During a dc corona discharge, the ions’ momentum will be transferred to the surrounding neutral molecules, inducing an ionic wind. The characteristics of corona discharge and the induced ionic wind are investigated experimentally and numerically under different polarities using a needle-to-ring electrode configuration. The morphology and mechanism of corona discharge, as well as the characteristics and mechanism of the ionic wind, are different when the needle serves as cathode or anode. Under the different polarities of the applied voltage, the ionic wind velocity has a linear relation with the overvoltage. The ionic wind is stronger but has a smaller active region for positive corona compared to that for negative corona under a similar condition. The involved physics are analyzed by theoretical deduction as well as simulation using a fluid model. The ionic wind of negative corona is mainly affected by negative ions. The discharge channel has a dispersed feature due to the dispersed field, and therefore the ionic wind has a larger active area. The ionic wind of positive corona is mainly affected by positive ions. The discharge develops in streamer mode, leading to a stronger ionic wind but a lower active area.

This paper explores the predominant mechanisms for the deviation of micro-gap dc gas breakdown and the transition between different mechanisms as the electrode separation d changing under a pin-to-plate electrode configuration using 2d3v particle-in-cell simulation with Monte Carlo collisions. The deviated breakdown characteristic curves as a function of d or gas pressure p are investigated and both present a plateau region. Through researching the position of discharge path, it is found that a self-modulation effect manages to maintain the breakdown voltage at the minimum value defined by Paschen’s curve in a certain d or p range and forms the plateau. The ranges of d and p for the plateau are also established. Theoretical calculation on the secondary electron emission coefficient induced by ion-enhanced field and determined by a surface roughness factor confirms that the ion-enhanced field emission effect affects the breakdown voltage significantly when d is below a critical value. The smaller the surface roughness factor is, the smaller the critical d will be. Under this effect, the breakdown voltage is decreased with d decreasing (also referred to as an increasing left branch with d increasing). Conclusively, the deviation characters of micro-gap gas breakdown are controlled by different mechanisms at different d ranges. The predominant mechanism for the deviation is the self-modulation effect, serving as the main reason for the plateau region, at moderate d of several micrometers and will transit to the ion-enhanced field emission effect, which is responsible for the increasing left branch at smaller d.

To understand the discharge characteristics under a gap of micrometers, the breakdown voltage and current-voltage curve are measured experimentally in a needle-to-plate electrode at a microscale gap of 3-50 μm in air. The effect of the needle radius and the gas pressure on the discharge characteristics are tested. The results show that when the gap is larger than 10 μm, the relation between the breakdown voltage and the gap looks like the Paschen curve; while below 10 μm, the breakdown voltage is nearly constant in the range of the tested gap. However, at the same gap distance, the breakdown voltage is still affected by the pressure and shows a trend similar to Paschen's law. The current-voltage characteristic in all the gaps is similar and follows the trend of a typical Townsend-to-glow discharge. A simple model is used to explain the non-normality of breakdown in the micro-gaps. The Townsend mechanism is suggested to control the breakdown process in this configuration before the gap reduces much smaller in air.

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