Simulation of pre-breakdown discharges in high-pressure air: II. Effect of surface protrusions

Analysis of deviations from the similarity law, observed at high and very high pressures in experiments on discharge ignition and breakdown in corona-like configurations, can serve as a useful, albeit inevitably indirect, source of information about microprotrusions on the surface of the electrodes. In this work, such analysis was performed by means of 2D numerical modelling. Conical or cylindrical protrusions on the surface of the inner electrode were studied and the kinetic scheme includes the electrons, one species of positive ions, and negative ions O-2(-), O-, and O-3(-). It is shown that the deviations from the similarity law, observed in the experiment, may indeed be attributed to enhanced ionization of air molecules in regions of amplified electric field near the microprotrusions. A qualitative agreement with the experiment in all the cases is achieved for protrusion heights of the order of 50 mu m. Such values may appear rather high, however there is no other explanation in sight at present. The enhancement of the field electron emission from the surface of the negative electrode due to the amplification of the electric field on the microprotrusion was estimated and found insignificant in the range of values of the protrusion aspect ratio where the enhanced ionization in the gas phase is already appreciable.

Automated summary

The deviations from the similarity law observed in experiments on discharge ignition and breakdown in corona-like configurations at high and very high pressures are attributed to enhanced ionization of air molecules near microprotrusions. The 2D numerical modeling study shows qualitative agreement with the experiment for protrusion heights of around 50 μm. The enhancement of field electron emission from the surface of the negative electrode due to electric field amplification on microprotrusions was found insignificant compared to enhanced ionization in the gas phase.