Analysis of the characteristics of microwave-enhanced laser-induced atmospheric air plasma and ablation plasma for Al target

Abstract

The investigation and comparison of air plasma generated directly in the absence as well as in the presence of solid target along with microwave enhancement were reported. The plasma characteristics including emissions, size, duration, and temperature were measured using high-speed camera, optical emission spectroscopy, and temperature approximation through spectral fitting. The laser energy required for ablation is much lower than the air breakdown energy but, the minimum energy density for air and alumina breakdown is inversely proportional to the focal lengths. The size and volume of the air plasma generated by the microwave and ablation plasma interactions were less than those of the microwave-enhanced laser-induced air plasma but was similar with standard laser-induced air plasma. When comparing the plasma lifetimes, both air plasmas (with and without a solid target) exceeded the duration of the microwave pulse input (approximately 1 ms). As the plasma lifetime is a function of electron density, shorter lifespan with the target ∼1500 ± 150 µs compared with the lifetime of the plasma without the target at ∼1800 ± 150 µs were observed. Similar results were observed for the time-series measurements of the OH and N2 second positive (N2PS) molecular emissions. OH and N2PS were not observed within the first 300 µs in the presence of the solid target; these air plasma components were only generated as a “by-product” of the microwave and ablation plasma. The rotational and vibrational temperatures were approximated by spectral fitting of simulated OH and N2PS spectra and the experimentally acquired spectra. The microwave increased the vibrational temperature in a constant rotational temperature during the expansion of the air plasma. The same trend is observed with the presence of the alumina target.