Microwave-enhanced laser-induced air plasma at atmospheric pressure

Abstract

This paper investigated how microwaves affect the temperature of laser-generated air plasma. The air breakdown threshold was experimentally characterized by focusing the 1064 nm YAG laser on varied condensing lens focal lengths. Increase in focal lengths increases the focused spot diameter of the laser and decreases the laser fluence. Large spot diameter required large amount of laser fluence for breakdown. However, the plasma generated with small spot sizes found to absorb higher laser energy in compared to the plasma generated with large spot size condition. In terms of energy density, the experimental threshold breakdown was generated between 2.6∼4.9 × 1011 W/cm2. The plasma formation was then observed under a high-speed camera. The area of intensity distribution increased with the input of microwaves owing to re-excitation and microwave absorption. This led to emission intensity measurements of the elusive stable electronically excited molecular nitrogen (N2 2nd positive system) and hydroxyl radical (OH). Without the input of microwave, these molecular and radical emissions were not observed. The OH and N2 2nd positive system emission intensities were then used to measure the rovibrational temperature using the synthetic spectrum method by SPECAIR. The rotational and vibrational temperatures were not found to be equal indicating non-equilibrium plasma. The nonequilibrium and nonthermal plasma was observed from after the initial laser air breakdown using the 2.6 × 1011 W/cm2, 1.0 kW microwave power, and 1.0 ms microwave pulse width. The microwaves were not found to affect the temporal changes in the rotational temperatures, demonstrating that the intensity enhancements and plasma sustainment were caused by re-excitation and not by microwave absorption.