Breakdown of atmospheric pressure microgaps at high excitation frequencies

Abstract

Microwave (mw) breakdown of atmospheric pressure microgaps is studied by a one-dimensional Particle-in-Cell Monte Carlo Collisions numerical model. The effect of both field electron emission and secondary electron emission (due to electron impact, ion impact, and primary electron reflection) from surfaces on the breakdown process is considered. For conditions where field emission is the dominant electron emission mechanism from the electrode surfaces, it is found that the breakdown voltage of mw microdischarge coincides with the breakdown voltage of direct-current (dc) microdischarge. When microdischarge properties are controlled by both field and secondary electron emission, breakdown voltage of mw microdischarge exceeds that of dc microdischarge. When microdischarge is controlled only by secondary electron emission, breakdown voltage of mw microdischarge is smaller than that of dc microdischarge. It is shown that if the interelectrode gap exceeds some critical value, mw microdischarge can be ignited only by electrons initially seeded within the gap volume. In addition, the influence of electron reflection and secondary emission due to electron impact is studied.

Gas breakdown and its scaling law in microgaps with multiple concentric cathode protrusions

“…The microgap discharge is ignited and maintained by field emission when the electric field is on the order of 1 V/nm, whereas secondary electron emission is more important for weaker electric fields. [17][18][19][20][21][22][23] In recent years, theoretical, numerical, and experimental works were conducted on microscale breakdown, including characterizing the breakdown mode transition, 24,25 controlling the plasma to microstructure interaction, 26,27 and promoting the microdischarge uniformity. [28][29][30] It can be expected that with the emerging advances in fabrication technologies, microdischarge devices designed with more diverse and complicated structures will be a reality for various targets of controlling discharge behaviors.…”

Gas breakdown in atmospheric pressure microgaps with a surface protrusion on the cathode

“…For a direct-current (DC) microdischarge, it is ignited and maintained by secondary electron emission when the gap distance is greater than $10 lm, while the dominant mechanism becomes electron field emission when the gap distance is less than a few microns, especially with microprotrusions on the cathode surface. [12][13][14] The presence of the protrusion on the electrode surface usually leads to enhancement of the local electric field. 15 When the electric field at the cathode surface reaches the order of 10 9 V/m, field emission starts to play a key role.…”

“…In the cases studied, since the magnitude of the cathode electric field (10 6 -10 7 V/m) is much less than the field emission threshold ($10 9 V/m or larger), field emission is ignored and the discharge is sustained by ion-impact secondary electron emission at the cathode. [12][13][14] The normal flux of electrons emitted by the cathode is related to the flux of incident ions by an effective secondary emission coefficient c, which is fixed at 0.1. 30,31 The equations of the discharge model are solved self-consistently to steady-state.…”

Paschen's curve in microgaps with an electrode surface protrusion

“…In the cases studied, the discharge is sustained by ionimpact secondary electron emission at the cathode, where field emission can be ignored since the shortest distance d min is no less than 50 lm and the maximum effective electric field including the modest field enhancement of the hemiellipsoidal tip is much smaller than 10 9 V/m. [16][17][18] The normal flux of electrons emitted by the cathode is related to the flux of incident ions by an effective secondary emission coefficient c, which is fixed at 0.1. 29,30 The equations of the model are solved self-consistently to reach steady-state, and the detailed description of the fluid model can be found in previous studies.…”