The breakdown voltage characteristics of compressed ambient air microdischarges from direct current to 10.2 MHz

This paper presents the results of the experimental and simulation studies of the breakdown characteristics of microdischarges in compressed ambient air under the influence of static and time-varying electric fields, up to radio-frequencies. The measurements were performed for sphere to plane geometry of the electrodes separated from 5 μm to 100 μm, within the pressure range 760 Torr to 3800 Torr. For gaps of the order of a few micrometers, it is not possible to properly obtain the left-hand side of the Paschen curve; and the Townsend mechanism is no longer suitable. Deviations are also observable in the direction of lower breakdown potential that appear at the right of the minimum of the direct-current breakdown voltage curves indicating that accumulated space charge plays an important role in enhanced field emission. The experimental data agree well with the simulation results obtained by a one-dimensional particle-in-cell/Monte Carlo collision code including field emission effects. Their fit to a simple formula describing the dependence of the breakdown voltage on the product of the pressure and the gap size is suggested. Based on the measured breakdown voltage curves, the effective secondary electron emission coefficients have been determined. This work especially focuses on the effect of the electrode surface degradation on the breakdown characteristics at high pressure and high frequency. It is observed that in the case of direct-current and low frequency discharges, there is no significant influence of the electrode surface degradation on the breakdown voltage and the effective yields. However, for higher frequencies, the breakdown voltages are lower and the effective yields are much higher after degradation. The obtained results represent our attempt to derive a preliminary understanding of the governing breakdown processes in compressed air microdischarges.