Post-arc electron density measurement in SF6 and SF6/CO2 mixture arcs using Thomson scattering

Abstract

SF₆ is extensively utilized in gas circuit breakers due to its superior arc-quenching capabilities. However, its strong greenhouse effect and relatively high liquefaction temperature have led to the adoption of buffering gases, such as CO₂, in combination with SF₆. Investigating the post-arc process in gas plasma is essential for assessing the arc-quenching performance of the gases and potential applications. Nevertheless, precise diagnostics during the post-arc phase remain challenging due to the non-equilibrium nature and poor stability of the plasma. This study developed a coherent laser Thomson scattering platform to measure the post-arc decay characteristics of gas arcs. A custom triple-grating spectrometer was incorporated to enhance the signal-to-noise ratio at low electron densities. The spatiotemporal electron density decay characteristics of pure SF₆ arcs under varying pressures and SF₆/CO₂ mixed-gas arcs with different mixing ratios were examined. The results show that, during the thermal recovery phase (0–70 μs), all gas arcs exhibit substantial energy dissipation, with the electron density at the arc center decreasing by two orders of magnitude. As pressure increases, the diameter of the pure SF₆ arc contracts, and the central electron density initially rises. With increasing pressure, a competition emerges between chemical reactions and diffusion-convection effects as the dominant factors influencing electron dissipation. The introduction of buffering gas reduces the recombination rate of positive ions and electrons, thereby decelerating the plasma decay rate. Both the initial electron density and plasma diameter diminish with increasing SF₆ content in the mixture.