Spatially and temporally resolved plasma formation on alumina target in microwave-enhanced laser-induced breakdown spectroscopy

Abstract

Microwave energy was applied to the laser-induced breakdown spectroscopy (LIBS) of laser-ablated Al plasma using a helical antenna. The process of generating an enhanced laser ablation plasma was visualized with a high-speed camera. The time and space evolution of the plasma were investigated by changing the laser power, microwave energy, and microwave energy duration. The obtained raw images were converted into 8-bit 255 grayscale and binary black and white (BW) images and were analyzed using Matlab. Three major periods of plasma evolution were established based on the key physical process involved. Laser energy was the primary parameter involved in period 1, where laser ablation and initial plasma formation occurred. The microwaves had little significance at the beginning of the ablation process, as suggested by the minimal changes in plasma size and intensity. The microwaves controlled the expansion of the plasma and sustained breakdown in period 2. The post ablation breakdown of the material by microwaves sustained the enlargement of the plasma. The events leading to plasma dissipation comprised period 3, where the nonthermal plasma was sustained for 500 μs after the microwave was turned off. The effects of the laser and microwave parameters in the timeline of these periods were discussed. Complementary emission intensity measurements were also presented.