Assessing the Role of Photon Processes in Facilitating Radio Frequency Breakdown of Air at Atmospheric Pressure in Millimeter Gaps

The behavior of the breakdown electric field versus gap lengths (in the 1–5-mm range) and at different frequencies in the 1–80-MHz span, has been studied numerically at atmospheric pressure. Unlike previous studies of radio frequency (RF) breakdown, the role of photon-emission processes is explicitly included and shown to be important for large-area electrode configurations. Numerical analysis based on Monte Carlo calculations is used to predict the breakdown thresholds. The simulations embed a statistical photon transport model, based on random selections of emission angles and times from excited atoms, as well as photoemission from the electrodes. Simulation results compare well with experimental data from our group, but only with the inclusion of photon processes. Though both photoemission and photoionization are included in the breakdown physics, the former is identified as the dominant process. The frequency behavior of breakdown fields is also assessed with the inclusion of photons, and the results reveal a U-shaped trend with increasing values for smaller gaps.

I. Introduction

Numerous studies of gas breakdown under radio frequency (RF) conditions have been reported in the literature [1], [2], [3], [4], [5], [6]. Applications of such breakdown include etching [7], [8], thin-film deposition [9], pollution control [10], biomedical engineering [11], pumping of gas lasers [12], etc. The use of atmospheric pressure plasmas has greatly expanded the scope and utility of plasma jets [13] and the cold plasma torch [14]. In this regard, a recent report on RF breakdown in air at atmospheric pressure for relatively larger gaps had been shown to match experimental data fairly well [15]. The present contribution expands on that previous report, with explicit inclusion of photoemission and photoionization. Results are compared with experiments that were performed on relatively large electrodes, a configuration that helps support and enhance the contribution of photoemission by providing an extended area for photon gathering.