Positive and negative streamers in ambient air: modeling evolution and velocities

TL;DR

This study models the evolution and velocities of positive and negative streamers in ambient air, revealing how their characteristics depend on electric fields and ionization, with results aligning with recent experiments.

Contribution

It provides a detailed simulation of streamer evolution in air, highlighting differences between positive and negative streamers and their dependence on physical parameters.

Findings

Positive streamer velocity correlates with radius and matches experimental data.

Negative streamer velocity is driven by electron drift and less affected by photo-ionization.

Negative streamer heads broaden and slow down due to electron drift effects.

Abstract

We simulate short positive and negative streamers in air at standard temperature and pressure. They evolve in homogeneous electric fields or emerge from needle electrodes with voltages of 10 to 20 kV. The streamer velocity at given streamer length depends only weakly on the initial ionization seed, except in the case of negative streamers in homogeneous fields. We characterize the streamers by length, head radius, head charge and field enhancement. We show that the velocity of positive streamers is mainly determined by their radius and in quantitative agreement with recent experimental results both for radius and velocity. The velocity of negative streamers is dominated by electron drift in the enhanced field; in the low local fields of the present simulations, it is little influenced by photo-ionization. Though negative streamer fronts always move at least with the electron drift velocity in the local field, this drift motion broadens the streamer head, decreases the field enhancement and ultimately leads to slower propagation or even extinction of the negative streamer.