Laser air plasma expansion by microwaves.

Utilizing microlasers and microwaves, our study examined the impact of microwaves on the expansion of air plasma. We applied microwaves to the air plasma generated by a microlaser, visualized its growth using a phone camera, and recorded plasma emissions using a high-resolution spectrometer. Software tools were then used to analyze these emissions for temperature changes and electron density. Notably, we noticed a 400-fold increase in plasma volume due to microwave enhancement, even though the microlaser operated at a modest energy level of 1 mJ. Simultaneously, we recorded an increase in temperature and a decrease in electron density when the plasma was subjected to microwaves, indicative of nonequilibrium plasmas. Further, a minor shift in electron temperature during microwave exposure pointed toward the ability of microwaves to sustain plasma characteristics over time. These findings suggest that the microwave application potentially improves the analytical performance of laser-induced breakdown spectroscopy.

Highly sensitive detection of sodium in aqueous solutions using laser-induced breakdown spectroscopy with liquid sheet jets.

Laser-induced breakdown spectroscopy (LIBS) combined with liquid jets was applied to the detection of trace sodium (Na) in aqueous solutions. The sensitivities of two types of liquid jets were compared: a liquid cylindrical jet with a diameter of 500 µm and a liquid sheet jet with a thickness of 20 µm. Compared with the cylindrical jet, the liquid sheet jet effectively reduced the splash from the laser-irradiated surface and produced long-lived luminous plasma. The limit of detection (LOD) of Na was determined to be 0.57 µg/L for the sheet jet and 10.5 µg/L for the cylindrical jet. The LOD obtained for the sheet jet was comparable to those obtained for commercially available inductively coupled plasma emission spectrometers.

Effect of liquid-sheet thickness on detection sensitivity for laser-induced breakdown spectroscopy of aqueous solution.

For aqueous-solution-based elemental analysis, we used a thin liquid sheet (µm-scale thickness) in laser-induced breakdown spectroscopy with nanosecond laser pulses. Laser-induced plasma is emitted by focusing a pulsed Nd:YAG laser (1064 nm) on a 5- to 80-µm-thick liquid sheet in air. To optimize the conditions for detecting elements, we studied how the signal-to-background ratio (SBR) for Hα Balmer and Na-neutral emission lines depends on the liquid-sheet thickness. The SBR of the Hα Balmer and Na-neutral lines was maximized for a sheet thickness of ~20 µm at the laser energy of 100 mJ. The hydrodynamics of liquid flow induced by the laser pulse was analyzed by laser flash shadowgraph imaging. Time-resolved observation of the hydrodynamics and plasma emission suggests that the dependence of the SBR on the liquid-sheet thickness is correlated with the volume of flowing liquid that interacts with the laser pulses.

Analysis of gadolinium oxide using microwave-enhanced fiber-coupled micro-laser-induced breakdown spectroscopy.

We report on the analysis of pure gadolinium oxide (Gd2O3) and its detection when mixed in surrogate nuclear debris using microwave-enhanced fiber-coupled micro-laser-induced breakdown spectroscopy (MWE-FC-MLIBS). The target application is remote analysis of nuclear debris containing uranium (U) inside the Fukushima Daiichi Nuclear Power Station. The surrogate nuclear debris used in this study contained gadolinium (Gd), cerium (Ce), zirconium (Zr), and iron (Fe). Ce is a surrogate for U, and Gd2O3 is an excellent hazard index because it is incorporated into some fuel rods. Gd detection is essential for assessing debris prior to the retrieval process. Surrogate debris was ablated by an 849 ps 1064 nm micro-laser under atmospheric pressure conditions while a helical antenna propagated 2.45 GHz 1.0 kW microwaves for 1.0 ms into the laser ablation, which was then characterized by a high-speed camera and high-resolution spectrometers. The results showed that microwave-induced plasma expansion led to enhanced emission signals of Gd I, Zr I, Fe I, Ce I, and Ce II. No self-absorption of Gd emissions was evident from the detection limit calibration graphs. Moreover, microwave irradiation decreased the standard deviations of the Gd and Ce emissions and lowered the Gd detection limit by 60%.

Laser ablation plasma expansion using microwaves.

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.

Antenna Characteristics of Helical Coil with 2.45 GHz Semiconductor Microwave for Microwave-Enhanced Laser-Induced Breakdown Spectroscopy (MW-LIBS).

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.

Isotope-selective Microscale Imaging of Radioactive Cs without Isobaric Interferences Using Sputtered Neutral Mass Spectrometry with Two-step Resonant Ionization Employing Newly-developed Ti:Sapphire Lasers.

The characterization of radionuclides in Fukushima is important to determine their origins and current state in the environment. Radionuclides exist as fine particles and are mixed with other constituents. A measurement method with both micro-imaging capability and highly selective element detection is necessary to analyze these particles. We developed such an imaging technique using a time-of-flight secondary ion mass spectrometry and wavelength-tunable Ti:Sapphire lasers for the resonance ionization of target elements without mass interference. This is called resonant laser ionization sputtered neutral mass spectrometry. The instrument has a high lateral resolution and a higher ionization selectivity using two-step resonance excitation of Cs with two lasers at different wavelengths. Optimization of the wavelength for resonance ionization using a Cs compound was performed, and a real environmental particle containing radioactive Cs was analyzed. Isotope images of three kinds of Cs were successfully obtained without interfere from Ba isotopes for the first time.