3D Particle-in-Cell Simulation of Positive Needle-to-Plane Streamer Discharge in SF6 with Field Ionization

SF₆ is a strong electronegative gas and used as insulating material in gas-insulated switchgear (GIS). For the efficient and reliable operation of GIS, it is necessary to investigate non-uniform field breakdown in SF₆. Generation of free electrons is an essential prerequisite for initiation and propagation of positive discharges. A 3D particle-in-cell MonteCarlo collision (PIC-MCC) model is used to investigate the initiation of positive needle-to-plane streamer discharge in SF₆ with field ionization at pressures ranging from 1 to 5 bar. Free electrons are generated from impurity molecules with low density and low ionization energy in SF₆ because of field ionization. The local electric field dependent electron generation rate is calculated using Zener's model. Two stages of tree like streamer discharge are observed. In first phase, a spherical shape inception cloud is formed around the needle tip, which eventually destabilizes into multiple branches in the next stage. With the increase in pressure, the decrease in particle density, number of branches and total propagation length of streamer discharge is observed. The simulation results show that field ionization could be the possible source of ionization in SF₆ in the presence of high electric field.