Three-Dimensional Simulation-Based Comparison of Streamer Initiation in SF₆/N₂ and SF₆/CO₂ for Different Mixing Ratios and Pressures

Being a greenhouse gas, SF₆ has significant potential to cause global warming. No alternative gas has been found so far that meets the required criteria. Ongoing research has narrowed down the candidates to some relatively environmentally friendly elementary gases such as N₂, CO₂, and their mixtures with a small percentage of SF₆ (10–20%). Streamers are important and play a deterministic role in the breakdown phenomenon. The inception and growth of streamer discharge depend on the generation of free electrons. Various ionization sources, including field ionization, Auger release of electrons, photoionization, and electron detachment from negative ions, have been employed in dielectric media. In this work, field ionization is considered a free-electron generation mechanism for streamer initiation. In field ionization, neutral molecules produce free electrons when extremely high electric fields are present near the needle electrode. A 3D particle model with field ionization is then used to investigate positive streamer initiation in SF₆/N₂ and SF₆/CO₂ for different mixing ratios at 1 and 5 bar. It was observed that for both mixtures, the number and the apparent length of streamer branching decreased with increasing SF₆ concentration and were minimal at 100% SF₆. The number of branches and the apparent length of streamers were higher in the case of SF₆/CO₂ compared with SF₆/N₂ mixtures, indicating a higher ionization rate for the SF₆/CO₂ mixture. With increasing pressure, the branching and length of the streamers for both mixtures decreased significantly. Although the field-ionization model is only suitable for very high electric fields in the vicinity of the needle tip, its validity is still questionable for uniform fields and at lower pressures.