Humidity as a critical parameter for predicting breakdown voltage in submicrometer electrode gaps in air

This experimental work reinvestigates the breakdown voltage in air for electrode gaps ranging from 0.10 to 6.00µm. Rarely addressed in the literature of the field, a special focus is placed on varying the gas humidity and its direct impact on the breakdown voltage. For short distances (<2µm), the results confirm significant deviations from the predictions of Paschen's law, with a linear increase of the breakdown voltage. The statistical approach of [B. Disson et al., Phys. Rev. E 112, 015202 (2025)2470-004510.1103/qqsd-kgb3] reveals breakdown characteristics in dry air (T_{dew}<-40^{∘}C) to be very analogous to the previous results obtained in argon gas. There are-at least-two competing elementary mechanisms responsible to trigger the breakdown: the electrons produced by field emission (FE-mechanism) and the Townsend avalanche (A-mechanism). Breakdowns following the FE-mechanism occur systematically around the electric field value of E_{FE}=0.86V/nm, which is very close to the value measured in argon. Unexpectedly, the gradual increase of humidity in air causes a counterintuitive evolution of the breakdown voltages. While in macroscopic discharges, water molecules in the gas are known to elevate the breakdown voltages compared to dry gas, the admixture of H_{2}O drastically reduces the breakdown voltages in microgaps, e.g., 0.86 to 0.26V/nm for dew points of T_{dew} <-40^{∘}C and 10^{∘}C, respectively. The water layers adsorbed by the surface of the electrodes as well as the formation of a Taylor cone under the effect of the high electric field are discussed. This supports the analysis to understand whether H_{2}O enhances the FE-mechanism and A-mechanism or if this is an additional contribution leading to the breakdown. More generally, this study sheds light on the crucial role of humidity in microgap breakdowns and calls into question measurements results reported in the literature where humidity conditions are not documented.