Underlying mechanism of the stagnation of positive streamers

Abstract

Several publications have shown that it is challenging to model the stagnation of positive streamers. They find that as the streamers propagate, the space charge region at the streamer tip reduces in size and that the associated electric field increases towards infinity. In a recent study on streamer propagation in unsteady airflow, we circumvented this problem by relaxing the local density and electric field approximation in the drift-diffusion model, commonly used in the past models. The results, recently published, show that the electric field remains bounded during the streamer propagation. In the present paper, we explore the process of stagnation further with a more rigorous approach. We confirm that the instability in the electric field is an immediate effect of the local density and field approximation and that an extended description of ionization stabilizes the electric field and leads to a decelerating streamer. Finally, we discuss the role of positive ions in the stagnation and we show that the stagnating streamer velocity decreases till it becomes comparable to the ion velocities in the streamer head. This causes a broadening of the streamer head which leads to a sharp decrease in the streamer tip electric field and the streamer stagnation.

A computational study of accelerating, steady and fading negative streamers in ambient air

“…Finally, we remark that negative streamer fading differs significantly from positive streamer stagnation. When positive streamers stagnate, their field enhancement tends to increase [31,32]. Only after their velocity has become comparable to the ion drift velocity, which is much lower than the electron drift velocity, do they lose their field enhancement.…”

“…Ion motion becomes significant only if the ion drift velocity is comparable to the streamer velocity or electron drift velocity. This is possible for positive streamers in air when they decelerate to a very low velocity comparable to the ion drift velocity, as shown in [31,32], for which ion motion should be considered. In contrast, for positive streamers in air, the recombination time can be lower than the attachment time due to a low internal electric field and high O + 4 density.…”

“…A fading negative streamer loses its field enhancement, but electrons near the streamer head continue their drift motion. In contrast, a stagnating positive streamer will eventually come to an approximate halt due to the relative immobility of positive ions [31,32], and its field enhancement tends to increase before stagnation. In [26], negative streamer deceleration was simulated in atmospheric air in weak uniform electric fields.…”

“…As pointed out in [33][34][35], the electric field can diverge when positive streamer stagnation is simulated using a fluid model with the local field approximation. Such problems can be avoided by using an extended fluid model [31] or by modifying the impact ionization source term [32].…”

A computational study of accelerating, steady and fading negative streamers in ambient air

“…As pointed out in [34][35][36], the electric field can diverge when positive streamer stagnation is simulated using a fluid model with the local field approximation. Such problems can be avoided by using an extended fluid model [31] or by modifying the impact ionization source term [32].…”