Simulations of positive streamers in air in different electric fields: steady motion of solitary streamer heads and the stability field

TL;DR

This study uses fluid simulations to analyze positive streamer behavior in air, revealing conditions for stable propagation at the so-called stability field and matching experimental observations of minimal streamers.

Contribution

It demonstrates that positive streamers can propagate steadily at a specific electric field, providing detailed insights into their velocity, radius, and stability conditions in air.

Findings

Streamers accelerate above 4.675 kV/cm and stagnate below.

Stable propagation occurs at approximately 4.675 kV/cm with consistent velocity and radius.

Electric field behind the streamer head returns to background levels within 1 cm.

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 10$^4$ m/s and an optical radius of 55 $\mu$m 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.