Laser-produced plasmas (LPPs) engulf exotic and complex conditions ranging in temperature, density, pressure, magnetic and electric fields, charge states, charged particle kinetics, and gas-phase reactions, based on the irradiation conditions, target geometries, and the background cover gas. The application potential of the LPP is so diverse that it generates considerable interest for both basic and applied research areas. Although most of the traditional characterization techniques developed for other plasma sources can be used to characterize the LPPs, care must be taken to interpret the results because of their small size, transient nature, and inhomogeneities. The existence of the large spatiotemporal density and temperature gradients often necessitates non-uniform weighted averaging over distance and time. Among the various plasma characterization tools, optical-based diagnostic tools play a key role in the accurate measurements of LPP parameters. The optical toolbox contains optical probing methods (Thomson scattering, shadowgraphy, Schlieren, interferometry, velocimetry, and deflectometry), optical spectroscopy (emission, absorption, and fluorescence), and passive and active imaging. Each technique is useful for measuring a specific property, and its use is limited to a certain time span during the LPP evolution because of the sensitivity issues related to the selected measuring tool. Therefore, multiple diagnostic tools are essential for a comprehensive insight into the entire plasma behavior. In recent times, the improvements in performance in the lasers and detector systems expanded the capability of the aforementioned passive and active diagnostics tools. This review provides an overview of optical diagnostic tools frequently employed for the characterization of the LPPs and emphasizes techniques, associated assumptions, and challenges. 

Optical diagnostics of laser-produced plasmas

Laser-produced plasmas (LPPs) engulf exotic and complex conditions ranging in temperature, density, pressure, magnetic and electric fields, charge states, charged particle kinetics, and gas-phase reactions, based on the irradiation conditions, target geometries, and the background cover gas. The application potential of the LPP is so diverse that it generates considerable interest for both basic and applied research areas. Although most of the traditional characterization techniques developed for other plasma sources can be used to characterize the LPPs, care must be taken to interpret the results because of their small size, transient nature, and inhomogeneities. The existence of the large spatiotemporal density and temperature gradients often necessitates non-uniform weighted averaging over distance and time. Among the various plasma characterization tools, optical-based diagnostic tools play a key role in the accurate measurements of LPP parameters. The optical toolbox contains optical probing methods (Thomson scattering, shadowgraphy, Schlieren, interferometry, velocimetry, and deflectometry), optical spectroscopy (emission, absorption, and fluorescence), and passive and active imaging. Each technique is useful for measuring a specific property, and its use is limited to a certain time span during the LPP evolution because of the sensitivity issues related to the selected measuring tool. Therefore, multiple diagnostic tools are essential for a comprehensive insight into the entire plasma behavior. In recent times, the improvements in performance in the lasers and detector systems expanded the capability of the aforementioned passive and active diagnostics tools. This review provides an overview of optical diagnostic tools frequently employed for the characterization of the LPPs and emphasizes techniques, associated assumptions, and challenges.

pdf/optical-diagnostics-of-laser-produced-plasmas-15d6noig.pdf

Optical emission spectroscopy of laser-produced plasmas, commonly known as laser-induced breakdown spectroscopy (LIBS), is an emerging analytical tool for rapid soil analysis. However, speciﬁc challenges with LIBS exist, such as matrix effects and quantiﬁcation issues, that require further study in the application of LIBS, particularly for analysis of heterogeneous samples such as soils. Advancements in the applications of Machine Learning (ML) methods can address some of these issues, advancing the potential for LIBS in soil analysis. This article aims to review the progress of LIBS application combined with ML methods, focusing on methodological approaches used in reducing matrix effect, feature selection, quantiﬁcation analysis, soil classiﬁcation, and self-absorption. The performance of various adopted ML approaches is discussed, including their shortcomings and advantages, to provide researchers with a clear picture of the current status of ML applications in LIBS for improving its analytical capability. The challenges and prospects of LIBS development in soil analysis are proposed, offering a path toward future research. This review article emphasize ML tools for LIBS soil analysis that are broadly relevant for other LIBS applications.

Progress Toward Machine Learning Methodologies for Laser-Induced Breakdown Spectroscopy With an Emphasis on Soil Analysis

We report spatio-temporal evolution of emission and absorption signatures of Al species in a nanosecond (ns) laserproduced plasma (LPP). The plasmas were generated from an Inconel target,which contained ∼ 0.4 wt. % Al, using 1064 nm, ≈ 6 ns full width half maximum pulses from an Nd:YAG laser at an Ar cover gas pressure of ≈ 34 Torr. The temporal distributions of the Al I (394.4 nm) transition were collected from various spatial points within the plasma employing time-of-flight (TOF) emission and laser absorption spectroscopy and they provide kinetics of the excited state and ground state population of the selected transition. The emission and absorption signatures showed multiple peaks in their temporal profiles, although they appeared at different spatial locations and times after the plasma onset. The absorption temporal profiles showed an early time signature representing shock wave propagation into the ambient gas. We also used emission and absorption spectral features for measuring various physical properties of the plasma. The absorption spectral profiles are utilized for measuring linewidths, column density and kinetic temperature while emission spectra were used to measure excitation temperature. A comparison between excitation and kinetic temperature were made at various spatial points in the plasma. Our results highlight that the TOF measurements provide a resourceful tool for showing the spatio-temporal LPP dynamics with higher spatial and temporal resolution than is possible with spectral measurements, but are difficult to interpret without additional information on excitation temperatures and linewidths. The combination of absorption and emission TOF and spectral measurements thus provides a more complete picture of LPP spatio-temporal dynamics than is possible using any one technique alone.

https://aip.scitation.org/doi/10.1063/5.0081597

Spatiotemporal evolution of emission and absorption signatures in a laser-produced plasma

Thermocouple Temperature Measurements in Metalized Explosive Fireballs

Optical spectroscopy and modeling of uranium gas-phase oxidation: Progress and perspectives

Measurement of temperature and water vapor concentration using laser absorption spectroscopy in kilogram-scale explosive fireballs

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Optical spectroscopy and modeling of uranium gas-phase oxidation: Progress and perspectives

Measurement of temperature and water vapor concentration using laser absorption spectroscopy in kilogram-scale explosive fireballs