A copper helical coil antenna was developed, characterized, and optimized for 2.45 GHz operations supplied by a microwave semiconductor oscillator. The application field of interest is laser-induced breakdown spectroscopy enhanced by microwave. Simulations using the Ansys HFSS demonstrate the superior localized E-field strength of the helical coil antenna, compared with other antenna-type structures. Simulation results show that E-field strength at the tip of the antenna has a logarithmic trend for increasing the coil pitch. The optimum pitch is 5 mm for a coil diameter of 6.5 mm upon consideration of the system compactness. Despite the antenna’s open-circuit end, the presence of target samples does not interfere with the E-field and H-field distribution of the antenna and the surrounding environment. Applications in microwave-enhanced laser-induced breakdown spectroscopy (MWLIBS) confirm the importance of the antenna reflector. The electric field strength was over 100 times higher than the previous capacitor-like antenna. The antenna configuration angle was then experimentally optimized for maximum enhancement effects in the spectrochemical analysis of Al₂O₃. The antenna angle of 60° from the laser beam propagation achieved maximum enhancement in the emission signal of Al I.

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 2^nd positive system) and hydroxyl radical (OH). Without the input of microwave, these molecular and radical emissions were not observed. The OH and N₂ 2^nd 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.

This study explores the potential of utilizing microwaves to sustain the expansion of transient laser ablation plasma of Zr target. By application of microwaves on the plasma, we observe a significant enhancement with a two to three order of magnitude increase in the plasma emission intensity, and 18 times increase in the plasma’s spatial volume. We investigate the temperature change of the plasma and observe that it decreases from 10,000 K to approximately 3000 K. Electron temperature decreased with volume expansion owing to increased surrounding air interaction, while the plasma can be sustained in air using microwaves. The increase in electron temperature during temperature drop is indicative of non-equilibrium plasma. Our results emphasize the contribution of microwaves in promoting enhanced emission and plasma formation at controlled, low temperature, thereby demonstrating the potential of microwaves to enhance the accuracy and performance of laser-induced breakdown spectroscopy. Importantly, our study suggests that microwaves could also mitigate the generation of toxic fumes and dust during ablation, a critical benefit when handling hazardous materials. The system we've developed is highly valuable for a range of applications, notably including the potential to reduce the possible emergence of toxic fumes during the decommissioning of nuclear debris.

A microwave-enhanced plasma generation technique was combined with laser-induced ignition to improve ignition characteristics. A locally intensified microwave field was formed near the laser-induced breakdown plasma. As the plasma absorbed the microwaves, the plasma emission intensity increased. The plasma lifetime could be controlled by changing the microwave oscillation duration. Furthermore, the microwave-enhanced laser-induced breakdown plasma improved the minimum ignition energy of the methane/air pre-mixture with just a small amount of absorbed microwave energy.