Fundamental processes for electric breakdown, i.e., electrode emission and bulk ionization, as well as the resultant Paschen’s law, are reviewed under various conditions. The effect of the ramping rate of applied voltage on breakdown is first introduced for macroscopic gaps, followed by showing the significant impact of the electric field nonuniformity due to gap geometry. The classical Paschen’s law assumes uniform electric field; a more general breakdown scaling law is illustrated for both DC and RF fields in geometrically similar gaps, based on the Townsend similarity theory. For a submillimeter gap, effects of electrode surface morphology with local field enhancement and electric shielding on the breakdown curve are discussed, including the most recent efforts. Breakdown characteristics and scaling laws in microgaps with both metallic and non-metallic (e.g., semiconductor) materials are detailed. For gap distance down to micro/nano scales, the breakdown characteristics and the breakdown mode transition from the secondary electron emission to the electric field emission or thermionic emission dominant regime. Additionally, the combined thermo-field emission regime is also reviewed. Previous efforts, including key simulations and experiments, have been devoted to diagnosing breakdown path evolution, measuring breakdown fields, and quantifying breakdown dependence on frequencies for gaps down to micro/nano scales. By summarizing and analyzing fundamental theories, recent progress, and on-going challenges, this tutorial review seeks to provide basic understanding and the state of the art of electric breakdown, which aids in advancing discoveries and promoting application prospects for discharge devices engineered in a wide range of regimes.

This Perspective outlines theoretical, simulation, and experimental studies linking electron emission and gas breakdown. Many studies have investigated field emission-driven microscale gas breakdown, including recent reviews [Garner et al., IEEE Trans. Plasma Sci. 48, 808–824 (2020); Fu et al., Plasma Res. Express 2, 013001 (2020)]. This Perspective focuses on generalizing field emission-driven microscale gas breakdown to consider the contribution of other forms of electron emission, specifically thermionic and space-charge limited. Recent theoretical studies have unified thermionic, field, and space-charge limited emission with and without collisions to derive “nexuses” where the individual solutions match, indicating transitions in the mechanisms. Reducing device size to nanoscale at atmospheric pressure leads to a transition from field emission to space-charge limited emission for nitrogen at ∼250 nm. This Perspective summarizes the derivation of these nexuses and future extensions. We next describe simulation and theoretical studies for field emission-driven microscale gas breakdown and highlight how the nexus theory may be integrated to account for temperature, space-charge, and pulse parameters. Finally, we summarize the development of optical techniques to assess microscale gas breakdown and recent nanoscale experiments at atmospheric pressure that suggest that space-charge may begin to contribute to field emission prior to gas breakdown. We highlight the combination of theory, simulation, and experiment to link electron emission and gas breakdown mechanisms across length, pressure, and temperature scales for applications that include vacuum electronics, pulsed power, and medicine.

In this Letter, the novel physics of higher harmonic (HH) generation of the normal electric field near a dielectric surface is reported in multipactor induced plasma ionization breakdown, as determined by kinetic particle-in-cell simulations. The observed HH frequency is around ten times the fundamental rf driving frequency, but lower than the electron plasma frequency. A theory is constructed which indicates that stream plasma interaction-induced instability is the mechanism of HH generation in the collisional regime. The HH frequency and its corresponding growth rate of the HH oscillation amplitude from the theory are in good agreement with kinetic particle-in-cell simulations.

Paschen's curve in microgaps with a hemi-ellipsoidal protrusion on the electrode surface is studied using a two-dimensional fluid model. The breakdown voltage is identified when the discharge enters the subnormal region, according to voltage-current characteristics. It is found that the breakdown in a microgap with a surface protrusion on the electrode can result in a combined Paschen's curve, which transits from long-gap (distance between the cathode and anode without the presence of protrusion) behavior at low pressure to short-gap (distance between the protrusion apex to the opposite electrode) behavior at high pressure. As gas pressure decreases, the length of the optimal discharge path increases, automatically moving from the top of the protrusion to its side surface and then to the wider non-protrusion electrode gap. The effects of the protrusion height and radius as well as the discharge polarity on the Paschen's curve are examined in detail. The effects of the protrusion aspect ratio on field enhancement are also considered. This work provides insights into the design of microgaps with controlled breakdown voltage across many orders in pressure via engineered electrode morphology.

We demonstrate a self-consistent and complete description of electron dynamics in a typical electropositive radio frequency magnetron sputtering (RFMS) argon discharge with a dielectric target. The electron dynamics, including the electron power absorption dynamics in one radio frequency (RF) period, is studied via a fully kinetic 2d3v particle-in-cell/Monte Carlo collision (PIC/MCC) electrostatic simulation. The interplay between the fundamental plasma parameters is analyzed through their spatiotemporal dynamics. Due to the influence of magnetic trap on the electron transport, a spatially dependent charging that perturbs the electric potential is observed on the dielectric target surface, resulting in a spatially dependent ion energy distribution along the target surface. The E × B drift-to-discharge current ratio is in approximate agreement with Bohm diffusion. The electron power absorption can be primarily decoupled into the positive Ohmic power absorption in the bulk plasma region and the negative pressure-induced power absorption near the target surface. Ohmic power absorption is the dominant electron power absorption mechanism, mostly contributed by the azimuthal electron current. The power absorption due to electron inertial effects is negligible on time-average. Both the maximum power absorption and dissipation of electrons appear in the bulk plasma region during the second half of the RF period, implying a strong electron trapping in magnetron discharges. The contribution of secondary electrons is negligible under typical RFMS discharge conditions.