3D particle-in-cell simulations of negative and positive streamers in C4F7N-CO2 mixtures

TL;DR

This study uses 3D particle-in-cell simulations to explore the behavior of negative and positive streamers in environmentally friendly C4F7N-CO2 mixtures, revealing their initiation conditions and growth patterns.

Contribution

It provides the first detailed 3D simulation analysis of streamer inception in C4F7N-CO2 mixtures, highlighting the effects of background ionization and the limitations of current photoionization models.

Findings

Negative streamers propagate near the critical field with short channels.

Positive streamers require background fields close to the critical field and free electrons.

Stochastic background ionization can generate chains of negative streamers.

Abstract

We investigate negative and positive streamers in C4F7N-CO2 mixtures through simulations. These mixtures are considered to be more environmentally friendly than the insulating gas SF6 that is widely used in high voltage technology. Simulations are performed using a 3D particle-in-cell model. Negative streamers can propagate when the background field is close to the critical field. We relate this to their short conductive channels, due to rapid electron attachment, which limits their field enhancement. Positive streamers also require a background field close to the critical field, and in addition a source of free electrons ahead of them. In our simulations these electrons are provided through an artificial stochastic background ionization process as no efficient photoionization process is known for these gases. In 3D, we can only simulate the early inception stage of positive discharges, due to the extremely high electric fields and electron densities that occur. Qualitative 2D Cartesian simulations show that the growth of these discharges is highly irregular, resulting from incoming negative streamers that connect to existing channels. The inclusion of a stochastic background ionization process also has an interesting effect on negative discharges: new streamers can be generated behind previous ones, thereby forming a chain of negative streamers.