebert, U. (Ute)
Abstract
We simulate and characterize positive streamers in ambient air in homogeneous background electric fields from 4.5 to 26 kV/cm in a 4 cm gap. They can accelerate or decelerate depending on the background electric field. Many experiments have shown that a streamer keeps propagating in a stable manner in the so-called stability field of 4.5 to 5 kV/cm. Our fluid streamer simulations in STP air show that: (1) In a homogeneous field larger than 4.675 kV/cm, a single streamer accelerates, and in a lower field, it decelerates and eventually stagnates with a small radius and very high field enhancement. (2) In a field of 4.675 kV/cm, the streamer head propagates with an approximately constant velocity of 6.7 x 104 m/s and an optical radius of 55 μm over distances of several centimeters as a stable coherent structure. These values for the radius and velocity agree well with measurements of so-called minimal streamers. (3) Behind the uniformly translating streamer head, the channel conductivity decreases due to electron attachment and recombination, and the electric field returns to its background value about 1 cm behind the head. The propagation behavior of the solitary streamer agrees with the original definition of the stability field, which is the homogeneous field in which a streamer can propagate with a constant speed and shape.